View source for History of life

{{Short description|none}}
{{Redirect-distinguish|History of evolution|History of evolutionary thought}}
{{Redirect|Prehistoric life|the book|Prehistoric Life (book){{!}}''Prehistoric Life'' (book)}}
{{For timeline|Timeline of the evolutionary history of life}}
{{Good article}}
{{Evolutionary biology}}

The '''history of life''' on [[Earth]] traces the processes by which living and extinct [[organism]]s evolved, from the earliest [[Abiogenesis|emergence of life]] to the present day. Earth formed about 4.5 billion years ago (abbreviated as ''Ga'', for ''[[gigaannum]]'') and evidence suggests that life emerged prior to 3.7 Ga.<ref name="Pearce 343–364">{{cite journal |last1=Pearce |first1=Ben K. D. |last2=Tupper |first2=Andrew S. |last3=Pudritz |first3=Ralph E. |author3-link=Ralph Pudritz |last4=Higgs |first4=Paul G. |display-authors=3 |date=March 1, 2018 |title=Constraining the Time Interval for the Origin of Life on Earth |journal=[[Astrobiology (journal)|Astrobiology]] |volume=18 |issue=3 |pages=343–364 |arxiv=1808.09460 |s2cid=4419671 |bibcode=2018AsBio..18..343P |doi=10.1089/ast.2017.1674 |issn=1531-1074 |pmid=29570409}}</ref><ref name="Rosing 674–676">{{cite journal |last=Rosing |first=Minik T. |date=January 29, 1999 |title=<sup>13</sup>C-Depleted Carbon Microparticles in &gt;3700-Ma Sea-Floor Sedimentary Rocks from West Greenland |journal=[[Science (journal)|Science]] |volume=283 |issue=5402 |pages=674–676 |bibcode=1999Sci...283..674R |doi=10.1126/science.283.5402.674 |issn=0036-8075 |pmid=9924024}}</ref><ref name="Ohtomo 25–28">{{cite journal |last1=Ohtomo |first1=Yoko |last2=Kakegawa |first2=Takeshi |last3=Ishida |first3=Akizumi |last4=Nagase |first4=Toshiro |last5=Rosing |first5=Minik T. |display-authors=3 |date=January 2014 |title=Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks |journal=[[Nature Geoscience]] |volume=7 |issue=1 |pages=25–28 |bibcode=2014NatGe...7...25O |doi=10.1038/ngeo2025 |issn=1752-0894}}</ref> The similarities among all known present-day [[species]] indicate that they have diverged through the process of [[evolution]] from a [[common ancestor]].<ref>{{harvnb|Futuyma|2005}}</ref>

The earliest clear evidence of life comes from [[biogenic]] [[Δ13C|carbon signatures]]<ref name="Rosing 674–676" /><ref name="Ohtomo 25–28" /> and [[stromatolite]] fossils<ref>{{cite journal |last1=Nutman |first1=Allen P. |last2=Bennett |first2=Vickie C. |last3=Friend |first3=Clark R. L. |last4=Van Kranendonk |first4=Martin J. |last5=Chivas |first5=Allan R. |display-authors=3 |date=September 22, 2016 |title=Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures |url=https://ro.uow.edu.au/cgi/viewcontent.cgi?article=5181&context=smhpapers |url-status=live |format=PDF |journal=[[Nature (journal)|Nature]] |volume=537 |issue=7621 |pages=535–538 |bibcode=2016Natur.537..535N |s2cid=205250494 |doi=10.1038/nature19355 |issn=0028-0836 |pmid=27580034 |archive-url=https://web.archive.org/web/20200102053302/https://ro.uow.edu.au/cgi/viewcontent.cgi?article=5181&context=smhpapers |archive-date=2020-01-02 |access-date=2020-02-17}}</ref> discovered in 3.7 billion-year-old [[metasediment]]ary rocks from western [[Greenland]]. In 2015, possible "remains of [[biotic life]]" were found in 4.1 billion-year-old rocks in [[Western Australia]].<ref name="AP-20151019">{{cite news |last=Borenstein |first=Seth |date=October 19, 2015 |title=Hints of life on what was thought to be desolate early Earth |url=https://apnews.com/e6be2537b4cd46ffb9c0585bae2b2e51 |url-status=live |work=[[Associated Press]] |archive-url=https://web.archive.org/web/20180712123134/https://apnews.com/e6be2537b4cd46ffb9c0585bae2b2e51 |archive-date=2018-07-12 |access-date=2020-02-17}}</ref><ref name="PNAS-20151124-pdf">{{cite journal |last1=Bell |first1=Elizabeth A. |last2=Boehnke |first2=Patrick |last3=Harrison |first3=T. Mark |last4=Mao |first4=Wendy L. |display-authors=3 |date=November 24, 2015 |title=Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon |url=https://www.pnas.org/content/pnas/early/2015/10/14/1517557112.full.pdf |url-status=live |journal=[[Proceedings of the National Academy of Sciences]] |volume=112 |issue=47 |pages=14518–14521 |bibcode=2015PNAS..11214518B |doi=10.1073/pnas.1517557112 |issn=0027-8424 |pmc=4664351 |pmid=26483481 |archive-url=https://web.archive.org/web/20200213002627/https://www.pnas.org/content/pnas/early/2015/10/14/1517557112.full.pdf |archive-date=2020-02-13 |access-date=2020-02-14 |doi-access=free}}</ref> There is further evidence of possibly the oldest forms of life  in the form of fossilized [[microorganism]]s in [[hydrothermal vent]] precipitates from the [[Nuvvuagittuq Belt]], that may have lived as early as 4.28 billion years ago, not long after the [[Origin of water on Earth#History of water on Earth|oceans formed]] 4.4 billion years ago, and after the [[Age of Earth|Earth formed]] 4.54 billion years ago.<ref name="NAT-20170301" /><ref name="NYT-20170301">{{cite news |last=Zimmer |first=Carl |author-link=Carl Zimmer |date=March 1, 2017 |title=Scientists Say Canadian Bacteria Fossils May Be Earth's Oldest |url=https://www.nytimes.com/2017/03/01/science/earths-oldest-bacteria-fossils.html |department=Matter |url-status=live |newspaper=[[The New York Times]] |location=New York |issn=0362-4331 |archive-url=https://web.archive.org/web/20200104080331/https://www.nytimes.com/2017/03/01/science/earths-oldest-bacteria-fossils.html |archive-date=2020-01-04 |access-date=2017-03-02}} "A version of this article appears in print on March 2, 2017, Section A, Page 9 of the New York edition with the headline: Artful Squiggles in Rocks May Be Earth's Oldest Fossils."</ref> These earliest fossils, however, may have originated from non-biological processes.<ref name="Pearce 343–364" /><ref>{{cite journal |last1=Papineau |first1=Dominic |last2=De Gregorio |first2=Bradley T. |last3=Cody |first3=George D. |last4=O'Neil |first4=J. |last5=Steele |first5=A. |last6=Stroud |first6=R. M. |last7=Fogel |first7=M. L. |display-authors=3 |date=June 2011 |title=Young poorly crystalline graphite in the &gt;3.8-Gyr-old Nuvvuagittuq banded iron formation |journal=[[Nature Geoscience]] |volume=4 |issue=6 |pages=376–379 |bibcode=2011NatGe...4..376P |doi=10.1038/ngeo1155 |issn=1752-0894}}</ref><ref name="PNAS-20151124-pdf" /><ref>{{cite journal |last1=Nemchin |first1=Alexander A. |last2=Whitehouse |first2=Martin J. |last3=Menneken |first3=Martina |last4=Geisler |first4=Thorsten |last5=Pidgeon |first5=Robert T. |last6=Wilde |first6=Simon A. |display-authors=3 |date=July 3, 2008 |title=A light carbon reservoir recorded in zircon-hosted diamond from the Jack Hills |journal=[[Nature (journal)|Nature]] |volume=454 |issue=7200 |pages=92–95 |bibcode=2008Natur.454...92N |doi=10.1038/nature07102 |issn=0028-0836 |pmid=18596808 |s2cid=4415308}}</ref>

[[Microbial mat]]s of coexisting [[bacteria]] and [[archaea]] were the dominant form of life in the early [[Archean]] eon, and many of the major steps in early evolution are thought to have taken place in this environment.<ref name="NisbetFowler1999">{{cite journal |last1=Nisbet |first1=Euan G. |last2=Fowler |first2=C. M. R. |author2-link=Mary Fowler (geologist) |date=December 7, 1999 |title=Archaean metabolic evolution of microbial mats |journal=[[Proceedings of the Royal Society B]] |volume=266 |issue=1436 |pages=2375–2382 |doi=10.1098/rspb.1999.0934 |issn=0962-8452 |pmc=1690475}}</ref> The evolution of [[photosynthesis]] by [[cyanobacteria]], around 3.5 Ga, eventually led to a buildup of its waste product, [[oxygen]], in the oceans. After free oxygen saturated all available [[reductant]] substances on the [[Earth's surface]], it built up in the atmosphere, leading to the [[Great Oxygenation Event]] around 2.4 Ga.<ref name="Anbar2007">{{cite journal |last1=Anbar |first1=Ariel D. |last2=Yun |first2=Duan |last3=Lyons |first3=Timothy W. |last4=Arnold |first4=Gail L. |last5=Kendall |first5=Brian |last6=Creaser |first6=Robert A. |last7=Kaufman |first7=Alan J. |last8=Gordon |first8=Gwyneth W. |last9=Scott |first9=Clinton |last10=Garvin |first10=Jessica |last11=Buick |first11=Roger |display-authors=3 |date=September 28, 2007 |title=A Whiff of Oxygen Before the Great Oxidation Event? |journal=[[Science (journal)|Science]] |volume=317 |issue=5846 |pages=1903–1906 |bibcode=2007Sci...317.1903A |doi=10.1126/science.1140325 |s2cid=25260892 |issn=0036-8075 |pmid=17901330}}</ref> The earliest evidence of [[eukaryote]]s (complex [[cell (biology)|cell]]s with [[organelle]]s) dates from 1.85 Ga,<ref>{{cite journal |last1=Knoll |first1=Andrew H. |author-link=Andrew H. Knoll |last2=Javaux |first2=Emmanuelle J. |last3=Hewitt |first3=David |last4=Cohen |first4=Phoebe |display-authors=3 |date=June 29, 2006 |title=Eukaryotic organisms in Proterozoic oceans |journal=[[Philosophical Transactions of the Royal Society B]] |volume=361 |issue=1470 |pages=1023–1038 |doi=10.1098/rstb.2006.1843 |issn=0962-8436 |pmc=1578724 |pmid=16754612}}</ref><ref name="Fedonkin2003" /> likely due to [[symbiogenesis]] between [[anaerobe|anaerobic]] archaea and [[aerobe|aerobic]] [[proteobacteria]] in co-adaptation against the new [[oxidative stress]]. While eukaryotes may have been present earlier, their diversification accelerated when aerobic [[cellular respiration]] by the [[endosymbiont]] [[mitochondria]] provided a more abundant source of [[biological energy]]. Around 1.6 Ga, some eukaryotes gained the ability to photosynthesize via endosymbiosis with cyanobacteria, and gave rise to various [[algae]] that eventually overtook cyanobacteria as the dominant [[primary producer]]s.

At around 1.7 Ga, [[multicellular organism]]s began to appear, with [[differentiated cell]]s performing specialised functions.<ref name="Bonner1999" /> While early organisms reproduced [[asexual reproduction|asexually]], the primary method of reproduction for the vast majority of [[macroscopic]] organisms, including almost all [[eukaryotes]] (which includes [[animal]]s and [[plant]]s), is [[sexual reproduction]], the fusion of male and female reproductive cells ([[gamete]]s) to create a [[zygote]].<ref>{{cite journal |last1=Otto |first1=Sarah P. |author1-link=Sarah Otto |last2=Lenormand |first2=Thomas |date=April 1, 2002 |title=Resolving the paradox of sex and recombination |journal=[[Nature Reviews Genetics]] |volume=3 |issue=4 |pages=252–261 |doi=10.1038/nrg761 |s2cid=13502795 |issn=1471-0056 |pmid=11967550}}</ref> The origin and [[evolution of sexual reproduction]] remain a puzzle for biologists, though it is thought to have evolved from a single-celled eukaryotic ancestor.<ref name="Letunic I and Bork P">{{cite web |url=https://itol.embl.de/ |title=iTOL: Interactive Tree of Life |last1=Letunic |first1=Ivica |last2=Bork |first2=Peer |author2-link=Peer Bork |publisher=[[European Molecular Biology Laboratory]] |location=Heidelberg, Germany |access-date=2015-07-21 |archive-date=2022-06-10 |archive-url=https://web.archive.org/web/20220610171813/https://itol.embl.de/ |url-status=live}}
*{{cite journal |last1=Letunic |first1=Ivica |last2=Bork |first2=Peer |author2-link=Peer Bork |date=January 1, 2007 |title=Interactive Tree Of Life (iTOL): an online tool for phylogenetic tree display and annotation |url=https://itol.embl.de/help/17050570.pdf |journal=[[Bioinformatics (journal)|Bioinformatics]] |volume=23 |issue=1 |pages=127–128 |doi=10.1093/bioinformatics/btl529 |issn=1367-4803 |pmid=17050570 |access-date=2015-07-21 |doi-access=free |archive-date=2022-05-19 |archive-url=https://web.archive.org/web/20220519215247/https://itol.embl.de/help/17050570.pdf |url-status=live}}
*{{cite journal |last1=Letunic |first1=Ivica |last2=Bork |first2=Peer |author2-link=Peer Bork |date=July 1, 2011 |title=Interactive Tree Of Life v2: online annotation and display of phylogenetic trees made easy |url=https://itol.embl.de/help/gkr201.pdf |journal=[[Nucleic Acids Research]] |volume=39 |issue=Suppl. 2 |pages=W475–W478 |doi=10.1093/nar/gkr201 |issn=0305-1048 |pmc=3125724 |pmid=21470960 |access-date=2015-07-21 |archive-date=2021-11-01 |archive-url=https://web.archive.org/web/20211101090511/https://itol.embl.de/help/gkr201.pdf |url-status=live}}</ref>

While [[microorganism]]s formed the earliest [[terrestrial ecosystem]]s at least 2.7 Ga, the [[evolution of plants]] from freshwater [[green algae]] dates back to about 1 billion years ago.<ref>{{cite journal |last1=Strother |first1=Paul K. |last2=Battison |first2=Leila |last3=Brasier |first3=Martin D. |author3-link=Martin Brasier |last4=Wellman |first4=Charles H. |display-authors=3 |date=May 26, 2011 |title=Earth's earliest non-marine eukaryotes |journal=[[Nature (journal)|Nature]] |volume=473 |issue=7348 |pages=505–509 |bibcode=2011Natur.473..505S |doi=10.1038/nature09943 |s2cid=4418860 |issn=0028-0836 |pmid=21490597}}</ref><ref>{{cite journal |last=Beraldi-Campesi |first=Hugo |date=February 23, 2013 |title=Early life on land and the first terrestrial ecosystems |journal=Ecological Processes |volume=2 |issue=1 |pages=1–17 |doi=10.1186/2192-1709-2-1 |doi-access=free |bibcode=2013EcoPr...2....1B |issn=2192-1709}}</ref> Microorganisms are thought to have paved the way for the inception of land plants in the [[Ordovician]] period. Land plants were so successful that they are thought to have contributed to the [[Late Devonian extinction event]]<ref name="AlgeoScheckler1998" /> as early tree ''[[Archaeopteris]]'' drew down CO<sub>2</sub> levels, leading to [[global cooling]] and lowered sea levels, while their roots increased rock weathering and nutrient run-offs which may have triggered [[algal bloom]] [[anoxic event]]s.

[[Bilateria]], animals having a left and a right side that are mirror images of each other, appeared by 555 Ma (million years ago).<ref>{{cite journal |last1=Fedonkin |first1=Mikhail A. |author1-link=Mikhail Fedonkin |last2=Simonetta |first2=Alberto |last3=Ivantsov |first3=Andrei Yu |date=January 1, 2007 |title=New data on ''Kimberella'', the Vendian mollusc-like organism (White Sea region, Russia): palaeoecological and evolutionary implications |url=https://www.monash.edu/__data/assets/pdf_file/0010/75673/fedonkin2.pdf |url-status=live |journal=Geological Society Special Publications |volume=286 |issue=1 |pages=157–179 |bibcode=2007GSLSP.286..157F |doi=10.1144/SP286.12 |issn=0375-6440 |s2cid=331187 |archive-url=https://web.archive.org/web/20170811081056/https://www.monash.edu/__data/assets/pdf_file/0010/75673/fedonkin2.pdf |archive-date=2017-08-11 |access-date=2020-02-18}}</ref> [[Ediacara biota]] appeared during the [[Ediacaran]] period,<ref>{{cite journal |last1=Jun-Yuan |first1=Chen |last2=Oliveri |first2=Paola |last3=Chia-Wei |first3=Li |author4=Gui-Qing Zhou |author5=Feng Gao |last6=Hagadorn |first6=James W. |last7=Peterson |first7=Kevin J. |last8=Davidson |first8=Eric H. |display-authors=3 |date=April 25, 2000 |title=Precambrian animal diversity: Putative phosphatized embryos from the Doushantuo Formation of China |journal=[[Proc. Natl. Acad. Sci. U.S.A.]] |volume=97 |issue=9 |pages=4457–4462 |bibcode=2000PNAS...97.4457C |doi=10.1073/pnas.97.9.4457 |issn=0027-8424 |pmid=10781044 |pmc=18256 |doi-access=free}}</ref> while [[vertebrate]]s, along with most other modern [[phylum|phyla]] originated about {{ma|525|Ma|Cambrian}} during the [[Cambrian explosion]].<ref name="D-G.Shu et al. 1999">{{cite journal |last1=D-G. |first1=Shu |last2=H-L. |first2=Luo |last3=Conway Morris |first3=Simon |author3-link=Simon Conway Morris |author4=X-L. Zhang |author5=S-X. Hu |author6=L. Chen |author7=J. Han |author8=M. Zhu |author9=Y. Li |author10=L-Z. Chen |display-authors=3 |date=November 4, 1999 |title=Lower Cambrian vertebrates from south China |url=http://www.bios.niu.edu/davis/bios458/Shu1.pdf |journal=[[Nature (journal)|Nature]] |volume=402 |pages=42–46 |doi=10.1038/46965 |issue=6757 |bibcode=1999Natur.402...42S |s2cid=4402854 |issn=0028-0836 |archive-url=https://web.archive.org/web/20090226122732/http://www.bios.niu.edu/davis/bios458/Shu1.pdf |archive-date=2009-02-26 |access-date=2015-01-22}}</ref> During the [[Permian]] period, [[synapsid]]s, including the ancestors of [[mammal]]s, dominated the land.<ref>{{cite web |url=http://www.csupomona.edu/~dfhoyt/classes/zoo138/SYNAPSID.HTML |title=Synapsid Reptiles |last=Hoyt |first=Donald F. |date=February 17, 1997 |website=ZOO 138 Vertebrate Zoology |publisher=[[California State Polytechnic University, Pomona]] |location=Pomona, CA |type=Lecture |archive-url=https://web.archive.org/web/20090520072737/http://www.csupomona.edu/~dfhoyt/classes/zoo138/SYNAPSID.HTML |archive-date=2009-05-20 |access-date=2015-01-22}}</ref>

The [[Permian–Triassic extinction event]] killed most complex species of its time, {{ma|252|Ma}}.<ref>{{cite web |url=https://science.nasa.gov/science-news/science-at-nasa/2002/28jan_extinction/ |url-status=live |title=The Great Dying |last=Barry |first=Patrick L. |date=January 28, 2002 |editor-last=Phillips |editor-first=Tony |work=Science@NASA |publisher=[[Marshall Space Flight Center]] |archive-url=https://web.archive.org/web/20100410015208/https://science.nasa.gov/science-news/science-at-nasa/2002/28jan_extinction/ |archive-date=2010-04-10 |access-date=2015-01-22}}</ref> During the recovery from this catastrophe, [[archosaur]]s became the most abundant land vertebrates;<ref name="TannerLucas2004">{{cite journal |last1=Tanner |first1=Lawrence H. |last2=Lucas |first2=Spencer G. |author2-link=Spencer G. Lucas |last3=Chapman |first3=Mary G. |date=March 2004 |title=Assessing the record and causes of Late Triassic extinctions |url=http://nmnaturalhistory.org/pdf_files/TJB.pdf |journal=[[Earth-Science Reviews]] |volume=65 |issue=1–2 |pages=103–139 |bibcode=2004ESRv...65..103T |doi=10.1016/S0012-8252(03)00082-5 |archive-url=https://web.archive.org/web/20071025225841/http://nmnaturalhistory.org/pdf_files/TJB.pdf |archive-date=2007-10-25 |access-date=2007-10-22}}</ref> one archosaur group, the [[dinosaur]]s, dominated the [[Jurassic]] and [[Cretaceous]] periods.<ref>{{harvnb|Benton|1997}}</ref> After the [[Cretaceous–Paleogene extinction event]] {{ma|Paleogene|Ma}} killed off the non-avian dinosaurs,<ref>{{cite journal |last1=Fastovsky |first1=David E. |last2=Sheehan |first2=Peter M. |date=March 2005 |title=The Extinction of the Dinosaurs in North America |url=https://www.geosociety.org/gsatoday/archive/15/3/pdf/i1052-5173-15-3-4.pdf |url-status=live |journal=[[GSA Today]] |volume=15 |issue=3 |pages=4–10 |doi=10.1130/1052-5173(2005)015<4:TEOTDI>2.0.CO;2 |issn=1052-5173 |archive-url=https://web.archive.org/web/20190322190338/https://www.geosociety.org/gsatoday/archive/15/3/pdf/i1052-5173-15-3-4.pdf |archive-date=2019-03-22 |access-date=2015-01-23}}</ref> mammals [[Adaptive radiation|increased rapidly in size and diversity]].<ref>{{cite news |last=Roach |first=John |date=June 20, 2007 |title=Dinosaur Extinction Spurred Rise of Modern Mammals |url=https://news.nationalgeographic.com/news/2007/06/070620-mammals-dinos.html |work=National Geographic News |location=Washington, D.C. |publisher=[[National Geographic Society]] |archive-url=https://web.archive.org/web/20080511161825/https://news.nationalgeographic.com/news/2007/06/070620-mammals-dinos.html |archive-date=2008-05-11 |access-date=2020-02-21}}
*{{cite journal |last1=Wible |first1=John R. |last2=Rougier |first2=Guillermo W. |last3=Novacek |first3=Michael J. |last4=Asher |first4=Robert J. |display-authors=3 |date=June 21, 2007 |title=Cretaceous eutherians and Laurasian origin for placental mammals near the K/T boundary |journal=[[Nature (journal)|Nature]] |volume=447 |issue=7147 |pages=1003–1006 |bibcode=2007Natur.447.1003W |doi=10.1038/nature05854 |issn=0028-0836 |pmid=17581585 |s2cid=4334424}}</ref> Such [[mass extinction]]s may have accelerated evolution by providing opportunities for new groups of organisms to diversify.<ref name="Van Valkenburgh 1999 463–493">{{cite journal |last=Van Valkenburgh |first=Blaire |author-link=Blaire Van Valkenburgh |date=May 1, 1999 |title=Major Patterns in the History of Carnivorous Mammals |url=https://zenodo.org/record/890156 |journal=[[Annual Review of Earth and Planetary Sciences]] |volume=27 |pages=463–493 |bibcode=1999AREPS..27..463V |doi=10.1146/annurev.earth.27.1.463 |issn=1545-4495 |access-date=August 3, 2019 |archive-date=February 29, 2020 |archive-url=https://web.archive.org/web/20200229201201/https://zenodo.org/record/890156 |url-status=live}}</ref>

Only a very small percentage of species have been identified: one estimate claims that Earth may have 1 trillion species, because "identifying every [[microorganism|microbial species]] on Earth presents a huge challenge."<ref name="NSF-2016002">{{cite press release |last1=Dybas |first1=Cheryl |last2=Fryling |first2=Kevin |date=May 2, 2016 |title=Researchers find that Earth may be home to 1 trillion species: Largest analysis of microbial data reveals that 99.999 percent of all species remain undiscovered |url=https://www.nsf.gov/news/news_summ.jsp?cntn_id=138446 |url-status=live |location=Alexandria, VA |publisher=[[National Science Foundation]] |id=News Release 16-052 |archive-url=https://web.archive.org/web/20160504111108/https://www.nsf.gov/news/news_summ.jsp?cntn_id=138446 |quote="Our results show that this leaves 100,000 times more microorganisms awaiting discovery -- and 100 million to be fully explored.<!--"Microbial biodiversity, it appears, is greater than we ever imagined."--> |archive-date=2016-05-04 |access-date=2016-12-11}}</ref><ref>{{cite journal |last1=Locey |first1=Kenneth J. |last2=Lennon |first2=Jay T. |date=May 24, 2016 |title=Scaling laws predict global microbial diversity |journal=[[Proc. Natl. Acad. Sci. U.S.A.]] |volume=113 |issue=21 |pages=5970–5975 |bibcode=2016PNAS..113.5970L |doi=10.1073/pnas.1521291113 |issn=0027-8424 |pmc=4889364 |pmid=27140646 |doi-access=free}}</ref> Only 1.75–1.8 million species have been named{{sfn|Chapman|2009}}<ref name="NYT-20141108-MJN">{{cite news |last=Novacek |first=Michael J. |date=November 8, 2014 |title=Prehistory's Brilliant Future |url=https://www.nytimes.com/2014/11/09/opinion/sunday/prehistorys-brilliant-future.html |url-status=live |archive-url=https://web.archive.org/web/20141110003127/https://www.nytimes.com/2014/11/09/opinion/sunday/prehistorys-brilliant-future.html |archive-date=2014-11-10 |access-date=2014-12-25 |newspaper=[[The New York Times]] |department=[[Sunday Review]] |location=New York |issn=0362-4331}} "A version of this article appears in print on November 9, 2014, Section SR, Page 6 of the New York edition with the headline: Prehistory's Brilliant Future."</ref> and 1.8 million documented in a central database.<ref name="col2016">{{cite web |year=2019 |title=Catalogue of Life: 2019 Annual Checklist |url=http://www.catalogueoflife.org/annual-checklist/2019/info/ac |url-status=live |archive-url=https://web.archive.org/web/20201007184209/http://www.catalogueoflife.org/annual-checklist/2019/info/ac |archive-date=2020-10-07 |access-date=2020-02-16 |publisher=[[Species 2000]]; [[Integrated Taxonomic Information System]]}}</ref> The currently living species represent less than one percent of all species that have ever lived on Earth.{{sfn|McKinney|1997|p=[https://books.google.com/books?id=4LHnCAAAQBAJ&pg=PA110 110]}}{{sfn|Stearns|Stearns|1999|p=[https://books.google.com/books?id=0BHeC-tXIB4C&q=99%20percent x]}}

{{Life timeline}}

== Earliest history of Earth ==
{{Graphical timeline

 | help=off
 | embedded=

 | title=History of Earth and its life
 | title-colour={{period color|Evolutionary history of life}}

 | from=-{{period start|Hadean}}
 | to=0

 | width=14
 | annotations-width=16

 | period1=Hadean
 | period1-text=[[Hadean]]
 | period2=Archean
 | period2-text=[[Archean]]
 | period3=Proterozoic
 | period3-text=[[Proterozoic|Protero<br />-zoic]]
 | period3-nudge-up=0.5
 | period4=Phanerozoic
 | period4-text=[[Phanerozoic|Phanero<br />-zoic]]
 | period4-nudge-up=0.1
 | period1-right=0.30
 | period2-right=0.30
 | period3-right=0.30
 | period4-right=0.30

 | period5      = Eoarchean
 | period5-text = [[Eoarchean|Eo]]
 | period6      = Paleoarchean
 | period6-text = [[Paleoarchean|Paleo]]
 | period7      = Mesoarchean
 | period7-text = [[Mesoarchean|Meso]]
 | period8      = Neoarchean
 | period8-text = [[Neoarchean|Neo]]

 | period5-left=0.305
 | period6-left=0.305
 | period7-left=0.305
 | period8-left=0.305
 | period5-right=0.55
 | period6-right=0.55
 | period7-right=0.55
 | period8-right=0.55

 | period9       = Paleoproterozoic
 | period9-text  = [[Paleoproterozoic|Paleo]]
 | period10      = Mesoproterozoic
 | period10-text = [[Mesoproterozoic|Meso]]
 | period11      = Neoproterozoic
 | period11-text = [[Neoproterozoic|Neo]]

 | period12      = Paleozoic
 | period12-text = [[Paleozoic|Paleo]]
 | period13      = Mesozoic
 | period13-text = [[Mesozoic|Meso]]
 | period14      = Cenozoic
 | period14-text = [[Cenozoic|Ceno]]

 | period9-left=0.305
 | period10-left=0.305
 | period11-left=0.305
 | period12-left=0.305
 | period13-left=0.305
 | period14-left=0.305
 | period9-right=0.55
 | period10-right=0.55
 | period11-right=0.55
 | period12-right=0.55
 | period13-right=0.55
 | period14-right=0.55

 | period1-border-width=0.04
 | period2-border-width=0.04
 | period3-border-width=0.04
 | period4-border-width=0.04
 | period5-border-width=0.04
 | period6-border-width=0.04
 | period7-border-width=0.04
 | period8-border-width=0.04
 | period9-border-width=0.04
 | period10-border-width=0.04
 | period11-border-width=0.04
 | period12-border-width=0.04
 | period13-border-width=0.04
 | period14-border-width=0.04
 | period15-border-width=0.04

 | period15 = 
 | period15-right=-0.01
 | period15-left=-0.08
 | period15-colour=#393
 | period15-text=
 | period15-border-colour=#c33

<!--black vertical bars -->
 | bar1-colour=black
 | bar1-left = 0.30
 | bar1-right = 0.305

 | bar2-colour=black
 | bar2-left=0.551
 | bar2-right = 0.554
 | bar2-from=-{{period start|eoarchean}}

