View source for Abiogenesis

{{Short description|Life arising from non-living matter}}
{{Redirect|Origin of life|non-scientific<!--Please do not attempt to change this without obtaining wide consensus first, the wording has been carefully checked, thanks--> views on the origins of life|Creation myth|the pre-scientific conception of macro-scale organisms randomly arising from non-living materials|Spontaneous generation}}
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{{Use dmy dates |date=June 2020}}
{{Use American English |date=December 2019}}
[[File:Origin of life stages.svg|thumb|upright=2<!--scale to make image labels readable-->|Stages in the origin of life range from the well-understood, such as the [[planetary habitability|habitable Earth]] and the abiotic synthesis of simple molecules, to the largely unknown, like the derivation of the [[last universal common ancestor]] (LUCA) with its complex molecular functionalities.<ref name="Walker Packard Cody 2017" />]]

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'''Abiogenesis''' is the natural process by which [[life]] arises from [[abiotic component|non-living matter]], such as simple [[organic compound]]s. The prevailing scientific<!--Please do not insert "theory", "speculative", "doubtful", etc., you can be blocked for pushing your point of view.--> [[hypothesis]] is that the transition from non-living to [[organism|living entities]] on Earth was not a single event, but a process of increasing complexity involving the formation of a [[planetary habitability|habitable planet]], the prebiotic synthesis of organic molecules, molecular [[self-replication]], [[self-assembly]], [[autocatalysis]], and the emergence of [[cell membrane]]s. The transition from non-life to life has never been observed experimentally, but many proposals have been made for different stages of the process.<!-- Please do not add refs here; this is a summary only. -->

The study of abiogenesis aims to determine how pre-life [[chemical reaction]]s gave rise to life under conditions strikingly different from those on Earth today. It primarily uses tools from [[biology]] and [[chemistry]], with more recent approaches attempting a synthesis of many sciences. Life functions through the specialized chemistry of [[carbon]] and water, and builds largely upon four key families of chemicals: [[lipid]]s for cell membranes, [[carbohydrate]]s such as sugars, [[amino acid]]s for protein metabolism, and [[nucleic acid]] [[DNA]] and [[RNA]] for the mechanisms of heredity. Any successful theory of abiogenesis must explain the origins and interactions of these classes of molecules.

Many approaches to abiogenesis investigate how self-replicating molecules, or their components, came into existence. Researchers generally think that current life descends from an [[RNA world]], although other self-replicating and self-catalyzing molecules may have preceded RNA. <!-- Please do not add refs here, this is a summary only -->Other approaches ([[Proto-metabolism|"metabolism-first" hypotheses]]) focus on understanding how [[catalysis]] in chemical systems on the early Earth might have provided the [[precursor (chemistry)|precursor molecules]] necessary for self-replication. The classic 1952 [[Miller–Urey experiment]] demonstrated that most amino acids, the chemical constituents of [[protein]]s, can be synthesized from [[inorganic compound]]s under conditions intended to replicate those of the [[Prebiotic atmosphere|early Earth]]. External sources of energy may have triggered these reactions, including [[lightning]], [[radiation]], atmospheric entries of micro-meteorites, and implosion of bubbles in sea and ocean waves. More recent research has found amino acids in meteorites, comets, asteroids, and star-forming regions of space.<!-- Please do not add refs here, this is a summary only -->

While the [[last universal common ancestor]] of all modern organisms (LUCA) is thought to have existed long after the origin of life, investigations into LUCA can guide research into early universal characteristics. A [[genomics]] approach has sought to characterize LUCA by identifying the genes shared by [[Archaea]] and [[Bacteria]], members of the two major branches of life (with [[Eukaryote]]s included in the archaean branch in the [[two-domain system]]). It appears there are 60 proteins common to all life and [[#Phylogeny and LUCA|355 prokaryotic genes]] that trace to LUCA; their functions imply that the LUCA was [[anaerobic respiration|anaerobic]] with the [[Wood–Ljungdahl pathway]], deriving energy by [[chemiosmosis]], and maintaining its hereditary material with DNA, the [[genetic code]], and [[ribosome]]s. Although the LUCA lived over 4&nbsp;billion years ago (4&nbsp;[[billion years|Gya]]), researchers believe it was far from the first form of life. Most evidence suggests that earlier cells might have had a leaky membrane and been powered by a naturally occurring [[proton gradient]] near a deep-sea white smoker [[hydrothermal vent]]; however, other evidence suggests instead that life may have originated inside the continental crust or in water at Earth's surface.<!-- Please do not add refs here, this is a summary only -->

Earth remains the only place in the [[universe]] known to harbor life. [[Earliest known life forms|Geochemical]] and [[Stromatolite|fossil evidence]] from the Earth informs most studies of abiogenesis. The [[age of Earth|Earth]] was formed at 4.54&nbsp;Gya, and the earliest evidence of life on Earth dates from at least 3.8&nbsp;Gya from [[Dresser Formation|Western Australia]]. Some studies have suggested that [[micropaleontology#Microfossils|fossil micro-organisms]] may have lived within hydrothermal vent precipitates dated 3.77 to 4.28&nbsp;Gya [[Nuvvuagittuq Greenstone Belt|from Quebec]], soon after [[Origin of water on Earth#History of water on Earth|ocean formation]] 4.4&nbsp;Gya during the [[Hadean]].<!-- Please do not add refs here, this is a summary only -->

== Overview ==

{{Further|Astrobiology}}

[[File:NASA on astrobiology.svg|thumb|right|upright=1.6|NASA's 2015 strategy for [[astrobiology]] aimed to solve the puzzle of the origin of life – how a fully functioning living system could emerge from non-living components – through research on the prebiotic origin of [[biochemistry|life's chemicals]], both in [[outer space|space]] and on [[planet]]s, as well as the functioning of early biomolecules to [[catalysis|catalyse]] reactions and support [[genetic code|inheritance]].<ref name="NASA strategy 2015" />]]

[[Life#Definitions|Life]] consists of reproduction with (heritable) variations.<ref>{{cite journal |last=Trifonov |first=Edward N. |author-link=Edward Trifonov |title=Vocabulary of Definitions of Life Suggests a Definition |date=17 March 2011 |journal=Journal of Biomolecular Structure and Dynamics |volume=29 |issue=2 |pages=259–266 |doi=10.1080/073911011010524992 |pmid=21875147 |s2cid=38476092 |doi-access=free}}</ref> [[NASA]] defines life as "a self-sustaining chemical system capable of [[evolution|Darwinian evolution]]."<ref>{{cite web |last=Voytek |first=Mary A. |author-link=Mary Voytek |title=About Life Detection |url=https://astrobiology.nasa.gov/research/life-detection/about/ |date=6 March 2021 |publisher=[[NASA]] |access-date=8 March 2021 |archive-date=16 August 2021 |archive-url=https://web.archive.org/web/20210816150806/https://astrobiology.nasa.gov/research/life-detection/about/ |url-status=live}}</ref> Such a system is complex; the [[last universal common ancestor]] (LUCA), presumably a single-celled organism which lived some 4 billion years ago, already had hundreds of [[gene]]s encoded in the [[DNA]] [[genetic code]] that is universal today. That in turn implies a suite of cellular machinery including [[messenger RNA]], [[transfer RNA]], and [[ribosome]]s to translate the code into [[protein]]s. Those proteins included [[enzyme]]s to operate its [[anaerobic respiration]] via the [[Wood–Ljungdahl pathway|Wood–Ljungdahl metabolic pathway]], and a [[DNA polymerase]] to replicate its genetic material.<ref name="Witzany 2016"/><ref name="AB-20141208"/>

The challenge for abiogenesis (origin of life)<ref>{{cite book |last=Oparin |first=Aleksandr Ivanovich |author-link=Alexander Oparin |translator-last=Morgulis |translator-first=Sergius |orig-year=1938 |title=The Origin of Life |url=https://books.google.com/books?id=Jv8psJCtI0gC |edition=2 |location=Mineola, New York |publisher=Courier |date=2003 |isbn=978-0-486-49522-4 |access-date=16 June 2018 |archive-date=2 April 2023 |archive-url=https://web.archive.org/web/20230402201809/https://books.google.com/books?id=Jv8psJCtI0gC |url-status=live}}</ref><ref name="Pereto" /><ref>Compare: {{cite journal |last=Scharf |first=Caleb |title=A Strategy for Origins of Life Research |date=18 December 2015 |journal=[[Astrobiology (journal)|Astrobiology]] |volume=15 |issue=12 |pages=1031–1042 |doi=10.1089/ast.2015.1113 |display-authors=etal |pmid=26684503 |pmc=4683543 |bibcode=2015AsBio..15.1031S |quote=What do we mean by the origins of life (OoL)? ... Since the early 20th century the phrase OoL has been used to refer to the events that occurred during the transition from non-living to living systems on Earth, i.e., the origin of terrestrial biology (Oparin, 1924; Haldane, 1929). The term has largely replaced earlier concepts such as abiogenesis (Kamminga, 1980; Fry, 2000).}}</ref> researchers is to explain how such a complex and tightly interlinked system could develop by evolutionary steps, as at first sight [[irreducible complexity|all its parts are necessary]] to enable it to function. For example, a cell, whether the LUCA or in a modern organism, copies its DNA with the DNA polymerase enzyme, which is itself produced by translating the DNA polymerase gene in the DNA. Neither the enzyme nor the DNA can be produced without the other.<ref name="Weiss Sousa Mrnjavac 2016" /> The likely answer to this challenge is that the evolutionary process could have involved molecular [[self-replication]], [[self-assembly]] such as of [[cell membrane]]s, and [[autocatalysis]] via RNA [[ribozyme]]s in an [[RNA world]] environment.<ref name="Witzany 2016">{{cite journal |last=Witzany |first=Guenther |title=Crucial steps to life: From chemical reactions to code using agents |journal=[[BioSystems]] |year=2016 |volume=140 |pages=49–57 |url=http://www.biocommunication.at/pdf/publications/biosystems_2016.pdf |doi=10.1016/j.biosystems.2015.12.007 |pmid=26723230 |bibcode=2016BiSys.140...49W |s2cid=30962295 |access-date=30 October 2018 |archive-date=31 October 2018 |archive-url=https://web.archive.org/web/20181031052532/http://www.biocommunication.at/pdf/publications/biosystems_2016.pdf |url-status=live}}</ref><ref name="AB-20141208">{{cite web |last=Howell |first=Elizabeth |title=How Did Life Become Complex, And Could It Happen Beyond Earth? |url=https://www.astrobio.net/origin-and-evolution-of-life/life-become-complex-happen-beyond-earth/ |date=8 December 2014 |work=[[Astrobiology Magazine]] |access-date=14 April 2022 |url-status=usurped |archive-url=https://web.archive.org/web/20180215024231/https://www.astrobio.net/origin-and-evolution-of-life/life-become-complex-happen-beyond-earth/ |archive-date=15 February 2018}}</ref><ref>{{cite book |last=Tirard |first=Stephane |title=Encyclopedia of Astrobiology |chapter=Abiogenesis |date=20 April 2015 |doi=10.1007/978-3-642-27833-4_2-4 |isbn=978-3-642-27833-4 |page=1 |quote=Thomas Huxley (1825–1895) used the term abiogenesis in an important text published in 1870. He strictly made the difference between spontaneous generation, which he did not accept, and the possibility of the evolution of matter from inert to living, without any influence of life. ... Since the end of the nineteenth century, evolutive abiogenesis means increasing complexity and evolution of matter from inert to living state in the abiotic context of evolution of primitive Earth.}}</ref> Nonetheless, the transition of non-life to life has never been observed experimentally, nor has there been a satisfactory chemical explanation.<ref>{{cite book |last1=Luisi |first1=Pier Luigi |title=The Emergence of Life: From Chemical Origins to Synthetic Biology |date=2018 |publisher=[[Cambridge University Press]] |isbn=9781108735506 |page=416 |quote=However, the turning point of non-life to life has never been put into one experimental set up. There are, of course, several hypotheses, and this plethora of ideas means already that we do not have a convincing one.}}</ref>

The preconditions to the development of a living cell like the LUCA are known, though disputed in detail: a habitable world is formed with a supply of minerals and liquid water. Prebiotic synthesis creates a range of simple organic compounds, which are assembled into polymers such as proteins and RNA. On the other side, the process after the LUCA is readily understood: biological evolution caused the development of a wide range of species with varied forms and biochemical capabilities. However, the derivation of the LUCA from simple components is far from understood.<ref name="Walker Packard Cody 2017">{{cite journal |last1=Walker |first1=Sara I. |last2=Packard |first2=N. |last3=Cody |first3=G. D. |title=Re-conceptualizing the origins of life |journal=[[Philosophical Transactions of the Royal Society A]] |volume=375 |issue=2109 |date=13 November 2017 |doi=10.1098/rsta.2016.0337 |page=20160337 |pmid=29133439 |pmc=5686397 |bibcode=2017RSPTA.37560337W}}</ref>

Although Earth remains the only place where life is known,<ref>{{cite journal |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19900013148.pdf |title=Extraterrestrial Life in the Universe |last=Graham |first=Robert W. |date=February 1990 |location=[[Glenn Research Center|Lewis Research Center]], Cleveland, Ohio |journal=[[NASA]] |volume=90 |page=22464 |bibcode=1990STIN...9022464G |type=NASA Technical Memorandum 102363 |access-date=2015-06-02 |url-status=live |archive-url=https://web.archive.org/web/20140903100534/http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19900013148.pdf |archive-date=3 September 2014}}</ref><ref>{{harvnb|Altermann|2009|p=xvii}}</ref> the science of [[astrobiology]] seeks evidence of life on other planets. The 2015 NASA strategy on the origin of life aimed to solve the puzzle by identifying interactions, intermediary structures and functions, energy sources, and environmental factors that contributed to evolvable macromolecular systems,<ref name="NASA strategy 2015">{{cite web |title=NASA Astrobiology Strategy |year=2015 |work=NASA |access-date=24 September 2017 |url=https://nai.nasa.gov/media/medialibrary/2015/10/NASA_Astrobiology_Strategy_2015_151008.pdf |archive-url=https://web.archive.org/web/20161222190306/https://nai.nasa.gov/media/medialibrary/2015/10/NASA_Astrobiology_Strategy_2015_151008.pdf |archive-date=22 December 2016}}</ref> and mapping the chemical landscape of potential primordial informational [[polymer]]s. The advent of such polymers was most likely a critical step in prebiotic chemical evolution.<ref name="NASA strategy 2015"/> Those polymers derived, in turn, from simple [[organic compound]]s such as [[nucleobase]]s, [[amino acid]]s, and [[sugar]]s, likely formed by reactions in the environment.<ref>{{harvnb|Oparin|1953|p=vi}}</ref><ref name="Pereto">{{cite journal |last=Peretó |first=Juli |year=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 |pmid=15906258 |access-date=1 June 2015  |archive-url=https://web.archive.org/web/20150824074726/http://www.im.microbios.org/0801/0801023.pdf |archive-date=24 August 2015}}</ref><ref>{{cite journal |last1=Warmflash |first1=David |last2=Warmflash |first2=Benjamin |date=November 2005 |title=Did Life Come from Another World? |journal=[[Scientific American]] |volume=293 |issue=5 |pages=64–71 |doi=10.1038/scientificamerican1105-64 |pmid=16318028 |bibcode=2005SciAm.293e..64W}}</ref><ref>{{harvnb|Yarus|2010|p=47}}</ref> A successful theory of the origin of life must explain how all these chemicals came into being.<ref>{{cite book |last1=Ward |first1=Peter |last2=Kirschvink |first2=Joe |author2-link=Joseph Kirschvink |date=2015 |title=A New History of Life: the radical discoveries about the origins and evolution of life on earth |publisher=[[Bloomsbury Press]] |pages=39–40 |isbn=978-1-60819-910-5}}</ref>

== Pre-1960s conceptual history ==

{{Main|History of research into the origin of life}}

[[File:Miller-Urey_experiment-en.svg|thumb|upright=1.6|The [[Miller–Urey experiment]] was a synthesis of small organic molecules in a mixture of simple gases in a thermal gradient created by heating (right) and cooling (left) the mixture at the same time, with electrical discharges.]]

=== Spontaneous generation ===

{{Main|Spontaneous generation}}

One ancient view of the origin of life, from [[Aristotle's biology|Aristotle]] until the 19th century, was of [[spontaneous generation]].<ref>{{harvnb|Sheldon|2005}}</ref> This held that "lower" animals such as insects were generated by decaying organic substances, and that life arose by chance.<ref>{{harvnb|Lennox|2001|pp=229–258}}</ref><ref name="Bernal 1967">{{harvnb|Bernal|1967}}</ref> This was questioned from the 17th century, in works like [[Sir Thomas Browne|Thomas Browne]]'s ''[[Pseudodoxia Epidemica]]''.<ref>{{cite journal |last=Balme |first=D. M. |author-link=David Mowbray Balme |year=1962 |title=Development of Biology in Aristotle and Theophrastus: Theory of Spontaneous Generation |journal=[[Phronesis (journal)|Phronesis]] |volume=7 |issue=1–2 |pages=91–104 |doi=10.1163/156852862X00052}}</ref><ref>{{harvnb|Ross|1652}}</ref> In 1665, [[Robert Hooke]] published the first drawings of a [[microorganism]]. In 1676, [[Antonie van Leeuwenhoek]] drew and described microorganisms, probably [[protozoa]] and [[bacteria]].<ref>{{harvnb|Dobell|1960}}</ref> Van Leeuwenhoek disagreed with spontaneous generation, and by the 1680s convinced himself, using experiments ranging from sealed and open meat incubation and the close study of insect reproduction, that the theory was incorrect.<ref>{{harvnb|Bondeson|1999}}</ref> In 1668 [[Francesco Redi]] showed that no [[maggot]]s appeared in meat when flies were prevented from laying eggs.<ref>{{cite web |last1=Levine |first1=R. |last2=Evers |first2=C. |title=The Slow Death of Spontaneous Generation (1668-1859) |url=http://www.accessexcellence.org/RC/AB/BC/Spontaneous_Generation.php |access-date=18 April 2013  |archive-url=https://web.archive.org/web/20080426191204/http://www.accessexcellence.org/RC/AB/BC/Spontaneous_Generation.php |archive-date=26 April 2008}}</ref> By the middle of the 19th century, spontaneous generation was considered disproven.<ref>{{harvnb|Oparin|1953|p=196}}</ref><ref>{{harvnb|Tyndall|1905|loc=IV, XII (1876), XIII (1878)}}</ref>

=== Panspermia ===

{{Main|Panspermia}}

Dating back to [[Anaxagoras]] in the 5th century BC, [[panspermia]]<ref>{{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 |doi=10.1128/MMBR.00016-09 |pmc=2832349 |pmid=20197502 |bibcode=2010MMBR...74..121H}}</ref> is the idea that [[life]] originated elsewhere in the [[universe]] and came to Earth. The modern version of panspermia holds that life may have been distributed to Earth by [[meteoroids]], [[asteroids]], [[comets]]<ref>{{cite journal |last=Wickramasinghe |first=Chandra |author-link=Chandra Wickramasinghe |title=Bacterial morphologies supporting cometary panspermia: a reappraisal |journal=[[International Journal of Astrobiology]] |year=2011 |volume=10 |issue=1 |pages=25–30 |doi=10.1017/S1473550410000157 |bibcode=2011IJAsB..10...25W |citeseerx=10.1.1.368.4449 |s2cid=7262449}}</ref> or [[small Solar System body|planetoids]].<ref>Rampelotto, P. H. (2010). "Panspermia: A promising field of research". In: Astrobiology Science Conference. Abs 5224.</ref> This shifts the origin of life to another heavenly body. The advantage is that life is not required to have formed on each planet it occurs on, but in a more limited set of locations, and then spread about the [[galaxy]] to other star systems.<ref>{{cite news |last=Chang |first=Kenneth |title=Visions of Life on Mars in Earth's Depths |url=https://www.nytimes.com/2016/09/13/science/south-african-mine-life-on-mars.html |date=12 September 2016 |work=[[The New York Times]] |access-date=12 September 2016 |url-status=live |archive-url=https://web.archive.org/web/20160912225220/http://www.nytimes.com/2016/09/13/science/south-african-mine-life-on-mars.html |archive-date=12 September 2016}}</ref> There is some interest in the possibility that [[Life on Mars|life originated on Mars]] and later transferred to Earth.<ref>{{cite news |url=https://www.smithsonianmag.com/air-space-magazine/life-may-have-been-more-likely-originate-mars-earth-180978021|title=Life May Have Been More Likely to Originate on Mars Than on Earth |first=Dirk |last=Schulze-Makuch |journal=[[Smithsonian (magazine)|Smithsonian Magazine]] |date=18 June 2021 |access-date=May 16, 2025}}</ref>

=== "A warm little pond": primordial soup ===

{{Main|Primordial soup}}

The idea that life originated from non-living matter in slow stages appeared in [[Herbert Spencer]]'s 1864–1867 book ''Principles of Biology'', and in [[William Turner Thiselton-Dyer]]'s 1879 paper "On spontaneous generation and evolution". On 1 February 1871 [[Charles Darwin]] wrote about these publications to [[Joseph Dalton Hooker|Joseph Hooker]], and set out his own speculation that the original spark of life may have been in a "warm little pond, with all sorts of [[ammonia]] and phosphoric [[Salt (chemistry)|salts]],—light, heat, electricity {{sic|hide=y|&c}} present, that a protein compound was chemically formed". Darwin explained that "at the present day such matter would be instantly devoured or absorbed, which would not have been the case before living creatures were formed."<ref>{{cite web |title=Letter no. 7471, Charles Darwin to Joseph Dalton Hooker, 1 February (1871) |website=Darwin Correspondence Project |url=https://www.darwinproject.ac.uk/letter/DCP-LETT-7471.xml |access-date=7 July 2020 |archive-date=7 July 2020 |archive-url=https://web.archive.org/web/20200707094423/https://www.darwinproject.ac.uk/letter/DCP-LETT-7471.xml |url-status=live}}</ref><ref>{{cite web |title=Origin and Evolution of Life on a Frozen Earth |last=Priscu |first=John C. |author-link=John Charles Priscu |publisher=[[National Science Foundation]] |location=Arlington County, Virginia |url=https://www.nsf.gov/news/special_reports/darwin/textonly/polar_essay1.jsp |access-date=1 March 2014 |url-status=live |archive-url=https://web.archive.org/web/20131218070241/http://www.nsf.gov/news/special_reports/darwin/textonly/polar_essay1.jsp |archive-date=18 December 2013}}</ref><ref>{{cite news |last=Marshall |first=Michael |title=Charles Darwin's hunch about early life was probably right |url=https://www.bbc.com/future/article/20201110-charles-darwin-early-life-theory |date=11 November 2020 |work=[[BBC News]] |access-date=11 November 2020 |archive-date=11 November 2020 |archive-url=https://web.archive.org/web/20201111015900/https://www.bbc.com/future/article/20201110-charles-darwin-early-life-theory |url-status=live}}</ref>

[[Alexander Oparin]] in 1924 and [[J. B. S. Haldane]] in 1929 proposed that the earliest cells slowly self-organized from a [[primordial soup]], the [[Primordial soup|Oparin–Haldane hypothesis]].<ref>{{cite journal |last=Bahadur |first=Krishna |year=1973 |title=Photochemical Formation of Self–sustaining Coacervates |journal=[[Proceedings of the Indian National Science Academy]] |volume=39 |issue=4 |pages=455–467 |doi=10.1016/S0044-4057(75)80076-1 |pmid=1242552 |url=http://www.dli.gov.in/rawdataupload/upload/insa/INSA_1/20005b73_455.pdf |archive-url=https://web.archive.org/web/20131019172800/http://www.dli.gov.in/rawdataupload/upload/insa/INSA_1/20005b73_455.pdf |archive-date=19 October 2013}}</ref><ref>{{cite journal |last=Bahadur |first=Krishna |year=1975 |title=Photochemical Formation of Self-Sustaining Coacervates |journal=[[International Journal of Medical Microbiology|Zentralblatt für Bakteriologie, Parasitenkunde, Infektionskrankheiten und Hygiene (Central Journal for Bacteriology, Parasitology, Infectious Diseases and Hygiene)]] |url=https://www.sciencedirect.com/science/article/abs/pii/S0044405775800761 |volume=130 |issue=3 |pages=211–218 |doi=10.1016/S0044-4057(75)80076-1 |oclc=641018092 |pmid=1242552 |access-date=13 December 2022 |archive-date= 13 December 2022 |archive-url=https://web.archive.org/web/20221213115635/https://www.sciencedirect.com/science/article/abs/pii/S0044405775800761 |url-status=live|url-access=subscription }}</ref> Haldane suggested that the Earth's prebiotic oceans consisted of a "hot dilute soup" in which organic compounds could have formed.<ref name="Bernal 1967"/><ref>{{harvnb|Bryson|2004|pp=300–302}}</ref> [[J. D. Bernal]] showed that such mechanisms could form most of the necessary molecules for life from inorganic precursors.<ref>{{harvnb|Bernal |1951}}</ref> In 1967, he suggested three "stages": the origin of biological [[monomer]]s; the origin of biological [[polymer]]s; and the evolution from molecules to cells.<ref name="Martin2003" /><ref>{{cite journal |last=Bernal |first=John Desmond |author-link=John Desmond Bernal |date=September 1949 |title=The Physical Basis of Life |journal= [[Proceedings of the Physical Society, Section A]] |volume=62 |issue=9 |pages=537–558 |bibcode=1949PPSA...62..537B |doi=10.1088/0370-1298/62/9/301 |s2cid= 83754271}}</ref>