<!-- Annotations, & occasional bar-->
|note1=[[Earth]] and [[Solar System]] formed
|note1-at=-4550
|note1-nudge-left=5.5
|note1-nudge-down=0.0 <!--- adj from 0.7 to 0.0 -- seems better --->
|note2= <!--- [[Giant impact hypothesis|Impact]] formed [[Moon]] -- overlaps other titles --->
|note2-at=-4510
|note2-nudge-left=5.5
|note2-nudge-down=0.3
|note3=Cool surface, oceans, atmosphere
|note3-at=-4450
|note3-nudge-left=5.5
|note3-nudge-up=0.0
|note4=[[Late Heavy Bombardment]]
|note4-at=-3900
|note4-nudge-left=5.5
|note4-nudge-down=0.0 <!--- adj from 0.3 to 0.0 -- seems better --->
| bar4-colour=gray
| bar4-left=0.555
| bar4-right = 0.60
| bar4-from=-4000
| bar4-to=-3800
|note5=[[Abiogenesis|Earliest evidence of life (-4100)]]
|note5-at=-4100
|note5-nudge-left=5.5
|note5-nudge-down=0.0
|note6=[[Oxygen catastrophe|Oxygenation]] of atmosphere
|note6-at=-2400
|note6-nudge-left=5.5
|note6-nudge-down=0.0 <!--- adj from 0.3 to 0.0 -- seems better --->
|note7=Earliest indisputable [[multicellular organism]]<ref name="PTRS-2015">{{cite journal |last=Erwin |first=Douglas H. |author-link=Douglas Erwin |date=December 19, 2015 |title=Early metazoan life: divergence, environment and ecology |journal=[[Philosophical Transactions of the Royal Society B]] |volume=370 |issue=1684 |doi=10.1098/rstb.2015.0036 |issn=0962-8436 |pmc=4650120 |pmid=26554036 |bibcode=2015RSPTB.37050036E |id=Article 20150036}}</ref><ref name="El Albani2010">{{cite journal |last1=El Albani |first1=Abderrazak |author1-link=Abderrazak El Albani |last2=Bengtson |first2=Stefan |last3=Canfield |first3=Donald E. |author3-link=Donald Canfield |last4=Bekker |first4=Andrey |last5=Macchiarelli |first5=Reberto |last6=Mazurier |first6=Arnaud |last7=Hammarlund |first7=Emma U. |last8=Boulvais |first8=Philippe |last9=Dupuy |first9=Jean-Jacques |display-authors=3 |date=July 1, 2010 |title=Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago |journal=[[Nature (journal)|Nature]] |volume=466 |issue=7302 |pages=100–104 |bibcode=2010Natur.466..100A |doi=10.1038/nature09166 |s2cid=4331375 |issn=0028-0836 |pmid=20596019}}</ref><ref>{{cite journal |url=https://www.cambridge.org/core/journals/paleobiology/article/bangiomorpha-pubescens-n-gen-n-sp-implications-for-the-evolution-of-sex-multicellularity-and-the-mesoproterozoicneoproterozoic-radiation-of-eukaryotes/E61F0C87E1F2CD5A622AEF57ADCC97EF |doi=10.1666/0094-8373(2000)026<0386:BPNGNS>2.0.CO;2 |title=Bangiomorpha pubescensn. Gen., n. Sp.: Implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes |year=2000 |last1=Butterfield |first1=Nicholas J. |journal=Paleobiology |volume=26 |issue=3 |pages=386–404 |bibcode=2000Pbio...26..386B |s2cid=36648568 |access-date=2022-11-09 |archive-date=2020-11-12 |archive-url=https://web.archive.org/web/20201112014759/https://www.cambridge.org/core/journals/paleobiology/article/bangiomorpha-pubescens-n-gen-n-sp-implications-for-the-evolution-of-sex-multicellularity-and-the-mesoproterozoicneoproterozoic-radiation-of-eukaryotes/E61F0C87E1F2CD5A622AEF57ADCC97EF |url-status=live|url-access=subscription }}</ref><ref>{{cite journal |last1=Butterfield |first1=Nicholas J. |title=Bangiomorpha pubescensn. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes |journal=Paleobiology |date=September 2000 |volume=26 |issue=3 |pages=386–404 |doi=10.1666/0094-8373(2000)026<0386:BPNGNS>2.0.CO;2 |bibcode=2000Pbio...26..386B |s2cid=36648568 |url=https://www.researchgate.net/publication/278693786 |access-date=22 December 2022 |language=en}}</ref>
|note7-at=-1047
|note7-nudge-left=5.5
|note7-nudge-down=0.0
|note8= <!--- Earliest known [[fungi]] -- overlaps other titles  --->
|note8-at=-0430
|note8-nudge-left=5.5
|note8-nudge-down=0.3
|note9= <!--- Earliest known [[cnidaria]]ns -- overlaps other titles --->
|note9-at=-580
|note9-nudge-left=5.5
|note9-nudge-down=0.7
|note10=[[Cambrian explosion]]
|note10-at=-530
|note10-nudge-left=5.5
|note10-nudge-down=0.4
|note11=Earliest land [[invertebrate]]s and [[plant]]s
|note11-at=-470
|note11-nudge-left=5.5
|note11-nudge-down=0.1
|note12=Earliest land [[vertebrate]]s
|note12-at=-345
|note12-nudge-left=5.5
|note12-nudge-down=0.1
|note13=Earliest known [[dinosaur]]
|note13-at=-225
|note13-nudge-left=5.5
|note13-nudge-down=0.0
|note14=Extinction of non-avian dinosaurs
|note14-at=-65
|note14-nudge-left=5.5
|note14-nudge-down=0.0 <!--- adj from 0.3 to 0.0 -- seems better --->

 | caption=Scale: [[Megaannum|Ma]] (Millions of years)

}}
{{main|History of Earth}}
The oldest [[meteorite]] fragments found on Earth are about 4.54 billion years old; this, coupled primarily with the dating of ancient [[lead]] deposits, has put the estimated age of Earth at around that time.<ref name="ageOfEarth">{{harvnb|Dalrymple|1991}}
*{{harvnb|Newman|2007}}
*{{cite journal |last=Dalrymple |first=G. Brent |author-link=Brent Dalrymple |year=2001 |title=The age of the Earth in the twentieth century: a problem (mostly) solved |url=https://sp.lyellcollection.org/content/190/1/205.abstract |url-status=live |journal=Geological Society Special Publication |volume=190 |issue=1 |pages=205–221 |bibcode=2001GSLSP.190..205D |doi=10.1144/GSL.SP.2001.190.01.14 |s2cid=130092094 |issn=0375-6440 |archive-url=https://web.archive.org/web/20200213004845/https://sp.lyellcollection.org/content/190/1/205.abstract |archive-date=2020-02-13 |access-date=2020-02-21|url-access=subscription }}</ref> The [[Moon]] has the same composition as Earth's [[Crust (geology)|crust]] but does not contain an [[iron]]-rich [[Planetary core|core]] like the Earth's. Many scientists think that about 40 million years after the formation of Earth, it collided with [[Giant-impact hypothesis|a body the size of Mars]], throwing crust material into the orbit that formed the Moon. Another [[hypothesis]] is that the Earth and Moon started to coalesce at the same time but the Earth, having a much stronger [[gravity]] than the early Moon, attracted almost all the iron particles in the area.<ref>{{cite journal |last1=Galimov |first1=Erik M. |author1-link=Erik M. Galimov |last2=Krivtsov |first2=Anton M. |date=December 2005 |title=Origin of the Earth—Moon system |url=https://www.ias.ac.in/article/fulltext/jess/114/06/0593-0600 |journal=[[Journal of Earth System Science]] |volume=114 |issue=6 |pages=593–600 |bibcode=2005JESS..114..593G |doi=10.1007/BF02715942 |s2cid=56094186 |issn=0253-4126 |citeseerx=10.1.1.502.314 |access-date=2020-02-22 |archive-date=2021-08-13 |archive-url=https://web.archive.org/web/20210813164632/https://www.ias.ac.in/article/fulltext/jess/114/06/0593-0600 |url-status=live}}</ref>

Until 2001, the oldest rocks found on Earth were about 3.8 billion years old,<ref>{{cite news |last=Thompson |first=Andrea |date=September 25, 2008 |title=Oldest Rocks on Earth Found |url=https://www.livescience.com/2896-oldest-rocks-earth.html |work=[[Live Science]] |publisher=[[Imaginova]] |location=Watsonville, CA |access-date=2015-01-23 |archive-date=2012-06-12 |archive-url=https://web.archive.org/web/20120612013656/https://www.livescience.com/2896-oldest-rocks-earth.html |url-status=live}}</ref><ref name="ageOfEarth" /> leading scientists to estimate that the Earth's surface had been molten until then. Accordingly, they named this part of Earth's history the [[Hadean]].<ref name="Cohen2000">{{cite journal |last1=Cohen |first1=Barbara A. |author1-link=Barbara Cohen (scientist) |last2=Swindle |first2=Timothy D. |last3=Kring |first3=David A. |date=December 1, 2000 |title=Support for the Lunar Cataclysm Hypothesis from Lunar Meteorite Impact Melt Ages |journal=[[Science (journal)|Science]] |volume=290 |issue=5497 |pages=1754–1756 |bibcode=2000Sci...290.1754C |doi=10.1126/science.290.5497.1754 |issn=0036-8075 |pmid=11099411}}</ref> However, analysis of [[zircon]]s formed 4.4 Ga indicates that Earth's crust solidified about 100 million years after the planet's formation and that the planet quickly acquired oceans and an [[atmosphere]], which may have been capable of supporting life.<ref>{{cite press release |author=<!--Staff writer(s); no by-line.--> |title=Early Earth Likely Had Continents And Was Habitable |url=http://www.colorado.edu/news/releases/2005/11/17/early-earth-likely-had-continents-and-was-habitable-says-new-study |location=Boulder, CO |publisher=[[University of Colorado]] |date=November 17, 2005 |access-date=2015-01-23 |archive-url=https://web.archive.org/web/20150124005824/http://www.colorado.edu/news/releases/2005/11/17/early-earth-likely-had-continents-and-was-habitable-says-new-study |archive-date=2015-01-24}}
*{{cite journal |last1=Harrison |first1=T. Mark |last2=Blichert-Toft |first2=Janne |author2-link=Janne Blichert-Toft |last3=Müller |first3=Wolfgang |last4=Albarede |first4=F. |last5=Holden |first5=P. |last6=Mojzsis |first6=Stephen J. |display-authors=3 |date=December 23, 2005 |title=Heterogeneous Hadean Hafnium: Evidence of Continental Crust at 4.4 to 4.5 Ga |journal=[[Science (journal)|Science]] |volume=310 |issue=5756 |pages=1947–1950 |doi=10.1126/science.1117926 |issn=0036-8075 |pmid=16293721 |bibcode=2005Sci...310.1947H |doi-access=free}}</ref><ref>{{cite journal |last1=Cavosie |first1=Aaron J. |last2=Valley |first2=John W. |last3=Wilde |first3=Simon A. |author4=Edinburgh Ion Microprobe Facility |date=July 15, 2005 |title=Magmatic δ<sup>18</sup>O in 4400–3900 Ma detrital zircons: A record of the alteration and recycling of crust in the Early Archean |journal=[[Earth and Planetary Science Letters]] |volume=235 |issue=3–4 |pages=663–681 |bibcode=2005E&PSL.235..663C |doi=10.1016/j.epsl.2005.04.028 |issn=0012-821X}}</ref><ref name="Garwood2012">{{cite journal |last=Garwood |first=Russell J. |year=2012 |title=Patterns In Palaeontology: The first 3 billion years of evolution |url=https://www.palaeontologyonline.com/articles/2012/patterns-in-palaeontology-the-first-3-billion-years-of-evolution/ |url-status=live |journal=Palaeontology Online |volume=2 |issue=Article 11 |pages=1–14 |archive-url=https://web.archive.org/web/20121209062437/https://www.palaeontologyonline.com/articles/2012/patterns-in-palaeontology-the-first-3-billion-years-of-evolution/ |archive-date=2012-12-09 |access-date=2020-02-25}}</ref>

Evidence from the Moon indicates that, from 4 to 3.8 Ga, it suffered a [[Late Heavy Bombardment]] by debris that was left over from the formation of the [[Solar System]], and the Earth should have experienced an even heavier bombardment due to its stronger gravity.<ref name="Cohen2000" /><ref name="space.com-bombardment">{{cite news |last=Britt |first=Robert Roy |date=July 24, 2002 |title=Evidence for Ancient Bombardment of Earth |url=http://www.space.com/scienceastronomy/planetearth/earth_bombarded_020724.html |work=Space.com |publisher=[[Imaginova]] |location=Watsonville, CA |archive-url=https://web.archive.org/web/20060415193738/http://space.com/scienceastronomy/planetearth/earth_bombarded_020724.html |archive-date=2006-04-15 |access-date=2015-01-23}}</ref> While there is no direct evidence of conditions on Earth 4 to 3.8 Ga, there is no reason to think that the Earth was not also affected by this late heavy bombardment.<ref>{{cite journal |last1=Valley |first1=John W. |last2=Peck |first2=William H. |last3=King |first3=Elizabeth M. |last4=Wilde |first4=Simon A. |display-authors=3 |date=April 1, 2002 |title=A cool early Earth |url=http://www.geology.wisc.edu/zircon/Valley2002Cool_Early_Earth.pdf |journal=[[Geology (journal)|Geology]] |volume=30 |issue=4 |pages=351–354 |bibcode=2002Geo....30..351V |doi=10.1130/0091-7613(2002)030<0351:ACEE>2.0.CO;2 |pmid=16196254 |issn=0091-7613 |access-date=2008-09-13 |archive-date=2008-12-16 |archive-url=https://web.archive.org/web/20081216211542/http://www.geology.wisc.edu/zircon/Valley2002Cool_Early_Earth.pdf |url-status=live}}</ref> This event may well have stripped away any previous atmosphere and oceans; in this case [[gas]]es and water from [[comet]] impacts may have contributed to their replacement, although [[outgassing]] from [[volcano]]es on Earth would have supplied at least half.<ref>{{cite journal |last1=Dauphas |first1=Nicolas |last2=Robert |first2=François |author2-link=François Robert |last3=Marty |first3=Bernard |date=December 2000 |title=The Late Asteroidal and Cometary Bombardment of Earth as Recorded in Water Deuterium to Protium Ratio |journal=[[Icarus (journal)|Icarus]] |volume=148 |issue=2 |pages=508–512 |bibcode=2000Icar..148..508D |doi=10.1006/icar.2000.6489 |issn=0019-1035}}</ref> However, if subsurface microbial life had evolved by this point, it would have survived the bombardment.<ref>{{cite web |url=https://astrobiology.nasa.gov/news/microbial-habitability-during-the-late-heavy-bombardment/ |url-status=live |title=Microbial Habitability During the Late Heavy Bombardment |last=Scalice |first=Daniella |editor-last=Fletcher |editor-first=Julie |date=May 20, 2009 |website=Astrobiology |publisher=[[NASA Astrobiology Institute#Program|NASA Astrobiology Program]] |location=Mountain View, CA |archive-url=https://archive.today/20150124225202/https://astrobiology.nasa.gov/articles/2009/5/20/microbial-habitability-during-the-late-heavy-bombardment/ |archive-date=2015-01-24 |access-date=2020-02-25}}
*{{cite journal |last1=Abramov |first1=Oleg |last2=Mojzsis |first2=Stephen J. |date=May 21, 2009 |title=Microbial habitability of the Hadean Earth during the late heavy bombardment |journal=[[Nature (journal)|Nature]] |volume=459 |issue=7245 |pages=419–422 |doi=10.1038/nature08015 |issn=0028-0836 |pmid=19458721 |bibcode=2009Natur.459..419A |s2cid=3304147}}</ref>

== Earliest evidence for life on Earth ==
{{main|Earliest known life forms}}
The earliest identified organisms were minute and relatively featureless, and their fossils looked like small rods that are very difficult to tell apart from structures that arise through abiotic physical processes. The oldest undisputed evidence of life on Earth, interpreted as fossilized bacteria, dates to 3 Ga.<ref name="BrasierMcLoughlinEtAl2006" /> Other finds in rocks dated to about 3.5 Ga have been interpreted as bacteria,<ref>{{cite journal |last=Schopf |first=J. William |author-link=J. William Schopf |date=April 30, 1993 |title=Microfossils of the Early Archean Apex Chert: New Evidence of the Antiquity of Life |journal=[[Science (journal)|Science]] |volume=260 |issue=5108 |pages=640–646 |bibcode=1993Sci...260..640S |doi=10.1126/science.260.5108.640 |s2cid=2109914 |issn=0036-8075 |pmid=11539831}}
*{{cite journal |last1=Altermann |first1=Wladyslaw |last2=Kazmierczak |first2=Józef |date=November 2003 |title=Archean microfossils: a reappraisal of early life on Earth |journal=Research in Microbiology |volume=154 |issue=9 |pages=611–617 |doi=10.1016/j.resmic.2003.08.006 |issn=0923-2508 |pmid=14596897 |doi-access=free}}</ref> with [[geochemical]] evidence also seeming to show the presence of life 3.8 Ga.<ref>{{cite journal |last1=Mojzsis |first1=Stephen J. |last2=Arrhenius |first2=Gustaf |last3=McKeegan |first3=Kevin D. |last4=Harrison |first4=T. Mark |last5=Nutman |first5=Allen P. |last6=Friend |first6=Clark R. L. |display-authors=3 |date=November 7, 1996 |title=Evidence for life on Earth before 3,800 million years ago |journal=[[Nature (journal)|Nature]] |volume=384 |issue=6604 |pages=55–59 |bibcode=1996Natur.384...55M |doi=10.1038/384055a0 |pmid=8900275 |issn=0028-0836 |hdl=2060/19980037618 |s2cid=4342620 |hdl-access=free}}</ref> However, these analyses were closely scrutinized, and non-biological processes were found which could produce all of the "signatures of life" that had been reported.<ref name="GrotzingerRothman1996">{{cite journal |last1=Grotzinger |first1=John P. |author1-link=John P. Grotzinger |last2=Rothman |first2=Daniel H. |date=October 3, 1996 |title=An abiotic model for stromatolite morphogenesis |journal=[[Nature (journal)|Nature]] |volume=383 |issue=6599 |pages=423–425 |bibcode=1996Natur.383..423G |doi=10.1038/383423a0 |s2cid=4325802 |issn=0028-0836}}</ref><ref>{{cite journal |last1=Fedo |first1=Christopher M. |last2=Whitehouse |first2=Martin J. |date=May 24, 2002 |title=Metasomatic Origin of Quartz-Pyroxene Rock, Akilia, Greenland, and Implications for Earth's Earliest Life |journal=[[Science (journal)|Science]] |volume=296 |issue=5572 |pages=1448–1452 |bibcode=2002Sci...296.1448F |doi=10.1126/science.1070336 |s2cid=10367088 |issn=0036-8075 |pmid=12029129}}
*{{cite journal |last1=Lepland |first1=Aivo |last2=van Zuilen |first2=Mark A. |last3=Arrhenius |first3=Gustaf |last4=Whitehouse |first4=Martin J. |last5=Fedo |first5=Christopher M. |display-authors=3 |date=January 2005 |title=Questioning the evidence for Earth's earliest life—Akilia revisited |journal=[[Geology (journal)|Geology]] |volume=33 |issue=1 |pages=77–79 |bibcode=2005Geo....33...77L |doi=10.1130/G20890.1 |issn=0091-7613}}</ref> While this does not prove that the structures found had a non-biological origin, they cannot be taken as clear evidence for the presence of life. Geochemical signatures from rocks deposited 3.4 Ga have been interpreted as evidence for life.<ref name="BrasierMcLoughlinEtAl2006">{{cite journal |last1=Brasier |first1=Martin |author1-link=Martin Brasier |last2=McLoughlin |first2=Nicola |last3=Green |first3=Owen |last4=Wacey |first4=David |display-authors=3 |date=June 2006 |title=A fresh look at the fossil evidence for early Archaean cellular life |url=http://physwww.mcmaster.ca/~higgsp/3D03/BrasierArchaeanFossils.pdf |url-status=live |journal=[[Philosophical Transactions of the Royal Society B]] |volume=361 |issue=1470 |pages=887–902 |doi=10.1098/rstb.2006.1835 |issn=0962-8436 |pmc=1578727 |pmid=16754605 |archive-url=https://web.archive.org/web/20070730080705/http://physwww.mcmaster.ca/~higgsp/3D03/BrasierArchaeanFossils.pdf |archive-date=2007-07-30 |access-date=2008-08-30}}</ref><ref>{{cite journal |last=Schopf |first=J. William |author-link=J. William Schopf |date=June 29, 2006 |title=Fossil evidence of Archaean life |journal=[[Philosophical Transactions of the Royal Society B]] |volume=361 |issue=1470 |pages=869–885 |doi=10.1098/rstb.2006.1834 |issn=0962-8436 |pmid=16754604 |pmc=1578735}}</ref>

Evidence for fossilized microorganisms considered to be 3.77 billion to 4.28 billion years old was found in the [[Nuvvuagittuq Greenstone Belt]] in Quebec, Canada,<ref name="NAT-20170301">{{cite journal |last1=Dodd |first1=Matthew S. |last2=Papineau |first2=Dominic |last3=Grenne |first3=Tor |last4=Slack |first4=John F. |last5=Rittner |first5=Martin |last6=Pirajno |first6=Franco |last7=O'Neil |first7=Jonathan |last8=Little |first8=Crispin T. S. |display-authors=3 |date=March 2, 2017 |title=Evidence for early life in Earth's oldest hydrothermal vent precipitates |url=https://eprints.whiterose.ac.uk/112179/1/ppnature21377_Dodd_for%20Symplectic.pdf |url-status=live |journal=[[Nature (journal)|Nature]] |volume=543 |issue=7643 |pages=60–64 |bibcode=2017Natur.543...60D |doi=10.1038/nature21377 |s2cid=2420384 |issn=0028-0836 |pmid=28252057 |archive-url=https://web.archive.org/web/20200213131959/https://eprints.whiterose.ac.uk/112179/1/ppnature21377_Dodd_for%20Symplectic.pdf |archive-date=2020-02-13 |access-date=2020-02-18 |doi-access=free}}</ref> although the evidence is disputed as inconclusive.<ref>{{cite news |last=Drake |first=Nadia |author-link=Nadia Drake |date=March 1, 2017 |title=This May Be the Oldest Known Sign of Life on Earth |url=https://www.nationalgeographic.com/news/2017/03/oldest-life-earth-iron-fossils-canada-vents-science/ |url-status=dead |work=National Geographic News |location=Washington, D.C. |publisher=[[National Geographic Society]] |archive-url=https://web.archive.org/web/20191023123727/https://www.nationalgeographic.com/news/2017/03/oldest-life-earth-iron-fossils-canada-vents-science/ |archive-date=2019-10-23 |access-date=2020-02-26}}</ref>

== Origins of life on Earth ==
{{PhylomapA|size=300px|caption=[[Evolutionary tree]] showing the divergence of modern species from their common ancestor in the center.<ref name="Ciccarelli">{{cite journal |last1=Ciccarelli |first1=Francesca D. |last2=Doerks |first2=Tobias |last3=von Mering |first3=Christian |last4=Creevey |first4=Christopher J. |last5=Snel |first5=Berend |last6=Bork |first6=Peer |author6-link=Peer Bork |display-authors=3 |date=March 3, 2006 |title=Toward Automatic Reconstruction of a Highly Resolved Tree of Life |url=https://bioinformatics.bio.uu.nl/pdf/Ciccarelli.s06-311.pdf |journal=[[Science (journal)|Science]] |volume=311 |issue=5765 |pages=1283–1287 |bibcode=2006Sci...311.1283C |doi=10.1126/science.1123061 |pmid=16513982 |citeseerx=10.1.1.381.9514 |s2cid=1615592 |archive-url=https://web.archive.org/web/20070824132444/https://bioinformatics.bio.uu.nl/pdf/Ciccarelli.s06-311.pdf |archive-date=2007-08-24}}</ref> The three [[Domain (biology)|domains]] are colored, with [[bacteria]] blue, [[archaea]] green, and [[eukaryote]]s red.}}
{{further|Evidence of common descent|Common descent|Homology (biology)}}
Most [[biologist]]s reason that all living organisms on Earth must share a single [[last universal ancestor]], because it would be virtually impossible that two or more separate lineages could have independently developed the many complex biochemical mechanisms common to all living organisms.<ref>{{cite journal |last=Mason |first=Stephen F. |author-link=Stephen Finney Mason |date=September 6, 1984 |title=Origins of biomolecular handedness |journal=[[Nature (journal)|Nature]] |volume=311 |issue=5981 |pages=19–23 |pmid=6472461 |doi=10.1038/311019a0 |bibcode=1984Natur.311...19M |s2cid=103653 |issn=0028-0836}}</ref><ref>{{cite magazine |last=Orgel |first=Leslie E. |author-link=Leslie Orgel |date=October 1994 |url=http://courses.washington.edu/biol354/The%20Origin%20of%20Life%20on%20Earth.pdf |url-status=live |title=The Origin of Life on the Earth |magazine=[[Scientific American]] |volume=271 |issue=4 |pages=76–83 |bibcode=1994SciAm.271d..76O |doi=10.1038/scientificamerican1094-76 |issn=0036-8733 |pmid=7524147 |archive-url=https://archive.today/20010124054500/http://proxy.arts.uci.edu/~nideffer/Hawking/early_proto/orgel.html |archive-date=2001-01-24 |access-date=2008-08-30}}</ref><!-- **** "After all, it is next to impossible for such universal traits to have evolved separately"- Orgel, cited **** -->

According to a different scenario<ref name="Kandler-1994">{{Cite book |last=Kandler |first=Otto |url=https://books.google.com/books?id=lOqdAwAAQBAJ&q=Bengtson+%22Early+Life+on+Earth%22++Nobel+Symposium+84 |title=Early Life on Earth. Nobel Symposium 84 |publisher=Columbia University Press |year=1994 |isbn=978-0-231-08088-0 |editor-last=Bengtson |editor-first=Stefan |location=New York |pages=152–160 |chapter=The early diversification of life |author-link=Otto Kandler |access-date=2023-03-19 |archive-date=2023-04-08 |archive-url=https://web.archive.org/web/20230408093724/https://books.google.com/books?id=lOqdAwAAQBAJ&q=Bengtson+%22Early+Life+on+Earth%22++Nobel+Symposium+84 |url-status=live}}</ref><ref name="Kandler-1995">{{Cite journal |last=Kandler |first=Otto |author-link=Otto Kandler |date=1995 |title=Cell Wall Biochemistry in Archaea and its Phylogenetic Implications |journal=Journal of Biological Physics |volume=20 |issue=1–4 |pages=165–169 |doi=10.1007/BF00700433 |s2cid=83906865}}</ref><ref name="Kandler-1998">{{Cite book |last=Kandler |first=Otto |title=Thermophiles: The keys to molecular evolution and the origin of life? |publisher=Taylor and Francis Ltd. |year=1998 |isbn=978-0-203-48420-3 |editor-last=Wiegel |editor-first=Jürgen |location=London |pages=19–31 |chapter=The early diversification of life and the origin of the three domains: A proposal |author-link=Otto Kandler |editor-last2=Adams |editor-first2=Michael W. W. |chapter-url=https://books.google.com/books?id=FtSzl4iastsC |access-date=2023-01-29 |archive-date=2023-02-25 |archive-url=https://web.archive.org/web/20230225201522/https://books.google.com/books?id=FtSzl4iastsC |url-status=live}}</ref> a single last universal ancestor, e.g. a "first cell" or a first individual precursor cell has never existed. Instead, the early biochemical evolution of life<ref name="Wächtershäuser-2000" /> led to diversification through the development of a multiphenotypical population of [[pre-cell]]s from which the precursor cells ([[protocell]]s) of the three [[domains of life]]<ref name="Woese-1990">{{Cite journal |last1=Woese |first1=Carl R. |author-link=Carl Woese |last2=Kandler |first2=Otto |author-link2=Otto Kandler |last3=Wheelis |first3=Mark |date=1990 |title=Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya |journal=Proc. Natl. Acad. Sci. USA |volume=87 |issue=12 |pages=4576–4579 |bibcode=1990PNAS...87.4576W |doi=10.1073/pnas.87.12.4576 |pmc=54159 |pmid=2112744 |doi-access=free}}</ref> emerged. Thus, the formation of cells was a successive process. See {{slink||Metabolism first: Pre-cells, successive cellularisation}}, below.

=== Independent emergence on Earth ===
{{main|Abiogenesis}}

Life on Earth is based on [[carbon]] and [[water]]. Carbon provides stable frameworks for complex chemicals and can be easily extracted from the environment, especially from [[carbon dioxide]].<ref name="Garwood2012" /> There is no other chemical element whose properties are similar enough to carbon's to be called an analogue; [[silicon]], the element directly below carbon on the [[periodic table]], does not form very many complex stable molecules, and because most of its compounds are water-insoluble and because [[silicon dioxide]] is a hard and abrasive solid in contrast to [[carbon dioxide]] at temperatures associated with living things, it would be more difficult for organisms to extract. The elements [[boron]] and [[phosphorus]] have more complex chemistries but suffer from other limitations relative to carbon. Water is an excellent [[solvent]] and has two other useful properties: the fact that ice floats enables aquatic organisms to survive beneath it in winter; and its molecules have [[electric charge|electrically negative]] and positive ends, which enables it to form a wider range of compounds than other solvents can. Other good solvents, such as [[ammonia]], are liquid only at such low temperatures that chemical reactions may be too slow to sustain life, and lack water's other advantages.<ref>{{harvnb|Bennett|2008|pp=[https://archive.org/details/beyondufossearch00benn/page/84 82–85]}}</ref> Organisms based on [[alternative biochemistry]] may, however, be possible on other planets.<ref>{{cite journal |last1=Schulze-Makuch |first1=Dirk |author1-link=Dirk Schulze-Makuch |last2=Irwin |first2=Louis N. |date=April 2006 |title=The prospect of alien life in exotic forms on other worlds |journal=[[Naturwissenschaften]] |volume=93 |issue=4 |pages=155–172 |bibcode=2006NW.....93..155S |doi=10.1007/s00114-005-0078-6 |s2cid=3207913 |issn=0028-1042 |pmid=16525788}}</ref>

Research on how life might have emerged from non-living chemicals focuses on three possible starting points: [[self-replication]], an organism's ability to produce offspring that are very similar to itself; metabolism, its ability to feed and repair itself; and external [[cell membrane]]s, which allow food to enter and waste products to leave, but exclude unwanted substances.<ref>{{cite journal |last=Peretó |first=Juli |date=March 2005 |title=Controversies on the origin of life |url=http://www.im.microbios.org/0801/0801023.pdf |journal=[[International Microbiology]] |volume=8 |issue=1 |pages=23–31 |issn=1139-6709 |pmid=15906258 |archive-url=https://web.archive.org/web/20070604201527/http://www.im.microbios.org/0801/0801023.pdf |archive-date=2007-06-04 |access-date=2007-10-07}}</ref> Research on abiogenesis still has a long way to go, since theoretical and [[empirical]] approaches are only beginning to make contact with each other.<ref>{{cite journal |last=Szathmáry |first=Eörs |author-link=Eörs Szathmáry |date=February 3, 2005 |title=In search of the simplest cell |journal=[[Nature (journal)|Nature]] |volume=433 |issue=7025 |pages=469–470 |bibcode=2005Natur.433..469S |doi=10.1038/433469a |s2cid=4360797 |pmid=15690023 |issn=0028-0836}}</ref><ref>{{cite journal |last1=Luisi |first1=Pier Luigi |author1-link=Pier Luigi Luisi |last2=Ferri |first2=Francesca |last3=Stano |first3=Pasquale |date=January 2006 |title=Approaches to semi-synthetic minimal cells: a review |journal=[[Naturwissenschaften]] |volume=93 |issue=1 |pages=1–13 |bibcode=2006NW.....93....1L |doi=10.1007/s00114-005-0056-z |s2cid=16567006 |issn=0028-1042 |pmid=16292523}}</ref>

==== Replication first: RNA world ====
{{main|Last universal common ancestor|RNA world}}
Even the simplest members of the [[Three-domain system|three modern domains]] of life use [[DNA]] to record their "recipes" and a complex array of [[RNA]] and [[protein]] molecules to "read" these instructions and use them for growth, maintenance and self-replication. The discovery that some RNA molecules can [[catalyze]] both their own replication and the construction of proteins led to the hypothesis of earlier life-forms based entirely on RNA.<ref>{{cite journal |last=Joyce |first=Gerald F. |author-link=Gerald Joyce |date=July 11, 2002 |title=The antiquity of RNA-based evolution |journal=[[Nature (journal)|Nature]] |volume=418 |issue=6894 |pages=214–221 |bibcode=2002Natur.418..214J |doi=10.1038/418214a |s2cid=4331004 |pmid=12110897 |issn=0028-0836}}</ref> These [[ribozyme]]s could have formed an [[RNA world]] in which there were individuals but no species, as [[mutation]]s and [[horizontal gene transfer]]s would have meant that offspring were likely to have different [[genome]]s from their parents, and [[evolution]] occurred at the level of genes rather than organisms.<ref name="Hoenigsberg2003">{{cite journal |last=Hoenigsberg |first=Hugo |date=December 30, 2003 |title=Evolution without speciation but with selection: LUCA, the Last Universal Common Ancestor in Gilbert's RNA world |url=http://www.funpecrp.com.br/gmr/year2003/vol4-2/gmr0070_full_text.htm |url-status=live |journal=[[Genetics and Molecular Research]] |volume=2 |issue=4 |pages=366–375 |issn=1676-5680 |pmid=15011140 |archive-url=https://web.archive.org/web/20040602133741/http://www.funpecrp.com.br/gmr/year2003/vol4-2/gmr0070_full_text.htm |archive-date=2004-06-02 |access-date=2008-08-30}}</ref> RNA would later have been replaced by DNA, which can build longer, more stable genomes, strengthening heritability and expanding the capabilities of individual organisms.<ref name="Hoenigsberg2003" /><ref>{{cite journal |last1=Trevors |first1=Jack T. |last2=Abel |first2=David L. |date=November 2004 |title=Chance and necessity do not explain the origin of life |journal=[[Cell Biology International]] |volume=28 |issue=11 |pages=729–739 |doi=10.1016/j.cellbi.2004.06.006 |issn=1065-6995 |pmid=15563395 |s2cid=30633352}}</ref><ref>{{cite journal |last1=Forterre |first1=Patrick |author1-link=Patrick Forterre |last2=Benachenhou-Lahfa |first2=Nadia |last3=Confalonieri |first3=Fabrice |last4=Duguet |first4=Michel |last5=Elie |first5=Christiane |last6=Labedan |first6=Bernard |editor1-last=Adoutte |editor1-first=André |editor2-last=Perasso |editor2-first=Roland |display-authors=3 |year=1992 |title=The nature of the last universal ancestor and the root of the tree of life, still open questions |journal=[[BioSystems]] |volume=28 |issue=1–3 |pages=15–32 |doi=10.1016/0303-2647(92)90004-I |issn=0303-2647 |pmid=1337989 |bibcode=1992BiSys..28...15F}} Part of a special issue: 9th Meeting of the International Society for Evolutionary Protistology, July 3–7, 1992, Orsay, France.</ref> Ribozymes remain as the main components of [[ribosome]]s, the "protein factories" in modern cells.<ref>{{cite journal |last=Cech |first=Thomas R. |author-link=Thomas Cech |date=August 11, 2000 |title=The Ribosome Is a Ribozyme |journal=[[Science (journal)|Science]] |volume=289 |issue=5481 |pages=878–879 |doi=10.1126/science.289.5481.878 |s2cid=24172338 |issn=0036-8075 |pmid=10960319}}</ref> Evidence suggests the first RNA molecules formed on Earth prior to 4.17 Ga.<ref>{{cite journal |last1=Pearce |first1=Ben K. D. |last2=Pudritz |first2=Ralph E. |author2-link=Ralph Pudritz |last3=Semenov |first3=Dmitry A. |last4=Henning |first4=Thomas K. |author4-link=Thomas Henning |display-authors=3 |date=October 24, 2017 |title=Origin of the RNA world: The fate of nucleobases in warm little ponds |journal=Proceedings of the National Academy of Sciences |volume=114 |issue=43 |pages=11327–11332 |bibcode=2017PNAS..11411327P |doi=10.1073/pnas.1710339114 |issn=0027-8424 |pmid=28973920 |pmc=5664528 |arxiv=1710.00434 |doi-access=free}}</ref>