=== Miller–Urey experiment ===

{{Main|Miller–Urey experiment}}

In 1952, [[Stanley L. Miller|Stanley Miller]] and [[Harold C. Urey|Harold Urey]] carried out a chemical experiment to demonstrate how organic molecules could have formed spontaneously from inorganic precursors under [[prebiotic atmosphere|prebiotic conditions]] like those posited by the Oparin–Haldane hypothesis. It used a highly [[redox|reducing]] (lacking oxygen) mixture of gases—[[methane]], [[ammonia]], and [[hydrogen gas|hydrogen]], with [[water vapor]]—to form organic monomers such as [[amino acid]]s.<ref>{{cite journal |last=Miller |first=Stanley L. |author-link=Stanley Miller |date=15 May 1953 |title=A Production of Amino Acids Under Possible Primitive Earth Conditions |journal=[[Science (journal)|Science]] |volume=117 |issue=3046 |pages=528–529 |bibcode=1953Sci...117..528M |doi=10.1126/science.117.3046.528 |pmid=13056598}}</ref><ref>{{cite journal |last1=Parker |first1=Eric T. |last2=Cleaves |first2=Henderson J. |last3=Dworkin |first3=Jason P. |last4=Glavin |first4=Daniel P. |last5=Callahan |first5=Michael |last6=Aubrey |first6=Andrew |last7=Lazcano |first7=Antonio |author7-link=Antonio Lazcano |last8=Bada |first8=Jeffrey L. |author8-link=Jeffrey L. Bada |display-authors=3 |date=5 April 2011 |title=Primordial synthesis of amines and amino acids in a 1958 Miller H<sub>2</sub>S-rich spark discharge experiment |journal=[[PNAS]] |volume=108 |issue=14 |pages=5526–5531 |bibcode=2011PNAS..108.5526P |doi=10.1073/pnas.1019191108 |pmc=3078417 |pmid=21422282 |doi-access=free}}</ref> Bernal said of the Miller–Urey experiment that "it is not enough to explain the formation of such molecules, what is necessary, is a physical-chemical explanation of the origins of these molecules that suggests the presence of suitable sources and sinks for free energy."<ref>{{harvnb|Bernal|1967|p=143}}</ref> However, current scientific consensus describes the primitive atmosphere as weakly reducing or neutral,<ref name="Cleaves 2008">{{cite journal |last1=Cleaves |first1=H. James |last2=Chalmers |first2=John H. |last3=Lazcano |first3=Antonio |author3-link=Antonio Lazcano |last4=Miller |first4=Stanley L. |last5=Bada |first5=Jeffrey L. |author5-link=Jeffrey L. Bada |display-authors=3 |date=April 2008 |title=A Reassessment of Prebiotic Organic Synthesis in Neutral Planetary Atmospheres |journal=[[Origins of Life and Evolution of Biospheres]] |volume=38 |issue=2 |pages=105–115 |bibcode=2008OLEB...38..105C |doi=10.1007/s11084-007-9120-3 |pmid=18204914 |s2cid=7731172}}</ref><ref>{{cite journal |last=Chyba |first=Christopher F. |author-link=Christopher Chyba |s2cid=93303848 |date=13 May 2005 |title=Rethinking Earth's Early Atmosphere |journal=[[Science (journal)|Science]] |volume=308 |issue=5724 |pages=962–963 |doi=10.1126/science.1113157 |pmid=15890865}}</ref> diminishing the amount and variety of amino acids that could be produced. The addition of [[iron]] and [[carbonate]] minerals, present in early oceans, produces a diverse array of amino acids.<ref name="Cleaves 2008" /> Later work has focused on two other potential reducing environments: [[outer space]] and deep-sea hydrothermal vents.<ref>{{harvnb|Barton|Briggs|Eisen|Goldstein|2007|pp=93–95}}</ref><ref>{{harvnb|Bada|Lazcano|2009|pp=56–57}}</ref><ref>{{cite journal |last1=Bada |first1=Jeffrey L. |author1-link=Jeffrey L. Bada |last2=Lazcano |first2=Antonio |author2-link=Antonio Lazcano |date=2 May 2003 |url=http://astrobiology.berkeley.edu/PDFs_articles/Bada_Science2003.pdf |title=Prebiotic Soup – Revisiting the Miller Experiment |journal=[[Science (journal)|Science]] |volume=300 |issue=5620 |pages=745–746 |doi=10.1126/science.1085145 |pmid=12730584 |s2cid=93020326 |access-date=2015-06-13 |url-status=live |archive-url=https://web.archive.org/web/20160304222002/http://astrobiology.berkeley.edu/PDFs_articles/Bada_Science2003.pdf |archive-date=4 March 2016}}</ref>
<!-- Please do not extend this section; it is a summary of the linked "main" article. Put your materials there, and THEN if absolutely necessary adjust this summary to match, very briefly. -->

== Producing a habitable Earth ==

{{Abiogenesis timeline}}

=== Evolutionary history ===

==== Early universe with first stars ====

{{See also|Chronology of the universe}}

Soon after the [[Big Bang]], roughly 14 Gya, the only chemical elements present in the universe were [[hydrogen]], [[helium]], and [[lithium]], the three lightest atoms in the periodic table. These elements gradually accreted and began orbiting in disks of gas and dust. Gravitational accretion of material at the hot and dense centers of these [[protoplanetary disk]]s formed stars by the fusion of hydrogen.<ref>{{Cite journal |last1=Madau |first1=Piero |last2=Dickinson |first2=Mark |date=2014-08-18 |title=Cosmic Star-Formation History |url=https://www.annualreviews.org/doi/10.1146/annurev-astro-081811-125615 |journal=Annual Review of Astronomy and Astrophysics |volume=52 |issue=1 |pages=415–486 |doi=10.1146/annurev-astro-081811-125615 |arxiv=1403.0007 |bibcode=2014ARA&A..52..415M |s2cid=658354 |access-date=8 December 2023 |archive-date=1 July 2022 |archive-url=https://web.archive.org/web/20220701214618/https://www.annualreviews.org/doi/10.1146/annurev-astro-081811-125615 |url-status=live}}</ref> Early stars were massive and short-lived, producing all the heavier elements by [[stellar nucleosynthesis]]. Such element formation proceeds to its most stable element Iron-[[Isotopes of iron|56]]. Heavier elements were formed during supernovae at the end of a star's lifecycle. [[Carbon]], currently the [[abundance of the chemical elements|fourth most abundant element]] in the universe, was formed mainly in [[white dwarf stars]].<ref>{{cite journal |last=Marigo |first=Paola |display-authors=|date=6 July 2020 |title=Carbon star formation as seen through the non-monotonic initial–final mass relation |url=https://www.nature.com/articles/s41550-020-1132-1 |url-status=live |journal=[[Nature Astronomy]] |volume=152 |issue=11 |pages=1102–1110 |arxiv=2007.04163 |bibcode=2020NatAs...4.1102M |doi=10.1038/s41550-020-1132-1 |s2cid=220403402 |archive-url=https://web.archive.org/web/20230216160258/https://www.nature.com/articles/s41550-020-1132-1 |archive-date=16 February 2023 |access-date=7 July 2020}}</ref> As these stars reached the end of their [[stellar life cycle|lifecycles]], they ejected heavier elements, including carbon and oxygen, throughout the universe. These allowed for the formation of rocky planets.<ref>{{cite web |title=WMAP- Life in the Universe |url=https://wmap.gsfc.nasa.gov/universe/uni_life.html |url-status=live |archive-url=https://web.archive.org/web/20230129215644/https://wmap.gsfc.nasa.gov/universe/uni_life.html |archive-date=29 January 2023 |access-date=27 September 2019}}</ref> According to the [[nebular hypothesis]], the [[Solar System]] began to form 4.6 Gya with the [[gravitational collapse]] of part of a giant [[molecular cloud]]. Most of the collapsing mass collected in the center, forming the [[Sun]], while the rest flattened into a [[protoplanetary disk]] out of which the [[planet]]s formed.<ref>{{cite web |title=Formation of Solar Systems: Solar Nebular Theory |url=http://www.astro.umass.edu/~myun/teaching/a100_old/solarnebulartheory.htm |url-status=live |archive-url=https://web.archive.org/web/20190927152503/http://www.astro.umass.edu/~myun/teaching/a100_old/solarnebulartheory.htm |archive-date=27 September 2019 |access-date=27 September 2019 |publisher=[[University of Massachusetts Amherst]]}}</ref>

==== Emergence of Earth ====

{{See also|Geological history of Earth|Circumstellar habitable zone|Prebiotic atmosphere}}

The [[age of the Earth]] is 4.54 Gya as found by radiometric dating of [[Calcium–aluminium-rich inclusion|calcium-aluminium-rich inclusions]] in [[carbonaceous chrondrite]] meteorites, the oldest material in the Solar System.<ref>{{cite web |date=9 July 2007 |title=Age of the Earth |url=https://pubs.usgs.gov/gip/geotime/age.html |url-status=live |archive-url=https://web.archive.org/web/20051223072700/http://pubs.usgs.gov/gip/geotime/age.html |archive-date=23 December 2005 |access-date=10 January 2006 |publisher=[[United States Geological Survey]]}}</ref><ref>{{harvnb|Dalrymple|2001|pp=205–221}}</ref> Earth, during the [[Hadean]] eon (from its formation until 4.031 Gya,) was at first inhospitable to life. During its formation, the Earth lost much of its initial mass, and so lacked the [[gravity]] to hold molecular hydrogen and the bulk of the original inert gases.<ref>{{harvnb|Fesenkov|1959|p=9}}</ref> Soon after initial accretion of Earth at 4.48 Gya, its collision with [[Theia (planet)|Theia]], a hypothesised impactor, is thought to have created the ejected debris that eventually formed the Moon.<ref>{{Cite journal |last1=Bottke |first1=W. F. |last2=Vokrouhlický |first2=D. |last3=Marchi |first3=S. |last4=Swindle |first4=T. |last5=Scott |first5=E. R. D. |last6=Weirich |first6=J. R. |last7=Levison |first7=H. |date=2015-04-17 |title=Dating the Moon-forming impact event with asteroidal meteorites |journal=Science |volume=348 |issue=6232 |pages=321–323 |doi=10.1126/science.aaa0602 |bibcode=2015Sci...348..321B |s2cid=206632612 |doi-access=free |pmid=25883354}}</ref> This impact removed the Earth's primary atmosphere, leaving behind clouds of viscous silicates and carbon dioxide. This unstable atmosphere was short-lived, soon condensing to form the bulk silicate Earth, leaving behind an atmosphere largely consisting of water vapor, [[nitrogen]], and [[carbon dioxide]], with smaller amounts of [[carbon monoxide]], hydrogen, and [[sulfur]] compounds.<ref>{{cite journal |last=Kasting |first=James F. |author-link=James Kasting |date=12 February 1993 |title=Earth's Early Atmosphere |url=http://wwwdca.iag.usp.br/www/material/fornaro/ACA410/Kasting%201993_EarthEarlyAtmos.pdf |journal=[[Science (journal)|Science]] |volume=259 |issue=5097 |pages=920–926 |bibcode=1993Sci...259..920K |doi=10.1126/science.11536547 |pmid=11536547 |s2cid=21134564 |archive-url=https://web.archive.org/web/20151010074651/http://wwwdca.iag.usp.br/www/material/fornaro/ACA410/Kasting%201993_EarthEarlyAtmos.pdf |archive-date=10 October 2015 |access-date=2015-07-28}}</ref><ref name="Follmann2009">{{cite journal |last1=Follmann |first1=Hartmut |last2=Brownson |first2=Carol |date=November 2009 |title=Darwin's warm little pond revisited: from molecules to the origin of life |journal=[[Naturwissenschaften]] |volume=96 |issue=11 |pages=1265–1292 |bibcode=2009NW.....96.1265F |doi=10.1007/s00114-009-0602-1 |pmid=19760276 |s2cid=23259886}}</ref> The solution of carbon dioxide in water is thought to have made the seas slightly [[acid]]ic, with a [[pH]] of about 5.5.<ref>{{cite journal |last=Morse |first=John |date=September 1998 |title=Hadean Ocean Carbonate Geochemistry |journal=Aquatic Geochemistry |volume=4 |issue=3/4 |pages=301–319 |bibcode=1998MinM...62.1027M |doi=10.1023/A:1009632230875 |s2cid=129616933}}</ref>