Although short self-replicating RNA molecules have been artificially produced in laboratories,<ref>{{cite journal |last1=Johnston |first1=Wendy K. |last2=Unrau |first2=Peter J. |last3=Lawrence |first3=Michael S. |last4=Glasner |first4=Margaret E. |last5=Bartel |first5=David P. |author5-link=David Bartel |display-authors=3 |date=May 18, 2001 |title=RNA-Catalyzed RNA Polymerization: Accurate and General RNA-Templated Primer Extension |url=http://www.dna.caltech.edu/courses/cs191/paperscs191/Science(292)1319.pdf |url-status=live |journal=[[Science (journal)|Science]] |volume=292 |issue=5520 |pages=1319–1325 |bibcode=2001Sci...292.1319J |doi=10.1126/science.1060786 |issn=0036-8075 |pmid=11358999 |citeseerx=10.1.1.70.5439 |s2cid=14174984 |archive-url=https://web.archive.org/web/20060909040050/http://www.dna.caltech.edu/courses/cs191/paperscs191/Science(292)1319.pdf |archive-date=2006-09-09}}</ref> doubts have been raised about whether natural non-biological synthesis of RNA is possible.<ref name="LevyMiller1998">{{cite journal |last1=Levy |first1=Matthew |last2=Miller |first2=Stanley L. |author2-link=Stanley Miller |date=July 7, 1998 |title=The stability of the RNA bases: Implications for the origin of life |journal=[[Proc. Natl. Acad. Sci. U.S.A.]] |volume=95 |issue=14 |pages=7933–7938 |bibcode=1998PNAS...95.7933L |doi=10.1073/pnas.95.14.7933 |issn=0027-8424 |pmc=20907 |pmid=9653118 |doi-access=free}}
*{{cite journal |last1=Larralde |first1=Rosa |last2=Robertson |first2=Michael P. |last3=Miller |first3=Stanley L. |author3-link=Stanley Miller |date=August 29, 1995 |title=Rates of decomposition of ribose and other sugars: Implications for chemical evolution |journal=[[Proc. Natl. Acad. Sci. U.S.A.]] |volume=92 |issue=18 |pages=8158–8160 |bibcode=1995PNAS...92.8158L |doi=10.1073/pnas.92.18.8158 |issn=0027-8424 |pmc=41115 |pmid=7667262 |doi-access=free}}
*{{cite journal |last=Lindahl |first=Tomas |author-link=Tomas Lindahl |date=April 22, 1993 |title=Instability and decay of the primary structure of DNA |journal=[[Nature (journal)|Nature]] |volume=362 |issue=6422 |pages=709–715 |bibcode=1993Natur.362..709L |doi=10.1038/362709a0 |s2cid=4283694 |pmid=8469282 |issn=0028-0836}}</ref> The earliest "ribozymes" may have been formed of simpler [[nucleic acid]]s such as [[Peptide nucleic acid|PNA]], [[Threose nucleic acid|TNA]] or [[Glycol nucleic acid|GNA]], which would have been replaced later by RNA.<ref>{{cite journal |last=Orgel |first=Leslie E. |author-link=Leslie Orgel |date=November 17, 2000 |title=A Simpler Nucleic Acid |journal=[[Science (journal)|Science]] |volume=290 |issue=5495 |pages=1306–1307 |doi=10.1126/science.290.5495.1306 |s2cid=83662769 |issn=0036-8075 |pmid=11185405}}</ref><ref>{{cite journal |last1=Nelson |first1=Kevin E. |last2=Levy |first2=Matthew |last3=Miller |first3=Stanley L. |author3-link=Stanley Miller |date=April 11, 2000 |title=Peptide nucleic acids rather than RNA may have been the first genetic molecule |journal=[[Proc. Natl. Acad. Sci. U.S.A.]] |volume=97 |issue=8 |pages=3868–3871 |bibcode=2000PNAS...97.3868N |doi=10.1073/pnas.97.8.3868 |issn=0027-8424 |pmc=18108 |pmid=10760258 |doi-access=free}}</ref>

In 2003, it was proposed that porous metal sulfide [[precipitates]] would assist RNA synthesis at about {{convert|100|°C|°F}} and ocean-bottom pressures near [[hydrothermal vent]]s. Under this hypothesis, [[lipid]] membranes would be the last major cell components to appear and, until then, the [[protocell]]s would be confined to the pores.<ref>{{cite journal |last1=Martin |first1=William |author1-link=William F. Martin |last2=Russell |first2=Michael J. |author2-link=Michael Russell (scientist) |date=January 29, 2003 |title=On the origins of cells: a hypothesis for the evolutionary transitions from abiotic geochemistry to chemoautotrophic prokaryotes, and from prokaryotes to nucleated cells |journal=[[Philosophical Transactions of the Royal Society B]] |volume=358 |issue=1429 |pages=59–85 |doi=10.1098/rstb.2002.1183 |issn=0962-8436 |pmc=1693102 |pmid=12594918}}</ref>

==== Membranes first: Lipid world ====
{{Annotated image | caption=Cross-section through a [[liposome]] | image=Liposome cross section.png | width=250 | height=160 | image-width = 125 | image-left=0 | image-top=0
| annotations =
{{Annotation|150|10|{{Color box|white|border=darkgray}} {{=}} water-attracting heads of [[lipid]] molecules}}
{{Annotation|150|90|{{Color box|#cd9922|border=darkgray}} {{=}} water-repellent tails}}
}}
It has been suggested that double-walled "bubbles" of lipids like those that form the external membranes of cells may have been an essential first step.<ref>{{cite journal |last1=Trevors |first1=Jack T. |last2=Psenner |first2=Roland |date=December 2001 |title=From self-assembly of life to present-day bacteria: a possible role for nanocells |journal=[[FEMS Microbiology Reviews]] |volume=25 |issue=5 |pages=573–582 |doi=10.1111/j.1574-6976.2001.tb00592.x |doi-access=free |issn=0168-6445 |pmid=11742692}}</ref> Experiments that simulated the conditions of the early Earth have reported the formation of lipids, and these can spontaneously form [[liposome]]s, double-walled "bubbles", and then reproduce themselves.<ref name="Garwood2012" /> Although they are not intrinsically information-carriers as nucleic acids are, they would be subject to [[natural selection]] for longevity and reproduction. Nucleic acids such as RNA might then have formed more easily within the liposomes than outside.<ref>{{cite journal |last1=Segré |first1=Daniel |last2=Ben-Eli |first2=Dafna |last3=Deamer |first3=David W. |author3-link=David W. Deamer |last4=Lancet |first4=Doron |author4-link=Doron Lancet |display-authors=3 |date=February 2001 |title=The Lipid World |url=https://www.weizmann.ac.il/molgen/Lancet/sites/molgen.Lancet/files/uploads/segre_lipid_world.pdf |url-status=live |journal=[[Origins of Life and Evolution of Biospheres]] |volume=31 |issue=1–2 |pages=119–145 |bibcode=2001OLEB...31..119S |doi=10.1023/A:1006746807104 |s2cid=10959497 |issn=0169-6149 |pmid=11296516 |archive-url=https://web.archive.org/web/20150626225745/https://www.weizmann.ac.il/molgen/Lancet/sites/molgen.Lancet/files/uploads/segre_lipid_world.pdf |archive-date=2015-06-26 |access-date=2020-02-28}}</ref>

==== The clay hypothesis ====
{{main|Graham Cairns-Smith#Clay hypothesis|RNA world}}

RNA is complex and there are doubts about whether it can be produced non-biologically in the wild.<ref name="LevyMiller1998" /> Some [[clay]]s, notably [[montmorillonite]], have properties that make them plausible accelerators for the emergence of an RNA world: they grow by self-replication of their [[crystal]]line pattern; they are subject to an analogue of natural selection, as the clay "species" that grows fastest in a particular environment rapidly becomes dominant; and they can catalyze the formation of RNA molecules.<ref>{{harvnb|Cairns-Smith|1968|pp=57–66}}</ref> Although this idea has not become the scientific consensus, it still has active supporters.<ref>{{cite journal |last=Ferris |first=James P. |author-link=James Ferris |date=June 1999 |title=Prebiotic Synthesis on Minerals: Bridging the Prebiotic and RNA Worlds |journal=[[The Biological Bulletin]] |volume=196 |issue=3 |pages=311–314 |doi=10.2307/1542957 |issn=0006-3185 |jstor=1542957 |pmid=10390828}} "This paper was originally presented at a workshop titled ''Evolution: A Molecular Point of View''."</ref>

Research in 2003 reported that [[montmorillonite]] could also accelerate the conversion of [[fatty acid]]s into "bubbles" and that the "bubbles" could encapsulate RNA attached to the clay. These "bubbles" can then grow by absorbing additional lipids and then divide. The formation of the earliest cells may have been aided by similar processes.<ref>{{cite journal |last1=Hanczyc |first1=Martin M. |last2=Fujikawa |first2=Shelly M. |last3=Szostak |first3=Jack W. |author3-link=Jack W. Szostak |date=October 24, 2003 |title=Experimental Models of Primitive Cellular Compartments: Encapsulation, Growth, and Division |journal=[[Science (journal)|Science]] |volume=302 |issue=5645 |pages=618–622 |bibcode=2003Sci...302..618H |doi=10.1126/science.1089904 |issn=0036-8075 |pmc=4484575 |pmid=14576428}}</ref>

A similar hypothesis presents self-replicating iron-rich clays as the progenitors of [[nucleotide]]s, lipids and [[amino acid]]s.<ref>{{cite journal |last=Hartman |first=Hyman |date=October 1998 |title=Photosynthesis and the Origin of Life |journal=[[Origins of Life and Evolution of Biospheres]] |volume=28 |issue=4–6 |pages=512–521 |bibcode=1998OLEB...28..515H |doi=10.1023/A:1006548904157 |s2cid=2464 |issn=0169-6149 |pmid=11536891}}</ref>

==== Metabolism first: Iron–sulfur world ====
{{main|Iron–sulfur world hypothesis}}

A series of experiments starting in 1997 showed that early stages in the formation of proteins from inorganic materials including [[carbon monoxide]] and [[hydrogen sulfide]] could be achieved by using [[iron(II) sulfide|iron sulfide]] and [[nickel sulfide]] as [[catalyst]]s. Most of the steps required temperatures of about {{convert|100|°C|°F}} and moderate pressures, although one stage required {{convert|250|°C|°F}} and a pressure equivalent to that found under {{convert|7|km|mi}} of rock. Hence it was suggested that self-sustaining synthesis of proteins could have occurred near hydrothermal vents.<ref name="Wächtershäuser-2000">{{cite journal |last=Wächtershäuser |first=Günter |author-link=Günter Wächtershäuser |date=August 25, 2000 |title=Life as We Don't Know It |journal=[[Science (journal)|Science]] |volume=289 |issue=5483 |pages=1307–1308 |doi=10.1126/science.289.5483.1307 |issn=0036-8075 |pmid=10979855 |s2cid=170713742}}</ref>{{clear}}

==== Metabolism first: Pre–cells (successive cellularisation) ====
In this scenario, the biochemical evolution of life<ref name="Wächtershäuser-2000" /> led to diversification through the development of a multiphenotypical population of [[pre-cell]]s,<ref name="Kandler-1994" /><ref name="Kandler-1995" /><ref name="Kandler-1998" /> i.e. evolving entities of primordial life with different characteristics and widespread [[horizontal gene transfer]]. 
[[File:Kandler 1998 Early diversification of life and pre-cell theory.svg|thumb|Early diversification of life with [[Otto Kandler|Kandler]]'s pre-cell theory (Kandler 1998, p. 22)<ref name="Kandler-1998" />]]
From this pre-cell population the founder groups A, B, C and then, from them, the precursor cells (here named proto-cells) of the three [[domains of life]]<ref name="Woese-1990" /> arose successively, leading first to the [[Bacteria|domain Bacteria]], then to the [[Archaea|domain Archea]] and finally to the [[Eukaryote|domain Eucarya]].

For the development of cells ([[Cellularization|cellularisation]]), the pre-cells had to be protected from their surroundings by envelopes (i.e. membranes, walls). For instance, the development of rigid [[cell wall]]s by the invention of [[peptidoglycan]] in bacteria (domain Bacteria) may have been a prerequisite for their successful survival, radiation and colonisation of virtually all habitats of the geosphere and hydrosphere.<ref name="Kandler-1998" />

This scenario may explain the quasi-random distribution of evolutionarily important features among the three domains and, at the same time, the existence of the most basic biochemical features (genetic code, set of protein amino acids etc.) in all three domains (unity of life), as well as the close relationship between the Archaea and the Eucarya. A scheme of the pre-cell scenario is shown in the adjacent figure,<ref name="Kandler-1998" /> where important evolutionary improvements are indicated by numbers.

=== Prebiotic environments ===

==== Geothermal springs ====
Wet-dry cycles at geothermal springs are shown to solve the problem of hydrolysis and promote the polymerization and vesicle encapsulation of biopolymers.<ref>{{Cite journal |last=Deamer |first=David |date=February 10, 2021 |title=Where Did Life Begin? Testing Ideas in Prebiotic Analogue Conditions |journal=Life |language=en |volume=11 |issue=2 |page=134 |doi=10.3390/life11020134 |pmid=33578711 |pmc=7916457 |bibcode=2021Life...11..134D |issn=2075-1729 |doi-access=free}}</ref><ref name="Damer-2020">{{Cite journal |last1=Damer |first1=Bruce |last2=Deamer |first2=David |date=2020-04-01 |title=The Hot Spring Hypothesis for an Origin of Life |journal=Astrobiology |volume=20 |issue=4 |pages=429–452 |doi=10.1089/ast.2019.2045 |issn=1531-1074 |pmc=7133448 |pmid=31841362 |bibcode=2020AsBio..20..429D}}</ref> The temperatures of geothermal springs are suitable for biomolecules.<ref>{{Cite journal |last1=Des Marais |first1=David J. |last2=Walter |first2=Malcolm R. |date=2019-12-01 |title=Terrestrial Hot Spring Systems: Introduction |journal=Astrobiology |volume=19 |issue=12 |pages=1419–1432 |doi=10.1089/ast.2018.1976 |issn=1531-1074 |pmc=6918855 |pmid=31424278 |bibcode=2019AsBio..19.1419D}}</ref> Silica minerals and metal sulfides in these environments have photocatalytic properties to catalyze biomolecules. Solar UV exposure also promotes the synthesis of biomolecules like RNA nucleotides.<ref name="Mulkidjanian-2012">{{Cite journal |last1=Mulkidjanian |first1=Armen Y. |last2=Bychkov |first2=Andrew Yu. |last3=Dibrova |first3=Daria V. |last4=Galperin |first4=Michael Y. |last5=Koonin |first5=Eugene V. |date=2012-04-03 |title=Origin of first cells at terrestrial, anoxic geothermal fields |journal=Proceedings of the National Academy of Sciences |language=en |volume=109 |issue=14 |pages=E821-30 |doi=10.1073/pnas.1117774109 |issn=0027-8424 |pmc=3325685 |pmid=22331915 |doi-access=free |bibcode=2012PNAS..109E.821M}}</ref><ref>{{Cite journal |last1=Patel |first1=Bhavesh H. |last2=Percivalle |first2=Claudia |last3=Ritson |first3=Dougal J. |last4=Duffy |first4=Colm D. |last5=Sutherland |first5=John D. |date=March 16, 2015 |title=Common origins of RNA, protein and lipid precursors in a cyanosulfidic protometabolism |journal=Nature Chemistry |language=en |volume=7 |issue=4 |pages=301–307 |doi=10.1038/nchem.2202 |pmid=25803468 |pmc=4568310 |bibcode=2015NatCh...7..301P |issn=1755-4349}}</ref> An analysis of hydrothermal veins at a 3.5 [[Bya|Gya]] (giga years ago or 1 billion years) geothermal spring setting were found to have elements required for the origin of life, which are potassium, boron, hydrogen, sulfur, phosphorus, zinc, nitrogen, and oxygen.<ref>{{Cite journal |last1=Van Kranendonk |first1=Martin J. |last2=Baumgartner |first2=Raphael |last3=Djokic |first3=Tara |last4=Ota |first4=Tsutomu |last5=Steller |first5=Luke |last6=Garbe |first6=Ulf |last7=Nakamura |first7=Eizo |date=2021-01-01 |title=Elements for the Origin of Life on Land: A Deep-Time Perspective from the Pilbara Craton of Western Australia |url=https://www.liebertpub.com/doi/abs/10.1089/ast.2019.2107 |journal=Astrobiology |volume=21 |issue=1 |pages=39–59 |doi=10.1089/ast.2019.2107 |pmid=33404294 |bibcode=2021AsBio..21...39V |s2cid=230783184 |issn=1531-1074 |access-date=2022-10-07 |archive-date=2023-07-16 |archive-url=https://web.archive.org/web/20230716080412/https://www.liebertpub.com/doi/abs/10.1089/ast.2019.2107 |url-status=live|url-access=subscription }}</ref> Mulkidjanian and colleagues find that such environments have identical ionic concentrations to the cytoplasm of modern cells.<ref name="Mulkidjanian-2012" /> Fatty acids in acidic or slightly alkaline geothermal springs assemble into vesicles after wet-dry cycles as there is a lower concentration of ionic solutes at geothermal springs since they are freshwater environments, in contrast to seawater which has a higher concentration of ionic solutes.<ref>{{Cite journal |last1=Milshteyn |first1=Daniel |last2=Damer |first2=Bruce |last3=Havig |first3=Jeff |last4=Deamer |first4=David |date=2018-05-10 |title=Amphiphilic Compounds Assemble into Membranous Vesicles in Hydrothermal Hot Spring Water but Not in Seawater |journal=Life |language=en |volume=8 |issue=2 |page=11 |doi=10.3390/life8020011 |pmid=29748464 |pmc=6027054 |bibcode=2018Life....8...11M |issn=2075-1729 |doi-access=free}}</ref> For organic compounds to be present at geothermal springs, they would have likely been transported by carbonaceous meteors. The molecules that fell from the meteors were then accumulated in geothermal springs. Geothermal springs can accumulate aqueous phosphate in the form of [[Phosphoric acid|phopshoric acid]]. Based on lab-run models, these concentrations of phoshate are insufficient to facilitate [[biosynthesis]].<ref name="Toner-2020">{{Cite journal |last1=Toner |first1=Jonathan D. |last2=Catling |first2=David C. |date=2020-01-14 |title=A carbonate-rich lake solution to the phosphate problem of the origin of life |journal=Proceedings of the National Academy of Sciences |language=en |volume=117 |issue=2 |pages=883–888 |doi=10.1073/pnas.1916109117 |issn=0027-8424 |pmc=6969521 |pmid=31888981 |bibcode=2020PNAS..117..883T |doi-access=free}}</ref> As for the evolutionary implications, freshwater heterotrophic cells that depended upon synthesized organic compounds later evolved photosynthesis because of the continuous exposure to sunlight as well as their cell walls with ion pumps to maintain their intracellular metabolism after they entered the oceans.<ref name="Damer-2020" />

==== Deep sea hydrothermal vents ====
Catalytic mineral particles and transition metal sulfides at these environments are capable of catalyzing organic compounds.<ref>{{Cite web |title=Origin of life: Chemistry of seabed's hot vents could explain emergence of life |url=https://www.sciencedaily.com/releases/2015/04/150427101635.htm |access-date=2022-10-07 |website=ScienceDaily |language=en |archive-date=2022-10-07 |archive-url=https://web.archive.org/web/20221007223205/https://www.sciencedaily.com/releases/2015/04/150427101635.htm |url-status=live}}</ref> Scientists simulated laboratory conditions that were identical to white smokers and successfully oligomerized RNA, measured to be 4 units long.<ref>{{Cite journal |last1=Burcar |first1=Bradley T. |last2=Barge |first2=Laura M. |last3=Trail |first3=Dustin |last4=Watson |first4=E. Bruce |last5=Russell |first5=Michael J. |last6=McGown |first6=Linda B. |date=2015-07-01 |title=RNA Oligomerization in Laboratory Analogues of Alkaline Hydrothermal Vent Systems |url=https://www.liebertpub.com/doi/abs/10.1089/ast.2014.1280 |journal=Astrobiology |volume=15 |issue=7 |pages=509–522 |doi=10.1089/ast.2014.1280 |pmid=26154881 |bibcode=2015AsBio..15..509B |issn=1531-1074 |access-date=2022-10-07 |archive-date=2020-06-03 |archive-url=https://web.archive.org/web/20200603131804/https://www.liebertpub.com/doi/abs/10.1089/ast.2014.1280 |url-status=live|url-access=subscription }}</ref> Long chain fatty acids can be synthesized via [[Fischer-Tropsch synthesis]].<ref name="Harrison-2022">{{Cite journal |last1=Harrison |first1=Stuart A. |last2=Palmeira |first2=Raquel Nunes |last3=Halpern |first3=Aaron |last4=Lane |first4=Nick |date=2022-11-01 |title=A biophysical basis for the emergence of the genetic code in protocells |journal=Biochimica et Biophysica Acta (BBA) - Bioenergetics |language=en |volume=1863 |issue=8 |page=148597 |doi=10.1016/j.bbabio.2022.148597 |pmid=35868450 |s2cid=250707510 |issn=0005-2728 |doi-access=free}}</ref> Another experiment that replicated conditions also similar white smokers, with long chain fatty acids present resulted in the assembly of vesicles.<ref>{{Cite journal |last1=Jordan |first1=Sean F. |last2=Rammu |first2=Hanadi |last3=Zheludev |first3=Ivan N. |last4=Hartley |first4=Andrew M. |last5=Maréchal |first5=Amandine |last6=Lane |first6=Nick |date=November 4, 2019 |title=Promotion of protocell self-assembly from mixed amphiphiles at the origin of life |url=https://eprints.bbk.ac.uk/id/eprint/29841/1/Jordan%20Lane%20Nat%20Ecol%20Evol%20accepted%20MS%202019.pdf |journal=Nature Ecology & Evolution |language=en |volume=3 |issue=12 |pages=1705–1714 |doi=10.1038/s41559-019-1015-y |pmid=31686020 |bibcode=2019NatEE...3.1705J |s2cid=207891212 |issn=2397-334X |access-date=October 10, 2022 |archive-date=October 10, 2022 |archive-url=https://web.archive.org/web/20221010214545/https://eprints.bbk.ac.uk/id/eprint/29841/1/Jordan%20Lane%20Nat%20Ecol%20Evol%20accepted%20MS%202019.pdf |url-status=live}}</ref> Exergonic reactions at hydrothermal vents are suggested to have been a source of free energy that promoted chemical reactions, synthesis of organic molecules, and are inducive to chemical gradients.<ref>{{Cite web |last1=Colín-García |first1=María |last2=Heredia |first2=Alejandro |last3=Cordero |first3=Guadalupe |last4=Camprubí |first4=Antoni |last5=Negrón-Mendoza |first5=Alicia |last6=Ortega-Gutiérrez |first6=Fernando |last7=Beraldi |first7=Hugo |last8=Ramos-Bernal |first8=Sergio |date=2016 |title=Hydrothermal vents and prebiotic chemistry: a review |url=http://boletinsgm.igeolcu.unam.mx/bsgm/index.php/component/content/article/309-sitio/articulos/cuarta-epoca/6803/1620-6803-13-colin |access-date=2022-10-07 |website=Boletín de la Sociedad Geológica Mexicana |language=en-gb |archive-date=2017-08-18 |archive-url=https://web.archive.org/web/20170818175803/http://boletinsgm.igeolcu.unam.mx/bsgm/index.php/component/content/article/309-sitio/articulos/cuarta-epoca/6803/1620-6803-13-colin |url-status=live}}</ref> In small rock pore systems, membranous structures between alkaline seawater and the acidic ocean would be conducive to natural proton gradients.<ref>{{Cite journal |last1=Lane |first1=Nick |last2=Allen |first2=John F. |last3=Martin |first3=William |date=2010-01-27 |title=How did LUCA make a living? Chemiosmosis in the origin of life |url=https://onlinelibrary.wiley.com/doi/abs/10.1002/bies.200900131 |journal=BioEssays |language=en |volume=32 |issue=4 |pages=271–280 |doi=10.1002/bies.200900131 |pmid=20108228 |access-date=2022-10-24 |archive-date=2022-10-24 |archive-url=https://web.archive.org/web/20221024030805/https://onlinelibrary.wiley.com/doi/abs/10.1002/bies.200900131 |url-status=live|url-access=subscription }}</ref> Nucleobase synthesis could occur by following universally conserved biochemical pathways by using metal ions as catalysts.<ref name="Harrison-2022" /> RNA molecules of 22 bases can be polymerized in alkaline hydrothermal vent pores. Thin pores are shown to only accumulate long polynucleotides whereas thick pores accumulate both short and long polynucleotides. Small mineral cavities or mineral gels could have been a compartment for abiogenic processes.<ref>{{Cite journal |last1=Westall |first1=F. |last2=Hickman-Lewis |first2=K. |last3=Hinman |first3=N. |last4=Gautret |first4=P. |last5=Campbell |first5=K.a. |last6=Bréhéret |first6=J.g. |last7=Foucher |first7=F. |last8=Hubert |first8=A. |last9=Sorieul |first9=S. |last10=Dass |first10=A.v. |last11=Kee |first11=T.p. |last12=Georgelin |first12=T. |last13=Brack |first13=A. |date=2018-03-01 |title=A Hydrothermal-Sedimentary Context for the Origin of Life |journal=Astrobiology |volume=18 |issue=3 |pages=259–293 |doi=10.1089/ast.2017.1680 |issn=1531-1074 |pmc=5867533 |pmid=29489386 |bibcode=2018AsBio..18..259W}}</ref><ref>{{Cite journal |last1=Lane |first1=Nick |last2=Martin |first2=William F. |date=2012-12-21 |title=The Origin of Membrane Bioenergetics |journal=Cell |language=en |volume=151 |issue=7 |pages=1406–1416 |doi=10.1016/j.cell.2012.11.050 |pmid=23260134 |s2cid=15028935 |issn=0092-8674 |doi-access=free}}</ref><ref>{{Cite journal |last1=Baaske |first1=Philipp |last2=Weinert |first2=Franz M. |last3=Duhr |first3=Stefan |last4=Lemke |first4=Kono H. |last5=Russell |first5=Michael J. |last6=Braun |first6=Dieter |date=2007-05-29 |title=Extreme accumulation of nucleotides in simulated hydrothermal pore systems |journal=Proceedings of the National Academy of Sciences |language=en |volume=104 |issue=22 |pages=9346–9351 |doi=10.1073/pnas.0609592104 |issn=0027-8424 |pmc=1890497 |pmid=17494767 |doi-access=free|bibcode=2007PNAS..104.9346B }}</ref> A genomic analysis supports this hypothesis as they found 355 genes that likely traced to [[LUCA]] upon 6.1 million sequenced prokaryotic genes. They reconstruct LUCA as a thermophilic anaerobe with a Wood-Ljungdahl pathway, implying an origin of life at white smokers. LUCA would also have exhibited other biochemical pathways such as [[gluconeogenesis]], [[Reverse Krebs cycle|reverse incomplete Krebs cycle]], [[glycolysis]], and the [[pentose phosphate pathway]], including biochemical reactions such as [[reductive amination]] and [[transamination]].<ref>{{Cite journal |last1=Weiss |first1=Madeline C. |last2=Sousa |first2=Filipa L. |last3=Mrnjavac |first3=Natalia |last4=Neukirchen |first4=Sinje |last5=Roettger |first5=Mayo |last6=Nelson-Sathi |first6=Shijulal |last7=Martin |first7=William F. |date=2016-07-25 |title=The physiology and habitat of the last universal common ancestor |url=https://www.almendron.com/tribuna/wp-content/uploads/2019/10/the-physiology-and-habitat-of-the-last-universal-common-ancestor.pdf |journal=Nature Microbiology |language=en |volume=1 |issue=9 |page=16116 |doi=10.1038/nmicrobiol.2016.116 |pmid=27562259 |s2cid=2997255 |issn=2058-5276 |access-date=2022-10-24 |archive-date=2023-01-29 |archive-url=https://web.archive.org/web/20230129185028/https://www.almendron.com/tribuna/wp-content/uploads/2019/10/the-physiology-and-habitat-of-the-last-universal-common-ancestor.pdf |url-status=live}}</ref><ref>{{Cite journal |last1=Weiss |first1=Madeline C. |last2=Preiner |first2=Martina |last3=Xavier |first3=Joana C. |last4=Zimorski |first4=Verena |last5=Martin |first5=William F. |date=2018-08-16 |title=The last universal common ancestor between ancient Earth chemistry and the onset of genetics |journal=PLOS Genetics |language=en |volume=14 |issue=8 |pages=e1007518 |doi=10.1371/journal.pgen.1007518 |issn=1553-7404 |pmc=6095482 |pmid=30114187 |doi-access=free}}</ref><ref name="Harrison-2022" /><ref>{{Cite journal |last1=Harrison |first1=Stuart A. |last2=Lane |first2=Nick |date=2018-12-12 |title=Life as a guide to prebiotic nucleotide synthesis |journal=Nature Communications |language=en |volume=9 |issue=1 |page=5176 |doi=10.1038/s41467-018-07220-y |pmid=30538225 |issn=2041-1723 |pmc=6289992 |bibcode=2018NatCo...9.5176H}}</ref>

===== Life "seeded" from elsewhere =====
{{main|Panspermia}}
The Panspermia hypothesis does not explain how life arose originally, but simply examines the possibility of its coming from somewhere other than Earth. The idea that life on Earth was "seeded" from elsewhere in the Universe dates back at least to the Greek philosopher [[Anaximander]] in the sixth century [[BCE]].<ref>{{harvnb|O'Leary|2008}}</ref> In the twentieth century it was proposed by the [[physical chemist]] [[Svante Arrhenius]],<ref name="Arrhenius1903">{{harvnb|Arrhenius|1980|p=32}}</ref> by the [[astronomer]]s [[Fred Hoyle]] and [[Chandra Wickramasinghe]],<ref name="HoyleWickramasinghe1979">{{cite journal |last1=Hoyle |first1=Fred |author1-link=Fred Hoyle |last2=Wickramasinghe |first2=Nalin C. |author2-link=Chandra Wickramasinghe |date=November 1979 |title=On the Nature of Interstellar Grains |journal=[[Astrophysics and Space Science]] |volume=66 |issue=1 |pages=77–90 |bibcode=1979Ap&SS..66...77H |doi=10.1007/BF00648361 |s2cid=115165958 |issn=0004-640X}}</ref> and by [[molecular biologist]] [[Francis Crick]] and chemist [[Leslie Orgel]].<ref name="CrickOrgel1973">{{cite journal |last1=Crick |first1=Francis H. |author1-link=Francis Crick |last2=Orgel |first2=Leslie E. |author2-link=Leslie Orgel |date=July 1973 |title=Directed Panspermia |journal=[[Icarus (journal)|Icarus]] |volume=19 |issue=3 |pages=341–346 |bibcode=1973Icar...19..341C |doi=10.1016/0019-1035(73)90110-3 |issn=0019-1035}}</ref>
[[File:ALH84001 structures.jpg|thumb|Potential bacterial fossils found on the [[Allan Hills 84001|Allan Hills Meteorite]]]]
There are three main versions of the "seeded from elsewhere" hypothesis: from elsewhere in our Solar System via fragments knocked into space by a large [[Meteoroid#Meteor|meteor]] impact, in which case the most credible sources are [[Mars]]<ref name="Scientific-American-panspermia">{{cite magazine |last1=Warmflash |first1=David |last2=Weiss |first2=Benjamin |date=November 2005 |title=Did Life Come From Another World? |magazine=[[Scientific American]] |volume=293 |issue=5 |pages=64–71 |bibcode=2005SciAm.293e..64W |doi=10.1038/scientificamerican1105-64 |issn=0036-8733 |pmid=16318028}}</ref> and [[Venus]];<ref>{{cite journal |last1=Wickramasinghe |first1=Nalin C. |author1-link=Chandra Wickramasinghe |last2=Wickramasinghe |first2=Janaki T. |date=September 2008 |title=On the possibility of microbiota transfer from Venus to Earth |journal=[[Astrophysics and Space Science]] |volume=317 |issue=1–2 |pages=133–137 |bibcode=2008Ap&SS.317..133W |citeseerx=10.1.1.470.2347 |doi=10.1007/s10509-008-9851-2 |s2cid=14623053 |issn=0004-640X}}</ref> by [[directed panspermia|alien visitors]], possibly as a result of accidental [[Interplanetary contamination|contamination]] by microorganisms that they brought with them;<ref name="CrickOrgel1973" /> and from outside the Solar System but by natural means.<ref name="Arrhenius1903" /><ref name="Scientific-American-panspermia" />

Experiments in low Earth orbit, such as [[EXOSTACK]], have demonstrated that some microorganism [[spore]]s can survive the shock of being catapulted into space and some can survive exposure to outer space radiation for at least 5.7 years.<ref name="Clancy">{{harvnb|Clancy|Brack|Horneck|2005}}</ref><ref name="Horneck">{{cite journal |last1=Horneck |first1=Gerda |last2=Klaus |first2=David M. |last3=Mancinelli |first3=Rocco L. |date=March 2010 |title=Space Microbiology |journal=[[Microbiology and Molecular Biology Reviews]] |volume=74 |issue=1 |pages=121–156 |bibcode=2010MMBR...74..121H |doi=10.1128/mmbr.00016-09 |pmc=2832349 |pmid=20197502}}</ref> Meteorite [[ALH84001]], which was once part of the [[Martian soil|Martian]] crust, shows evidence of carbonate-globules with texture and size indicative of terrestrial bacterial activity.<ref>{{Cite journal |last1=McKay |first1=David S. |last2=Gibson |first2=Everett K. |last3=Thomas-Keprta |first3=Kathie L. |last4=Vali |first4=Hojatollah |last5=Romanek |first5=Christopher S. |last6=Clemett |first6=Simon J. |last7=Chillier |first7=Xavier D. F. |last8=Maechling |first8=Claude R. |last9=Zare |first9=Richard N. |date=1996-08-16 |title=Search for Past Life on Mars: Possible Relic Biogenic Activity in Martian Meteorite ALH84001 |url=https://www.science.org/doi/10.1126/science.273.5277.924 |journal=Science |language=en |volume=273 |issue=5277 |pages=924–930 |doi=10.1126/science.273.5277.924 |pmid=8688069 |bibcode=1996Sci...273..924M |s2cid=40690489 |issn=0036-8075 |access-date=2023-02-26 |archive-date=2023-02-21 |archive-url=https://web.archive.org/web/20230221231234/https://www.science.org/doi/10.1126/science.273.5277.924 |url-status=live|url-access=subscription }}</ref> Scientists are divided over the likelihood of life arising independently on Mars,<ref>{{cite news |url=https://www.space.com/4267-claim-martian-life-called-bogus.html |url-status=live |title=Claim of Martian Life Called 'Bogus' |last=Than |first=Ker |date=August 23, 2007 |work=Space.com |publisher=[[Imaginova]] |location=Watsonville, CA |archive-url=https://web.archive.org/web/20110508014308/https://www.space.com/4267-claim-martian-life-called-bogus.html |archive-date=2011-05-08 |access-date=2015-01-25}}</ref> or on other planets in our [[galaxy]].<ref name="Scientific-American-panspermia" />

===== Carbonate-rich lakes =====
One theory traces the [[origins of life]] to the abundant carbonate-rich lakes which would have dotted the [[early Earth]]. [[Phosphate]] would have been an essential cornerstone to the origin of life since it is a critical component of [[nucleotide]]s, [[phospholipid]]s, and [[adenosine triphosphate]].<ref name="Schwartz-2006">{{Cite journal |last=Schwartz |first=Alan W. |date=2006-09-07 |title=Phosphorus in prebiotic chemistry |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=361 |issue=1474 |pages=1743–1749 |doi=10.1098/rstb.2006.1901 |pmid=17008215 |pmc=1664685 |issn=0962-8436}}</ref> Phosphate is often depleted in natural environments due to its uptake by microbes and its affinity for [[calcium]] ions. In a process called '[[apatite]] precipitation', free phosphate ions react with the calcium ions abundant in water to precipitate out of solution as apatite minerals.<ref name="Schwartz-2006" /> When attempting to simulate prebiotic [[phosphorylation]], scientists have only found success when using phosphorus levels far above modern day natural concentrations.<ref name="Toner-2020" />

This problem of low phosphate is solved in [[carbonate]]-rich environments. When in the presence of carbonate, calcium readily reacts to form [[calcium carbonate]] instead of apatite minerals.<ref name="Nathan-1981">{{Cite journal |last1=Nathan |first1=Yaacov |last2=Sass |first2=Eytan |date=November 1981 |title=Stability relations of apatites and calcium carbonates |journal=Chemical Geology |volume=34 |issue=1–2 |pages=103–111 |doi=10.1016/0009-2541(81)90075-9 |bibcode=1981ChGeo..34..103N |issn=0009-2541}}</ref> With the free calcium ions removed from [[Solution (chemistry)|solution]], phosphate ions are no longer precipitated from solution.<ref name="Nathan-1981" /> This is specifically seen in lakes with no inflow, since no new calcium is introduced into the water body.<ref name="Toner-2020" /> After all of the calcium is sequestered into calcium carbonate ([[calcite]]), phosphate concentrations are able to increase to levels necessary for facilitating [[biomolecule]] creation.<ref>{{Cite journal |last=Gulbrandsen |first=R. A. |date=1969-06-01 |title=Physical and chemical factors in the formation of marine apatite |journal=Economic Geology |volume=64 |issue=4 |pages=365–382 |doi=10.2113/gsecongeo.64.4.365 |bibcode=1969EcGeo..64..365G |issn=1554-0774}}</ref>

Though carbonate-rich lakes have [[Base (chemistry)|alkaline]] chemistry in modern times, models suggest that carbonate lakes had a pH low enough for prebiotic synthesis when placed in the acidifying context of Earth's early [[Early atmosphere|carbon dioxide rich atmosphere]].<ref name="Toner-2020" /> Rainwater rich in [[carbonic acid]] [[Weathering|weathered]] the rock on the surface of the Earth at rates far greater than today.<ref>{{Cite journal |last1=Zahnle |first1=K. |last2=Schaefer |first2=L. |last3=Fegley |first3=B. |date=2010-06-23 |title=Earth's Earliest Atmospheres |journal=Cold Spring Harbor Perspectives in Biology |volume=2 |issue=10 |pages=a004895 |doi=10.1101/cshperspect.a004895 |pmid=20573713 |pmc=2944365 |issn=1943-0264}}</ref> With high phosphate influx, no phosphate precipitation, and no [[microbial]] usage of phosphate at this time, models show phosphate reached concentrations approximately 100 times greater than they are today.<ref name="Toner-2020" /> Modeled [[pH]] and phosphate levels of early Earth carbonate-rich lakes nearly match the conditions used in current laboratory experiments on the origin of life.<ref name="Toner-2020" />

Similar to the process predicted by [[Geothermal springs|geothermal]] [https://www.liebertpub.com/doi/10.1089/ast.2019.2045 hot spring hypotheses], changing lake levels and wave action deposited phosphorus-rich brine onto dry shore and marginal pools.<ref name="Damer-2020" /> This drying of the solution promotes [[polymerization]] reactions and removes enough water to promote phosphorylation, a process integral to biological energy storage and transfer.<ref name="Toner-2020" /><ref name="Damer-2020" /><ref>{{Cite journal |last1=Ross |first1=David |last2=Deamer |first2=David |date=2016-07-26 |title=Dry/Wet Cycling and the Thermodynamics and Kinetics of Prebiotic Polymer Synthesis |journal=Life |volume=6 |issue=3 |page=28 |doi=10.3390/life6030028 |pmid=27472365 |pmc=5041004 |bibcode=2016Life....6...28R |issn=2075-1729 |doi-access=free}}</ref> When washed away by further precipitation and wave action, researchers concluded these newly formed biomolecules may have washed back into the lake - allowing the first prebiotic syntheses on Earth to occur.<ref name="Toner-2020" />

== Environmental and evolutionary impact of microbial mats ==
{{main|Microbial mat|Great Oxidation Event}}
[[File:Stromatolites in Sharkbay.jpg|right|thumb|Modern [[stromatolite]]s in [[Shark Bay]], [[Western Australia]]]]
Microbial mats are multi-layered, multi-species colonies of bacteria and other organisms that are generally only a few millimeters thick, but still contain a wide range of chemical environments, each of which favors a different set of microorganisms.<ref name="KrumbeinBrehmEtAl2003">{{harvnb|Krumbein|Brehm|Gerdes|Gorbushina|2003|pp=1–28}}</ref> To some extent each mat forms its own [[food chain]], as the by-products of each group of microorganisms generally serve as "food" for adjacent groups.<ref name="RisattiEtAl1994">{{cite journal |last1=Risatti |first1=J. Bruno |last2=Capman |first2=William C. |last3=Stahl |first3=David A. |date=October 11, 1994 |title=Community structure of a microbial mat: The phylogenetic dimension |journal=[[Proc. Natl. Acad. Sci. U.S.A.]] |volume=91 |issue=21 |pages=10173–10177 |bibcode=1994PNAS...9110173R |doi=10.1073/pnas.91.21.10173 |issn=0027-8424 |pmc=44980 |pmid=7937858 |doi-access=free}}</ref>
[[File:Yorgia_trace.jpg|thumb|left| Traces like [[Epibaion]] from the [[Ediacaran]] represent trace fossils of feeding and movement by members of the phylum [[proarticulata]]]]
[[Stromatolite]]s are stubby pillars built as microorganisms in mats slowly migrate upwards to avoid being smothered by sediment deposited on them by water.<ref name="KrumbeinBrehmEtAl2003" /> There has been vigorous debate about the validity of alleged stromatolite fossils from before 3 Ga,<ref>{{cite journal |author=<!--Staff writer(s); no by-line.--> |date=June 8, 2006 |title=Biodiversity rocks |url=https://www.nature.com/nature/journal/v441/n7094/edsumm/e060608-01.html |journal=[[Nature (journal)|Nature]] |type=Editor's summary |volume=441 |issue=7094 |issn=0028-0836 |archive-url=https://web.archive.org/web/20060619105759/https://www.nature.com/nature/journal/v441/n7094/edsumm/e060608-01.html |archive-date=2006-06-19 |access-date=2020-03-03}}
*{{cite journal |last=Awramik |first=Stanley M. |author-link=Stanley Awramik |date=June 8, 2006 |title=Respect for stromatolites |journal=[[Nature (journal)|Nature]] |volume=441 |issue=7094 |pages=700–701 |doi=10.1038/441700a |pmid=16760962 |s2cid=4429617 |issn=0028-0836}}
*{{harvnb|Allwood|Walter|Kamber et al.|2006}}</ref> with critics arguing that they could have been formed by non-biological processes.<ref name="GrotzingerRothman1996" /> In 2006, another find of stromatolites was reported from the same part of Australia, in rocks dated to 3.5 Ga.<ref>{{cite journal |last1=Allwood |first1=Abigail C. |last2=Walter |first2=Malcolm R. |last3=Kamber |first3=Balz S. |last4=Marshall |first4=Craig P. |last5=Burch |first5=Ian W. |display-authors=3 |date=June 8, 2006 |title=Stromatolite reef from the Early Archaean era of Australia |journal=[[Nature (journal)|Nature]] |volume=441 |issue=7094 |pages=714–718 |bibcode=2006Natur.441..714A |doi=10.1038/nature04764 |s2cid=4417746 |pmid=16760969 |issn=0028-0836 |ref={{harvid|Allwood|Walter|Kamber et al.|2006}}}}</ref><!-- previous version **** 
[[Stromatolite]]s are stubby pillars built as microbes in mats slowly migrate upwards to avoid being smothered by sediment deposited on them by water.<ref name="KrumbeinBrehmEtAl2003" /> Although earlier reports of fossilized stromatolites from about 3.5 Ga were criticized on the grounds that the structures in the rocks could have been produced by non-biological processes,<ref name="GrotzingerRothman1996" /> in 2006 another find of stromatolites was reported from the same part of Australia, in rocks also dated to 3.5 Ga.<ref>{{cite journal |author=Allwood, A.C., Walter, M.R., Kamber, B.S., Marshall1, C.P. and Burch, I.W. |title=Stromatolite reef from the Early Archaean era of Australia |journal=[[Nature (journal)|Nature]] |volume=441 |pages=714–718 |date=June 2006 |doi=10.1038/nature04764 |url=http://www.nature.com/nature/journal/v441/n7094/abs/nature04764.html |access-date=2008-08-31 |pmid=16760969 |issue=7094 |issn=0028-0836 |bibcode=2006Natur.441..714A}}</ref> **** -->

In modern underwater mats the top layer often consists of photosynthesizing [[cyanobacteria]] which create an oxygen-rich environment, while the bottom layer is oxygen-free and often dominated by hydrogen sulfide emitted by the organisms living there.<ref name="RisattiEtAl1994" /> Oxygen is toxic to organisms that are not adapted to it, but greatly increases the [[metabolic]] efficiency of oxygen-adapted organisms;<ref name="Abele2002">{{cite journal |last=Abele |first=Doris |author-link=Doris Abele |date=November 7, 2002 |title=Toxic oxygen: The radical life-giver |url=https://epic.awi.de/id/eprint/6249/1/Abe2002b.pdf |journal=[[Nature (journal)|Nature]] |volume=420 |issue=6911 |page=27 |bibcode=2002Natur.420...27A |doi=10.1038/420027a |issn=0028-0836 |pmid=12422197 |access-date=2020-03-03 |s2cid=4317378 |archive-date=2022-05-12 |archive-url=https://web.archive.org/web/20220512081832/https://epic.awi.de/id/eprint/6249/1/Abe2002b.pdf |url-status=live}}</ref><ref>{{cite web |last=Westerdahl |first=Becky B. |year=2007 |title=Introduction to Aerobic Respiration |url=http://trc.ucdavis.edu/biosci10v/bis10v/week3/06aerobicrespirintro.html |archive-url=https://web.archive.org/web/20071029120120/http://trc.ucdavis.edu/biosci10v/bis10v/week3/06aerobicrespirintro.html |archive-date=2007-10-29 |access-date=2008-07-14 |website=Biological Science 10V |publisher=[[University of California, Davis]] |location=Davis, CA |type=Lecture}}

* Textbook used for lecture: {{harvnb|Starr|Evers|Starr|2007}}.</ref> [[Oxygen#Photosynthesis and respiration|oxygenic photosynthesis]] by bacteria in mats increased biological productivity by a factor of between 100 and 1,000. The source of hydrogen atoms used by oxygenic photosynthesis is water, which is much more plentiful than the geologically produced [[reducing agents]] required by the earlier non-oxygenic photosynthesis.<ref name="Blankenship2001">{{cite journal |last=Blankenship |first=Robert E. |author1-link=Robert E. Blankenship |date=January 1, 2001 |title=Molecular evidence for the evolution of photosynthesis |journal=[[Trends (journals)|Trends in Plant Science]] |volume=6 |issue=1 |pages=4–6 |doi=10.1016/S1360-1385(00)01831-8 |pmid=11164357 |bibcode=2001TPS.....6....4B}}</ref> From this point onwards life itself produced significantly more of the resources it needed than did geochemical processes.<ref name="HoehlerBeboutMarais2001">{{cite journal |last1=Hoehler |first1=Tori M. |last2=Bebout |first2=Brad M. |last3=Des Marais |first3=David J. |date=July 19, 2001 |title=The role of microbial mats in the production of reduced gases on the early Earth |journal=[[Nature (journal)|Nature]] |volume=412 |issue=6844 |pages=324–327 |doi=10.1038/35085554 |issn=0028-0836 |pmid=11460161 |bibcode=2001Natur.412..324H |s2cid=4365775}}</ref>

Oxygen became a significant component of Earth's atmosphere about 2.4 Ga.<ref>{{cite journal |last1=Goldblatt |first1=Colin |last2=Lenton |first2=Timothy M. |author2-link=Tim Lenton |last3=Watson |first3=Andrew J. |author3-link=Andrew Watson (scientist) |year=2006 |title=The Great Oxidation at ~2.4 Ga as a bistability in atmospheric oxygen due to UV shielding by ozone |url=https://www.cosis.net/abstracts/EGU06/00770/EGU06-J-00770.pdf |url-status=live |journal=Geophysical Research Abstracts |volume=8 |issue=770 |issn=1029-7006 |id=SRef-ID: 1607-7962/gra/EGU06-A-00770 |archive-url=https://web.archive.org/web/20070926163715/https://www.cosis.net/abstracts/EGU06/00770/EGU06-J-00770.pdf |archive-date=2007-09-26 |access-date=2020-03-05}}</ref> Although eukaryotes may have been present much earlier,<ref name="GlansdorffXuLabedan2008">{{cite journal |last1=Glansdorff |first1=Nicolas |last2=Ying |first2=Xu |last3=Labedan |first3=Bernard |date=July 9, 2008 |title=The Last Universal Common Ancestor: emergence, constitution and genetic legacy of an elusive forerunner |journal=[[Biology Direct]] |volume=3 |issue=29 |page=29 |doi=10.1186/1745-6150-3-29 |issn=1745-6150 |pmc=2478661 |pmid=18613974 |doi-access=free}}</ref><ref name="BrocksLoganEtAl1999">{{cite journal |last1=Brocks |first1=Jochen J. |last2=Logan |first2=Graham A. |last3=Buick |first3=Roger |last4=Summons |first4=Roger E. |author4-link=Roger Everett Summons |display-authors=3 |date=August 13, 1999 |title=Archean Molecular Fossils and the Early Rise of Eukaryotes |journal=[[Science (journal)|Science]] |volume=285 |issue=5430 |pages=1033–1036 |doi=10.1126/science.285.5430.1033 |issn=0036-8075 |pmid=10446042 |bibcode=1999Sci...285.1033B |citeseerx=10.1.1.516.9123}}</ref> the oxygenation of the atmosphere was a prerequisite for the evolution of the most complex eukaryotic cells, from which all multicellular organisms are built.<ref name="HedgesBlairEtAl2004">{{cite journal |last1=Hedges |first1=S. Blair |author1-link=Stephen Blair Hedges |last2=Blair |first2=Jaime E. |last3=Venturi |first3=Maria L. |last4=Shoe |first4=Jason L. |display-authors=3 |date=January 28, 2004 |title=A molecular timescale of eukaryote evolution and the rise of complex multicellular life |journal=[[BMC Evolutionary Biology]] |volume=4 |page=2 |doi=10.1186/1471-2148-4-2 |issn=1471-2148 |pmc=341452 |pmid=15005799 |doi-access=free}}</ref> The boundary between oxygen-rich and oxygen-free layers in microbial mats would have moved upwards when photosynthesis shut down overnight, and then downwards as it resumed on the next day. This would have created [[selection pressure]] for organisms in this intermediate zone to acquire the ability to tolerate and then to use oxygen, possibly via [[endosymbiosis]], where one organism lives inside another and both of them benefit from their association.<ref name="NisbetFowler1999" />

Cyanobacteria have the most complete biochemical "toolkits" of all the mat-forming organisms. Hence they are the most self-sufficient, well-adapted to strike out on their own both as floating mats and as the first of the [[phytoplankton]], provide the basis of most marine food chains.<ref name="NisbetFowler1999" />

== Diversification of eukaryotes ==

{{main|Eukaryote}}

===Chromatin, nucleus, endomembrane system, and mitochondria===

Eukaryotes may have been present long before the oxygenation of the atmosphere,<ref name="GlansdorffXuLabedan2008" /> but most modern eukaryotes require oxygen, which is used by their [[mitochondria]] to fuel the production of [[Adenosine triphosphate|ATP]], the internal energy supply of all known cells.<ref name="HedgesBlairEtAl2004" /> In the 1970s, a vigorous debate concluded that eukaryotes emerged as a result of a sequence of endosymbiosis between [[prokaryote]]s. For example: a [[predator]]y microorganism invaded a large prokaryote, probably an [[archaea]]n, but instead of killing its prey, the attacker took up residence and evolved into mitochondria; one of these [[Chimera (genetics)|chimera]]s later tried to swallow a photosynthesizing cyanobacterium, but the victim survived inside the attacker and the new combination became the ancestor of [[plant]]s; and so on. After each endosymbiosis, the partners eventually eliminated unproductive duplication of genetic functions by re-arranging their genomes, a process which sometimes involved transfer of genes between them.<ref>{{harvnb|Margulis|1981}}</ref><ref>{{cite journal |last1=Vellai |first1=Tibor |last2=Vida |first2=Gábor |date=August 7, 1999 |title=The origin of eukaryotes: the difference between prokaryotic and eukaryotic cells |journal=[[Proceedings of the Royal Society B]] |volume=266 |issue=1428 |pages=1571–1577 |doi=10.1098/rspb.1999.0817 |issn=0962-8452 |pmc=1690172 |pmid=10467746}}</ref><ref>{{cite journal |last1=Selosse |first1=Marc-André |last2=Abert |first2=Béatrice |last3=Godelle |first3=Bernard |date=March 1, 2001 |title=Reducing the genome size of organelles favours gene transfer to the nucleus |journal=[[Trends in Ecology & Evolution]] |volume=16 |issue=3 |pages=135–141 |doi=10.1016/S0169-5347(00)02084-X |issn=0169-5347 |pmid=11179577}}</ref> Another hypothesis proposes that mitochondria were originally [[sulfur]]- or [[hydrogen]]-metabolising endosymbionts, and became oxygen-consumers later.<ref>{{cite journal |last1=Pisani |first1=Davide |last2=Cotton |first2=James A. |last3=McInerney |first3=James O. |author3-link=James O. McInerney |date=August 2007 |title=Supertrees Disentangle the Chimerical Origin of Eukaryotic Genomes |journal=[[Molecular Biology and Evolution]] |volume=24 |issue=8 |pages=1752–1760 |doi=10.1093/molbev/msm095 |doi-access=free |issn=0737-4038 |pmid=17504772}}</ref> On the other hand, mitochondria might have been part of eukaryotes' original equipment.<ref>{{cite journal |last1=Gray |first1=Michael W. |last2=Burger |first2=Gertraud |last3=Lang |first3=B. Franz |date=March 5, 1999 |title=Mitochondrial Evolution |journal=[[Science (journal)|Science]] |volume=283 |issue=5407 |pages=1476–1481 |bibcode=1999Sci...283.1476G |doi=10.1126/science.283.5407.1476 |issn=0036-8075 |pmid=10066161 |pmc=3428767}}
*{{cite journal |last=Gray |first=Michael W. |date=September 2012 |title=Mitochondrial Evolution |journal=Cold Spring Harbor Perspectives in Biology |volume=4 |issue=9 |doi=10.1101/cshperspect.a011403 |page=a011403 |issn=1943-0264 |pmc=3428767 |pmid=22952398}}</ref>

There is a debate about when eukaryotes first appeared: the presence of [[sterane]]s in Australian [[shale]]s may indicate eukaryotes at 2.7 Ga;<ref name="BrocksLoganEtAl1999" /> however, an analysis in 2008 concluded that these chemicals infiltrated the rocks less than 2.2 Ga and prove nothing about the origins of eukaryotes.<ref>{{cite journal |last1=Rasmussen |first1=Birger |last2=Fletcher |first2=Ian R. |last3=Brocks |first3=Jochen J. |last4=Kilburn |first4=Matt R. |display-authors=3 |date=October 23, 2008 |title=Reassessing the first appearance of eukaryotes and cyanobacteria |journal=[[Nature (journal)|Nature]] |volume=455 |issue=7216 |pages=1101–1104 |bibcode=2008Natur.455.1101R |doi=10.1038/nature07381 |s2cid=4372071 |issn=0028-0836 |pmid=18948954}}</ref> Fossils of the [[algae]] ''[[Grypania]]'' have been reported in 1.85 billion-year-old rocks (originally dated to 2.1 Ga but later revised<ref name="Fedonkin2003" />), indicating that eukaryotes with organelles had already evolved.<ref>{{cite journal |last1=Tsu-Ming |first1=Han |last2=Runnegar |first2=Bruce |author2-link=Bruce Runnegar |date=July 10, 1992 |title=Megascopic eukaryotic algae from the 2.1-billion-year-old negaunee iron-formation, Michigan |journal=[[Science (journal)|Science]] |volume=257 |issue=5067 |pages=232–235 |bibcode=1992Sci...257..232H |doi=10.1126/science.1631544 |issn=0036-8075 |pmid=1631544}}</ref> A diverse collection of fossil algae were found in rocks dated between 1.5 and 1.4 Ga.<ref>{{cite journal |last1=Javaux |first1=Emmanuelle J. |last2=Knoll |first2=Andrew H. |author2-link=Andrew H. Knoll |last3=Walter |first3=Malcolm R. |date=July 2004 |title=TEM evidence for eukaryotic diversity in mid-Proterozoic oceans |journal=[[Geobiology (journal)|Geobiology]] |volume=2 |issue=3 |pages=121–132 |doi=10.1111/j.1472-4677.2004.00027.x |bibcode=2004Gbio....2..121J |s2cid=53600639 |issn=1472-4677}}</ref> The earliest known fossils of [[fungi]] date from 1.43 Ga.<ref name="Butterfield2005">{{cite journal |last=Butterfield |first=Nicholas J. |date=Winter 2005 |title=Probable Proterozoic fungi |url=https://pubs.geoscienceworld.org/paleobiol/article-abstract/31/1/165/86457/Probable-Proterozoic-fungi?redirectedFrom=fulltext |url-status=live |journal=[[Paleobiology (journal)|Paleobiology]] |volume=31 |issue=1 |pages=165–182 |doi=10.1666/0094-8373(2005)031<0165:PPF>2.0.CO;2 |bibcode=2005Pbio...31..165B |s2cid=86332371 |issn=0094-8373 |archive-url=https://web.archive.org/web/20181223150938/https://pubs.geoscienceworld.org/paleobiol/article-abstract/31/1/165/86457/Probable-Proterozoic-fungi?redirectedFrom=fulltext |archive-date=2018-12-23 |access-date=2020-03-10|url-access=subscription }}</ref>

=== Plastids ===
[[Plastid]]s, the superclass of [[organelle]]s of which [[chloroplast]]s are the best-known exemplar, are thought to have originated from [[endosymbiotic]] cyanobacteria. The symbiosis evolved around 1.5 Ga and enabled eukaryotes to carry out [[Carbon fixation#Oxygenic photosynthesis|oxygenic photosynthesis]].<ref name="HedgesBlairEtAl2004"/> Three evolutionary lineages of photosynthetic plastids <!--there are many other non-photosynthetic kinds-->have since emerged: chloroplasts in [[green algae]] and plants, [[rhodoplast]]s in [[red algae]] and [[cyanelle]]s in the [[glaucophyte]]s.<ref>{{cite book |last=Wise |first=Robert R. |chapter=1. The Diversity of Plastid Form and Function |title=Advances in Photosynthesis and Respiration |publisher=Springer |date=2006 |volume=23 |pages=3–26 |doi=10.1007/978-1-4020-4061-0_1 |isbn=978-1-4020-4060-3}}</ref> Not long after this primary endosymbiosis of plastids, rhodoplasts and chloroplasts were passed down to other [[bikont]]s, establishing a eukaryotic assemblage of phytoplankton by the end of the [[Neoproterozoic]] Eon.