Condensation to form liquid [[Origin of water on Earth|oceans]] is theorised to have occurred as early as the Moon-forming impact.<ref>{{Cite journal |last1=Sleep |first1=Norman H. |last2=Zahnle |first2=Kevin J. |last3=Lupu |first3=Roxana E. |date=2014-09-13 |title=Terrestrial aftermath of the Moon-forming impact |journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences |volume=372 |issue=2024 |pages=20130172 |doi=10.1098/rsta.2013.0172 |pmid=25114303 |bibcode=2014RSPTA.37230172S |s2cid=6902632 |doi-access=free}}</ref><ref>{{Cite journal |last1=Morse |first1=John W. |last2=Mackenzie |first2=Fred T. |date=1998 |title=Hadean Ocean Carbonate Geochemistry |url=http://link.springer.com/10.1023/A:1009632230875 |journal=Aquatic Geochemistry |volume=4 |issue=3/4 |pages=301–319 |doi=10.1023/A:1009632230875 |bibcode=1998AqGeo...4..301M |s2cid=129616933 |access-date=8 December 2023 |archive-date=31 January 2024 |archive-url=https://web.archive.org/web/20240131154932/https://link.springer.com/article/10.1023/A:1009632230875 |url-status=live|url-access=subscription }}</ref> This scenario is supported by the dating of 4.404 Gya [[zircon]] crystals with high [[Δ18O|''δ''<sup>18</sup>O]] values from metamorphosed [[quartzite]] of [[Narryer Gneiss terrane|Mount Narryer]] in Western Australia.<ref>{{Cite journal |last1=Crowley |first1=James L. |last2=Myers |first2=John S. |last3=Sylvester |first3=Paul J |last4=Cox |first4=Richard A. |date=May 2005 |title=Detrital Zircon from the Jack Hills and Mount Narryer, Western Australia: Evidence for Diverse >4.0 Ga Source Rocks |url=https://www.journals.uchicago.edu/doi/10.1086/428804 |journal=The Journal of Geology |volume=113 |issue=3 |pages=239–263 |doi=10.1086/428804 |bibcode=2005JG....113..239C |s2cid=140715676 |access-date=8 December 2023 |archive-date=16 December 2023 |archive-url=https://web.archive.org/web/20231216004103/https://www.journals.uchicago.edu/doi/10.1086/428804 |url-status=live|url-access=subscription }}</ref><ref>{{cite journal |last1=Wilde |first1=Simon A. |last2=Valley |first2=John W. |last3=Peck |first3=William H. |last4=Graham |first4=Colin M. |date=11 January 2001 |title=Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4&nbsp;Gyr ago |url=http://www.geology.wisc.edu/~valley/zircons/Wilde2001Nature.pdf |url-status=live |journal=[[Nature (journal)|Nature]] |volume=409 |issue=6817 |pages=175–178 |bibcode=2001Natur.409..175W |doi=10.1038/35051550 |pmid=11196637 |s2cid=4319774 |archive-url=https://web.archive.org/web/20150605132344/http://www.geology.wisc.edu/~valley/zircons/Wilde2001Nature.pdf |archive-date=5 June 2015 |access-date=3 June 2015}}</ref> The Hadean atmosphere has been characterized as a "gigantic, productive outdoor chemical laboratory," similar to volcanic gases today which still support some abiotic chemistry. Despite the likely increased volcanism from early plate tectonics, the Earth may have been a predominantly water world between 4.4 and 4.3 Gya. It is debated whether crust was exposed above this ocean.<ref>{{Cite journal |last=Korenaga |first=Jun |date=December 2008 |title=Plate tectonics, flood basalts and the evolution of Earth's oceans |journal=Terra Nova |volume=20 |issue=6 |pages=419–439 |doi=10.1111/j.1365-3121.2008.00843.x |bibcode=2008TeNov..20..419K |s2cid=36766331 |doi-access=free}}</ref> Immediately after the Moon-forming impact, Earth likely had little if any continental crust, a turbulent atmosphere, and a [[hydrosphere]] subject to intense [[ultraviolet]] light from a [[T Tauri star|T Tauri stage Sun]]. It was also affected by [[Cosmic ray|cosmic radiation]], and continued asteroid and [[comet]] impacts.<ref>{{cite journal |last1=Rosing |first1=Minik T. |last2=Bird |first2=Dennis K. |last3=Sleep |first3=Norman H. |last4=Glassley |first4=William |last5=Albarède |first5=Francis |author-link5=Francis Albarède |display-authors=3 |date=22 March 2006 |title=The rise of continents – An essay on the geologic consequences of photosynthesis |url=https://www.researchgate.net/publication/223066196 |url-status=live |journal=[[Palaeogeography, Palaeoclimatology, Palaeoecology]] |volume=232 |issue=2–4 |pages=99–113 |bibcode=2006PPP...232...99R |doi=10.1016/j.palaeo.2006.01.007 |archive-url=https://web.archive.org/web/20150714073656/http://www.researchgate.net/profile/Francis_Albarede/publication/223066196_The_rise_of_continentsAn_essay_on_the_geologic_consequences_of_photosynthesis/links/00b7d51766c442f58b000000.pdf |archive-date=14 July 2015 |access-date=2015-06-08}}</ref>

The [[Late Heavy Bombardment]] hypothesis posits that a period of intense impact occurred at 4.1 to 3.8&nbsp;Gya during the Hadean and early [[Archean]] eons.<ref>{{Cite journal |last1=Tera |first1=Fouad |last2=Papanastassiou |first2=D.A. |last3=Wasserburg |first3=G.J. |date=April 1974 |title=Isotopic evidence for a terminal lunar cataclysm |journal=Earth and Planetary Science Letters |volume=22 |issue=1 |pages=1–21 |doi=10.1016/0012-821x(74)90059-4 |bibcode=1974E&PSL..22....1T |url=https://www.sciencedirect.com/science/article/abs/pii/0012821X74900594 |archive-date=31 January 2024 |archive-url=https://web.archive.org/web/20240131154923/https://www.sciencedirect.com/science/article/abs/pii/0012821X74900594?via%3Dihub |url-status=live|url-access=subscription }}</ref><ref>{{Cite journal |last=Stoffler |first=D. |date=2006-01-01 |title=Cratering History and Lunar Chronology |journal=Reviews in Mineralogy and Geochemistry |volume=60 |issue=1 |pages=519–596 |doi=10.2138/rmg.2006.60.05 |bibcode=2006RvMG...60..519S |url=https://pubs.geoscienceworld.org/msa/rimg/article-abstract/60/1/519/140783/Cratering-History-and-Lunar-Chronology?redirectedFrom=fulltext |archive-date=31 January 2024 |archive-url=https://web.archive.org/web/20240131154819/https://pubs.geoscienceworld.org/msa/rimg/article-abstract/60/1/519/140783/Cratering-History-and-Lunar-Chronology?redirectedFrom=fulltext |url-status=live|url-access=subscription }}</ref> Originally it was thought that the Late Heavy Bombardment was a single cataclysmic impact event occurring at 3.9 Gya; this would have had the potential to sterilise Earth by volatilising liquid oceans and blocking sunlight needed for photosynthesis, delaying the earliest possible emergence of life.<ref>{{Cite journal |last1=Sleep |first1=Norman H. |last2=Zahnle |first2=Kevin J. |last3=Kasting |first3=James F. |last4=Morowitz |first4=Harold J. |date=December 1989 |title=Annihilation of ecosystems by large asteroid impacts on the early Earth |journal=Nature |volume=342 |issue=6246 |pages=139–142 |doi=10.1038/342139a0 |pmid=11536616 |bibcode=1989Natur.342..139S |s2cid=1137852 |url=https://www.nature.com/articles/342139a0 |archive-date=31 January 2024 |archive-url=https://web.archive.org/web/20240131154923/https://www.nature.com/articles/342139a0 |url-status=live|url-access=subscription }}</ref> More recent research questioned the intensity of the Late Heavy Bombardment and its potential for sterilisation. If it was not one giant impact but a period of raised impact rate, it would have had much less destructive power.<ref>{{Cite journal |last1=Fassett |first1=Caleb I. |last2=Minton |first2=David A. |date=2013-06-23 |title=Impact bombardment of the terrestrial planets and the early history of the Solar System |journal=Nature Geoscience |volume=6 |issue=7 |pages=520–524 |doi=10.1038/ngeo1841 |bibcode=2013NatGe...6..520F |url=https://www.nature.com/articles/ngeo1841 |archive-date=31 January 2024 |archive-url=https://web.archive.org/web/20240131154819/https://www.nature.com/articles/ngeo1841 |url-status=live|url-access=subscription }}</ref><ref>{{Cite journal |last1=Abramov |first1=Oleg |last2=Mojzsis |first2=Stephen J. |date=May 2009 |title=Microbial habitability of the Hadean Earth during the late heavy bombardment |journal=Nature |volume=459 |issue=7245 |pages=419–422 |doi=10.1038/nature08015 |pmid=19458721 |bibcode=2009Natur.459..419A |s2cid=3304147 |url=https://www.nature.com/articles/nature08015 |archive-date=31 January 2024 |archive-url=https://web.archive.org/web/20240131154926/https://www.nature.com/articles/nature08015 |url-status=live|url-access=subscription }}</ref> The 3.9 Gya date arose from dating of [[Apollo program|Apollo mission sample returns]] collected mostly near the [[Mare Imbrium|Imbrium Basin]], biasing the age of recorded impacts.<ref>{{Cite journal |last1=Boehnke |first1=Patrick |last2=Harrison |first2=T. Mark |date=2016-09-12 |title=Illusory Late Heavy Bombardments |journal=Proceedings of the National Academy of Sciences |volume=113 |issue=39 |pages=10802–10806 |doi=10.1073/pnas.1611535113 |pmid=27621460 |pmc=5047187 |bibcode=2016PNAS..11310802B |doi-access=free}}</ref> Impact modelling of the lunar surface reveals that rather than a cataclysmic event at 3.9 Gya, multiple small-scale, short-lived periods of bombardment likely occurred.<ref>{{Cite journal |last=Zellner |first=Nicolle E. B. |date=2017-05-03 |title=Cataclysm No More: New Views on the Timing and Delivery of Lunar Impactors |journal=Origins of Life and Evolution of Biospheres |volume=47 |issue=3 |pages=261–280 |doi=10.1007/s11084-017-9536-3 |pmid=28470374 |pmc=5602003 |arxiv=1704.06694 |bibcode=2017OLEB...47..261Z}}</ref> Terrestrial data backs this idea by showing multiple periods of ejecta in the rock record both before and after the 3.9 Gya marker, suggesting that the early Earth was subject to continuous impacts with less impact on extinction.<ref>{{Cite journal |last1=Lowe |first1=Donald R. |last2=Byerly |first2=Gary R. |date=2018-04-01 |title=The terrestrial record of Late Heavy Bombardment |journal=New Astronomy Reviews |volume=81 |pages=39–61 |doi=10.1016/j.newar.2018.03.002 |bibcode=2018NewAR..81...39L |doi-access=free}}</ref>

If life evolved in the ocean at depths of more than ten meters, it would have been shielded both from late impacts and the then high levels of ultraviolet radiation from the sun. Geothermically heated oceanic crust could have yielded far more organic compounds through deep [[hydrothermal vent]]s than the [[Miller–Urey experiment]]s indicated.<ref>{{harvnb|Davies|1999|p=155}}</ref> The available energy is maximized at 100–150&nbsp;°C, the temperatures at which [[Hyperthermophile|hyperthermophilic]] bacteria and [[Thermoacidophile|thermoacidophilic]] [[archaea]] live.<ref>{{harvnb|Bock|Goode|1996}}</ref>

==== Earliest evidence of life ====

[[File:Stromatolithe Paléoarchéen - MNHT.PAL.2009.10.1.jpg|thumb|upright=1.4|If [[banded iron formation]] rocks of Archaean age (like these from Australia) are fossilized [[stromatolite]]s, they would be among the earliest life-forms.<ref name="NAT-20170301" /><ref name="García-Ruiz 2017" />]]
[[File:Stromatolites in Sharkbay.jpg|thumb|upright=1.4|Modern stromatolites in [[Shark Bay]], created by photosynthetic [[cyanobacteria]]]]
{{main|Earliest known life forms}}

Life most likely emerged on Earth between 3.48 and 4.32 Gya. Minimum age estimates are based on evidence from the [[geologic record]]. In 2017, the earliest physical evidence of life was reported to consist of [[microbialite]]s in the [[Nuvvuagittuq Greenstone Belt]] of Northern Quebec, in [[banded iron formation]] rocks at least 3.77 and possibly as old as 4.32 Gya. The micro-organisms could have lived within hydrothermal vent precipitates, soon after the 4.4 Gya [[Origin of water on Earth#History of water on Earth|formation of oceans]] during the Hadean. The microbes resemble modern hydrothermal vent bacteria, supporting the view that abiogenesis began in such an environment.<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 |title=Evidence for early life in Earth's oldest hydrothermal vent precipitates |journal=[[Nature (journal)|Nature]] |date=1 March 2017 |volume=543 |issue=7643 |pages=60–64 |doi=10.1038/nature21377 |doi-access=free |pmid=28252057 |bibcode=2017Natur.543...60D |url=http://eprints.whiterose.ac.uk/112179/ |access-date=2 March 2017 |url-status=live |archive-url=https://web.archive.org/web/20170908201821/http://eprints.whiterose.ac.uk/112179/ |archive-date=8 September 2017}}</ref> However, later research disputed this interpretation of the data, stating that the observations may be better explained by abiotic processes in silica-rich waters,<ref name="García-Ruiz 2017">{{Cite journal |last1=García-Ruiz |first1=Juan Manuel |last2=Nakouzi |first2=Elias |last3=Kotopoulou |first3=Electra |last4=Tamborrino |first4=Leonardo |last5=Steinbock |first5=Oliver |date=2017-03-03 |title=Biomimetic mineral self-organization from silica-rich spring waters |journal=Science Advances  |volume=3 |issue=3 |pages=e1602285 |doi=10.1126/sciadv.1602285 |pmc=5357132 |pmid=28345049|bibcode=2017SciA....3E2285G}}</ref> "chemical gardens,"<ref>{{Cite journal |last=McMahon |first=Sean |date=2019-12-04 |title=Earth's earliest and deepest purported fossils may be iron-mineralized chemical gardens |journal=Proceedings of the Royal Society B: Biological Sciences |volume=286 |issue=1916 |pages=20192410 |doi=10.1098/rspb.2019.2410 |pmc=6939263 |pmid=31771469}}</ref> circulating hydrothermal fluids,<ref>{{Cite journal |last1=Johannessen |first1=Karen C. |last2=McLoughlin |first2=Nicola |last3=Vullum |first3=Per Erik |last4=Thorseth |first4=Ingunn H. |date=January 2020 |title=On the biogenicity of Fe-oxyhydroxide filaments in silicified low-temperature hydrothermal deposits: Implications for the identification of Fe-oxidizing bacteria in the rock record |url=https://onlinelibrary.wiley.com/doi/10.1111/gbi.12363 |journal=Geobiology |volume=18 |issue=1 |pages=31–53 |doi=10.1111/gbi.12363 |pmid=31532578 |bibcode=2020Gbio...18...31J |hdl=11250/2632364 |hdl-access=free}}</ref> or volcanic ejecta.<ref>{{Cite journal |last1=Wacey |first1=David |last2=Saunders |first2=Martin |last3=Kong |first3=Charlie |date=April 2018 |title=Remarkably preserved tephra from the 3430 Ma Strelley Pool Formation, Western Australia: Implications for the interpretation of Precambrian microfossils |url=http://dx.doi.org/10.1016/j.epsl.2018.01.021 |journal=Earth and Planetary Science Letters |volume=487 |pages=33–43 |doi=10.1016/j.epsl.2018.01.021 |bibcode=2018E&PSL.487...33W|url-access=subscription }}</ref>

Biogenic [[graphite]] has been found in 3.7 Gya metasedimentary rocks from southwestern [[Greenland]]<ref>{{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}}</ref> and in [[microbial mat]] fossils from 3.49&nbsp;Gya [[chert]]s in the [[Pilbara craton|Pilbara]] region of [[Western Australia]].<ref name="AST-20131108">{{cite journal |last1=Noffke |first1=Nora |author1-link=Nora Noffke |last2=Christian |first2=Daniel |last3=Wacey |first3=David |last4=Hazen |first4=Robert M. |author-link4=Robert Hazen |date=16 November 2013 |title=Microbially Induced Sedimentary Structures Recording an Ancient Ecosystem in the ''ca.'' 3.48 Gyo Dresser Formation, Pilbara, Western Australia |journal=[[Astrobiology (journal)|Astrobiology]] |volume=13 |issue=12 |pages=1103–1124 |bibcode=2013AsBio..13.1103N |doi=10.1089/ast.2013.1030 |pmc=3870916 |pmid=24205812}}</ref> Evidence of early life in rocks from [[Akilia]] Island, near the [[Isua Greenstone Belt|Isua supracrustal belt]] in southwestern Greenland, dating to 3.7&nbsp;Gya, have shown biogenic [[carbon isotope]]s.<ref>{{harvnb|Davies|1999}}</ref> In other parts of the Isua supracrustal belt, graphite inclusions trapped within [[garnet]] crystals are connected to the other elements of life: oxygen, nitrogen, and possibly phosphorus in the form of [[phosphate]], providing further evidence for life 3.7&nbsp;Gya.<ref>{{cite journal |last1=Hassenkam |first1=T. |last2=Andersson |first2=M. P. |last3=Dalby |first3=K. N. |last4=Mackenzie |first4=D. M. A. |last5=Rosing |first5=M.T. |title=Elements of Eoarchean life trapped in mineral inclusions |journal=[[Nature (journal)|Nature]] |doi=10.1038/nature23261 |pmid=28738409 |volume=548 |issue=7665 |pages=78–81 |year=2017 |bibcode=2017Natur.548...78H |s2cid=205257931}}</ref> In the [[Pilbara]] region of Western Australia, compelling evidence of early life was found in [[pyrite]]-bearing sandstone in a fossilized beach, with rounded tubular cells that oxidized sulfur by photosynthesis in the absence of oxygen.<ref>{{cite journal |last=O'Donoghue |first=James |date=21 August 2011 |title=Oldest reliable fossils show early life was a beach |journal=[[New Scientist]] |doi=10.1016/S0262-4079(11)62064-2 |volume=211 |page=13 |url=https://www.newscientist.com/article/dn20813-oldest-reliable-fossils-show-early-life-was-a-beach/ |url-status=live |archive-url=https://web.archive.org/web/20150630201918/http://www.newscientist.com/article/dn20813-oldest-reliable-fossils-show-early-life-was-a-beach.html |archive-date=30 June 2015}}</ref><ref>{{cite journal |last1=Wacey |first1=David |last2=Kilburn |first2=Matt R. |last3=Saunders |first3=Martin |last4=Cliff |first4=John |last5=Brasier |first5=Martin D. |author-link5=Martin Brasier |display-authors=3 |date=October 2011 |title=Microfossils of sulphur-metabolizing cells in 3.4-billion-year-old rocks of Western Australia |journal=[[Nature Geoscience]] |volume=4 |issue=10 |pages=698–702 |bibcode=2011NatGe...4..698W |doi=10.1038/ngeo1238}}</ref> Carbon isotope ratios on graphite inclusions from the Jack Hills zircons suggest that life could have existed on Earth from 4.1&nbsp;Gya.<ref>{{Cite journal |last1=Bell |first1=Elizabeth A. |last2=Boehnke |first2=Patrick |last3=Harrison |first3=T. Mark |last4=Mao |first4=Wendy L. |date=2015-11-24 |title=Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon |journal=Proceedings of the National Academy of Sciences |volume=112 |issue=47 |pages=14518–14521 |doi=10.1073/pnas.1517557112 |pmc=4664351 |pmid=26483481 |bibcode=2015PNAS..11214518B |doi-access=free}}</ref>

The Pilbara region of Western Australia contains the [[Dresser Formation]] with rocks 3.48 Gya, including layered structures called [[stromatolite]]s. Their modern counterparts are created by photosynthetic micro-organisms including [[cyanobacteria]].<ref>{{cite journal |last1=Baumgartner |first1=Rafael |last2=Van Kranendonk |first2=Martin |last3= Wacey |first3=David |last4=Fiorentini |first4=Marco |last5=Saunders |first5=Martin |last6=Caruso |first6=Caruso |last7=Pages |first7=Anais |last8=Homann |first8=Martin |last9= Guagliardo |first9=Paul |display-authors=3 |year=2019 |title=Nano−porous pyrite and organic matter in 3.5-billion-year-old stromatolites record primordial life |journal= [[Geology (journal)|Geology]] |volume=47 |issue=11 |pages=1039–1043 |doi=10.1130/G46365.1 |bibcode=2019Geo....47.1039B |s2cid=204258554 |url= https://discovery.ucl.ac.uk/id/eprint/10087275/1/Baumgartner%20et%20al%202019%20accepted.pdf |access-date=10 January 2021 |archive-date=5 December 2020 |archive-url=https://web.archive.org/web/20201205130046/https://discovery.ucl.ac.uk/id/eprint/10087275/1/Baumgartner%20et%20al%202019%20accepted.pdf |url-status=live}}</ref> These lie within undeformed hydrothermal-sedimentary strata; their texture indicates a biogenic origin. Parts of the Dresser formation preserve [[hot spring]]s on land, but other regions seem to have been shallow seas.<ref name="Djokic Van Kranendonk 2017">{{cite journal |last1=Djokic |first1=Tara |last2=Van Kranendonk |first2=Martin J. |last3=Campbell |first3=Kathleen A. |last4=Walter |first4=Malcolm R. |last5=Ward |first5=Colin R. |date=9 May 2017 |title=Earliest signs of life on land preserved in ca. 3.5 Gao hot spring deposits |journal=[[Nature Communications]] |volume=8 |page=15263 |bibcode=2017NatCo...815263D |doi=10.1038/ncomms15263 |pmc=5436104 |pmid=28486437}}</ref> A molecular clock analysis suggests the LUCA emerged prior to 3.9 Gya.<ref>{{Cite journal |last1=Betts |first1=Holly C. |last2=Puttick |first2=Mark N. |last3=Clark |first3=James W. |last4=Williams |first4=Tom A. |last5=Donoghue |first5=Philip C. J. |last6=Pisani |first6=Davide |date=20 August 2018 |title=Integrated genomic and fossil evidence illuminates life's early evolution and eukaryote origin |journal=Nature Ecology & Evolution |volume=2 |issue=10 |pages=1556–1562 |doi=10.1038/s41559-018-0644-x |pmid=30127539|pmc=6152910 |bibcode=2018NatEE...2.1556B}}</ref>

== Producing molecules: prebiotic synthesis ==

{{further|Primordial soup|Nucleosynthesis}}

All [[chemical element]]s derive from stellar nucleosynthesis except for hydrogen and some helium and lithium. Basic chemical ingredients of life – the [[carbon-hydrogen bond|carbon-hydrogen molecule]] (CH), the carbon-hydrogen positive ion (CH+) and the carbon ion (C+) – can be produced by [[ultraviolet light]] from stars.<ref name="NASA-20161012">{{cite web |last=Landau |first=Elizabeth |title=Building Blocks of Life's Building Blocks Come From Starlight |url=http://www.jpl.nasa.gov/news/news.php?feature=6645 |date=12 October 2016 |work=[[NASA]] |access-date=13 October 2016 |url-status=live |archive-url=https://web.archive.org/web/20161013135018/http://www.jpl.nasa.gov/news/news.php?feature=6645 |archive-date=13 October 2016}}</ref> Complex molecules, including organic molecules, form naturally both in space and on planets.<ref name="Ehrenfreund2010" /> Organic molecules on the early Earth could have had either terrestrial origins, with organic molecule synthesis driven by impact shocks or by other energy sources, such as ultraviolet light, [[organic redox reaction|redox]] coupling, or electrical discharges; or extraterrestrial origins ([[panspermia#Origination and distribution of organic molecules: Pseudo-panspermia|pseudo-panspermia]]), with organic molecules formed in [[interstellar cloud|interstellar dust clouds]] raining down on to the planet.<ref>{{cite journal |last1=Geballe |first1=Thomas R. |last2=Najarro |first2=Francisco |last3=Figer |first3=Donald F. |author-link3=Donald Figer |last4=Schlegelmilch |first4=Barret W. |last5=de la Fuente |first5=Diego |display-authors=3 |date=10 November 2011 |title=Infrared diffuse interstellar bands in the Galactic Centre region |journal=[[Nature (journal)|Nature]] |volume=479 |issue=7372 |pages=200–202 |arxiv=1111.0613 |bibcode=2011Natur.479..200G |doi=10.1038/nature10527 |pmid=22048316 |s2cid=17223339 |ref=none}}</ref><ref>{{harvnb|Klyce|2001}}</ref>