== Sexual reproduction and multicellular organisms ==

=== Evolution of sexual reproduction ===
{{Main|Evolution of sexual reproduction}}

The defining characteristics of [[sexual reproduction]] in eukaryotes are [[meiosis]] and [[fertilization]], resulting in [[genetic recombination]], giving offspring 50% of their genes from each parent.<ref name="Jokela2001" /> By contrast, in [[asexual reproduction]] there is no recombination, but occasional [[horizontal gene transfer]]. Bacteria also exchange DNA by [[bacterial conjugation]], enabling the spread of resistance to [[antibiotic resistance|antibiotic]]s and other [[toxin]]s, and the ability to utilize new [[metabolite]]s.<ref>{{harvnb|Holmes|Jobling|1996}}</ref> However, conjugation is not a means of reproduction, and is not limited to members of the same species&nbsp;– there are cases where bacteria transfer DNA to plants and animals.<ref name="Christie, P. J. 2001 294–305">{{cite journal |last=Christie |first=Peter J. |date=April 2001 |title=Type IV secretion: intercellular transfer of macromolecules by systems ancestrally related to conjugation machines |journal=[[Molecular Microbiology (journal)|Molecular Microbiology]] |volume=40 |issue=2 |pages=294–305 |doi=10.1046/j.1365-2958.2001.02302.x |issn=0950-382X |pmc=3922410 |pmid=11309113}}</ref>

On the other hand, bacterial [[Transformation (genetics)|transformation]] is clearly an adaptation for transfer of DNA between bacteria of the same species. This is a complex process involving the products of numerous bacterial genes and can be regarded as a bacterial form of sex.<ref>{{cite journal |last1=Michod |first1=Richard E. |last2=Bernstein |first2=Harris |last3=Nedelcu |first3=Aurora M. |date=May 2008 |title=Adaptive value of sex in microbial pathogens |url=http://www.hummingbirds.arizona.edu/Faculty/Michod/Downloads/IGE%20review%20sex.pdf |journal=[[Infection, Genetics and Evolution]] |volume=8 |issue=3 |pages=267–285 |doi=10.1016/j.meegid.2008.01.002 |pmid=18295550 |bibcode=2008InfGE...8..267M |issn=1567-1348 |access-date=2013-07-21 |archive-date=2020-05-11 |archive-url=https://web.archive.org/web/20200511153411/http://www.hummingbirds.arizona.edu/Faculty/Michod/Downloads/IGE%20review%20sex.pdf |url-status=live}}</ref><ref>{{cite journal |last1=Bernstein |first1=Harris |last2=Bernstein |first2=Carol |date=July 2010 |title=Evolutionary Origin of Recombination during Meiosis |journal=[[BioScience]] |volume=60 |issue=7 |pages=498–505 |doi=10.1525/bio.2010.60.7.5 |s2cid=86663600 |issn=0006-3568}}</ref> This process occurs naturally in at least 67 prokaryotic species (in seven different phyla).<ref>{{cite journal |last1=Johnsborg |first1=Ola |last2=Eldholm |first2=Vegard |last3=Håvarstein |first3=Leiv Sigve |date=December 2007 |title=Natural genetic transformation: prevalence, mechanisms and function |journal=Research in Microbiology |volume=158 |issue=10 |pages=767–778 |doi=10.1016/j.resmic.2007.09.004 |issn=0923-2508 |pmid=17997281 |doi-access=free}}</ref> Sexual reproduction in eukaryotes may have evolved from bacterial transformation.<ref name="novapublishers.com">{{harvnb|Bernstein|Bernstein|Michod|2012|pp=1–50}}</ref>

The disadvantages of sexual reproduction are well-known: the genetic reshuffle of recombination may break up favorable combinations of genes; and since males do not directly increase the number of offspring in the next generation, an asexual population can out-breed and displace in as little as 50 generations a sexual population that is equal in every other respect.<ref name="Jokela2001">{{cite encyclopedia |last1=Neiman |first1=Maurine |last2=Jokela |first2=Jukka |encyclopedia=[[Encyclopedia of Life Sciences]] |year=2010 |publisher=[[John Wiley & Sons]] |location=Hoboken, NJ |isbn=978-0-470-01617-6 |doi=10.1002/9780470015902.a0001716.pub2 |chapter=Sex: Advantage}}</ref> Nevertheless, the great majority of animals, plants, fungi and [[protist]]s reproduce sexually. There is strong evidence that sexual reproduction arose early in the history of eukaryotes and that the genes controlling it have changed very little since then.<ref>{{cite journal |last1=Ramesh |first1=Marilee A. |last2=Malik |first2=Shehre-Banoo |last3=Logsdon |first3=John M. Jr. |date=January 26, 2005 |title=A Phylogenomic Inventory of Meiotic Genes: Evidence for Sex in ''Giardia'' and an Early Eukaryotic Origin of Meiosis |journal=[[Current Biology]] |volume=15 |issue=2 |pages=185–191 |doi=10.1016/j.cub.2005.01.003 |s2cid=17013247 |issn=0960-9822 |pmid=15668177 |doi-access=free |bibcode=2005CBio...15..185R}}</ref> How sexual reproduction evolved and survived is an unsolved puzzle.<ref name="OttoGerstein2006">{{cite journal |last1=Otto |first1=Sarah P. |author1-link=Sarah Otto |last2=Gerstein |first2=Aleeza C. |date=August 2006 |title=Why have sex? The population genetics of sex and recombination |journal=[[Biochemical Society Transactions]] |volume=34 |issue=4 |pages=519–522 |doi=10.1042/BST0340519 |issn=0300-5127 |pmid=16856849}}</ref>

[[File:Horodyskia per Fedonkin 2003.png|thumb|right|''[[Horodyskia]]'' may have been an early [[metazoan]],<ref name="Fedonkin2003" /> or a [[colony (biology)|colonial]] [[foraminifera]]n.<ref name="DongEtAl2008" /> It apparently re-arranged itself into fewer but larger main masses as the sediment grew deeper round its base.<ref name="Fedonkin2003" />]]

The [[Red Queen hypothesis]] suggests that sexual reproduction provides protection against [[parasite]]s, because it is easier for parasites to evolve means of overcoming the defenses of genetically identical [[cloning|clone]]s than those of sexual species that present moving targets, and there is some experimental evidence for this. However, there is still doubt about whether it would explain the survival of sexual species if multiple similar clone species were present, as one of the clones may survive the attacks of parasites for long enough to out-breed the sexual species.<ref name="Jokela2001" /> Furthermore, contrary to the expectations of the Red Queen hypothesis, Kathryn A. Hanley et al. found that the prevalence, abundance and mean intensity of mites was significantly higher in sexual geckos than in asexuals sharing the same habitat.<ref>{{cite journal |last1=Hanley |first1=Kathryn A. |last2=Fisher |first2=Robert N. |last3=Case |first3=Ted J. |date=June 1995 |title=Lower Mite Infestations in an Asexual Gecko Compared With Its Sexual Ancestors |journal=[[Evolution (journal)|Evolution]] |volume=49 |issue=3 |pages=418–426 |doi=10.2307/2410266 |issn=0014-3820 |jstor=2410266 |pmid=28565091}}</ref> In addition, biologist Matthew Parker, after reviewing numerous genetic studies on plant disease resistance, failed to find a single example consistent with the concept that pathogens are the primary selective agent responsible for sexual reproduction in the host.<ref>{{cite journal |last=Parker |first=Matthew A. |date=September 1994 |title=Pathogens and sex in plants |journal=[[Evolutionary Ecology (journal)|Evolutionary Ecology]] |volume=8 |issue=5 |pages=560–584 |doi=10.1007/bf01238258 |bibcode=1994EvEco...8..560P |s2cid=31756267 |issn=0269-7653}}</ref>

[[Alexey Kondrashov]]'s ''deterministic mutation hypothesis'' (DMH) assumes that each organism has more than one harmful mutation and that the combined effects of these mutations are more harmful than the sum of the harm done by each individual mutation. If so, sexual recombination of genes will reduce the harm that bad mutations do to offspring and at the same time eliminate some bad mutations from the [[gene pool]] by isolating them in individuals that perish quickly because they have an above-average number of bad mutations. However, the evidence suggests that the DMH's assumptions are shaky because many species have on average less than one harmful mutation per individual and no species that has been investigated shows evidence of [[synergy]] between harmful mutations.<ref name="Jokela2001" />

The random nature of recombination causes the relative abundance of alternative traits to vary from one generation to another. This [[genetic drift]] is insufficient on its own to make sexual reproduction advantageous, but a combination of genetic drift and natural selection may be sufficient. When chance produces combinations of good traits, natural selection gives a large advantage to lineages in which these traits become genetically linked. On the other hand, the benefits of good traits are neutralized if they appear along with bad traits. Sexual recombination gives good traits the opportunities to become linked with other good traits, and mathematical models suggest this may be more than enough to offset the disadvantages of sexual reproduction.<ref name="OttoGerstein2006" /> Other combinations of hypotheses that are inadequate on their own are also being examined.<ref name="Jokela2001" />
<!--
The following hypotheses attempt to explain how and why sex evolved:
*It may have enabled organisms to repair genetic damage.<ref name="dna-repair">{{cite journal |author=Bernstein, H., Byerly, H., Hopf, F. and Michod, R. |title=Origin of sex |year=1984 |journal=Journal of Theoretical Biology |volume=110 |issue=3 |pages=323–351 |pmid=6209512 |doi=10.1016/S0022-5193(84)80178-2}}</ref> The most primitive form of sex may have been one organism repairing damaged DNA by replicating an undamaged strand from a similar organism.<ref name="Judson2002">{{cite book |author=Judson, O. |title=Dr. Tatiana's sex advice to all creation |url=https://archive.org/details/drtatianassexadv00juds |url-access=registration |publisher=Metropolitan Books |location=New York |year=2002 |isbn=0-8050-6331-5 |pages=[https://archive.org/details/drtatianassexadv00juds/page/233 233–4]}}</ref>
*Sexual reproduction may have originated from [[Selfish DNA|selfish parasitic genetic elements]] propagating themselves by transfer to new hosts.<ref name="hickey">{{cite journal |author=Hickey, D. |title=Selfish DNA: A Sexually-Transmitted Nuclear Parasite |journal=Genetics |volume=101 |issue=3–4 |pages=519–31 |year=1982 |pmid=6293914 |pmc=1201875}}</ref>
*It may have evolved from [[cannibalism]], where some of the victim's DNA was incorporated into the cannibal organism.<ref name="Judson2002"/>
*Sexual reproduction may have evolved from ancient [[haloarchaea]] through a combination of [[jumping genes]], and swapping [[plasmid]]s.<ref>{{cite journal |title=Extreme Microbes |first1=S. |last1=DasSarma |doi=10.1511/2007.65.1024 |journal=American Scientist |year=2007 |volume=95 |issue=3 |pages=224–231}}</ref>
*Or it may have evolved as a form of [[vaccination]] in which infected hosts exchanged weakened [[symbiotic]] copies of parasitic DNA as protection against more virulent versions. The [[meiosis]] stage of sexual reproduction may then have evolved as a way of removing the symbiotes.<ref name="Sterrer">{{cite journal |author=Sterrer, W. |title=On the origin of sex as vaccination |journal=Journal of Theoretical Biology |volume=216 |pages=387–96 |year=2002 |doi=10.1006/jtbi.2002.3008 |pmid=12151256 |issue=4}}</ref>
[[Bacteria]] also exchange [[DNA]] by [[bacterial conjugation]], the benefits of which include resistance to [[antibiotic resistance|antibiotics]] and other [[toxin]]s, and the ability to utilize new [[metabolite]]s.<ref>{{cite book |last1=Holmes |first1=Randall K. |last2=Jobling |first2=Michael G. |year=1996 |chapter=Genetics |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK7908/ |editor-last=Baron |editor-first=Samuel |title=Medical Microbiology |edition=4th |location=Galveston, TX |publisher=[[University of Texas Medical Branch]] |at=Exchange of Genetic Information |isbn=0-9631172-1-1 |lccn=95050499 |oclc=33838234 |pmid=21413277}}</ref> However, conjugation is not a means of reproduction and is not limited to members of the same species, and there are cases where bacteria transfer DNA to plants and animals.<ref name="Christie, P. J. 2001 294–305"/> Nevertheless, it may be an example of the "selfish genetic element" hypothesis, as it transfers DNA by means of such a "selfish gene", the [[F-plasmid]].<ref name="Judson2002"/>
-->

The adaptive function of sex remains a major unresolved issue in biology. The competing models to explain it were reviewed by John A. Birdsell and [[Christopher Wills]].<ref>{{harvnb|Birdsell|Wills|2003|pp=27–137}}</ref> The hypotheses discussed above all depend on the possible beneficial effects of random genetic variation produced by genetic recombination. An alternative view is that sex arose and is maintained as a process for repairing DNA damage, and that the genetic variation produced is an occasionally beneficial byproduct.<ref name="novapublishers.com" /><ref>{{harvnb|Bernstein|Hopf|Michod|1987|pp=323–370}}</ref>

=== Multicellularity ===
{{main|Multicellular organism}}
The simplest definitions of "multicellular", for example "having multiple cells", could include [[Colony (biology)|colonial]] cyanobacteria like ''[[Nostoc]]''. Even a technical definition such as "having the same genome but different types of cell" would still include some [[genera]] of the green algae [[Volvox]], which have cells that specialize in reproduction.<ref>{{cite journal |last1=Bell |first1=Graham |author1-link=Graham Bell (biologist) |last2=Mooers |first2=Arne O. |date=March 1997 |title=Size and complexity among multicellular organisms |url=http://biology.mcgill.ca/faculty/bell/articles/70.BellMooers_1997_BJLS60.pdf |journal=[[Biological Journal of the Linnean Society]] |volume=60 |issue=3 |pages=345–363 |doi=10.1111/j.1095-8312.1997.tb01500.x |issn=0024-4066 |access-date=2015-02-02 |doi-access=free |archive-date=2016-03-05 |archive-url=https://web.archive.org/web/20160305034347/http://biology.mcgill.ca/faculty/bell/articles/70.BellMooers_1997_BJLS60.pdf |url-status=live}}</ref> Multicellularity evolved independently in organisms as diverse as [[sponge]]s and other animals, fungi, plants, [[brown algae]], cyanobacteria, [[slime mold]]s and [[myxobacteria]].<ref name="Fedonkin2003">{{cite journal |last=Fedonkin |first=Mikhail A. |author-link=Mikhail Fedonkin |date=March 31, 2003 |title=The origin of the Metazoa in the light of the Proterozoic fossil record |url=http://www.vend.paleo.ru/pub/Fedonkin_2003.pdf |journal=Paleontological Research |volume=7 |issue=1 |pages=9–41 |doi=10.2517/prpsj.7.9 |bibcode=2003PalRe...7....9F |s2cid=55178329 |issn=1342-8144 |archive-url=https://web.archive.org/web/20090226122725/http://www.vend.paleo.ru/pub/Fedonkin_2003.pdf |archive-date=2009-02-26 |access-date=2008-09-02}}</ref><ref>{{cite journal |last=Kaiser |first=Dale |author-link=A. Dale Kaiser |date=December 2001 |title=Building a Multicellular Organism |journal=[[Annual Review of Genetics]] |volume=35 |pages=103–123 |doi=10.1146/annurev.genet.35.102401.090145 |issn=0066-4197 |pmid=11700279}}</ref> For the sake of brevity, this article focuses on the organisms that show the greatest specialization of cells and variety of cell types, although this approach to the [[evolution of biological complexity]] could be regarded as "rather [[anthropocentric]]".<ref name="Bonner1999">{{cite journal |last=Bonner |first=John Tyler |author-link=John Tyler Bonner |date=January 7, 1998 |title=The origins of multicellularity |journal=[[Integrative Biology]] |volume=1 |issue=1 |pages=27–36 |doi=10.1002/(SICI)1520-6602(1998)1:1<27::AID-INBI4>3.0.CO;2-6 |issn=1757-9694}}</ref>

[[File:Slime mold solves maze.png|thumb|right|A [[slime mold]] solves a maze. The mold (yellow) explored and filled the maze (left). When the researchers placed sugar (red) at two separate points, the mold concentrated most of its mass there and left only the most efficient connection between the two points (right).<ref name="NakagakiYamadaTóth2000" />]]
The initial advantages of multicellularity may have included: more efficient sharing of nutrients that are digested outside the cell,<ref name="Koschwanezetal2011">{{cite journal |last1=Koschwanez |first1=John H. |last2=Foster |first2=Kevin R. |last3=Murray |first3=Andrew W. |date=August 9, 2011 |title=Sucrose Utilization in Budding Yeast as a Model for the Origin of Undifferentiated Multicellularity |journal=[[PLOS Biology]] |volume=9 |issue=8 |page=e1001122 |doi=10.1371/journal.pbio.1001122 |issn=1544-9173 |pmid=21857801 |pmc=3153487 |doi-access=free}}</ref> increased resistance to predators, many of which attacked by engulfing; the ability to resist currents by attaching to a firm surface; the ability to reach upwards to filter-feed or to obtain sunlight for photosynthesis;<ref name="Butterfield2000" /> the ability to create an internal environment that gives protection against the external one;<ref name="Bonner1999" /> and even the opportunity for a group of cells to behave "intelligently" by sharing information.<ref name="NakagakiYamadaTóth2000">{{cite journal |last1=Nakagaki |first1=Toshiyuki |last2=Yamada |first2=Hiroyasu |last3=Tóth |first3=Ágota |date=September 28, 2000 |title=Maze-solving by an amoeboid organism |journal=[[Nature (journal)|Nature]] |volume=407 |issue=6803 |page=470 |doi=10.1038/35035159 |bibcode=2000Natur.407..470N |s2cid=205009141 |pmid=11028990 |issn=0028-0836 |doi-access=free}}</ref> These features would also have provided opportunities for other organisms to diversify, by creating more varied environments than flat microbial mats could.<ref name="Butterfield2000" />

Multicellularity with differentiated cells is beneficial to the organism as a whole but disadvantageous from the point of view of individual cells, most of which lose the opportunity to reproduce themselves. In an asexual multicellular organism, rogue cells which retain the ability to reproduce may take over and reduce the organism to a mass of undifferentiated cells. Sexual reproduction eliminates such rogue cells from the next generation and therefore appears to be a prerequisite for complex multicellularity.<ref name="Butterfield2000" />

The available evidence indicates that eukaryotes evolved much earlier but remained inconspicuous until a rapid diversification around 1 Ga. The only respect in which eukaryotes clearly surpass bacteria and archaea is their capacity for variety of forms, and sexual reproduction enabled eukaryotes to exploit that advantage by producing organisms with multiple cells that differed in form and function.<ref name="Butterfield2000" />

By comparing the composition of transcription factor families and regulatory network motifs between unicellular organisms and multicellular organisms, scientists found there are many novel transcription factor families and three novel types of regulatory network motifs in multicellular organisms, and novel family transcription factors are preferentially wired into these novel network motifs which are essential for multicullular development. These results propose a plausible mechanism for the contribution of novel-family transcription factors and novel network motifs to the origin of multicellular organisms at transcriptional regulatory level.<ref name="MBE_1767">{{cite journal |last1=Jinpu |first1=Jin |last2=Kun |first2=He |last3=Xing |first3=Tang |last4=Zhe |first4=Li |last5=Le |first5=Lv |last6=Yi |first6=Zhao |last7=Jingchu |first7=Luo |last8=Ge |first8=Gao |display-authors=3 |title=An ''Arabidopsis'' transcriptional regulatory map reveals distinct functional and evolutionary features of novel transcription factors |journal=[[Molecular Biology and Evolution]] |date=July 2015 |volume=32 |issue=7 |pages=1767–1773 |doi=10.1093/molbev/msv058 |issn=0737-4038 |pmc=4476157 |pmid=25750178}}</ref>

=== Fossil evidence ===
Fungi-like fossils were found in vesicular basalt dating back to the [[Paleoproterozoic]] Era around 2.4 billion years ago.<ref>{{cite journal |last1=Bengtson |first1=Stefan |last2=Rasmussen |first2=Birger |last3=Ivarsson |first3=Magnus |last4=Muhling |first4=Janet |last5=Broman |first5=Curt |last6=Marone |first6=Federica |last7=Stampanoni |first7=Marco |last8=Bekker |first8=Andrey |s2cid=25586788 |title=Fungus-like mycelial fossils in 2.4-billion-year-old vesicular basalt |journal=[[Nature Ecology & Evolution]] |date=24 April 2017 |volume=1 |issue=6 |pages=0141 |doi=10.1038/s41559-017-0141 |pmid=28812648 |bibcode=2017NatEE...1..141B |hdl=20.500.11937/67718 |hdl-access=free |url=https://escholarship.org/uc/item/4883d4qh |access-date=15 July 2019 |archive-url=https://web.archive.org/web/20190715234418/https://escholarship.org/uc/item/4883d4qh |archive-date=15 July 2019 |url-status=live}}</ref> The controversial [[Francevillian biota]] fossils, dated to 2.1 Ga, are the earliest known fossil organisms that are clearly multicellular, if they are indeed fossils.<ref name="El Albani2010"/> They may have had differentiated cells.<ref name="Dickey2010">{{cite magazine |last=Dickey |first=Gwyneth |date=July 31, 2010 |title=Evidence for earlier multicellular life |url=http://www.sciencenewsdigital.org/sciencenews/20100731?pg=19#pg19 |format=PNG |magazine=[[Science News]] |volume=178 |issue=3 |page=17 |doi=10.1002/scin.5591780322 |issn=0036-8423 |access-date=2015-02-02}}</ref> Another early multicellular fossil, ''[[Qingshania]]'', dated to 1.7 Ga, appears to consist of virtually identical cells. The red algae called ''[[Bangiomorpha]]'', dated at 1.2 Ga, is the earliest known organism that certainly has differentiated, specialized cells, and is also the oldest known sexually reproducing organism.<ref name="Butterfield2000">{{cite journal |last=Butterfield |first=Nicholas J. |date=Summer 2000 |title=''Bangiomorpha pubescens'' n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes |url=http://paleobiol.geoscienceworld.org/content/26/3/386.abstract |journal=[[Paleobiology (journal)|Paleobiology]] |volume=26 |issue=3 |pages=386–404 |doi=10.1666/0094-8373(2000)026<0386:BPNGNS>2.0.CO;2 |bibcode=2000Pbio...26..386B |s2cid=36648568 |issn=0094-8373 |access-date=2015-02-01 |archive-date=2016-10-23 |archive-url=https://web.archive.org/web/20161023233131/http://paleobiol.geoscienceworld.org/content/26/3/386.abstract |url-status=live|url-access=subscription }}</ref> The 1.43 billion-year-old fossils interpreted as fungi appear to have been multicellular with differentiated cells.<ref name="Butterfield2005" /> The "string of beads" organism ''[[Horodyskia]]'', found in rocks dated from 1.5 Ga to 900 Ma, may have been an early metazoan;<ref name="Fedonkin2003" /> however, it has also been interpreted as a colonial [[foraminifera]]n.<ref name="DongEtAl2008">{{cite journal |author1=Lin Dong |author2=Shuhai Xiao |author3=Bing Shen |author4=Chuanming Zhou |display-authors=3 |date=January 1, 2008 |title=Silicified ''Horodyskia'' and ''Palaeopascichnus'' from upper Ediacaran cherts in South China: tentative phylogenetic interpretation and implications for evolutionary stasis |journal=[[Journal of the Geological Society]] |volume=165 |issue=1 |pages=367–378 |bibcode=2008JGSoc.165..367D |doi=10.1144/0016-76492007-074 |s2cid=129309037 |issn=0016-7649}}</ref>
{{clear}}

== Emergence of animals ==
{{further|Animal|Ediacaran biota|Cambrian explosion|Burgess Shale-type fauna|Crown group#Stem groups}}
<div style="width:40%; border:solid 1px silver; padding:2px; margin:2px; float:right;">
<div style="width:auto; border:solid 1px silver; padding:5px">
{{clade
|label1=
|1={{clade
   |label1=[[ParaHoxozoa]]
   |1={{clade
      |label1=[[Bilateria]]ns
      |1={{clade
         |1={{clade
            |1=[[Deuterostome]]s
            |label2=[[Protostome]]s
            |2={{clade
               |1=[[Ecdysozoa]]
               |2=[[Spiralia]]
               }}
            }}
         |2=[[Xenacoelomorpha]]
         }}
      |2=[[Cnidaria]]
      |3=[[Placozoa]]
      }}
   |2=[[Ctenophora]]
   |3=[[Porifera]]
   }}
}}
</div>The animal family tree<ref>{{cite book |last1=Giribet |first1=Gonzalo |last2=Edgecombe |first2=Gregory |title=The Invertebrate Tree of Life |date=3 March 2020 |publisher=Princeton University Press}}</ref></div>

Animals are multicellular eukaryotes,<ref group=note>[[Myxozoa]] were thought to be an exception, but are now thought to be heavily modified members of the [[Cnidaria]]. {{cite journal |last1=Jímenez-Guri |first1=Eva |last2=Philippe |first2=Hervé |last3=Okamura |first3=Beth |last4=Holland |first4=Peter W. H. |display-authors=3 |date=July 6, 2007 |title=''Buddenbrockia'' Is a Cnidarian Worm |journal=[[Science (journal)|Science]] |volume=317 |issue=5834 |pages=116–118 |bibcode=2007Sci...317..116J |doi=10.1126/science.1142024 |s2cid=5170702 |issn=0036-8075 |pmid=17615357}}</ref> and are distinguished from plants, algae, and fungi by lacking [[cell wall]]s.<ref>{{cite web |url=http://micro.magnet.fsu.edu/cells/animalcell.html |title=Animal Cell Structure |last=Davidson |first=Michael W. |author-link=Michael W. Davidson |date=May 26, 2005 |website=Molecular Expressions |publisher=[[Florida State University]] |location=Tallahassee, FL |access-date=2008-09-03 |archive-date=2007-09-20 |archive-url=https://web.archive.org/web/20070920235924/http://micro.magnet.fsu.edu/cells/animalcell.html |url-status=live}}</ref> All animals are [[motile]],<ref>{{cite web |url=http://employees.csbsju.edu/SSAUPE/biol116/Zoology/digestion.htm |title=Concepts of Biology |last=Saupe |first=Stephen G. |date=January 3, 2004 |website=Concepts of Biology (BIOL116) |publisher=[[College of Saint Benedict and Saint John's University]] |location=St. Joseph, MN |type=Lecture |access-date=2008-09-03 |archive-date=2007-11-21 |archive-url=https://web.archive.org/web/20071121084100/http://employees.csbsju.edu/SSAUPE/biol116/Zoology/digestion.htm |url-status=live}}</ref> if only at certain life stages. All animals except sponges have bodies differentiated into separate [[Tissue (biology)|tissue]]s, including [[muscle]]s, which move parts of the animal by contracting, and [[nervous system|nerve tissue]], which transmits and processes signals.<ref>{{harvnb|Hinde|2001|pp=28–57}}</ref> In November 2019, researchers reported the discovery of ''[[Caveasphaera]]'', a [[multicellular organism]] found in 609-million-year-old rocks, that is not easily defined as an animal or non-animal, which may be related to one of the earliest instances of [[animal evolution]].<ref name="EA-20191127">{{cite press release |last=Chen |first=Xiaozheng |title=Researchers say animal-like embryos preceded animal appearance |url=https://www.eurekalert.org/pub_releases/2019-11/caos-rsa112719.php |url-status=live |date=November 27, 2019 |publisher=[[American Association for the Advancement of Science]] |agency=[[EurekAlert!]] |archive-url=https://web.archive.org/web/20191128151859/https://www.eurekalert.org/pub_releases/2019-11/caos-rsa112719.php |archive-date=2019-11-28 |access-date=2019-11-28}}</ref><ref name="NYT-20191127">{{cite news |last=Zimmer |first=Carl |author-link=Carl Zimmer |title=Is This the First Fossil of an Embryo? - Mysterious 609-million-year-old balls of cells may be the oldest animal embryos — or something else entirely. |url=https://www.nytimes.com/2019/11/27/science/fossil-embryo-paleontology-caveaspharea.html |date=27 November 2019 |newspaper=[[The New York Times]] |location=New York |department=Matter |issn=0362-4331 |access-date=28 November 2019 |archive-date=1 January 2022 |archive-url=https://ghostarchive.org/archive/20220101/https://www.nytimes.com/2019/11/27/science/fossil-embryo-paleontology-caveaspharea.html |url-status=live}}</ref> Fossil studies of ''Caveasphaera'' have suggested that animal-like embryonic development arose much earlier than the oldest clearly defined animal fossils.<ref name="EA-20191127" /> and may be consistent with studies suggesting that animal evolution may have begun about 750 million years ago.<ref name="NYT-20191127" /><ref name="BE-20161205">{{cite journal |last1=Cunningham |first1=John A. |last2=Liu |first2=Alexander G. |last3=Bengtson |first3=Stefan |last4=Donoghue |first4=Philip C. J. |author4-link=Philip Donoghue |display-authors=3 |title=The origin of animals: Can molecular clocks and the fossil record be reconciled? |date=January 2017 |journal=[[BioEssays]] |volume=39 |issue=1 |pages=e201600120 |doi=10.1002/bies.201600120 |doi-access=free |issn=0265-9247 |pmid=27918074}}</ref>

Nonetheless, the earliest widely accepted animal fossils are the rather modern-looking [[cnidaria]]ns (the group that includes [[jellyfish]], [[sea anemone]]s and ''[[Hydra (genus)|Hydra]]''), possibly from around {{ma|580|Ma}}, although fossils from the [[Doushantuo Formation]] can only be dated approximately. Their presence implies that the cnidarian and [[bilateria]]n lineages had already diverged.<ref>{{cite journal |last1=Jun-Yuan |first1=Chen |last2=Oliveri |first2=Paola |last3=Feng |first3=Gao |last4=Dornbos |first4=Stephen Q. |author5=Chia-Wei Li |last6=Bottjer |first6=David J. |last7=Davidson |first7=Eric H. |author7-link=Eric H. Davidson |date=August 1, 2002 |title=Precambrian Animal Life: Probable Developmental and Adult Cnidarian Forms from Southwest China |url=https://pantherfile.uwm.edu/sdornbos/www/PDF%27s/Chen%20et%20al.%202002.pdf |journal=[[Developmental Biology]] |volume=248 |issue=1 |pages=182–196 |doi=10.1006/dbio.2002.0714 |issn=0012-1606 |pmid=12142030 |access-date=2015-02-04 |display-authors=3 |archive-url=https://web.archive.org/web/20130526203745/https://pantherfile.uwm.edu/sdornbos/www/PDF%27s/Chen%20et%20al.%202002.pdf |archive-date=May 26, 2013}}</ref>

The Ediacara biota, which flourished for the last 40 million years before the start of the [[Cambrian]],<ref name="Grazhdankin2004">{{cite journal |last=Grazhdankin |first=Dima |date=June 2004 |title=Patterns of distribution in the Ediacaran biotas: facies versus biogeography and evolution |journal=[[Paleobiology (journal)|Paleobiology]] |volume=30 |issue=2 |pages=203–221 |doi=10.1666/0094-8373(2004)030<0203:PODITE>2.0.CO;2 |bibcode=2004Pbio...30..203G |s2cid=129376371 |issn=0094-8373}}</ref> were the first animals more than a very few centimetres long. Many were flat and had a "quilted" appearance, and seemed so strange that there was a proposal to classify them as a separate [[Kingdom (biology)|kingdom]], [[Vendozoa]].<ref name="Seilacher1992">{{cite journal |last=Seilacher |first=Adolf |author-link=Adolf Seilacher |date=August 1992 |title=Vendobionta and Psammocorallia: lost constructions of Precambrian evolution |url=http://jgs.lyellcollection.org/content/149/4/607.abstract |journal=[[Journal of the Geological Society]] |volume=149 |issue=4 |pages=607–613 |doi=10.1144/gsjgs.149.4.0607 |issn=0016-7649 |access-date=2015-02-04 |bibcode=1992JGSoc.149..607S |s2cid=128681462 |archive-date=2022-04-22 |archive-url=https://web.archive.org/web/20220422011953/https://jgs.lyellcollection.org/content/149/4/607.abstract |url-status=live|url-access=subscription }}</ref> Others, however, have been interpreted as early [[mollusc]]s (''[[Kimberella]]''<ref name="Martin2000">{{cite journal |last1=Martin |first1=Mark W. |last2=Grazhdankin |first2=Dmitriy V. |last3=Bowring |first3=Samuel A. |author3-link=Samuel Bowring |last4=Evans |first4=David A. D. |last5=Fedonkin |first5=Mikhail A. |author5-link=Mikhail Fedonkin |last6=Kirschvink |first6=Joseph L. |date=May 5, 2000 |title=Age of Neoproterozoic Bilaterian Body and Trace Fossils, White Sea, Russia: Implications for Metazoan Evolution |journal=[[Science (journal)|Science]] |volume=288 |issue=5467 |pages=841–845 |doi=10.1126/science.288.5467.841 |bibcode=2000Sci...288..841M |issn=0036-8075 |pmid=10797002 |display-authors=3}}</ref><ref name="FedonkinWaggoner1997">{{cite journal |last1=Fedonkin |first1=Mikhail A. |author1-link=Mikhail Fedonkin |last2=Waggoner |first2=Benjamin M. |date=August 28, 1997 |title=The late Precambrian fossil ''Kimberella'' is a mollusc-like bilaterian organism |journal=[[Nature (journal)|Nature]] |volume=388 |issue=6645 |pages=868–871 |bibcode=1997Natur.388..868F |doi=10.1038/42242 |s2cid=4395089 |issn=0028-0836 |doi-access=free}}</ref>), [[echinoderm]]s (''[[Arkarua]]''<ref>{{cite journal |last1=Mooi |first1=Rich |last2=David |first2=Bruno |date=December 1998 |title=Evolution Within a Bizarre Phylum: Homologies of the First Echinoderms |journal=[[American Zoologist]] |volume=38 |issue=6 |pages=965–974 |doi=10.1093/icb/38.6.965 |issn=0003-1569 |doi-access=free}}</ref>), and [[arthropod]]s (''[[Spriggina]]'',<ref>{{cite conference |url=https://gsa.confex.com/gsa/2003AM/finalprogram/abstract_62056.htm |title=Spriggina ''is a trilobitoid ecdysozoan'' |first=Mark A. S. |last=McMenamin |author-link=Mark McMenamin |date=September 2003 |conference=Geoscience Horizons Seattle 2003 |conference-url=https://www.geosociety.org/meetings/2003/ |volume=35 |issue=6 |series=Abstracts with Programs |publisher=[[Geological Society of America]] |location=Boulder, CO |page=105 |oclc=249088612 |access-date=2007-11-24 |archive-url=https://web.archive.org/web/20160412064305/https://gsa.confex.com/gsa/2003AM/finalprogram/abstract_62056.htm |archive-date=2016-04-12 |issn=0016-7592}} Paper No. 40-2 presented at the Geological Society of America's 2003 Seattle Annual Meeting (November 2–5, 2003) on November 2, 2003, at the [[Washington State Convention Center]].</ref> ''[[Parvancorina]]''<ref>{{cite journal |last1=Jih-Pai |first1=Lin |last2=Gon |first2=Samuel M. III |last3=Gehling |first3=James G. |last4=Babcock |first4=Loren E. |last5=Yuan-Long |first5=Zhao |last6=Xing-Liang |first6=Zhang |last7=Shi-Xue |first7=Hu |last8=Jin-Liang |first8=Yuan |last9=Mei-Yi |first9=Yu |last10=Jin |first10=Peng |display-authors=3 |year=2006 |title=A ''Parvancorina''-like arthropod from the Cambrian of South China |journal=[[Historical Biology]] |volume=18 |issue=1 |pages=33–45 |doi=10.1080/08912960500508689 |bibcode=2006HBio...18...33L |s2cid=85821717 |issn=0891-2963}}</ref>). There is still debate about the classification of these specimens, mainly because the diagnostic features which allow taxonomists to classify more recent organisms, such as similarities to living organisms, are generally absent in the Ediacarans. However, there seems little doubt that ''Kimberella'' was at least a [[triploblastic]] bilaterian animal, in other words, an animal significantly more complex than the cnidarians.<ref name="Butterfield2006">{{cite journal |last=Butterfield |first=Nicholas J. |date=December 2006 |title=Hooking some stem-group 'worms': fossil lophotrochozoans in the Burgess Shale |journal=[[BioEssays]] |volume=28 |issue=12 |pages=1161–1166 |doi=10.1002/bies.20507 |issn=0265-9247 |pmid=17120226 |s2cid=29130876}}</ref>