=== Observed extraterrestrial organic molecules ===

{{see also|List of interstellar and circumstellar molecules|Pseudo-panspermia}}

An organic compound is a chemical whose molecules contain carbon. Carbon is abundant in the Sun, stars, comets, and in the [[atmosphere]]s of most planets of the Solar System.<ref name="NASA-20140221">{{cite web |url=http://www.nasa.gov/ames/need-to-track-organic-nano-particles-across-the-universe-nasas-got-an-app-for-that/ |title=Need to Track Organic Nano-Particles Across the Universe? NASA's Got an App for That |last=Hoover |first=Rachel |date=21 February 2014 |website=[[Ames Research Center]] |publisher=[[NASA]] |access-date=22 June 2015 |url-status=live |archive-url=https://web.archive.org/web/20150906061428/http://www.nasa.gov/ames/need-to-track-organic-nano-particles-across-the-universe-nasas-got-an-app-for-that/ |archive-date=6 September 2015}}</ref> Organic compounds are relatively common in space, formed by "factories of complex molecular synthesis" which occur in molecular clouds and [[circumstellar envelope]]s, and chemically evolve after reactions are initiated mostly by [[ionizing radiation]].<ref name="Ehrenfreund2010">{{cite journal |last1=Ehrenfreund |first1=Pascale |last2=Cami |first2=Jan |date=December 2010 |title=Cosmic carbon chemistry: from the interstellar medium to the early Earth. |journal=[[Cold Spring Harbor Perspectives in Biology]] |volume=2 |issue=12 |page=a002097 |doi=10.1101/cshperspect.a002097 |pmc=2982172 |pmid=20554702}}</ref><ref>{{cite journal |last1=Goncharuk |first1=Vladislav V. |last2=Zui |first2=O. V. |date=February 2015 |title=Water and carbon dioxide as the main precursors of organic matter on Earth and in space |journal=Journal of Water Chemistry and Technology |volume=37 |issue=1 |pages=2–3 |doi=10.3103/S1063455X15010026 |bibcode=2015JWCT...37....2G |s2cid=97965067}}</ref><ref>{{cite journal |last1=Abou Mrad |first1=Ninette |last2=Vinogradoff |first2=Vassilissa |last3=Duvernay |first3=Fabrice |last4=Danger |first4=Grégoire |last5=Theulé |first5=Patrice |last6=Borget |first6=Fabien |last7=Chiavassa |first7=Thierry |display-authors=3 |year=2015 |title=Laboratory experimental simulations: Chemical evolution of the organic matter from interstellar and cometary ice analogs |url=http://popups.ulg.ac.be/0037-9565/index.php?id=4621&file=1 |journal=Bulletin de la Société Royale des Sciences de Liège |volume=84 |pages=21–32 |bibcode=2015BSRSL..84...21A |access-date=6 April 2015 |url-status=live |archive-url=https://web.archive.org/web/20150413050621/http://popups.ulg.ac.be/0037-9565/index.php?id=4621&file=1 |archive-date=13 April 2015}}</ref> [[Purine]] and [[pyrimidine]] nucleobases including [[guanine]], [[adenine]], [[cytosine]], [[uracil]], and [[thymine]] have been found in [[meteorite]]s. These could have provided the materials for DNA and [[RNA]] to form on the [[early Earth]].<ref name="NC-20220426">{{cite journal |last=Oba |first=Yasuhiro |display-authors=|title=Identifying the wide diversity of extraterrestrial purine and pyrimidine nucleobases in carbonaceous meteorites |date=26 April 2022 |journal=[[Nature Communications]] |volume=13 |number=2008 |page=2008 |doi=10.1038/s41467-022-29612-x |pmid=35473908 |pmc=9042847 |bibcode=2022NatCo..13.2008O |s2cid=248402205}}</ref> The amino acid [[glycine]] was found in material ejected from comet [[81P/Wild|Wild&nbsp;2]]; it had earlier been detected in meteorites.<ref>{{cite news |last=<!--Staff writer(s); no by-line.--> |date=18 August 2009 |title='Life chemical' detected in comet |url=http://news.bbc.co.uk/2/hi/science/nature/8208307.stm |work=BBC News |location=London |access-date=23 June 2015 |url-status=live |archive-url=https://web.archive.org/web/20150525071228/http://news.bbc.co.uk/2/hi/science/nature/8208307.stm |archive-date=25 May 2015}}</ref> Comets are encrusted with dark material, thought to be a [[tar]]-like organic substance formed from simple carbon compounds under ionizing radiation. A rain of material from comets could have brought such complex organic molecules to Earth.<ref>{{cite journal |last1=Thompson |first1=William Reid |last2=Murray |first2=B. G. |last3=Khare |first3=Bishun Narain |author-link3=Bishun Khare |last4=Sagan |first4=Carl |author4-link=Carl Sagan |date=30 December 1987 |title=Coloration and darkening of methane clathrate and other ices by charged particle irradiation: Applications to the outer solar system |journal=[[Journal of Geophysical Research]] |volume=92 |issue=A13 |pages=14933–14947 |bibcode=1987JGR....9214933T |doi=10.1029/JA092iA13p14933 |pmid=11542127}}</ref><ref>{{cite journal |last1=Goldman |first1=Nir |last2=Tamblyn |first2=Isaac |date=20 June 2013 |title=Prebiotic Chemistry within a Simple Impacting Icy Mixture |journal=[[Journal of Physical Chemistry A]] |volume=117 |issue=24 |pages=5124–5131 |doi=10.1021/jp402976n |pmid=23639050 |bibcode=2013JPCA..117.5124G |osti=1466173 |s2cid=5144843 |url=http://nparc.nrc-cnrc.gc.ca/eng/view/fulltext/?id=e89d2ac7-4cf8-40e0-bcc9-3c53f68ed70a |access-date=29 August 2019 |archive-date=21 July 2018 |archive-url=https://web.archive.org/web/20180721183453/http://nparc.nrc-cnrc.gc.ca/eng/view/fulltext/?id=e89d2ac7-4cf8-40e0-bcc9-3c53f68ed70a |url-status=live}}</ref><ref name="Follmann2009" /> It is estimated that during the Late Heavy Bombardment, meteorites may have delivered up to five million [[ton]]s of organic prebiotic elements to Earth per year.<ref name="Follmann2009" /> Currently 40,000 tons of cosmic dust falls to Earth each year.<ref>{{Cite web|url=https://www.astronomy.com/science/how-much-dust-falls-on-earth-each-year-does-it-affect-our-planets-gravity/|title=How much dust falls on Earth each year? Does it affect our planet's gravity? &#124; Astronomy.com|first=Astronomy|last=Staff|date=July 28, 2014}}</ref>

==== Polycyclic aromatic hydrocarbons====

[[File:PIA22568-CatsPawNebula-Spitzer-20181023.jpg|thumb|upright=1.25|The [[Cat's Paw Nebula]] is inside the [[Milky Way Galaxy]], in the [[constellation]] [[Scorpius]].<br />Green areas show regions where radiation from hot stars collided with large molecules and small dust grains called "[[polycyclic aromatic hydrocarbon]]s" (PAHs), causing them to [[fluoresce]]. [[Spitzer Space Telescope]], 2018.]]

[[Polycyclic aromatic hydrocarbon]]s (PAH) are the most common and abundant polyatomic molecules in the [[observable universe]], and are a major store of carbon.<ref name="NASA-20140221" /><ref>{{cite web |url=http://www.astrochem.org/pahdb/ |title=NASA Ames PAH IR Spectroscopic Database |publisher=NASA |access-date=17 June 2015 |url-status=live |archive-url=https://web.archive.org/web/20150629185734/http://www.astrochem.org/pahdb/ |archive-date=29 June 2015}}</ref><ref name="AJ-20051010">{{cite journal |last1=Hudgins |first1=Douglas M. |last2=Bauschlicher |first2=Charles W. Jr. |last3=Allamandola |first3=Louis J. |date=10 October 2005 |title=Variations in the Peak Position of the 6.2 μm Interstellar Emission Feature: A Tracer of N in the Interstellar Polycyclic Aromatic Hydrocarbon Population |journal=[[The Astrophysical Journal]] |volume=632 |pages=316–332 |issue=1 |bibcode=2005ApJ...632..316H |doi=10.1086/432495 |citeseerx=10.1.1.218.8786 |s2cid=7808613}}</ref><ref name="CD">{{cite web |last1=Des Marais |first1=David J. |last2=Allamandola |first2=Louis J. |last3=Sandford |first3=Scott |author-link3=Scott Sandford |last4=Mattioda |first4=Andrew |last5=Gudipati |first5=Murthy |last6=Roser |first6=Joseph |last7=Bramall |first7=Nathan |last8=Nuevo |first8=Michel |last9=Boersma |first9=Christiaan |last10=Bernstein |first10=Max |last11=Peeters |first11=Els |first14=Jason |last14=Dworkin |first13=Jamie Elsila |last13=Cook |first12=Jan |last12=Cami |display-authors=3 |year=2009 |title=Cosmic Distribution of Chemical Complexity |website=Ames Research Center |publisher=NASA |location=Mountain View, California |url=http://amesteam.arc.nasa.gov/Research/cosmic.html |access-date=24 June 2015  |archive-url=https://web.archive.org/web/20140227184503/http://amesteam.arc.nasa.gov/Research/cosmic.html |archive-date=27 February 2014}}</ref> They seem to have formed shortly after the Big Bang,<ref name="SP-20051018" /><ref name="AJ-20051010" /><ref name="CD" /> and are associated with [[star formation|new stars]] and [[exoplanet]]s.<ref name="NASA-20140221" /> They are a likely constituent of Earth's primordial sea.<ref name="SP-20051018">{{cite news |last=Carey |first=Bjorn |date=18 October 2005 |title=Life's Building Blocks 'Abundant in Space' |url=http://www.space.com/1686-life-building-blocks-abundant-space.html |website=Space.com |location=Watsonville, California |publisher=[[Imaginova]] |access-date=23 June 2015 |url-status=live |archive-url=https://web.archive.org/web/20150626223942/http://www.space.com/1686-life-building-blocks-abundant-space.html |archive-date=26 June 2015}}</ref><ref name="AJ-20051010" /><ref name="CD" /> PAHs have been detected in [[nebula]]e,<ref name="AJL-20101120">{{cite journal |last1=García-Hernández |first1=Domingo. A. |last2=Manchado |first2=Arturo |last3=García-Lario |first3=Pedro |last4=Stanghellini |first4=Letizia |last5=Villaver |first5=Eva |last6=Shaw |first6=Richard A. |last7=Szczerba |first7=Ryszard |last8=Perea-Calderón |first8=Jose Vicente |display-authors=3 |date=20 November 2010 |title=Formation of Fullerenes in H-Containing Planetary Nebulae |journal=[[The Astrophysical Journal Letters]] |volume=724 |issue=1 |pages=L39–L43 |arxiv=1009.4357 |bibcode=2010ApJ...724L..39G |doi=10.1088/2041-8205/724/1/L39 |s2cid=119121764}}</ref> and in the [[interstellar medium]], in comets, and in meteorites.<ref name="NASA-20140221" />

A star, HH 46-IR, resembling the sun early in its life, is surrounded by a disk of material which contains molecules including cyanide compounds, [[hydrocarbon]]s, and carbon monoxide. PAHs in the interstellar medium can be transformed through [[hydrogenation]], [[oxygenate|oxygenation]], and [[hydroxylation]] to more complex organic compounds used in living cells.<ref name="AJL-20120901">{{cite journal |last1=Gudipati |first1=Murthy S. |last2=Yang |first2=Rui |date=1 September 2012 |title=In-situ Probing of Radiation-induced Processing of Organics in Astrophysical Ice Analogs – Novel Laser Desorption Laser Ionization Time-of-flight Mass Spectroscopic Studies |journal=[[The Astrophysical Journal Letters]] |volume=756 |issue=1 |bibcode=2012ApJ...756L..24G |doi=10.1088/2041-8205/756/1/L24 |page=L24 |s2cid=5541727}}</ref>

==== Nucleobases and nucleotides ====

{{further|Nucleobase|Nucleotide}}

Organic compounds introduced on Earth by [[cosmic dust|interstellar dust particles]] can help to form complex molecules, thanks to their peculiar [[catalysis|surface-catalytic]] activities.<ref name="Lincei">{{cite journal |last=Gallori |first=Enzo |title=Astrochemistry and the origin of genetic material |journal=Rendiconti Lincei |date=June 2011 |volume=22 |issue=2 |pages=113–118 |doi=10.1007/s12210-011-0118-4 |s2cid=96659714}} "Paper presented at the Symposium 'Astrochemistry: molecules in space and time' (Rome, 4–5 November 2010), sponsored by Fondazione 'Guido Donegani', Accademia Nazionale dei Lincei."</ref><ref>{{cite journal |last=Martins |first=Zita |author-link=Zita Martins |date=February 2011 |title=Organic Chemistry of Carbonaceous Meteorites |journal=[[Elements (journal)|Elements]] |volume=7 |issue=1 |pages=35–40 |doi=10.2113/gselements.7.1.35|bibcode=2011Eleme...7...35M}}</ref> The RNA component uracil and related molecules, including [[xanthine]], in the [[Murchison meteorite]] were likely formed extraterrestrially, as suggested by studies of <sup>12</sup>C/<sup>13</sup>C [[natural abundance|isotopic ratios]].<ref name="Murch_base">{{cite journal |last1=Martins |first1=Zita |last2=Botta |first2=Oliver |last3=Fogel |first3=Marilyn L. |last4=Sephton |first4=Mark A. |last5=Glavin |first5=Daniel P. |last6=Watson |first6=Jonathan S. |last7=Dworkin |first7=Jason P. |last8=Schwartz |first8=Alan W. |last9=Ehrenfreund |first9=Pascale |display-authors=3 |date=15 June 2008 |title=Extraterrestrial nucleobases in the Murchison meteorite |journal=[[Earth and Planetary Science Letters]] |volume=270 |issue=1–2 |pages=130–136 |bibcode=2008E&PSL.270..130M |arxiv=0806.2286 |doi=10.1016/j.epsl.2008.03.026 |s2cid=14309508}}</ref> NASA studies of meteorites suggest that all four DNA nucleobases (adenine, guanine and related organic molecules) have been formed in outer space.<ref name="Lincei" /><ref name="Callahan">{{cite journal |last1=Callahan |first1=Michael P. |last2=Smith |first2=Karen E. |last3=Cleaves |first3=H. James II |last4=Ruzica |first4=Josef |last5=Stern |first5=Jennifer C. |last6=Glavin |first6=Daniel P. |last7=House |first7=Christopher H. |last8=Dworkin |first8=Jason P. |display-authors=3 |date=23 August 2011 |title=Carbonaceous meteorites contain a wide range of extraterrestrial nucleobases |journal=[[PNAS]] |volume=108 |issue=34 |pages=13995–13998 |bibcode=2011PNAS..10813995C |doi=10.1073/pnas.1106493108 |pmc=3161613 |pmid=21836052 |doi-access=free}}</ref><ref name="Steigerwald">{{cite web |url=http://www.nasa.gov/topics/solarsystem/features/dna-meteorites.html |title=NASA Researchers: DNA Building Blocks Can Be Made in Space |last=Steigerwald |first=John |date=8 August 2011 |work=[[Goddard Space Flight Center]] |publisher=NASA |access-date=23 June 2015 |url-status=live |archive-url=https://web.archive.org/web/20150623004556/http://www.nasa.gov/topics/solarsystem/features/dna-meteorites.html |archive-date=23 June 2015}}</ref> The [[cosmic dust]] permeating the universe contains complex organics ("amorphous organic solids with a mixed [[aromaticity|aromatic]]–[[aliphatic compound|aliphatic]] structure") that could be created rapidly by stars.<ref name="Nature-20111026">{{cite journal |last1=Kwok |first1=Sun |author-link=Sun Kwok |last2=Zhang |first2=Yong |date=3 November 2011 |title=Mixed aromatic–aliphatic organic nanoparticles as carriers of unidentified infrared emission features |journal=[[Nature (journal)|Nature]] |volume=479 |issue=7371 |pages=80–83 |bibcode=2011Natur.479...80K |doi=10.1038/nature10542 |pmid=22031328 |s2cid=4419859}}</ref> [[Glycolaldehyde]], a sugar molecule and RNA precursor, has been detected in regions of space including around [[protostar]]s and on meteorites.<ref>{{cite journal |last1=Jørgensen |first1=Jes K. |last2=Favre |first2=Cécile |last3=Bisschop |first3=Suzanne E. |last4=Bourke |first4=Tyler L. |last5=van Dishoeck |first5=Ewine F. |author-link5=Ewine van Dishoeck |last6=Schmalzl |first6=Markus |display-authors=3 |date=2012 |title=Detection of the simplest sugar, glycolaldehyde, in a solar-type protostar with ALMA |url=http://www.eso.org/public/archives/releases/sciencepapers/eso1234/eso1234a.pdf |journal=[[The Astrophysical Journal Letters]] |volume=757 |issue=1 |arxiv=1208.5498 |bibcode=2012ApJ...757L...4J |doi=10.1088/2041-8205/757/1/L4 |access-date=2015-06-23 |page=L4 |s2cid=14205612 |url-status=live |archive-url=https://web.archive.org/web/20150924021240/http://www.eso.org/public/archives/releases/sciencepapers/eso1234/eso1234a.pdf |archive-date=24 September 2015}}</ref><ref name="PNAS-20191113">{{cite journal |last1=Furukawa |first1=Yoshihiro |last2=Chikaraishi |first2=Yoshito |last3=Ohkouchi |first3=Naohiko |last4=Ogawa |first4=Nanako O. |last5=Glavin |first5=Daniel P. |last6=Dworkin |first6=Jason P. |last7=Abe |first7=Chiaki |last8=Nakamura |first8=Tomoki |display-authors=3 |date=2019-11-13 |title=Extraterrestrial ribose and other sugars in primitive meteorites |journal=[[PNAS]] |volume=116 |issue=49 |pages=24440–24445 |doi=10.1073/pnas.1907169116 |pmid=31740594 |pmc=6900709 |bibcode=2019PNAS..11624440F |doi-access=free}}</ref>

=== Laboratory synthesis ===

As early as the 1860s, experiments demonstrated that biologically relevant molecules can be produced from interaction of simple carbon sources with abundant inorganic catalysts. The spontaneous formation of complex polymers from abiotically generated monomers under the conditions posited by the "soup" theory is not straightforward. Besides the necessary basic organic monomers, compounds that would have prohibited the formation of polymers were also formed in high concentration during the [[Miller–Urey experiment]] and [[Joan Oró]] experiments.<ref>{{cite journal |last1=Oró |first1=Joan |last2=Kimball |first2=Aubrey P. |date=February 1962 |title=Synthesis of purines under possible primitive earth conditions: II. Purine intermediates from hydrogen cyanide |journal=[[Archives of Biochemistry and Biophysics]] |volume=96 |issue=2 |pages=293–313 |doi=10.1016/0003-9861(62)90412-5 |pmid=14482339}}</ref> Biology uses essentially 20 amino acids for its coded protein enzymes, representing a very small subset of the structurally possible products. Since life tends to use whatever is available, an explanation is needed for why the set used is so small.<ref>{{cite journal |last1=Cleaves II |first1=Henderson |year=2010 |title=The origin of the biologically coded amino acids |journal=[[Journal of Theoretical Biology]] |volume=263 |issue=4 |pages=490–498 |doi=10.1016/j.jtbi.2009.12.014 |pmid=20034500 |bibcode=2010JThBi.263..490C}}</ref> Formamide is attractive as a medium that potentially provided a source of amino acid derivatives from simple aldehyde and nitrile feedstocks.<ref>{{cite journal |last=Green |first=Nicholas J. |last2=Russell |first2=David A. |last3=Tanner |first3=Sasha H. |last4=Sutherland |first4=John D. |title=Prebiotic Synthesis of N-Formylaminonitriles and Derivatives in Formamide |journal=Journal of the American Chemical Society |year=2023 |volume=145 |issue=19 |pages=10533–10541 |doi=10.1021/jacs.2c13306 |pmid=37146260 |doi-access=free |pmc=10197134 |bibcode=2023JAChS.14510533G}}</ref>

==== Sugars ====

[[File:Formose.png|thumb|upright=1.5|The Breslow catalytic cycle for formaldehyde dimerization and C2-C6 sugar formation]]

[[Alexander Butlerov]] showed in 1861 that the [[formose reaction]] created sugars including tetroses, pentoses, and hexoses when [[formaldehyde]] is heated under basic conditions with divalent metal ions like calcium. R. Breslow proposed that the reaction was autocatalytic in 1959.<ref>{{cite journal |last=Breslow |first=R. |year=1959 |title=On the Mechanism of the Formose Reaction |journal=[[Tetrahedron Letters]] |volume=1 |issue=21 |pages=22–26 |doi=10.1016/S0040-4039(01)99487-0}}</ref>

==== Nucleobases ====

Nucleobases, such as guanine and adenine, can be synthesized from simple carbon and nitrogen sources, such as [[hydrogen cyanide]] (HCN) and ammonia.<ref>{{cite journal |last=Oró |first=Joan |author-link=Joan Oró |date=16 September 1961 |title=Mechanism of Synthesis of Adenine from Hydrogen Cyanide under Possible Primitive Earth Conditions |journal=[[Nature (journal)|Nature]] |volume=191 |issue=4794 |pages=1193–1194 |bibcode=1961Natur.191.1193O |doi=10.1038/1911193a0 |pmid=13731264 |s2cid=4276712}}</ref> [[Formamide]] produces all four ribonucleotides when warmed with terrestrial minerals. It is ubiquitous, produced by the reaction of water and HCN. It can be concentrated by the evaporation of water.<ref name="Saladino2012">{{cite journal |last1=Saladino |first1=Raffaele |last2=Crestini |first2=Claudia |last3=Pino |first3=Samanta |last4=Costanzo |first4=Giovanna |last5=Di Mauro |first5=Ernesto |display-authors=3 |date=March 2012 |title=Formamide and the origin of life. |journal=[[Physics of Life Reviews]] |volume=9 |issue=1 |pages=84–104 |bibcode=2012PhLRv...9...84S |doi=10.1016/j.plrev.2011.12.002 |pmid=22196896 |hdl=2108/85168 |url=https://art.torvergata.it/bitstream/2108/85168/1/PoLRev%202012.pdf |hdl-access=free |access-date=29 August 2019 |archive-date=27 January 2023 |archive-url=https://web.archive.org/web/20230127162359/https://art.torvergata.it/bitstream/2108/85168/1/PoLRev%202012.pdf |url-status=live}}</ref><ref name="Saladino2012b">{{cite journal |last1=Saladino |first1=Raffaele |last2=Botta |first2=Giorgia |last3=Pino |first3=Samanta |last4=Costanzo |first4=Giovanna |last5=Di Mauro |first5=Ernesto |display-authors=3 |date=July 2012 |title=From the one-carbon amide formamide to RNA all the steps are prebiotically possible |journal=[[Biochimie]] |volume=94 |issue=7 |pages=1451–1456 |doi=10.1016/j.biochi.2012.02.018 |pmid=22738728 |hdl=11573/515604}}</ref> HCN is poisonous only to [[aerobic organism]]s, which did not yet exist. It can contribute to chemical processes such as the synthesis of the amino acid glycine.<ref name="Follmann2009"/>

DNA and RNA components including uracil, cytosine and thymine can be synthesized under outer space conditions, using starting chemicals such as pyrimidine found in meteorites. Pyrimidine may have been formed in [[red giant]] stars or in interstellar dust and gas clouds.<ref name="NASA-20150303">{{cite web |url=http://www.nasa.gov/content/nasa-ames-reproduces-the-building-blocks-of-life-in-laboratory |title=NASA Ames Reproduces the Building Blocks of Life in Laboratory |editor-last=Marlaire |editor-first=Ruth |date=3 March 2015 |work=Ames Research Center |publisher=NASA |access-date=5 March 2015 |url-status=live |archive-url=https://web.archive.org/web/20150305083306/http://www.nasa.gov/content/nasa-ames-reproduces-the-building-blocks-of-life-in-laboratory/ |archive-date=5 March 2015}}</ref> All four RNA-bases may be synthesized from formamide in high-energy density events like extraterrestrial impacts.<ref>{{cite journal |last1=Ferus |first1=Martin |last2=Nesvorný |first2=David |last3=Šponer |first3=Jiří |last4=Kubelík |first4=Petr |last5=Michalčíková |first5=Regina |last6=Shestivská |first6=Violetta |last7=Šponer |first7=Judit E. |last8=Civiš |first8=Svatopluk |display-authors=3 |year=2015 |title=High-energy chemistry of formamide: A unified mechanism of nucleobase formation |journal=[[PNAS]] |volume=112 |issue=3 |pages=657–662 |doi=10.1073/pnas.1412072111 |pmid=25489115 |bibcode=2015PNAS..112..657F |pmc=4311869 |doi-access=free}}</ref> Several ribonucleotides for RNA formation have been synthesized in a laboratory environment which replicates [[Prebiotic world|prebiotic conditions]] via [[Autocatalysis|autocatalytic formose reaction]].<ref>{{Cite journal |last1=Tran |first1=Quoc Phuong |last2=Yi |first2=Ruiqin |last3=Fahrenbach |first3=Albert C. |date=2023-09-13 |title=Towards a prebiotic chemoton – nucleotide precursor synthesis driven by the autocatalytic formose reaction |journal=Chemical Science |volume=14 |issue=35 |pages=9589–9599 |doi=10.1039/D3SC03185C |doi-access=free |pmid=37712016 |pmc=10498504}}</ref>