The [[small shelly fauna]] are a very mixed collection of fossils found between the Late Ediacaran and [[Middle Cambrian]] periods. The earliest, ''[[Cloudina]]'', shows signs of successful defense against predation and may indicate the start of an [[evolutionary arms race]]. Some tiny Early Cambrian shells almost certainly belonged to molluscs, while the owners of some "armor plates", ''[[Halkieria]]'' and ''[[Microdictyon]]'', were eventually identified when more complete specimens were found in Cambrian [[lagerstätte]]n that preserved soft-bodied animals.<ref name="Bengtson2004">{{harvnb|Bengtson|2004|pp=67–78}}</ref>

[[File:20191108 Opabinia regalis.png|thumb|left|''[[Opabinia]]'' made the largest single contribution to modern interest in the Cambrian explosion.<ref>{{harvnb|Gould|1989|pp=124–136}}</ref>]]
In the 1970s there was already a debate about whether the emergence of the modern phyla was "explosive" or gradual but hidden by the shortage of [[Precambrian]] animal fossils.<ref name="Bengtson2004" /> A re-analysis of fossils from the [[Burgess Shale]] lagerstätte increased interest in the issue when it revealed animals, such as ''[[Opabinia]]'', which did not fit into any known [[phylum]]. At the time these were interpreted as evidence that the modern phyla had evolved very rapidly in the Cambrian explosion and that the Burgess Shale's "weird wonders" showed that the Early Cambrian was a uniquely experimental period of animal evolution.<ref>{{harvnb|Gould|1989}}</ref> Later discoveries of similar animals and the development of new theoretical approaches led to the conclusion that many of the "weird wonders" were evolutionary "aunts" or "cousins" of modern groups<ref name="Budd2003">{{cite journal |last=Budd |first=Graham E. |author-link=Graham Budd |date=February 2003 |title=The Cambrian Fossil Record and the Origin of the Phyla |journal=[[Integrative and Comparative Biology]] |volume=43 |issue=1 |pages=157–165 |doi=10.1093/icb/43.1.157 |doi-access=free |issn=1540-7063 |pmid=21680420}}</ref>—for example that ''Opabinia'' was a member of the [[lobopod]]s, a group which includes the ancestors of the arthropods, and that it may have been closely related to the modern [[tardigrade]]s.<ref name="Budd1996">{{cite journal |last=Budd |first=Graham E. |author-link=Graham Budd |date=March 1996 |title=The morphology of Opabinia regalis and the reconstruction of the arthropod stem-group |journal=[[Lethaia]] |volume=29 |issue=1 |pages=1–14 |doi=10.1111/j.1502-3931.1996.tb01831.x |bibcode=1996Letha..29....1B |issn=0024-1164}}</ref> Nevertheless, there is still much debate about whether the Cambrian explosion was really explosive and, if so, how and why it happened and why it appears unique in the history of animals.<ref name="Marshall2006">{{cite journal |last=Marshall |first=Charles R. |author-link=Charles R. Marshall |s2cid=85623607 |date=May 30, 2006 |title=Explaining the Cambrian 'Explosion' of Animals |journal=Annual Review of Earth and Planetary Sciences |volume=34 |pages=355–384 |bibcode=2006AREPS..34..355M |doi=10.1146/annurev.earth.33.031504.103001 |issn=1545-4495}}</ref>
{{clear}}

===Deuterostomes and the first vertebrates===
[[File:Acanthodes BW.jpg|thumb|right|[[Acanthodii|Acanthodian]]s were among the earliest vertebrates with [[jaw]]s.<ref>{{cite encyclopedia |last1=Janvier |first1=Philippe |author-link=Philippe Janvier |encyclopedia=[[Encyclopedia of Life Sciences]] |year=2001 |publisher=[[John Wiley & Sons]] |location=Hoboken, NJ |isbn=978-0-470-01617-6 |doi=10.1038/npg.els.0001531 |chapter=Vertebrata (Vertebrates)}}</ref> ]]
{{Main|Chordate|Evolution of fish}}
{{See also|Chordate genomics}}
Most of the animals at the heart of the Cambrian explosion debate were [[protostome]]s, one of the two main groups of complex animals. The other major group, the [[deuterostome]]s, contains [[invertebrate]]s such as [[starfish]] and [[sea urchin]]s (echinoderms), as well as [[chordate]]s (see below). Many echinoderms have hard [[calcite]] "shells", which are fairly common from the Early Cambrian small shelly fauna onwards.<ref name="Bengtson2004" /> Other deuterostome groups are soft-bodied, and most of the significant Cambrian deuterostome fossils come from the [[Chengjiang fauna]], a lagerstätte in [[China]].<ref>{{cite magazine |last=Conway Morris |first=Simon |author-link=Simon Conway Morris |date=August 2, 2003 |title=Once we were worms |url=http://cas.bellarmine.edu/tietjen/Evolution/once_we_were_worms.htm |magazine=[[New Scientist]] |volume=179 |issue=2406 |page=34 |issn=0262-4079 |archive-url=https://web.archive.org/web/20080725103609/http://cas.bellarmine.edu/tietjen/Evolution/once_we_were_worms.htm |archive-date=2008-07-25 |access-date=2008-09-05}}</ref> The chordates are another major deuterostome group: animals with a distinct dorsal nerve cord. Chordates include soft-bodied invertebrates such as [[tunicate]]s as well as vertebrates—animals with a backbone. While tunicate fossils predate the Cambrian explosion,<ref name="Jun-Yuan2003">{{cite journal |author1=Jun-Yuan Chen |author2=Di-Ying Huang |author3=Qing-Qing Peng |author4=Hui-Mei Chi |author5=Xiu-Qiang Wang |author6=Man Feng |date=July 8, 2003 |title=The first tunicate from the Early Cambrian of South China |journal=[[Proc. Natl. Acad. Sci. U.S.A.]] |volume=100 |issue=14 |pages=8314–8318 |bibcode=2003PNAS..100.8314C |doi=10.1073/pnas.1431177100 |issn=0027-8424 |pmc=166226 |pmid=12835415 |display-authors=3 |doi-access=free}}</ref> the Chengjiang fossils ''[[Haikouichthys]]'' and ''[[Myllokunmingia]]'' appear to be true vertebrates,<ref name="D-G.Shu et al. 1999" /> and ''Haikouichthys'' had distinct [[vertebrae]], which may have been slightly [[biomineralization|mineralized]].<ref>{{cite journal |author1=D.-G. Shu |last2=Conway Morris |first2=Simon |author2-link=Simon Conway Morris |author3=J. Han |author4=Z.-F. Zhang |author5=K. Yasui |last6=Janvier |first6=Philippe |author6-link=Philippe Janvier |author7=L. Chen |author8=X.-L. Zhang |author9=J.-N. Liu |author10=Y. Li |author11=H.-Q. Liu |date=January 30, 2003 |title=Head and backbone of the Early Cambrian vertebrate ''Haikouichthys'' |journal=[[Nature (journal)|Nature]] |volume=421 |issue=6922 |pages=526–529 |bibcode=2003Natur.421..526S |doi=10.1038/nature01264 |s2cid=4401274 |issn=0028-0836 |pmid=12556891 |display-authors=3}}</ref> Vertebrates with [[jaw]]s, such as the [[acanthodian]]s, first appeared in the Late [[Ordovician]].<ref>{{harvnb|Sansom|Smith|Smith|2001|pp=156–171}}</ref>

==Colonization of land==
{{See also|Earliest known life forms}}
Adaptation to life on land is a major challenge: all land organisms need to avoid drying-out and all those above microscopic size must create special structures to withstand gravity; [[respiration (physiology)|respiration]] and [[gas exchange]] systems have to change; reproductive systems cannot depend on water to carry [[egg]]s and [[sperm]] towards each other.<ref name="CowenHistLifeEd3P120To122">{{harvnb|Cowen|2000|pp=120–122}}</ref><ref name="Selden2001" /><ref name="Garwood">{{cite journal |last1=Garwood |first1=Russell J. |last2=Edgecombe |first2=Gregory D. |author2-link=Gregory Edgecombe |date=September 2011 |title=Early Terrestrial Animals, Evolution, and Uncertainty |journal=Evolution: Education and Outreach |volume=4 |issue=3 |pages=489–501 |doi=10.1007/s12052-011-0357-y |doi-access=free |issn=1936-6434}}</ref> Although the earliest good evidence of land plants and animals dates back to the Ordovician period ({{ma|488|444|Ma}}), and a number of microorganism lineages made it onto land much earlier,<ref name="Battistuzzi2004">{{cite journal |last1=Battistuzzi |first1=Fabia U. |last2=Feijao |first2=Andreia |last3=Hedges |first3=S. Blair |date=November 9, 2004 |title=A genomic timescale of prokaryote evolution: insights into the origin of methanogenesis, phototrophy, and the colonization of land |journal=[[BMC Evolutionary Biology]] |volume=4 |page=44 |doi=10.1186/1471-2148-4-44 |issn=1471-2148 |pmc=533871 |pmid=15535883 |doi-access=free}}</ref><ref>{{harvnb|Weber|Büdel|Belnap|2016|pp=37–54|loc=Chapter 3: "Terrestrial Ecosystems in the Precambrian" by Hugo Beraldi-Campesi and [[Gregory Retallack|Gregory J. Retallack]]}}. {{doi|10.1007/978-3-319-30214-0_3}}: "...terrestrial ecosystems were indeed present, full of life, and functional since the Archean."</ref> modern land [[ecosystem]]s only appeared in the Late [[Devonian]], about {{ma|385|359|Ma}}.<ref name="Shear2000" /> In May 2017, evidence of the [[Earliest known life forms#Earliest life forms|earliest known life on land]] may have been found in 3.48-billion-year-old [[geyserite]] and other related mineral deposits (often found around [[hot spring]]s and [[geyser]]s) uncovered in the [[Pilbara craton|Pilbara Craton]] of [[Western Australia]].<ref name="UNSW-20170510">{{cite press release |last=Smith |first=Deborah |date=May 10, 2017 |title=Oldest evidence of life on land found in 3.48 billion-year-old Australian rocks |url=https://newsroom.unsw.edu.au/news/science-tech/oldest-evidence-life-land-found-348-billion-year-old-australian-rocks |location=Sydney, Australia |publisher=UNSW Media |access-date=2020-07-14}}</ref><ref name="NC-20170509">{{cite journal |last1=Djokic |first1=Tara |last2=Van Kranendonk |first2=Martin J. |last3=Campbell |first3=Kathleen A. |author3-link=Kathleen Ann Campbell |last4=Walter |first4=Malcolm R. |last5=Ward |first5=Colin R. |display-authors=3 |title=Earliest signs of life on land preserved in ca. 3.5 Ga hot spring deposits |date=May 9, 2017 |journal=[[Nature Communications]] |volume=8 |page=15263 |bibcode=2017NatCo...815263D |doi=10.1038/ncomms15263 |issn=2041-1723 |pmid=28486437 |pmc=5436104}}</ref> In July 2018, scientists reported that the earliest life on land may have been [[bacteria]] living on land 3.22 billion years ago.<ref name="NG-20180723">{{cite journal |last1=Homann |first1=Martin |last2=Sansjofre |first2=Pierre |last3=Van Zuilen |first3=Mark |display-authors=et al. |date=September 2018 |title=Microbial life and biogeochemical cycling on land 3,220 million years ago |journal=[[Nature Geoscience]] |volume=11 |issue=9 |pages=665–671 |bibcode=2018NatGe..11..665H |doi=10.1038/s41561-018-0190-9 |s2cid=134935568 |issn=1752-0894 |url=https://hal.univ-brest.fr/hal-01901955/file/Homann%20et%20al.%202018%20-%20accepted-1.pdf |access-date=2021-05-10 |archive-date=2021-05-09 |archive-url=https://web.archive.org/web/20210509082017/https://hal.univ-brest.fr/hal-01901955/file/Homann%20et%20al.%202018%20-%20accepted-1.pdf |url-status=live}}</ref> In May 2019, scientists reported the discovery of a [[fossil]]ized [[fungus]], named ''[[Ourasphaira giraldae]]'', in the [[Canadian Arctic]], that may have grown on land a billion years ago, well before [[plant]]s were living on land.<ref name="NYT-20190522">{{cite news |last=Zimmer |first=Carl |author-link=Carl Zimmer |title=A Billion-Year-Old Fungus May Hold Clues to Life's Arrival on Land |url=https://www.nytimes.com/2019/05/22/science/fungi-fossils-plants.html |url-status=live |date=May 22, 2019 |department=Matter |newspaper=[[The New York Times]] |location=New York |issn=0362-4331 |archive-url=https://web.archive.org/web/20190523011853/https://www.nytimes.com/2019/05/22/science/fungi-fossils-plants.html |archive-date=2019-05-23 |access-date=2019-05-23}} "A version of this article appears in print on May 28, 2019, Section D, Page 3 of the New York edition with the headline: Finding a Clue to Life's Arrival."</ref><ref name="NAT-20190522">{{cite journal |last1=Loron |first1=Corentin C. |last2=François |first2=Camille |last3=Rainbird |first3=Robert H. |last4=Turner |first4=Elizabeth C. |last5=Borensztajn |first5=Stephan |last6=Javaux |first6=Emmanuelle J. |display-authors=3 |title=Early fungi from the Proterozoic era in Arctic Canada |journal=[[Nature (journal)|Nature]] |volume=570 |issue=7760 |pages=232–235 |date=June 13, 2019 |issn=0028-0836 |bibcode=2019Natur.570..232L |doi=10.1038/s41586-019-1217-0 |s2cid=162180486 |pmid=31118507}}</ref><ref name="Ars Technica 2019">{{cite web |last=Timmer |first=John |title=Billion-year-old fossils may be early fungus |website=[[Ars Technica]] |date=May 22, 2019 |url=https://arstechnica.com/science/2019/05/billion-year-old-fossils-may-be-early-fungus/ |url-status=live |archive-url=https://web.archive.org/web/20190522222400/https://arstechnica.com/science/2019/05/billion-year-old-fossils-may-be-early-fungus/ |archive-date=2019-05-22 |access-date=May 23, 2019}}</ref>

===Evolution of terrestrial antioxidants===

Oxygen began to accumulate in Earth's atmosphere over 3 Ga, as a by-product of [[photosynthesis]] in cyanobacteria (blue-green algae). However, oxygen produces destructive chemical [[oxidation]] which was toxic to most previous organisms. Protective endogenous antioxidant enzymes and exogenous dietary antioxidants helped to prevent oxidative damage. For example, brown algae accumulate inorganic mineral [[antioxidant]]s such as [[rubidium]], [[vanadium]], [[zinc]], iron, [[copper]], [[molybdenum]], [[selenium]] and [[iodine]], concentrated more than 30,000 times more than in seawater. Most marine mineral antioxidants act in the cells as essential [[trace element]]s in [[redox]] and antioxidant [[Metalloprotein|metalloenzymes]].{{citation needed|date=April 2021}}

When plants and animals began to enter rivers and land about 500 Ma, environmental deficiency of these marine mineral antioxidants was a challenge to the evolution of terrestrial life.<ref name="Venturi_2011">{{cite journal |last=Venturi |first=Sebastiano |date=September 2011 |title=Evolutionary Significance of Iodine |journal=Current Chemical Biology |volume=5 |issue=3 |pages=155–162 |doi=10.2174/187231311796765012 |doi-broken-date=16 December 2024 |issn=2212-7968}}</ref><ref>{{cite journal |last=Crockford |first=Susan J. |author-link=Susan J. Crockford |date=August 2009 |title=Evolutionary roots of iodine and thyroid hormones in cell-cell signaling |journal=[[Integrative and Comparative Biology]] |volume=49 |issue=2 |pages=155–166 |doi=10.1093/icb/icp053 |doi-access=free |issn=1557-7023 |pmid=21669854}}</ref> Terrestrial plants slowly optimized the production of new endogenous antioxidants such as [[ascorbic acid]], [[polyphenol]]s, [[flavonoid]]s, [[tocopherol]]s, etc.

A few of these appeared more recently, in the last 200–50 Ma, in [[fruit]]s and [[flower]]s of [[angiosperm]] plants.{{citation needed|date=April 2021}} In fact, angiosperms (the dominant type of plant today) and most of their antioxidant pigments evolved during the [[Late Jurassic]] period. Plants employ antioxidants to defend their structures against [[reactive oxygen species]] produced during photosynthesis. Animals are exposed to the same oxidants, and they have evolved endogenous enzymatic antioxidant systems.<ref>{{cite journal |last1=Venturi |first1=Sebastiano |last2=Donati |first2=Francesco M. |last3=Venturi |first3=Alessandro |last4=Venturi |first4=Mattia |display-authors=3 |date=August 2000 |title=Environmental Iodine Deficiency: A Challenge to the Evolution of Terrestrial Life? |journal=[[Thyroid (journal)|Thyroid]] |volume=10 |issue=8 |pages=727–729 |doi=10.1089/10507250050137851 |issn=1050-7256 |pmid=11014322}}</ref> [[Iodine]] in the form of the iodide ion I<sup>−</sup> is the most primitive and abundant electron-rich essential element in the diet of marine and terrestrial organisms; it acts as an [[electron donor]] and has this ancestral antioxidant function in all iodide-concentrating cells, from primitive marine algae to terrestrial vertebrates.<ref>{{cite journal |last1=Küpper |first1=Frithjof C. |last2=Carpenter |first2=Lucy J. |author2-link=Lucy Carpenter |last3=McFiggans |first3=Gordon B. |last4=Palmer |first4=Carl J. |last5=Waite |first5=Tim J. |last6=Boneberg |first6=Eva-Maria |last7=Woitsch |first7=Sonja |last8=Weiller |first8=Markus |last9=Abela |first9=Rafael |date=May 13, 2008 |title=Iodide accumulation provides kelp with an inorganic antioxidant impacting atmospheric chemistry |journal=[[Proc. Natl. Acad. Sci. U.S.A.]] |volume=105 |issue=19 |pages=6954–6958 |bibcode=2008PNAS..105.6954K |doi=10.1073/pnas.0709959105 |issn=0027-8424 |pmc=2383960 |pmid=18458346 |display-authors=3 |doi-access=free}}</ref>

===Evolution of soil===

Before the colonization of land there was no [[soil]], a combination of mineral particles and decomposed [[organic matter]]. Land surfaces were either bare rock or shifting sand produced by [[weathering]]. Water and dissolved nutrients would have drained away very quickly.<ref name="Shear2000" /> In the [[Sub-Cambrian peneplain]] in Sweden, for example, maximum depth of [[kaolin]]itization by [[Neoproterozoic]] [[weathering]] is about 5 m, while nearby kaolin deposits developed in the [[Mesozoic]] are [[Sub-Mesozoic hilly peneplains|much thicker]].<ref name=Lidmar-Bergstrometal2013>{{cite journal |last1=Lidmar-Bergström |first1=Karna |last2=Bonow |first2=Johan M. |last3=Japsen |first3=Peter |author-link=Karna Lidmar-Bergström |date=January 2013 |title=Stratigraphic Landscape Analysis and geomorphological paradigms: Scandinavia as an example of Phanerozoic uplift and subsidence |journal=[[Global and Planetary Change]] |volume=100 |pages=153–171 |doi=10.1016/j.gloplacha.2012.10.015 |bibcode=2013GPC...100..153L |issn=0921-8181}}</ref> It has been argued that in the late Neoproterozoic [[sheet wash]] was a dominant process of erosion of surface material due to the [[Evolutionary history of plants|lack of plants]] on land.<ref name=Karna1993>{{cite journal |last1=Lidmar-Bergström |first1=Karna |author1-link=Karna Lidmar-Bergström |date=December 1993 |title=Denudation surfaces and tectonics in the southernmost part of the Baltic Shield |journal=[[Precambrian Research]] |volume=64 |issue=1–4 |pages=337–345 |doi=10.1016/0301-9268(93)90086-H |bibcode=1993PreR...64..337L |issn=0301-9268}}</ref>
[[File:Lichen.jpg|thumb|right|[[Lichen]]s growing on [[concrete]]]]
Films of cyanobacteria, which are not plants but use the same photosynthesis mechanisms, have been found in modern deserts in areas unsuitable for [[vascular plant]]s. This suggests that microbial mats may have been the first organisms to colonize dry land, possibly in the Precambrian. Mat-forming cyanobacteria could have gradually evolved resistance to desiccation as they spread from the seas to [[intertidal zone]]s and then to land.<ref name="Shear2000">[[#Gee 2000|Shear 2000]], "The Early Development of Terrestrial Ecosystems," pp. [https://books.google.com/books?id=ZJe_Dmdbm-QC&pg=PA169 169–184] {{Webarchive|url=https://web.archive.org/web/20230405034427/https://books.google.com/books?id=ZJe_Dmdbm-QC&pg=PA169 |date=2023-04-05 }}</ref> [[Lichen]]s, which are [[symbiotic]] combinations of a fungus (almost always an [[ascomycete]]) and one or more photosynthesizers (green algae or cyanobacteria),<ref name="Hawksworth2001">{{cite encyclopedia |last=Hawksworth |first=David L. |author-link=David Leslie Hawksworth |encyclopedia=[[Encyclopedia of Life Sciences]] |year=2002 |publisher=[[John Wiley & Sons]] |location=Hoboken, NJ |isbn=978-0-470-01617-6 |doi=10.1038/npg.els.0000368 |chapter=Lichens |s2cid=241563883}}</ref> are also important colonizers of lifeless environments,<ref name="Shear2000" /> and their ability to break down rocks contributes to [[soil formation]] where plants cannot survive.<ref name="Hawksworth2001" /> The earliest known ascomycete fossils date from {{ma|423|419|Ma}} in the [[Silurian]].<ref name="Shear2000" />

Soil formation would have been very slow until the appearance of burrowing animals, which mix the mineral and organic components of soil and whose [[feces]] are a major source of organic components.<ref name="Shear2000" /> Burrows have been found in Ordovician sediments, and are attributed to [[annelid]]s (worms) or arthropods.<ref name="Shear2000" /><ref>{{cite journal |last1=Retallack |first1=Gregory J. |author1-link=Gregory Retallack |last2=Feakes |first2=Carolyn R. |date=January 2, 1987 |title=Trace Fossil Evidence for Late Ordovician Animals on Land |journal=[[Science (journal)|Science]] |volume=235 |issue=4784 |pages=61–63 |bibcode=1987Sci...235...61R |doi=10.1126/science.235.4784.61 |s2cid=37351505 |issn=0036-8075 |pmid=17769314}}</ref>

===Plants and the Late Devonian wood crisis===
{{main|Evolutionary history of plants}}

[[File:Cooksonia pertoni.png|thumb|upright=0.7|Reconstruction of ''[[Cooksonia]]'', a [[vascular plant]] from the [[Silurian]]]]
[[File:Gilboa.jpg|thumb|upright=0.7|Fossilized [[tree]]s from the Middle [[Devonian]] [[Gilboa Fossil Forest]]]]
In aquatic algae, almost all cells are capable of photosynthesis and are nearly independent. Life on land requires plants to become internally more complex and specialized: photosynthesis is most efficient at the top; roots extract water and nutrients from the ground; and the intermediate parts support and transport.<ref name="CowenHistLifeEd3P120To122" /><ref name="KenrickCrane1997" />

Spores of land plants resembling [[liverworts]] have been found in Middle Ordovician rocks from {{ma|476|Ma|Ordovician}}. [[Wenlock (Silurian)|Middle Silurian]] rocks from {{ma|430|Ma|Silurian}} contain fossils of true plants, including [[clubmoss]]es such as ''[[Baragwanathia]]''; most were under {{convert|10|cm|in}} high, and some appear closely related to [[vascular plant]]s, the group that includes [[tree]]s.<ref name="KenrickCrane1997">{{cite journal |last1=Kenrick |first1=Paul |last2=Crane |first2=Peter R. |author2-link=Peter Crane |date=September 4, 1997 |title=The origin and early evolution of plants on land |url=http://biology.kenyon.edu/courses/biol112/Biol112WebPage/Syllabus/Topics/Week%207/land%20plants.pdf |journal=[[Nature (journal)|Nature]] |volume=389 |issue=6646 |pages=33–39 |bibcode=1997Natur.389...33K |doi=10.1038/37918 |s2cid=3866183 |issn=0028-0836 |access-date=2015-02-10 |archive-date=2016-03-03 |archive-url=https://web.archive.org/web/20160303232910/http://biology.kenyon.edu/courses/biol112/Biol112WebPage/Syllabus/Topics/Week%207/land%20plants.pdf |url-status=live}}</ref>

By the Late Devonian {{ma|370|Ma|Devonian}}, abundant trees such as ''[[Archaeopteris]]'' bound the soil so firmly that they changed river systems from mostly [[braided river|braided]] to mostly [[meander]]ing.<ref>[[#Briggs and Crowther 2001|Scheckler 2001]], "Afforestation—the First Forests," pp. 67–70</ref> This caused the "Late Devonian wood crisis" because:<ref>The phrase "Late Devonian wood crisis" is used at {{cite web |url=http://palaeos.com/vertebrates/tetrapoda/acanthostega.html |title=Tetrapoda: ''Acanthostega'' |website=[[Palaeos]] |access-date=2015-02-10 |archive-date=2021-08-15 |archive-url=https://web.archive.org/web/20210815082817/http://palaeos.com/vertebrates/tetrapoda/acanthostega.html |url-status=live}}</ref> 
* They removed more carbon dioxide from the atmosphere, reducing the [[greenhouse effect]] and thus causing an [[ice age]] in the [[Carboniferous]] period.<ref name="AlgeoScheckler1998">{{cite journal |last1=Algeo |first1=Thomas J. |last2=Scheckler |first2=Stephen E. |date=January 29, 1998 |title=Terrestrial-marine teleconnections in the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic events |journal=[[Philosophical Transactions of the Royal Society B]] |volume=353 |issue=1365 |pages=113–130 |doi=10.1098/rstb.1998.0195 |issn=0962-8436 |pmc=1692181}}</ref> This did not repeat in later ecosystems, since the carbon dioxide "locked up" in wood was returned to the atmosphere by decomposition of dead wood, but the earliest fossil evidence of fungi that can decompose wood also comes from the Late Devonian.<ref>{{cite journal |last1=Taylor |first1=Thomas N. |last2=Osborn |first2=Jeffrey M. |date=February 1996 |title=The importance of fungi in shaping the paleoecosystem |journal=[[Review of Palaeobotany and Palynology]] |volume=90 |issue=3–4 |pages=249–262 |doi=10.1016/0034-6667(95)00086-0 |bibcode=1996RPaPa..90..249T |issn=0034-6667}}</ref>
* The increasing depth of plants' roots led to more washing of nutrients into rivers and seas by rain. This caused [[algal bloom]]s whose high consumption of oxygen caused [[anoxic event]]s in deeper waters, increasing the extinction rate among deep-water animals.<ref name="AlgeoScheckler1998" />

===Land invertebrates===

Animals had to change their feeding and [[excretory]] systems, and most land animals developed [[internal fertilization]] of their eggs.<ref name="Garwood" /> The difference in [[refractive index]] between water and air required changes in their eyes. On the other hand, in some ways movement and breathing became easier, and the better transmission of high-frequency sounds in the air encouraged the development of [[hearing]].<ref name="Selden2001" />

The oldest animal with evidence of air-breathing, although not being the oldest myriapod fossil record, is ''[[Pneumodesmus]]'', an [[archipolypoda]]n [[millipede]] from the Early [[Devonian]], about {{ma|414|Ma}}.<ref>{{Cite journal |last1=Brookfield |first1=M. E. |last2=Catlos |first2=E. J. |last3=Suarez |first3=S. E. |date=2021-10-03 |title=Myriapod divergence times differ between molecular clock and fossil evidence: U/Pb zircon ages of the earliest fossil millipede-bearing sediments and their significance |url=https://www.tandfonline.com/doi/full/10.1080/08912963.2020.1762593 |journal=Historical Biology |language=en |volume=33 |issue=10 |pages=2014–2018 |doi=10.1080/08912963.2020.1762593 |bibcode=2021HBio...33.2014B |s2cid=238220137 |issn=0891-2963 |access-date=2023-06-23 |archive-date=2023-06-23 |archive-url=https://web.archive.org/web/20230623173237/https://www.tandfonline.com/doi/full/10.1080/08912963.2020.1762593 |url-status=live|url-access=subscription }}</ref> Its air-breathing, terrestrial nature is evidenced by the presence of [[Spiracle (arthropods)|spiracle]]s, the openings to [[Trachea#Invertebrates|tracheal systems]].<ref name=Shear2010>{{cite journal |last1=Shear |first1=William A. |author1-link=William Shear |last2=Edgecombe |first2=Gregory D. |author2-link=Gregory Edgecombe |date=March–May 2010 |title=The geological record and phylogeny of the Myriapoda |journal=Arthropod Structure & Development |volume=39 |issue=2–3 |pages=174–190 |doi=10.1016/j.asd.2009.11.002 |issn=1467-8039 |pmid=19944188 |bibcode=2010ArtSD..39..174S}}</ref> However, some earlier [[trace fossil]]s from the Cambrian-Ordovician boundary about {{ma|490|Ma|Cambrian}} are interpreted as the tracks of large [[amphibian|amphibious]] arthropods on coastal [[sand dune]]s, and may have been made by [[euthycarcinoid]]s,<ref>{{cite journal |last1=MacNaughton |first1=Robert B. |last2=Cole |first2=Jennifer M. |last3=Dalrymple |first3=Robert W. |last4=Braddy |first4=Simon J. |last5=Briggs |first5=Derek E. G. |author5-link=Derek Briggs |last6=Lukie |first6=Terrence D. |display-authors=3 |date=May 2002 |title=First steps on land: Arthropod trackways in Cambrian-Ordovician eolian sandstone, southeastern Ontario, Canada |journal=[[Geology (journal)|Geology]] |volume=30 |issue=5 |pages=391–394 |bibcode=2002Geo....30..391M |doi=10.1130/0091-7613(2002)030<0391:FSOLAT>2.0.CO;2 |issn=0091-7613}}</ref> which are thought to be evolutionary "aunts" of [[myriapod]]s.<ref>{{cite journal |last1=Vaccari |first1=N. Emilio |last2=Edgecombe |first2=Gregory D. |author2-link=Gregory Edgecombe |last3=Escudero |first3=C. |date=July 29, 2004 |title=Cambrian origins and affinities of an enigmatic fossil group of arthropods |journal=[[Nature (journal)|Nature]] |volume=430 |issue=6999 |pages=554–557 |bibcode=2004Natur.430..554V |doi=10.1038/nature02705 |s2cid=4419235 |issn=0028-0836 |pmid=15282604}}</ref> Other trace fossils from the Late Ordovician a little over {{ma|445|Ma|Ordovician}} probably represent land invertebrates, and there is clear evidence of numerous arthropods on coasts and [[alluvial plain]]s shortly before the Silurian-Devonian boundary, about {{ma|415|Ma|Silurian}}, including signs that some arthropods ate plants.<ref>{{cite journal |last1=Buatois |first1=Luis A. |last2=Mangano |first2=M. Gabriela |last3=Genise |first3=Jorge F. |last4=Taylor |first4=Thomas N. |display-authors=3 |date=June 1998 |title=The Ichnologic Record of the Continental Invertebrate Invasion: Evolutionary Trends in Environmental Expansion, Ecospace Utilization, and Behavioral Complexity |journal=[[PALAIOS]] |volume=13 |issue=3 |pages=217–240 |bibcode=1998Palai..13..217B |doi=10.2307/3515447 |issn=0883-1351 |jstor=3515447}}</ref> Arthropods were well [[pre-adapted]] to colonise land, because their existing jointed exoskeletons provided protection against desiccation, support against gravity and a means of locomotion that was not dependent on water.<ref name="Garwood" /><ref name="CowenHistLifeEd3P126">{{harvnb|Cowen|2000|p=126}}</ref>

The [[Fossil#Dating|fossil record]] of other major invertebrate groups on land is poor: none at all for non-[[parasitic]] [[flatworm]]s, [[nematode]]s or [[nemertea]]ns; some parasitic nematodes have been fossilized in [[amber]]; annelid worm fossils are known from the Carboniferous, but they may still have been aquatic animals; the earliest fossils of [[gastropod]]s on land date from the Late Carboniferous, and this group may have had to wait until [[leaf litter]] became abundant enough to provide the moist conditions they need.<ref name="Selden2001">[[#Briggs and Crowther 2001|Selden 2001]], "Terrestrialization of Animals," pp. 71–74</ref>

The earliest confirmed fossils of flying [[insect]]s date from the Late Carboniferous, but it is thought that insects developed the ability to fly in the Early Carboniferous or even Late Devonian. This gave them a wider range of [[ecological niche]]s for feeding and breeding, and a means of escape from predators and from unfavorable changes in the environment.<ref>{{harvnb|Grimaldi|Engel|2005|pp=155–160}}</ref> About 99% of modern insect species fly or are descendants of flying species.<ref>{{harvnb|Grimaldi|Engel|2005|p=12}}</ref>

===Amphibians===
{{main|Tetrapod|Evolution of tetrapods}}
[[File:Acanthostega BW.jpg|thumb|left|''[[Acanthostega]]'' changed views about the early evolution of [[tetrapod]]s.<ref name="Clack2005" />]]

<div style="border:solid 1px silver; padding:2px; margin:2px; float:right;">
<div style="width:auto; border:solid 1px silver; padding:5px">
{{clade
| label1="Fish"
| 1={{clade
     |1=[[Osteolepiformes]]
     |2={{clade
         | 1=[[Panderichthyida]]e
         | 2={{clade
              |1=[[Obruchevichthys|Obruchevichthidae]]
              | 2={{clade
                   |1=''[[Acanthostega]]''
                   | 2={{clade
                        |1=''[[Ichthyostega]]''
                        | 2={{clade
                             |1=''[[Tulerpeton]]''
                             |2={{clade
                                 |1=Early [[labyrinthodont]]s
                                 |2={{clade
                                    |1=[[Anthracosauria]]
                                    |2=[[Amniote]]s
                                    }}
                                }}
                            }}
                       }}
                  }}
             }}
        }}
    }}
}}
</div>Family tree of [[tetrapod]]s<ref name="AhlbergMilner1994" /></div>

[[Tetrapod]]s, vertebrates with four limbs, evolved from other [[rhipidistia]]n fish over a relatively short timespan during the Late Devonian ({{ma|370|360|Ma|Devonian}}).<ref>{{cite journal |last1=Gordon |first1=Malcolm S. |last2=Graham |first2=Jeffrey B. |last3=Wang |first3=Tobias |date=September–October 2004 |title=Introduction to the Special Collection: Revisiting the Vertebrate Invasion of the Land |journal=[[Physiological and Biochemical Zoology]] |volume=77 |issue=5 |pages=697–699 |doi=10.1086/425182 |s2cid=83750933 |issn=1522-2152}}</ref> The early groups are grouped together as [[Labyrinthodontia]]. They retained aquatic, fry-like [[tadpole]]s, a system still seen in [[Lissamphibia|modern amphibians]].

Iodine and [[Triiodothyronine|T4/T3]] stimulate the amphibian metamorphosis and the [[evolution of nervous systems]] transforming the aquatic, vegetarian tadpole into a "more evolved" terrestrial, carnivorous frog with better neurological, visuospatial, olfactory and cognitive abilities for hunting.<ref name="Venturi_2011" /> The new hormonal action of T3 was made possible by the formation of T3-receptors in the cells of vertebrates. First, about 600–500 million years ago, the alpha T3-receptors with a metamorphosing action appeared in primitive chordates and then, about 250–150 million years ago, the beta T3-receptors with metabolic and thermogenetic actions appeared in birds and mammals.<ref>[[#Cunnane and Stewart 2010|Venturi and Bégin 2010]], "Thyroid Hormone, Iodine and Human Brain Evolution," pp. [https://books.google.com/books?id=gfkRnv20GtsC&pg=PA105 105–124] {{Webarchive|url=https://web.archive.org/web/20230716080927/https://books.google.com/books?id=gfkRnv20GtsC&pg=PA105 |date=2023-07-16 }}</ref>

From the 1950s to the early 1980s it was thought that tetrapods evolved from fish that had already acquired the ability to crawl on land, possibly in order to go from a pool that was drying out to one that was deeper. However, in 1987, nearly complete fossils of ''[[Acanthostega]]'' from about {{ma|363|Ma|Devonian}} showed that this Late Devonian [[Transitional fossil|transitional]] animal had [[leg]]s and both [[lung]]s and [[gill]]s, but could never have survived on land: its limbs and its wrist and ankle joints were too weak to bear its weight; its ribs were too short to prevent its lungs from being squeezed flat by its weight; its fish-like tail fin would have been damaged by dragging on the ground. The current hypothesis is that ''Acanthostega'', which was about {{convert|1|m|ft}} long, was a wholly aquatic predator that hunted in shallow water<!-- , using its limbs to hold on to vegetation while it lay in ambush -->. Its skeleton differed from that of most fish, in ways that enabled it to raise its head to breathe air while its body remained submerged, including: its jaws show modifications that would have enabled it to gulp air; the bones at the back of its skull are locked together, providing strong attachment points for muscles that raised its head; the head is not joined to the [[shoulder girdle]] and it has a distinct neck.<ref name="Clack2005">{{cite magazine |last=Clack |first=Jennifer A. |author-link=Jenny Clack |date=December 2005 |title=Getting a Leg Up on Land |magazine=[[Scientific American]] |volume=293 |issue=6 |pages=100–107 |bibcode=2005SciAm.293f.100C |doi=10.1038/scientificamerican1205-100 |issn=0036-8733 |pmid=16323697}}</ref>

The Devonian proliferation of land plants may help to explain why air breathing would have been an advantage: leaves falling into streams and rivers would have encouraged the growth of aquatic vegetation; this would have attracted grazing invertebrates and small fish that preyed on them; they would have been attractive prey but the environment was unsuitable for the big marine predatory fish; air-breathing would have been necessary because these waters would have been short of oxygen, since warm water holds less [[dissolved oxygen]] than cooler marine water and since the decomposition of vegetation would have used some of the oxygen.<ref name="Clack2005" />

Later discoveries revealed earlier transitional forms between ''Acanthostega'' and completely fish-like animals.<ref>{{cite journal |last1=Daeschler |first1=Edward B. |author1-link=Ted Daeschler |last2=Shubin |first2=Neil H. |author2-link=Neil Shubin |last3=Jenkins |first3=Farish A. Jr. |author3-link=Farish Jenkins |date=April 6, 2006 |title=A Devonian tetrapod-like fish and the evolution of the tetrapod body plan |url=https://www.nature.com/articles/nature04639.pdf |journal=[[Nature (journal)|Nature]] |volume=440 |pages=757–763 |issue=7085 |bibcode=2006Natur.440..757D |doi=10.1038/nature04639 |s2cid=4413217 |issn=0028-0836 |pmid=16598249 |access-date=2020-07-26 |doi-access=free |archive-date=2022-06-29 |archive-url=https://web.archive.org/web/20220629160054/https://www.nature.com/articles/nature04639.pdf |url-status=live}}</ref> Unfortunately, there is then a gap ([[Romer's gap]]) of about 30 Ma between the fossils of ancestral tetrapods and Middle Carboniferous fossils of vertebrates that look well-adapted for life on land, during which only some fossils are found, which had five digits in the terminating point of the four limbs, showing true or crown [[tetrapods]] appeared in the gap around 350 Ma. Some of the fossils after this gap look as if the animals which they belonged to were early relatives of modern amphibians, all of which need to keep their skins moist and to lay their eggs in water, while others are accepted as early relatives of the [[amniote]]s, whose waterproof skin and egg membranes enable them to live and breed far from water.<ref name="AhlbergMilner1994">{{cite journal |last1=Ahlberg |first1=Per E. |author1-link=Per E. Ahlberg |last2=Milner |first2=Andrew R. |date=April 7, 1994 |title=The origin and early diversification of tetrapods |journal=[[Nature (journal)|Nature]] |volume=368 |issue=6471 |pages=507–514 |bibcode=1994Natur.368..507A |doi=10.1038/368507a0 |s2cid=4369342 |issn=0028-0836}}</ref> The [[Carboniferous Rainforest Collapse]] may have paved the way for amniotes to become dominant over amphibians.

===Reptiles===
{{further|Evolution of dinosaurs|Origin of birds|Evolution of mammals}}

<div style="border:solid 1px silver; padding:2px; margin:2px; float:right;">
<div style="width:auto; border:solid 1px silver; padding:5px">
{{clade
 |label1=[[Amniote]]s
 |1={{clade
    |label1=[[Synapsid]]s
    |1={{clade
        |1=Early synapsids (extinct)
        |label2=[[Pelycosaur]]s
        |2={{clade
            |1=Extinct pelycosaurs
            |label2=[[Therapsid]]s
            |2={{clade
                |1=Extinct therapsids
                |label2=[[Mammaliaformes]]
                |2={{clade
                    |label1=
                    |1=Extinct mammaliaforms
                    |label2=
                    |2=[[Mammal]]s
                   }}
               }}
           }}
       }}
    |label2=[[Sauropsid]]s
    |2={{clade
        |1={{clade
            |1=[[Anapsid]]s; whether [[turtle]]s belong here is debated<ref>{{cite journal |last1=deBraga |first1=Michael |last2=Rieppel |first2=Olivier |date=July 1997 |title=Reptile phylogeny and the interrelationships of turtles |journal=[[Zoological Journal of the Linnean Society]] |volume=120 |issue=3 |pages=281–354 |doi=10.1111/j.1096-3642.1997.tb01280.x |doi-access=free |issn=0024-4082}}</ref>
            |label2=
            |2={{clade
               |1=[[Captorhinidae]] and [[Protorothyrididae]] (extinct)
               |label2=[[Diapsid]]s
               |2={{clade
                   |1=[[Araeoscelidia]] (extinct)
                   |label2=
                   |2={{clade
                       |label1=
                       |1=[[Squamata]] ([[lizard]]s and [[snake]]s)
                       |label2=[[Archosaur]]s
                       |2={{clade
                           |1=Extinct archosaurs
                           |2=[[Crocodilia]]ns
                           |label3=
                           |3={{clade
                               |1=[[Pterosaur]]s (extinct)
                               |label2=[[Dinosaur]]s
                               |2={{clade
                                   |label11=[[Saurischia]]ns
                                   |1={{clade
                                       |label1=[[Theropod]]s
                                       |1={{clade 
                                           |label1=
                                           |1=Extinct<br />theropods
                                           |label2=
                                           |2=[[Bird]]s
                                          }}
                                       |2=[[Sauropod]]s<br />(extinct)
                                      }}
                                   |label2=
                                   |2=[[Ornithischia]]ns (extinct)
                                  }}
                              }}
                          }}
                      }}
                  }}
               }}
           }}
       }}
   }}
}}
</div>Possible family tree of [[dinosaur]]s, [[bird]]s and [[mammal]]s<ref name="BentonDonoghue2007" /><ref name="Benton1999" /></div>
Amniotes, whose eggs can survive in dry environments, probably evolved in the Late Carboniferous period ({{ma|330|Permian|Ma}}). The earliest fossils of the two surviving amniote groups, [[synapsid]]s and [[sauropsid]]s, date from around {{ma|313|Ma|Carboniferous}}.<ref name="BentonDonoghue2007">{{cite journal |last1=Benton |first1=Michael J. |author1-link=Michael Benton |last2=Donoghue |first2=Philip C. J. |date=January 2007 |title=Paleontological Evidence to Date the Tree of Life |journal=[[Molecular Biology and Evolution]] |volume=24 |issue=1 |pages=26–53 |doi=10.1093/molbev/msl150 |doi-access=free |issn=0737-4038 |pmid=17047029}}</ref><ref name="Benton1999">{{cite journal |last=Benton |first=Michael J. |author-link=Michael Benton |date=May 1990 |title=Phylogeny of the major tetrapod groups: Morphological data and divergence dates |journal=[[Journal of Molecular Evolution]] |volume=30 |issue=5 |pages=409–424 |doi=10.1007/BF02101113 |issn=0022-2844 |pmid=2111854 |bibcode=1990JMolE..30..409B |s2cid=35082873}}</ref> The synapsid [[pelycosaur]]s and their descendants the [[therapsid]]s are the most common land vertebrates in the best-known Permian (298.9 to 251.9 Ma) fossil beds. However, at the time these were all in [[temperate]] zones at middle [[latitude]]s, and there is evidence that hotter, drier environments nearer the Equator were dominated by sauropsids and amphibians.<ref>{{cite journal |last1=Sidor |first1=Christian A. |author1-link=Christian Sidor |last2=O'Keefe |first2=F. Robin |last3=Damiani |first3=Ross |last4=Steyer |first4=J. Sébastien |last5=Smith |first5=Roger M. H. |last6=Larsson |first6=Hans C. E. |last7=Sereno |first7=Paul C. |author7-link=Paul Sereno |last8=Ide |first8=Oumarou |last9=Maga |first9=Abdoulaye |date=April 14, 2005 |title=Permian tetrapods from the Sahara show climate-controlled endemism in Pangaea |journal=[[Nature (journal)|Nature]] |volume=434 |issue=7035 |pages=886–889 |bibcode=2005Natur.434..886S |doi=10.1038/nature03393 |s2cid=4416647 |pmid=15829962 |display-authors=3 |issn=0028-0836 |url=http://doc.rero.ch/record/15308/files/PAL_E2607.pdf |access-date=December 18, 2022 |archive-date=February 28, 2023 |archive-url=https://web.archive.org/web/20230228205842/https://doc.rero.ch/record/15308/files/PAL_E2607.pdf |url-status=live}}</ref>

The [[Permian–Triassic extinction event]] wiped out almost all land vertebrates,<ref>{{cite journal |last1=Smith |first1=Roger |last2=Botha |first2=Jennifer |date=September–October 2005 |title=The recovery of terrestrial vertebrate diversity in the South African Karoo Basin after the end-Permian extinction |journal=[[Comptes Rendus Palevol]] |volume=4 |issue=6–7 |pages=623–636 |doi=10.1016/j.crpv.2005.07.005 |bibcode=2005CRPal...4..623S |issn=1631-0683}}</ref> as well as the great majority of other life.<ref>{{harvnb|Benton|2005}}</ref> During the slow recovery from this catastrophe, estimated to have taken 30 million years,<ref name="SahneyBenton2008">{{cite journal |last1=Sahney |first1=Sarda |last2=Benton |first2=Michael J. |author2-link=Michael Benton |date=April 7, 2008 |title=Recovery from the most profound mass extinction of all time |journal=[[Proceedings of the Royal Society B]] |volume=275 |pages=759–765 |issue=1636 |doi=10.1098/rspb.2007.1370 |issn=0962-8452 |pmc=2596898 |pmid=18198148}}</ref> a previously obscure sauropsid group became the most abundant and diverse terrestrial vertebrates: a few fossils of [[archosauriformes]] ("ruling lizard forms") have been found in Late Permian rocks,<ref>{{harvnb|Gauthier|Cannatella|de Queiroz|Kluge|1989|p=[https://repository.si.edu/bitstream/handle/10088/4689/VZ_1989GauthieretalHierLife.pdf 345]}}</ref> but, by the [[Middle Triassic]], archosaurs were the dominant land vertebrates. Dinosaurs distinguished themselves from other archosaurs in the Late Triassic, and became the dominant land vertebrates of the Jurassic and Cretaceous periods ({{ma|Jurassic|Paleogene|Ma}}).<ref>{{cite journal |last=Benton |first=Michael J. |author-link=Michael Benton |date=March 1983 |title=Dinosaur Success in the Triassic: A Noncompetitive Ecological Model |url=http://palaeo.gly.bris.ac.uk/Benton/reprints/1983success.pdf |journal=[[The Quarterly Review of Biology]] |volume=58 |issue=1 |pages=29–55 |jstor=2828101 |access-date=2008-09-08 |doi=10.1086/413056 |s2cid=13846947 |issn=0033-5770 |archive-url=https://web.archive.org/web/20080911075351/http://palaeo.gly.bris.ac.uk/Benton/reprints/1983success.pdf |archive-date=2008-09-11}}</ref>

=== Birds ===
During the Late Jurassic, birds evolved from small, predatory [[theropod]] dinosaurs.<ref name="padian2004">{{harvnb|Padian|2004|pp=210–231}}</ref> The first birds inherited teeth and long, bony tails from their dinosaur ancestors,<ref name="padian2004" /> but some had developed horny, toothless [[beak]]s by the very Late Jurassic<ref>{{cite journal |last1=Hou |first1=Lian-hai |last2=Zhou |first2=Zhonghe |author2-link=Zhou Zhonghe |last3=Martin |first3=Larry D. |author3-link=Larry Martin |last4=Feduccia |first4=Alan |author4-link=Alan Feduccia |display-authors=3 |date=October 19, 1995 |title=A beaked bird from the Jurassic of China |journal=[[Nature (journal)|Nature]] |volume=377 |pages=616–618 |issue=6550 |bibcode=1995Natur.377..616H |doi=10.1038/377616a0 |s2cid=4357707 |issn=0028-0836}}</ref> and short [[pygostyle]] tails by the [[Early Cretaceous]].<ref>{{cite journal |last1=Clarke |first1=Julia A. |author1-link=Julia Clarke |last2=Zhou |first2=Zhonghe |author2-link=Zhou Zhonghe |last3=Zhang |first3=Fucheng |date=March 2006 |title=Insight into the evolution of avian flight from a new clade of Early Cretaceous ornithurines from China and the morphology of ''Yixianornis grabaui'' |journal=[[Journal of Anatomy]] |volume=208 |issue=3 |pages=287–308 |doi=10.1111/j.1469-7580.2006.00534.x |issn=1469-7580 |pmc=2100246 |pmid=16533313}}</ref>

=== Mammals ===
While the archosaurs and dinosaurs were becoming more dominant in the Triassic, the [[mammaliaform]] successors of the therapsids evolved into small, mainly nocturnal [[insectivore]]s. This ecological role may have promoted the [[evolution of mammals]], for example nocturnal life may have accelerated the development of [[endotherm]]y ("warm-bloodedness") and hair or fur.<ref name="RubenJones2000">{{cite journal |last1=Ruben |first1=John A. |author1-link=John Ruben |last2=Jones |first2=Terry D. |date=August 2000 |title=Selective Factors Associated with the Origin of Fur and Feathers |journal=[[American Zoologist]] |volume=40 |issue=4 |pages=585–596 |doi=10.1093/icb/40.4.585 |doi-access=free |issn=0003-1569}}</ref> By {{ma|195|Ma|Jurassic}} in the [[Early Jurassic]] there were animals that were very like today's mammals in a number of respects.<ref>{{cite journal |last1=Zhe-Xi |first1=Luo |author1-link=Zhe-Xi Luo |last2=Crompton |first2=Alfred W. |author2-link=Alfred W. Crompton |last3=Ai-Lin |first3=Sun |date=May 25, 2001 |title=A New Mammaliaform from the Early Jurassic and Evolution of Mammalian Characteristics |journal=[[Science (journal)|Science]] |volume=292 |issue=5521 |pages=1535–1540 |bibcode=2001Sci...292.1535L |doi=10.1126/science.1058476 |s2cid=8738213 |issn=0036-8075 |pmid=11375489}}</ref> Unfortunately, there is a gap in the fossil record throughout the Middle Jurassic.<ref name="Cifelli2001">{{cite journal |last=Cifelli |first=Richard L. |date=November 2001 |title=Early mammalian radiations |url=http://jpaleontol.geoscienceworld.org/content/75/6/1214.extract |journal=[[Journal of Paleontology]] |volume=75 |issue=6 |pages=1214–1226 |doi=10.1666/0022-3360(2001)075<1214:EMR>2.0.CO;2 |bibcode=2001JPal...75.1214C |s2cid=85882683 |issn=0022-3360 |access-date=2015-02-16 |archive-date=2020-05-30 |archive-url=https://web.archive.org/web/20200530172145/https://pubs.geoscienceworld.org/jpaleontol/article-abstract/75/6/1214/83323/EARLY-MAMMALIAN-RADIATIONS?redirectedFrom=fulltext |url-status=live}}</ref> However, fossil teeth discovered in [[Madagascar]] indicate that the split between the lineage leading to [[monotreme]]s and the one leading to other living mammals had occurred by {{ma|167|Ma|Jurassic}}.<ref>{{cite journal |last1=Flynn |first1=John J. |last2=Parrish |first2=J. Michael |last3=Rakotosamimanana |first3=Berthe |author3-link=Berthe Rakotosamimanana |last4=Simpson |first4=William F. |last5=Wyss |first5=André R. |author5-link=Andre Wyss |date=September 2, 1999 |title=A Middle Jurassic mammal from Madagascar |journal=[[Nature (journal)|Nature]] |volume=401 |issue=6748 |pages=57–60 |bibcode=1999Natur.401...57F |doi=10.1038/43420 |s2cid=40903258 |display-authors=3 |issn=0028-0836}}</ref> After dominating land vertebrate niches for about 150 Ma, the non-avian dinosaurs perished in the Cretaceous–Paleogene extinction event ({{ma|Paleogene|Ma}}) along with many other groups of organisms.<ref name="MacLeod">{{cite journal |last1=MacLeod |first1=Norman |last2=Rawson |first2=Peter F. |last3=Forey |first3=Peter L. |last4=Banner |first4=Frederick T. |last5=Boudagher-Fadel |first5=Marcelle K. |last6=Bown |first6=Paul R. |last7=Burnett |first7=Jackie A. |last8=Chambers |first8=Paul |last9=Culver |first9=Stephen |last10=Evans |first10=Susan E. |last11=Jeffery |first11=Charlotte |last12=Kaminski |first12=Michael A. |last13=Lord |first13=Allan R. |last14=Milner |first14=Angela C. |last15=Milner |first15=Andrew R. |last16=Morris |first16=Noel |last17=Owen |first17=Ellis |last18=Rosen |first18=Brian R. |last19=Smith |first19=Andrew B. |last20=Taylor |first20=Paul D. |last21=Urquhart |first21=Elspeth |last22=Young |first22=Jeremy R. |date=April 1997 |title=The Cretaceous–Tertiary biotic transition |url=http://jgs.geoscienceworld.org/content/154/2/265.abstract |journal=[[Journal of the Geological Society]] |volume=154 |issue=2 |pages=265–292 |doi=10.1144/gsjgs.154.2.0265 |issn=0016-7649 |access-date=2015-02-16 |display-authors=3 |bibcode=1997JGSoc.154..265M |s2cid=129654916 |archive-date=2020-05-30 |archive-url=https://web.archive.org/web/20200530172144/https://pubs.geoscienceworld.org/jgs/article-abstract/154/2/265/93802/The-Cretaceous-Tertiary-biotic-transition?redirectedFrom=fulltext |url-status=live|url-access=subscription }}</ref> Mammals throughout the time of the dinosaurs had been restricted to a narrow range of [[taxa]], sizes and shapes, but increased rapidly in size and diversity after the extinction,<ref>{{cite journal |last=Alroy |first=John |author-link=John Alroy |date=March 1999 |title=The Fossil Record of North American Mammals: Evidence for a Paleocene Evolutionary Radiation |journal=[[Systematic Biology]] |volume=48 |issue=1 |pages=107–118 |doi=10.1080/106351599260472 |doi-access=free |issn=1063-5157 |pmid=12078635}}</ref><ref>{{cite journal |last1=Archibald |first1=J. David |last2=Deutschman |first2=Douglas H. |date=June 2001 |title=Quantitative Analysis of the Timing of the Origin and Diversification of Extant Placental Orders |url=http://www.bio.sdsu.edu/faculty/archibald/ArchDeut01JME8p107.pdf |journal=Journal of Mammalian Evolution |volume=8 |issue=2 |pages=107–124 |doi=10.1023/A:1011317930838 |issn=1064-7554 |s2cid=15581162 |access-date=2015-02-16 |url-status=live |archive-url=https://web.archive.org/web/20090709140528/http://www.bio.sdsu.edu/faculty/archibald/ArchDeut01JME8p107.pdf |archive-date=2009-07-09}}</ref> with [[bat]]s taking to the air within 13&nbsp;million years,<ref>{{cite journal |last1=Simmons |first1=Nancy B. |author-link1=Nancy Simmons |last2=Seymour |first2=Kevin L. |last3=Habersetzer |first3=Jörg |last4=Gunnell |first4=Gregg F. |display-authors=3 |date=February 14, 2008 |title=Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation |journal=[[Nature (journal)|Nature]] |volume=451 |pages=818–821 |issue=7180 |bibcode=2008Natur.451..818S |doi=10.1038/nature06549 |s2cid=4356708 |hdl-access=free |issn=0028-0836 |pmid=18270539 |hdl=2027.42/62816}}</ref> and [[cetacea]]ns to the sea within 15&nbsp;million years.<ref>{{harvnb|Thewissen|Madar|Hussain|1996}}</ref>
{{clear}}

=== Flowering plants ===
{{main|Flowering plant}}

{| align="right" border="0" cellspacing="0" cellpadding="3"
|- valign="top"
|<div style="width:auto; border:solid 1px silver; padding:2px; margin:2px;">
<div style="width:auto; border:solid 1px silver; padding:5px">
{{clade
  |label1=[[Gymnosperm]]s
  |1={{clade
      |1={{clade     
          |1={{clade     
              |1={{clade     
                  |1=[[Gnetum|Gnetales]]<br />(gymnosperm)
                  |2=''[[Welwitschia]]''<br />(gymnosperm)
                 }}
              |2=''[[Ephedra (plant)|Ephedra]]''<br />(gymnosperm)
             }}
          |2=[[Bennettitales]]
          |3=[[Angiosperm]]s<br />(flowering plants)
         }}
     }}
}}
</div>One possible family tree of [[flowering plant]]s<ref name="CraneFriisPedersen2000" /></div>
|<div style="width:auto; border:solid 1px silver; padding:2px; margin:2px;">
<div style="width:auto; border:solid 1px silver; padding:5px">
{{clade
  |label1=[[Gymnosperm]]s
  |1={{clade
      |1=[[Angiosperm]]s<br />(flowering plants)
      |2={{clade
           |1={{clade
                |1={{clade
                    |1=[[Cycad]]s<br />(gymnosperm)
                    |2=[[Bennettitales]]
                  }}
               |2=''[[Ginkgo]]''
             }}
          |2={{clade
              |1=[[Gnetales]]<br />(gymnosperm)
              |2=[[Conifer]]s<br />(gymnosperm)
             }}
         }}
     }}
}}
</div>Another possible family tree<ref name="Crepet2000" /></div>
|}
The first flowering plants appeared around 130 Ma.<ref>{{cite encyclopedia |encyclopedia=[[Encyclopædia Britannica Online]] |title=evolution: plant timeline |url=https://www.britannica.com/EBchecked/media/24/Significant-events-in-plant-evolution |year=1996 |publisher=[[Encyclopædia Britannica, Inc.]] |isbn=978-0-691-18550-7 |oclc=42796406 |archive-url=https://web.archive.org/web/20150327154926/https://www.britannica.com/EBchecked/media/24/Significant-events-in-plant-evolution |archive-date=2015-03-27 |access-date=2020-10-23}}</ref> The 250,000 to 400,000 species of flowering plants outnumber all other ground plants combined, and are the dominant vegetation in most terrestrial ecosystems. There is fossil evidence that flowering plants diversified rapidly in the Early Cretaceous, from {{ma|130|90|Ma|Cretaceous}},<ref name="CraneFriisPedersen2000">[[#Gee 2000|Crane, Friis and Pedersen 2000]], "The Origin and Early Diversification of Angiosperms," pp. [https://books.google.com/books?id=ZJe_Dmdbm-QC&pg=PA233 233–250] {{Webarchive|url=https://web.archive.org/web/20230716080907/https://books.google.com/books?id=ZJe_Dmdbm-QC&pg=PA233 |date=2023-07-16 }}</ref><ref name="Crepet2000">{{cite journal |last=Crepet |first=William L. |date=November 21, 2000 |title=Progress in understanding angiosperm history, success, and relationships: Darwin's abominably 'perplexing phenomenon' |journal=[[Proc. Natl. Acad. Sci. U.S.A.]] |volume=97 |issue=24 |pages=12939–12941 |bibcode=2000PNAS...9712939C |doi=10.1073/pnas.97.24.12939 |issn=0027-8424 |pmc=34068 |pmid=11087846 |doi-access=free}}</ref> and that their rise was associated with that of [[pollinating]] insects.<ref name="Crepet2000" /> Among modern flowering plants ''[[Magnoliaceae|Magnolia]]'' are thought to be close to the common ancestor of the group.<ref name="CraneFriisPedersen2000" /> However, paleontologists have not succeeded in identifying the earliest stages in the evolution of flowering plants.<ref name="CraneFriisPedersen2000" /><ref name="Crepet2000" />
{{clear}}

=== Social insects ===
[[File:Termite cathedral mounds in a bushfire blackened tropical savanna.jpg|thumb|right|upright|These [[termite]] mounds have survived a bush fire.]]
{{main|Eusociality}}

The social insects are remarkable because the great majority of individuals in each colony are sterile. This appears contrary to basic concepts of evolution such as natural selection and the [[Gene-centered view of evolution|selfish gene]]. In fact, there are very few [[eusocial]] insect species: only 15 out of approximately 2,600 living [[family (biology)|families]] of insects contain eusocial species, and it seems that eusociality has evolved independently only 12 times among arthropods, although some eusocial lineages have diversified into several families. Nevertheless, social insects have been spectacularly successful; for example although [[ant]]s and [[termite]]s account for only about 2% of known insect species, they form over 50% of the total mass of insects. Their ability to control a territory appears to be the foundation of their success.<ref name="WilsonHölldobler2005">{{cite journal |last1=Wilson |first1=Edward O. |author1-link=E. O. Wilson |last2=Hölldobler |first2=Bert |author2-link=Bert Hölldobler |date=September 20, 2005 |title=Eusociality: Origin and consequences |journal=[[Proc. Natl. Acad. Sci. U.S.A.]] |volume=102 |issue=38 |pages=13367–13371 |bibcode=2005PNAS..10213367W |doi=10.1073/pnas.0505858102 |issn=0027-8424 |pmc=1224642 |pmid=16157878 |doi-access=free}}</ref>

The sacrifice of breeding opportunities by most individuals has long been explained as a consequence of these species' unusual [[haplodiploid]] method of [[sex-determination system|sex determination]], which has the paradoxical consequence that two sterile worker daughters of the same queen share more genes with each other than they would with their offspring if they could breed.<ref>{{cite journal |last1=Hughes |first1=William O. H. |last2=Oldroyd |first2=Benjamin P. |last3=Beekman |first3=Madeleine |last4=Ratnieks |first4=Francis L. W. |date=May 30, 2008 |title=Ancestral Monogamy Shows Kin Selection Is Key to the Evolution of Eusociality |journal=[[Science (journal)|Science]] |volume=320 |issue=5880 |pages=1213–1216 |bibcode=2008Sci...320.1213H |doi=10.1126/science.1156108 |s2cid=20388889 |issn=0036-8075 |pmid=18511689}}</ref> However, [[E. O. Wilson]] and [[Bert Hölldobler]] argue that this explanation is faulty: for example, it is based on [[kin selection]], but there is no evidence of [[nepotism]] in colonies that have multiple queens. Instead, they write, eusociality evolves only in species that are under strong pressure from predators and competitors, but in environments where it is possible to build "fortresses"; after colonies have established this security, they gain other advantages through co-operative [[foraging]]. In support of this explanation they cite the appearance of eusociality in [[Blesmol|bathyergid]] mole rats,<ref name="WilsonHölldobler2005" /> which are not haplodiploid.<ref>{{cite journal |last=Lovegrove |first=Barry G. |date=January 1991 |title=The evolution of eusociality in molerats (Bathyergidae): a question of risks, numbers, and costs |journal=[[Behavioral Ecology and Sociobiology]] |volume=28 |issue=1 |pages=37–45 |doi=10.1007/BF00172137 |bibcode=1991BEcoS..28...37L |s2cid=36466393 |issn=0340-5443}}</ref>

The earliest fossils of insects have been found in Early Devonian rocks from about {{ma|400|Ma|Devonian}}, which preserve only a few varieties of flightless insect. The [[Mazon Creek fossil beds|Mazon Creek lagerstätten]] from the Late Carboniferous, about {{ma|300|Ma|Carboniferous}}, include about 200 species, some gigantic by modern standards, and indicate that insects had occupied their main modern ecological niches as [[herbivore]]s, [[detritivore]]s and insectivores. Social termites and ants first appeared in the Early Cretaceous, and advanced social bees have been found in Late Cretaceous rocks but did not become abundant until the Middle [[Cenozoic]].<ref>{{harvnb|Labandeira|Eble|1999}}</ref>

=== Humans ===
{{main|Human evolution}}

The idea that, along with other life forms, modern-day humans evolved from an ancient, common ancestor was proposed by [[Robert Chambers (journalist)|Robert Chambers]] in 1844 and taken up by [[Charles Darwin]] in 1871.<ref>{{cite web |url=https://darwin200.christs.cam.ac.uk/node/90 |title=Darwin & The Descent of Man |last=Montgomery |first=Stephen |year=2009 |website=Charles Darwin & Evolution |publisher=[[Christ's College, Cambridge|Christ's College]] |location=Cambridge |access-date=2020-11-19 |url-status=unfit |archive-url=https://web.archive.org/web/20190620081336/https://darwin200.christs.cam.ac.uk/node/90 |archive-date=2019-06-20}}</ref> Modern humans evolved from a lineage of upright-walking [[ape]]s that has been traced back over {{ma|6|Ma}} to ''[[Sahelanthropus]]''.<ref>{{cite journal |last1=Brunet |first1=Michel |author1-link=Michel Brunet (paleontologist) |last2=Guy |first2=Franck |last3=Pilbeam |first3=David |author3-link=David Pilbeam |last4=Mackaye |first4=Hassane Taisso |last5=Likius |first5=Andossa |last6=Ahounta |first6=Djimdoumalbaye |last7=Beauvilain |first7=Alain |last8=Blondel |first8=Cécile |date=July 11, 2002 |title=A new hominid from the Upper Miocene of Chad, Central Africa |journal=[[Nature (journal)|Nature]] |volume=418 |issue=6894 |pages=145–151 |bibcode=2002Natur.418..145B |s2cid=1316969 |doi=10.1038/nature00879 |pmid=12110880 |display-authors=3 |issn=0028-0836 |url=http://doc.rero.ch/record/13388/files/PAL_E190.pdf |access-date=December 18, 2022 |archive-date=February 25, 2023 |archive-url=https://web.archive.org/web/20230225204437/https://doc.rero.ch/record/13388/files/PAL_E190.pdf |url-status=live}}</ref> The first known [[stone tool]]s were made about {{ma|2.5|Ma}}, apparently by ''[[Australopithecus garhi]]'', and were found near animal bones that bear scratches made by these tools.<ref>{{cite journal |last1=de Heinzelin |first1=Jean |author1-link=Jean de Heinzelin de Braucourt |last2=Clark |first2=J. Desmond |author2-link=J. Desmond Clark |last3=White |first3=Tim |author3-link=Tim D. White |last4=Hart |first4=William |last5=Renne |first5=Paul |author5-link=Paul Renne |last6=WoldeGabriel |first6=Giday |author6-link=Giday WoldeGabriel |last7=Beyene |first7=Yonas |last8=Vrba |first8=Elisabeth |author8-link=Elisabeth Vrba |date=April 23, 1999 |title=Environment and Behavior of 2.5-Million-Year-Old Bouri Hominids |journal=[[Science (journal)|Science]] |volume=284 |issue=5414 |pages=625–629 |doi=10.1126/science.284.5414.625 |issn=0036-8075 |pmid=10213682 |display-authors=3 |bibcode=1999Sci...284..625D}}</ref> The earliest [[hominine]]s had [[chimpanzee]]-sized brains, but there has been a fourfold increase in the last 3&nbsp;Ma; a statistical analysis suggests that hominine brain sizes depend almost completely on the date of the fossils, while the species to which they are assigned has only slight influence.<ref>{{cite journal |last1=De Miguel |first1=Carmen |last2=Henneberg |first2=Maciej |author2-link=Maciej Henneberg |year=2001 |title=Variation in hominid brain size: How much is due to method? |journal=[[Homo: Journal of Comparative Human Biology]] |volume=52 |issue=1 |pages=3–58 |doi=10.1078/0018-442X-00019 |issn=0018-442X |pmid=11515396|hdl=2440/5362 |hdl-access=free }}</ref> There is a long-running debate about whether modern humans evolved [[Multiregional origin of modern humans|all over the world]] simultaneously from existing advanced hominines or are descendants of a [[Recent African origin of modern humans|single small population in Africa]], which then migrated all over the world less than 200,000&nbsp;years ago and replaced previous hominine species.<ref>{{harvnb|Leakey|1994|pp=87–89}}</ref> There is also debate about whether anatomically modern humans had an intellectual, cultural and technological [[Behavioral modernity|"Great Leap Forward"]] under 40,000–50,000&nbsp;years ago and, if so, whether this was due to neurological changes that are not visible in fossils.<ref name="Klein 1995">{{cite journal |last=Klein |first=Richard |title=Anatomy, behavior, and modern human origins |journal=Journal of World Prehistory |date=1995 |volume=9 |issue=2 |pages=167–198 |doi=10.1007/bf02221838 |s2cid=10402296}}</ref><ref>{{cite journal |last=Mellars |first=Paul |author-link=Paul Mellars |date=June 20, 2006 |title=Why did modern human populations disperse from Africa ''ca.'' 60,000 years ago? A new model |journal=[[Proc. Natl. Acad. Sci. U.S.A.]] |volume=103 |pages=9381–9386 |issue=25 |bibcode=2006PNAS..103.9381M |doi=10.1073/pnas.0510792103 |issn=0027-8424 |pmid=16772383 |pmc=1480416 |doi-access=free}}</ref>

== Mass extinctions ==
{{main|Extinction event}}
{{annotated image/Extinction|caption=Apparent extinction intensity, i.e. the fraction of [[genera]] going extinct at any given time, as reconstructed from the [[Fossil#Dating|fossil record]]. (Graph not meant to include the recent, ongoing [[Holocene extinction]] event).}}

Life on Earth has suffered occasional mass extinctions at least since {{ma|542|Ma}}. Although they were disasters at the time, mass extinctions have sometimes accelerated the evolution of life on Earth. When dominance of particular ecological niches passes from one group of organisms to another, it is rarely because the new dominant group is "superior" to the old and usually because an extinction event eliminates the old dominant group and makes way for the new one.<ref name="Van Valkenburgh 1999 463–493" /><ref>{{harvnb|Benton|2005a|loc=Chapter 6: "Tetrapods of the Triassic"}}</ref>
{{Phanerozoic biodiversity |float=right}}
The fossil record appears to show that the gaps between mass extinctions are becoming longer and that the average and background rates of extinction are decreasing. Both of these phenomena could be explained in one or more ways:<ref name="MacLeod2001">{{cite web |url=http://www.firstscience.com/SITE/ARTICLES/macleod.asp |url-status=live |title=Extinction! |last=MacLeod |first=Norman |year=1999 |website=FirstScience.com |archive-url=https://web.archive.org/web/20000819004853/http://www.firstscience.com/site/articles/macleod.asp |archive-date=2000-08-19 |access-date=2020-12-25}}</ref>
* The oceans may have become more hospitable to life over the last 500&nbsp;Ma and less vulnerable to mass extinctions: dissolved oxygen became more widespread and penetrated to greater depths; the development of life on land reduced the run-off of nutrients and hence the risk of [[eutrophication]] and anoxic events; and marine ecosystems became more diversified so that food chains were less likely to be disrupted.<ref>{{cite journal |last=Martin |first=Ronald E. |date=June 1995 |title=Cyclic and secular variation in microfossil biomineralization: clues to the biogeochemical evolution of Phanerozoic oceans |journal=[[Global and Planetary Change]] |volume=11 |issue=1–2 |pages=1–23 |bibcode=1995GPC....11....1M |doi=10.1016/0921-8181(94)00011-2 |issn=0921-8181}}</ref><ref>{{cite journal |last=Martin |first=Ronald E. |date=June 1996 |title=Secular Increase in Nutrient Levels through the Phanerozoic: Implications for Productivity, Biomass, and Diversity of the Marine Biosphere |journal=[[PALAIOS]] |volume=11 |issue=3 |pages=209–219 |bibcode=1996Palai..11..209M |doi=10.2307/3515230 |issn=0883-1351 |jstor=3515230}}</ref>
* Reasonably complete fossils are very rare, most extinct organisms are represented only by partial fossils, and complete fossils are rarest in the oldest rocks. So paleontologists have mistakenly assigned parts of the same organism to different genera, which were often defined solely to accommodate these finds—the story of ''[[Anomalocaris]]'' is an example of this. The risk of this mistake is higher for older fossils because these are often both unlike parts of any living organism and poorly conserved. Many of the "superfluous" genera are represented by fragments which are not found again and the "superfluous" genera appear to become extinct very quickly.<ref name="MacLeod2001" />

[[Biodiversity]] in the fossil record, which is "...the number of distinct genera alive at any given time; that is, those whose first occurrence predates and whose last occurrence postdates that time"<ref name="RohdeMuller2005" /> shows a different trend: a fairly swift rise from {{ma|542|400|Ma}}; a slight decline from {{ma|400|200|Ma}}, in which the devastating Permian–Triassic extinction event is an important factor; and a swift rise from {{ma|200|Ma}} to the present.<ref name="RohdeMuller2005">{{cite journal |last1=Rohde |first1=Robert A. |author1-link=Robert Rohde |last2=Muller |first2=Richard A. |author2-link=Richard A. Muller |date=March 10, 2005 |title=Cycles in fossil diversity |url=http://muller.lbl.gov/papers/Rohde-Muller-Nature.pdf |journal=[[Nature (journal)|Nature]] |volume=434 |issue=7030 |pages=208–210 |bibcode=2005Natur.434..208R |doi=10.1038/nature03339 |s2cid=32520208 |pmid=15758998 |issn=0028-0836 |access-date=2008-09-22 |archive-date=2008-10-03 |archive-url=https://web.archive.org/web/20081003221929/http://muller.lbl.gov/papers/Rohde-Muller-Nature.pdf |url-status=live}}</ref>
{{clear}}

== See also ==

{{div col|colwidth=22em}}
* [[History of evolutionary thought]]
* ''[[On the Origin of Species]]''
* [[Phylogenetics]]
* [[Speciation]]
* [[Taxonomy of commonly fossilised invertebrates]]
* ''[[Treatise on Invertebrate Paleontology]]''
* [[Viral evolution]]
{{div col end}}

== Footnotes ==
{{Reflist|group=note}}

== References ==
{{Reflist|30em}}

== Bibliography ==

{{Refbegin|30em}}
* {{cite book |last=Arrhenius |first=Svante |author-link=Svante Arrhenius |year=1980 |orig-date=Arrhenius paper originally published 1903 |chapter=The Propagation of Life in Space |editor-last=Goldsmith |editor-first=Donald |title=The Quest for Extraterrestrial Life: A Book of Readings |others=Foreword by Sir [[Fred Hoyle]] |location=Mill Valley, CA |publisher=University Science Books |bibcode=1980qel..book...32A |isbn=978-0-935702-02-6 |lccn=79057423 |oclc=7121102}}
* {{cite book |last=Bengtson |first=Stefan |year=2004 |chapter=Early Skeletal Fossils |editor1-last=Lipps |editor1-first=Jere H. |editor1-link=Jere H. Lipps |editor2-last=Waggoner |editor2-first=Benjamin M. |title=Neoproterozoic-Cambrian Biological Revolutions: Presented as a Paleontological Society Short Course at the Annual Meeting of the Geological Society of America, Denver, Colorado, November 6, 2004 |series=Paleontological Society Papers |volume=10 |location=New Haven, CT |publisher=Yale University Reprographics & Imaging Service; [[Paleontological Society]] |oclc=57481790}}
* {{cite book |last=Bennett |first=Jeffrey O. |year=2008 |title=Beyond UFOs: The Search for Extraterrestrial Life and Its Astonishing Implications for Our Future |location=Princeton, NJ |publisher=[[Princeton University Press]] |isbn=978-0-691-13549-6 |lccn=2007037872 |oclc=172521761 |bibcode=2008buse.book.....B |url=https://archive.org/details/beyondufossearch00benn}}
* {{cite book |last=Benton |first=Michael J. |author-link=Michael Benton |year=1997 |title=Vertebrate Palaeontology |edition=2nd |location=London |publisher=[[Chapman & Hall]] |isbn=978-0-412-73800-5 |oclc=37378512 |title-link=Vertebrate Palaeontology (Benton)}}
* {{cite book |last=Benton |first=Michael J. |year=2005 |orig-date=Originally published 2003 |title=When Life Nearly Died: The Greatest Mass Extinction of All Time |edition=1st paperback |location=London |publisher=[[Thames & Hudson]] |isbn=978-0-500-28573-2 |lccn=2002109744 |oclc=62145244}}
* {{cite book |last=Benton |first=Michael J. |year=2005 |title=Vertebrate Palaeontology |edition=3rd |location=Malden, MA |publisher=[[Blackwell Science]] |isbn=978-0-632-05637-8 |lccn=2003028152 |oclc=53970617 |ref={{harvid|Benton|2005a}}}}
* {{cite book |last1=Bernstein |first1=Harris |last2=Bernstein |first2=Carol |last3=Michod |first3=Richard E. |year=2012 |chapter=DNA Repair as the Primary Adaptive Function of Sex in Bacteria and Eukaryotes |chapter-url=https://www.novapublishers.com/catalog/product_info.php?products_id=31918 |editor1-last=Kimura |editor1-first=Sakura |editor2-last=Shimizu |editor2-first=Sora |title=DNA Repair: New Research |location=Hauppauge, NY |publisher=[[Nova Science Publishers]] |isbn=978-1-62100-808-8 |lccn=2011038504 |oclc=828424701 |access-date=2013-07-21 |archive-date=2013-10-29 |archive-url=https://web.archive.org/web/20131029202307/https://www.novapublishers.com/catalog/product_info.php?products_id=31918}}
* {{cite book |last1=Bernstein |first1=Harris |last2=Hopf |first2=Frederic A. |last3=Michod |first3=Richard E. |year=1987 |chapter=The Molecular Basis of the Evolution of Sex |editor-last=Scandalios |editor-first=John G. |title=Molecular Genetics of Development |volume=24 |pages=323–70 |series=Advances in Genetics |location=San Diego, CA |publisher=[[Academic Press]] |isbn=978-0-12-017624-3 |oclc=646754753 |pmid=3324702 |doi=10.1016/s0065-2660(08)60012-7}}
* {{cite book |last1=Birdsell |first1=John A. |last2=Wills |first2=Christopher |author2-link=Christopher Wills |year=2003 |chapter=The Evolutionary Origin and Maintenance of Sexual Recombination: A Review of Contemporary Models |editor1-last=MacIntyre |editor1-first=Ross J. |editor2-last=Clegg |editor2-first=Michael T. |title=Evolutionary Biology |volume=33 |location=New York |publisher=[[Springer Science+Business Media]] |isbn=978-1-4419-3385-0 |oclc=751583918}}
* {{cite book |editor1-last=Briggs |editor1-first=Derek E. G. |editor1-link=Derek Briggs |editor2-last=Crowther |editor2-first=Peter R. |year=2001 |title=Palaeobiology II |others=Foreword by [[Euan Clarkson|E. N. K. Clarkson]] |location=Malden, MA |publisher=Blackwell Science |isbn=978-0-632-05149-6 |lccn=0632051477 |oclc=43945263 |ref=Briggs and Crowther 2001}}
* {{cite book |last=Cairns-Smith |first=A. G. |author-link=Graham Cairns-Smith |year=1968 |chapter=An Approach to a Blueprint for a Primitive Organism |editor-last=Waddington |editor-first=C. H |editor-link=C. H. Waddington |title=Towards a Theoretical Biology |location=Edinburgh, Scotland |publisher=[[Edinburgh University Press]] |volume=1 |isbn=978-0-85224-018-2 |lccn=71419832 |oclc=230043266}}
* {{cite book |last=Chapman |first=Arthur D. |year=2009 |title=Numbers of Living Species in Australia and the World |edition=2nd |location=Canberra |publisher=[[Australian Biological Resources Study]] |url=https://www.environment.gov.au/science/abrs/publications/other/numbers-living-species/ |isbn=978-0-642-56860-1 |oclc=780539206 |access-date=2016-11-06 |archive-date=2016-12-25 |archive-url=https://web.archive.org/web/20161225064434/http://www.environment.gov.au/science/abrs/publications/other/numbers-living-species |url-status=live}}
* {{cite book |last1=Clancy |first1=Paul |last2=Brack |first2=André |last3=Horneck |first3=Gerda |year=2005 |title=Looking for Life, Searching the Solar System |location=Cambridge; New York |publisher=[[Cambridge University Press]] |isbn=978-0-521-82450-7 |lccn=2006271630 |oclc=57574490}}
* {{cite book |last=Cowen |first=Richard |year=2000 |title=History of Life |edition=3rd |location=Malden, MA |publisher=Blackwell Science |isbn=978-0-632-04444-3 |lccn=99016542 |oclc=47011068}}
* {{cite book |editor1-last=Cunnane |editor1-first=Stephen C. |editor2-last=Stewart |editor2-first=Kathlyn M. |year=2010 |title=Environmental Influences on Human Brain Evolution |publisher=[[John Wiley & Sons]] |isbn=978-0-470-45268-4 |lccn=2009047979 |oclc=940843566 |ref=Cunnane and Stewart 2010}}
* {{cite book |last=Dalrymple |first=G. Brent |author-link=Brent Dalrymple |year=1991 |title=The Age of the Earth |location=Stanford, CA |publisher=[[Stanford University Press]] |isbn=978-0-8047-1569-0 |lccn=90047051 |oclc=22347190}}
* {{cite book |last=Futuyma |first=Douglas J. |author-link=Douglas J. Futuyma |year=2005 |title=Evolution |location=Sunderland, MA |publisher=[[Sinauer Associates]] |isbn=978-0-87893-187-3 |lccn=2004029808 |oclc=57311264 |url=https://archive.org/details/evolution0000futu}}
* {{cite book |last1=Gauthier |first1=Jacques |author1-link=Jacques Gauthier |last2=Cannatella |first2=David C. |last3=de Queiroz |first3=Kevin |last4=Kluge |first4=Arnold G. |author4-link=Arnold G. Kluge |last5=Rowe |first5=Timothy |year=1989 |chapter=Tetrapod phylogeny |editor1-last=Fernholm |editor1-first=Bo |editor2-last=Bremer |editor2-first=Kåre |editor2-link=Kåre Bremer |editor3-last=Jörnvall |editor3-first=Hans |title=The Hierarchy of Life: Molecules and Morphology in Phylogenetic Analysis |series=International Congress Series |location=Amsterdam, the Netherlands; New York |publisher=[[Elsevier|Excerpta Medica/Elsevier Science Publishers B.V. (Biomedical Division)]] |volume=824 |isbn=978-0-444-81073-1 |lccn=89001132 |oclc=19129518 |display-authors=3}} "Proceedings from Nobel Symposium 70 held at [[Alfred Nobel]]'s [[Björkborn Manor|Björkborn]], [[Karlskoga]], Sweden, August 29 – September 2, 1988"
* {{cite book |editor-last=Gee |editor-first=Henry |editor-link=Henry Gee |year=2000 |title=Shaking the Tree: Readings from ''Nature'' in the History of Life |location=Chicago, IL |publisher=[[University of Chicago Press]] |isbn=978-0-226-28497-2 |lccn=99049796 |oclc=42476104 |ref=Gee 2000}}
* {{cite book |last=Gould |first=Stephen Jay |author-link=Stephen Jay Gould |year=1989 |title=Wonderful Life: The Burgess Shale and the Nature of History |edition=1st |location=New York |publisher=[[W. W. Norton & Company]] |isbn=978-0-393-02705-1 |lccn=88037469 |oclc=18983518 |title-link=Wonderful Life (book) |bibcode=1989wlbs.book.....G}}
* {{cite book |last1=Grimaldi |first1=David |author1-link=David Grimaldi (entomologist) |last2=Engel |first2=Michael S. |author2-link=Michael S. Engel |year=2005 |title=Evolution of the Insects |location=Cambridge; New York |publisher=Cambridge University Press |isbn=978-0-521-82149-0 |lccn=2004054605 |oclc=56057971}}
* {{cite book |last=Hinde |first=Rosalind T. |year=2001 |chapter=The Cnidaria and Ctenophora |editor-last=Anderson |editor-first=D. T. |editor-link=Donald Thomas Anderson |title=Invertebrate Zoology |edition=2nd |location=Melbourne; New York |publisher=[[Oxford University Press]] |isbn=978-0-19-551368-4 |lccn=2002276846 |oclc=49663129}}
* {{cite book |last1=Holmes |first1=Randall K. |last2=Jobling |first2=Michael G. |year=1996 |chapter=Genetics |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK7908/ |editor-last=Baron |editor-first=Samuel |title=Medical Microbiology |edition=4th |location=Galveston, TX |publisher=[[University of Texas Medical Branch]] |at=Exchange of Genetic Information |isbn=978-0-9631172-1-2 |lccn=95050499 |oclc=33838234 |pmid=21413277 |access-date=2017-08-31 |archive-date=2022-04-03 |archive-url=https://web.archive.org/web/20220403115956/https://www.ncbi.nlm.nih.gov/books/NBK7908/ |url-status=live}}
* {{cite book |last1=Krumbein |first1=Wolfgang E. |last2=Brehm |first2=Ulrike |last3=Gerdes |first3=Gisela |last4=Gorbushina |first4=Anna A. |last5=Levit |first5=George |last6=Palinska |first6=Katarzyna A. |year=2003 |chapter=Biofilm, Biodictyon, Biomat Microbialites, Oolites, Stromatolites Geophysiology, Global Mechanism, Parahistology |chapter-url=http://134.106.242.33/krumbein/htdocs/Archive/397/Krumbein_397.pdf |editor1-last=Krumbein |editor1-first=Wolfgang E. |editor2-last=Paterson |editor2-first=David M. |editor3-last=Zavarzin |editor3-first=Georgii A. |title=Fossil and Recent Biofilms: A Natural History of Life on Earth |location=Dordrecht, the Netherlands |publisher=[[Springer Science+Business Media|Kluwer Academic Publishers]] |isbn=978-1-4020-1597-7 |lccn=2003061870 |oclc=52901566 |archive-url=https://web.archive.org/web/20070106201614/http://134.106.242.33/krumbein/htdocs/Archive/397/Krumbein_397.pdf |display-authors=3 |archive-date=2007-01-06}}
* {{cite book |last1=Labandeira |first1=Conrad C. |last2=Eble |first2=Gunther J. |date=1999 |chapter=The Fossil Record of Insect Diversity and Disparity |chapter-url=http://www.santafe.edu/media/workingpapers/00-08-044.pdf |editor1-last=Anderson |editor1-first=John M. |editor2-last=Thackeray |editor2-first=John Francis |editor3-last=Van Wyk |editor3-first=B. |editor4-last=De Wit |editor4-first=M. |title=Towards Gondwana Alive: Promoting biodiversity & stemming the Sixth Extinction |location=Pretoria |publisher=Gondwana Alive Society |isbn=978-1-919795-43-0 |lccn=2001385090 |oclc=44822625 |display-editors=2 |access-date=2015-02-17 |archive-date=2016-04-12 |archive-url=https://web.archive.org/web/20160412065128/http://www.santafe.edu/media/workingpapers/00-08-044.pdf}} "Preview booklet for 'Gondwana alive : biodiversity and the evolving terrestrial biosphere', book planned for September 2000, and associated projects."
* {{cite book |last=Leakey |first=Richard |author-link=Richard Leakey |year=1994 |title=The Origin of Humankind |series=Science Masters Series |location=New York |publisher=[[Basic Books]] |isbn=978-0-465-03135-1 |lccn=94003617 |oclc=30739453 |url=https://archive.org/details/originofhumankin00leak}}
* {{cite book |last=Margulis |first=Lynn |author-link=Lynn Margulis |year=1981 |title=Symbiosis in Cell Evolution: Life and its Environment on the Early Earth |url=https://archive.org/details/symbiosisincelle00marg |url-access=registration |location=San Francisco, CA |publisher=[[W. H. Freeman and Company]] |isbn=978-0-7167-1256-5 |lccn=80026695 |oclc=6982472}}
* {{cite book |last=McKinney |first=Michael L. |year=1997 |chapter=How do rare species avoid extinction? A paleontological view |editor1-last=Kunin |editor1-first=William E. |editor2-last=Gaston |editor2-first=Kevin J. |title=The Biology of Rarity: Causes and consequences of rare—common differences |edition=1st |location=London; New York |publisher=[[Chapman & Hall]] |isbn=978-0-412-63380-5 |lccn=96071014 |oclc=36442106}}
* {{cite book |last1=Miller |first1=G. Tyler |last2=Spoolman |first2=Scott E. |year=2012 |title=Environmental Science |edition=14th |location=Belmont, CA |publisher=[[Brooks/Cole]] |isbn=978-1-111-98893-7 |lccn=2011934330 |oclc=741539226}}
* {{cite web |url=http://pubs.usgs.gov/gip/geotime/age.html |title=Age of the Earth |last=Newman |first=William L. |date=July 9, 2007 |website=Geologic Time |publisher=Publications Services, [[USGS]] |location=Reston, VA |oclc=18792528 |access-date=2008-08-29 |archive-date=2005-12-23 |archive-url=https://web.archive.org/web/20051223072700/http://pubs.usgs.gov/gip/geotime/age.html |url-status=live}}
* {{cite book |last=O'Leary |first=Margaret R. |year=2008 |title=Anaxagoras and the Origin of Panspermia Theory |location=Bloomington, IN |publisher=[[iUniverse]] |isbn=978-0-595-49596-2 |oclc=757322661}}
* {{cite book |last=Padian |first=Kevin |author-link=Kevin Padian |year=2004 |chapter=Basal Avialae |editor1-last=Weishampel |editor1-first=David B. |editor1-link=David B. Weishampel |editor2-last=Dodson |editor2-first=Peter |editor2-link=Peter Dodson |editor3-last=Osmólska |editor3-first=Halszka |editor3-link=Halszka Osmólska |title=The Dinosauria |edition=2nd |location=Berkeley |publisher=[[University of California Press]] |isbn=978-0-520-24209-8 |lccn=2004049804 |oclc=55000644 |title-link=The Dinosauria}}
* {{cite book |last1=Sansom |first1=Ivan J. |author1-link=Ivan Sansom |last2=Smith |first2=Moya M. |last3=Smith |first3=M. Paul |author3-link=M. Paul Smith |year=2001 |chapter=The Ordovician radiation of vertebrates |editor-last=Ahlberg |editor-first=Per Erik |editor-link=Per E. Ahlberg |title=Major Events in Early Vertebrate Evolution: Palaeontology, phylogeny, genetics and development |series=Systematics Association special volume series |volume=61 |location=London; New York |publisher=[[Taylor & Francis]] |isbn=978-0-415-23370-5 |lccn=00062919 |oclc=51667292}}
* {{cite book |last1=Starr |first1=Cecie |last2=Evers |first2=Christine A. |last3=Starr |first3=Lisa |year=2007 |title=Biology Today and Tomorrow with Physiology |edition=2nd |location=Southbank, Australia; Belmont, CA |publisher=[[Thomson Corporation|Thomson Brooks/Cole]] |isbn=978-0-495-01654-0 |lccn=2005936176 |oclc=68043255}}
* {{cite book |last1=Stearns |first1=Beverly Peterson |last2=Stearns |first2=Stephen C. |author2-link=Stephen C. Stearns |year=1999 |title=Watching, from the Edge of Extinction |url=https://archive.org/details/isbn_9780300084696 |url-access=registration |location=New Haven, CT |publisher=[[Yale University Press]] |isbn=978-0-300-08469-6 |lccn=98034087 |oclc=803522914}}
* {{cite book |last1=Thewissen |first1=J. G. M. |last2=Madar |first2=S. I. |last3=Hussain |first3=S. T. |year=1996 |title=Ambulocetus natans, an Eocene cetacean (Mammalia) from Pakistan |series=Courier Forschungsinstitut Senckenberg |volume=191 |location=Frankfurt |publisher=[[Senckenbergische Naturforschende Gesellschaft]] |isbn=978-3-929907-32-2 |lccn=97151576 |oclc=36463214}}
* {{cite book |editor1-last=Weber |editor1-first=Bettina |editor2-last=Büdel |editor2-first=Burkhard |editor3-last=Belnap |editor3-first=Jayne |year=2016 |title=Biological Soil Crusts: An Organizing Principle in Drylands |series=Ecological Studies (Analysis and Synthesis) |volume=226 |publisher=Springer International Publishing |isbn=978-3-319-30212-6 |lccn=2016941282 |oclc=995222892}}
{{Refend}}

== Further reading ==

* {{cite book |last=Dawkins |first=Richard |author-link=Richard Dawkins |year=1989 |title=The Selfish Gene |edition=New |location=Oxford; New York |publisher=Oxford University Press |isbn=978-0-19-286092-7 |lccn=89016077 |oclc=20012195 |title-link=The Selfish Gene |ref=none}}
* {{cite book |last=Dawkins |first=Richard |year=2004 |title=The Ancestor's Tale: A Pilgrimage to the Dawn of Life |location=Boston |publisher=[[Houghton Mifflin Company]] |isbn=978-0-618-00583-3 |lccn=2004059864 |oclc=56617123 |title-link=The Ancestor's Tale |ref=none}}
* {{cite book |editor1-last=Ruse |editor1-first=Michael |editor1-link=Michael Ruse |editor2-last=Travis |editor2-first=Joseph |editor2-link=Joseph Travis |year=2009 |title=Evolution: The First Four Billion Years |others=Foreword by [[Edward O. Wilson]] |location=Cambridge, MA |publisher=[[Belknap Press of Harvard University Press]] |isbn=978-0-674-03175-3 |lccn=2008030270 |oclc=225874308 |url=https://archive.org/details/evolutionfirstfo00mich |ref=none}}
* {{cite book |last1=Smith |first1=John Maynard |author1-link=John Maynard Smith |last2=Szathmáry |first2=Eörs |author2-link=Eörs Szathmáry |year=1997 |orig-date=1995; Oxford: [[W. H. Freeman and Company|W. H. Freeman/Spektrum]] |title=The Major Transitions in Evolution |location=Oxford; New York |publisher=Oxford University Press |isbn=978-0-19-850294-4 |lccn=94026965 |oclc=715217397 |title-link=The Major Transitions in Evolution |ref=none}}

== External links ==

* {{cite web |url=http://darwin-online.org.uk/ |title=The Complete Work of Charles Darwin Online |editor-last=van Wyhe |editor-first=John |editor-link=John van Wyhe |access-date=2015-02-23 |ref=none}}
* {{cite web |url=http://www.rationalrevolution.net/articles/understanding_evolution.htm |title=Understanding Evolution: History, Theory, Evidence, and Implications |last=Price |first=R. G. |website=rationalrevolution.net |access-date=2015-02-23 |ref=none}}
* {{cite web |url=http://www.fossilmuseum.net/Evolution.htm |title=Evolution |website=The Virtual Fossil Museum |access-date=2015-02-22}} General information on evolution compiled by Roger Perkins.
* {{cite web |url=https://evolution.berkeley.edu/evolibrary/home.php |title=Understanding Evolution |publisher=[[University of California, Berkeley]] |access-date=2020-12-25 |archive-date=2021-11-10 |archive-url=https://web.archive.org/web/20211110135435/https://evolution.berkeley.edu/evolibrary/home.php |url-status=dead}}
* {{cite web |url=https://www.nationalacademies.org/evolution |title=Evolution Resources |publisher=[[National Academies of Sciences, Engineering, and Medicine|National Academies]] |location=Washington, D.C. |access-date=2020-12-25}}
* {{cite web |url=https://tellapallet.com/tree-of-life/ |title=Tree of Life |access-date=2020-12-25 |url-status=live |archive-url=https://web.archive.org/web/20201215235925/https://tellapallet.com/tree-of-life/ |archive-date=2020-12-15}} Tree of life diagram by Neal Olander.
* {{cite magazine |url=https://www.newscientist.com/article-topic/evolution/ |title=Evolution |magazine=[[New Scientist]] |access-date=2020-12-25}}
* {{HowStuffWorks|page=life/evolution/evolution|name=How Evolution Works|author=[[Marshall Brain|Brain, Marshall]]}}
* {{cite web |url=https://www2.palomar.edu/anthro/synthetic/default.htm |title=Modern Theories of Evolution: An Introduction to the Concepts and Theories That Led to Our Current Understanding of Evolution |publisher=[[Palomar College]] |access-date=2020-12-25}} Tutorial created by Dennis O'Neil.

{{Evolution}}
{{Nature}}
{{Earth}}
{{Portal bar|Evolutionary biology|Biology}}

{{DEFAULTSORT:History Of Life}}
[[Category:Evolution-related timelines]]
[[Category:Paleontology]]
[[Category:Evolutionary biology]]
[[Category:Earth sciences]]
[[Category:Geochronology]]