Other pathways for synthesizing bases from inorganic materials have been reported.<ref name="Basile1984">{{cite journal |last1=Basile |first1=Brenda |last2=Lazcano |first2 = Antonio |author2-link=Antonio Lazcano |last3=Oró |first3=Joan |year=1984 |title=Prebiotic syntheses of purines and pyrimidines |journal=[[Advances in Space Research]] |volume = 4 |issue=12 |pages=125–131 |bibcode=1984AdSpR...4l.125B |doi=10.1016/0273-1177(84)90554-4 |pmid=11537766}}</ref> Freezing temperatures assist the synthesis of purines, by concentrating key precursors such as HCN.<ref>{{cite journal |last=Orgel |first=Leslie E. |date=August 2004 |title = Prebiotic Adenine Revisited: Eutectics and Photochemistry |journal=[[Origins of Life and Evolution of Biospheres]] |volume=34 |issue=4 |pages=361–369 |bibcode= 2004OLEB...34..361O |doi=10.1023/B:ORIG.0000029882.52156.c2 |pmid=15279171 |s2cid=4998122}}</ref> However, while adenine and guanine require freezing conditions, cytosine and uracil may require boiling temperatures.<ref>{{cite journal |last1=Robertson |first1=Michael P. |last2=Miller |first2=Stanley L. |date=29 June 1995 |title = An efficient prebiotic synthesis of cytosine and uracil |journal=Nature |volume=375 |issue=6534 |pages=772–774 |bibcode=1995Natur.375..772R |doi= 10.1038/375772a0 |pmid = 7596408 |s2cid=4351012}}</ref> Seven amino acids and eleven types of nucleobases formed in ice when ammonia and [[cyanide]] were left in a freezer for 25 years.<ref>{{cite journal |last=Fox |first=Douglas |date=1 February 2008 |title=Did Life Evolve in Ice? |journal=[[Discover (magazine)|Discover]] |url=https://www.discovermagazine.com/planet-earth/did-life-evolve-in-ice |access-date=2008-07-03 |url-status=live |archive-url=https://web.archive.org/web/20080630043228/http://discovermagazine.com/2008/feb/did-life-evolve-in-ice |archive-date=30 June 2008}}</ref><ref name = jeffreybada>{{cite journal |last1=Levy |first1=Matthew |last2=Miller |first2=Stanley L. |last3=Brinton |first3=Karen |last4=Bada |first4=Jeffrey L. |author4-link=Jeffrey L. Bada |date=June 2000 |title=Prebiotic Synthesis of Adenine and Amino Acids Under Europa-like Conditions |journal=[[Icarus (journal)|Icarus]] |volume=145 |issue=2 |pages=609–613 |bibcode=2000Icar..145..609L |doi=10.1006/icar.2000.6365 |pmid=11543508}}</ref> S-[[triazine]]s (alternative nucleobases), pyrimidines including cytosine and uracil, and adenine can be synthesized by subjecting a urea solution to freeze-thaw cycles under a reductive atmosphere with spark discharges.<ref>{{cite journal |last1=Menor-Salván |first1=César |last2=Ruiz-Bermejo |first2= Marta |last3=Guzmán |first3=Marcelo I. |last4=Osuna-Esteban |first4=Susana |last5=Veintemillas-Verdaguer |first5=Sabino |display-authors=3 |date=20 April 2009 |title=Synthesis of Pyrimidines and Triazines in Ice: Implications for the Prebiotic Chemistry of Nucleobases |journal=[[Chemistry: A European Journal]] |volume=15 |issue=17 |pages=4411–4418 |doi=10.1002/chem.200802656 |pmid=19288488}}</ref> The unusual speed of these low-temperature reactions is due to [[Eutectic system|eutectic freezing]], which crowds impurities in microscopic pockets of liquid within the ice.<ref>{{cite journal |last1=Roy |first1=Debjani |last2=Najafian |first2=Katayoun |last3=von Ragué Schleyer |first3=Paul |author-link3=Paul von Ragué Schleyer |date=30 October 2007 |title=Chemical evolution: The mechanism of the formation of adenine under prebiotic conditions |journal=[[PNAS]] |volume=104 |issue=44 |pages=17272–17277 |bibcode= 2007PNAS..10417272R |doi=10.1073/pnas.0708434104 |pmc=2077245 |pmid=17951429 |doi-access=free}}</ref>

==== Peptides ====

Prebiotic peptide synthesis could have occurred by several routes.  Some center on high temperature/concentration conditions in which condensation becomes energetically favorable, while others use plausible prebiotic condensing agents.<ref name=":0">{{cite journal |last1=Frenkel-Pinter |first1=Moran |last2=Samanta |first2=Mousumi |last3=Ashkenasy |first3=Gonen |last4=Leman |first4=Luke J. |date=2020-06-10 |title=Prebiotic Peptides: Molecular Hubs in the Origin of Life |url=https://pubs.acs.org/doi/10.1021/acs.chemrev.9b00664 |journal=Chemical Reviews |volume=120 |issue=11 |pages=4707–4765 |doi=10.1021/acs.chemrev.9b00664 |pmid=32101414 |bibcode=2020ChRv..120.4707F|url-access=subscription }}</ref>

Experimental evidence for the formation of peptides in uniquely concentrated environments is bolstered by work suggesting that wet-dry cycles and the presence of specific salts can greatly increase spontaneous condensation of glycine into poly-glycine chains.<ref>{{cite journal |last1=Campbell |first1=Thomas D. | last2=Febrian | first2=Rio | last3=McCarthy | first3=Jack T. |last4=Kleinschmidt |first4=Holly E. |last5=Forsythe | first5=Jay G. |last6=Bracher |first6=Paul J. |title=Prebiotic condensation through wet–dry cycling regulated by deliquescence |date=2019 |journal=Nature Communications |volume=10 |issue=1 |page=4508 |doi=10.1038/s41467-019-11834-1 |pmc=6778215 |pmid=31586058 |bibcode=2019NatCo..10.4508C}}</ref> Other work suggests that while mineral surfaces, such as those of pyrite, calcite, and rutile catalyze peptide condensation, they also catalyze their hydrolysis. The authors suggest that additional chemical activation or coupling would be necessary to produce peptides at sufficient concentrations.  Thus, mineral surface catalysis, while important, is not sufficient alone for peptide synthesis.<ref>{{cite journal |last1=Marshall-Bowman |first1=Karina |last2=Ohara |first2=Shohei |last3=Sverjensky |first3=Dimitri A. |last4=Hazen |first4=Robert M. |last5=Cleaves |first5=H. James |date=October 2010 |title=Catalytic peptide hydrolysis by mineral surface: Implications for prebiotic chemistry |journal=Geochimica et Cosmochimica Acta |volume=74 |issue=20 |pages=5852–5861 |doi=10.1016/j.gca.2010.07.009 |bibcode=2010GeCoA..74.5852M}}</ref>

Many prebiotically plausible condensing/activating agents have been identified, including the following: cyanamide, dicyanamide, dicyandiamide, diaminomaleonitrile, urea, trimetaphosphate, NaCl, CuCl<sub>2,</sub> (Ni,Fe)S, CO, carbonyl sulfide (COS), carbon disulfide (CS<sub>2</sub>)<sub>,</sub> SO<sub>2,</sub> and diammonium phosphate (DAP).<ref name=":0"/>

A 2024 experiment used a sapphire substrate with a web of thin cracks under a heat flow, mimicking [[Hydrothermal vent|deep-ocean vents]], to concentrate prebiotically-relevant building blocks from a dilute mixture by up to three orders of magnitude. This could help to create biopolymers such as peptides.<ref>{{cite journal |last1=Matreux |first1=Thomas |last2=Aikkila |first2=Paula |last3=Scheu |first3=Bettina |last4=Braun |first4=Dieter |last5=Mast |first5=Christof B. |date=April 2024 |title=Heat flows enrich prebiotic building blocks and enhance their reactivity |journal=Nature |volume=628 |issue=8006 |pages=110–116 |doi=10.1038/s41586-024-07193-7 |pmc=10990939 |pmid=38570715 |bibcode=2024Natur.628..110M}}</ref> A similar role has been suggested for clays.<ref>{{cite journal |last=Paecht-Horowitz |first=Mella |date=1974-01-01 |title=The possible role of clays in prebiotic peptide synthesis |journal=Origins of Life |volume=5 |issue=1 |pages=173–187 |doi=10.1007/BF00927022 |pmid=4842069 |bibcode=1974OrLi....5..173P}}</ref>

The prebiotic synthesis of peptides from simpler molecules such as CO, NH<sub>3</sub> and C, skipping the step of amino acid formation, is also very efficient.<ref>{{cite journal |last1=Krasnokutski |first1=S. A. |last2=Chuang |first2=K.-J. |last3=Jäger |first3=C. |last4=Ueberschaar |first4=N. |last5=Henning |first5=Th. |date=2022-02-10 |title=A pathway to peptides in space through the condensation of atomic carbon |url=https://www.nature.com/articles/s41550-021-01577-9 |journal=Nature Astronomy |volume=6 |issue=3 |pages=381–386 |doi=10.1038/s41550-021-01577-9 |arxiv=2202.12170 |bibcode=2022NatAs...6..381K}}</ref><ref>{{Cite journal |last1=Krasnokutski |first1=Serge A. |last2=Jäger |first2=Cornelia |last3=Henning |first3=Thomas |last4=Geffroy |first4=Claude |last5=Remaury |first5=Quentin B. |last6=Poinot |first6=Pauline |date=2024-04-19 |title=Formation of extraterrestrial peptides and their derivatives |journal=Science Advances |volume=10 |issue=16 |pages=eadj7179 |doi=10.1126/sciadv.adj7179 |pmc=11023503 |pmid=38630826 |arxiv=2405.00744 |bibcode=2024SciA...10J7179K}}</ref>

== Producing protocells==

{{further|Gard model|Self-organization#Biology|Cellularization}}

[[File:Phospholipids aqueous solution structures.svg|thumb|upright|The three main structures composed of [[phospholipid]]s form spontaneously by [[self-assembly]] in solution: the [[liposome]] (a closed bilayer), the [[micelle]] and the bilayer.]]

The largest unanswered question in evolution is how simple protocells first arose and differed in reproductive contribution to the following generation, thus initiating evolution. The [[Gard model|lipid world]] theory postulates that the first self-replicating object was [[lipid]]-like.<ref>{{cite web |url=http://www.weizmann.ac.il/molgen/Lancet/research/prebiotic-evolution |title=Systems Prebiology-Studies of the origin of Life |last=Lancet |first=Doron |date=30 December 2014 |website=The Lancet Lab |publisher=Department of Molecular Genetics; [[Weizmann Institute of Science]] |location=Rehovot, Israel |access-date=26 June 2015 |url-status=live |archive-url=https://web.archive.org/web/20150626180507/http://www.weizmann.ac.il/molgen/Lancet/research/prebiotic-evolution |archive-date=26 June 2015}}</ref><ref>{{cite journal |last1=Segré |first1=Daniel |last2=Ben-Eli |first2=Dafna |last3=Deamer |first3=David W. |last4=Lancet |first4=Doron |date=February 2001 |title=The Lipid World |url=http://www.weizmann.ac.il/molgen/Lancet/sites/molgen.Lancet/files/uploads/segre_lipid_world.pdf |journal=[[Origins of Life and Evolution of Biospheres]] |volume=31 |issue=1–2 |pages=119–145 |doi=10.1023/A:1006746807104 |pmid=11296516 |bibcode=2001OLEB...31..119S |s2cid=10959497 |url-status=live |archive-url=https://web.archive.org/web/20150626225745/http://www.weizmann.ac.il/molgen/Lancet/sites/molgen.Lancet/files/uploads/segre_lipid_world.pdf |archive-date=26 June 2015}}</ref> Phospholipids form [[lipid bilayer]]s (as in cell membranes) in water while under agitation. These molecules were not present on early Earth, but other membrane-forming [[amphiphile|amphiphilic]] long-chain molecules were. These bodies may expand by insertion of additional lipids, and may spontaneously split into two [[offspring]] of similar size and composition. Lipid bodies may have provided sheltering envelopes for information storage, allowing the evolution of information-storing polymers like RNA. Only one or two types of vesicle-forming amphiphiles have been studied.<ref name="Chen 2010"/> There is an enormous number of possible arrangements of lipid bilayer membranes, and those with the best reproductive characteristics would have converged toward a hypercycle reaction,<ref>{{cite journal |last1=Eigen |first1=Manfred |author-link1=Manfred Eigen |last2=Schuster |first2=Peter |author-link2=Peter Schuster |date=November 1977 |title=The Hypercycle. A Principle of Natural Self-Organization. Part A: Emergence of the Hypercycle |url=http://jaguar.biologie.hu-berlin.de/~wolfram/pages/seminar_theoretische_biologie_2007/literatur/schaber/Eigen1977Naturwissenschaften64.pdf |journal=[[Naturwissenschaften]] |volume=64 |issue=11 |pages=541–65 |bibcode=1977NW.....64..541E |doi=10.1007/bf00450633 |pmid=593400 |s2cid=42131267 |archive-url=https://web.archive.org/web/20160303194728/http://jaguar.biologie.hu-berlin.de/~wolfram/pages/seminar_theoretische_biologie_2007/literatur/schaber/Eigen1977Naturwissenschaften64.pdf |archive-date=3 March 2016 |ref=none}}

* {{cite journal |last1=Eigen |first1=Manfred |author1-link=Manfred Eigen |last2=Schuster |first2=Peter |author2-link=Peter Schuster |year=1978 |title=The Hypercycle. A Principle of Natural Self-Organization. Part B: The Abstract Hypercycle |url=http://jaguar.biologie.hu-berlin.de/~wolfram/pages/seminar_theoretische_biologie_2007/literatur/schaber/Eigen1978Naturwissenschaften65a.pdf |journal=[[Naturwissenschaften]] |volume=65 |issue=1 |pages=7–41 |bibcode=1978NW.....65....7E |doi=10.1007/bf00420631 |s2cid=1812273 |archive-url=https://web.archive.org/web/20160303203325/http://jaguar.biologie.hu-berlin.de/~wolfram/pages/seminar_theoretische_biologie_2007/literatur/schaber/Eigen1978Naturwissenschaften65a.pdf |archive-date=3 March 2016 |ref=none}}
* {{cite journal |last1=Eigen |first1=Manfred |author1-link=Manfred Eigen |last2=Schuster |first2=Peter |author-link2=Peter Schuster |date=July 1978 |title=The Hypercycle. A Principle of Natural Self-Organization. Part C: The Realistic Hypercycle |url=http://jaguar.biologie.hu-berlin.de/~wolfram/pages/seminar_theoretische_biologie_2007/literatur/schaber/Eigen1978Naturwissenschaften65b.pdf |journal=[[Naturwissenschaften]] |volume=65 |issue=7 |pages=341–369 |bibcode=1978NW.....65..341E |doi=10.1007/bf00439699 |s2cid=13825356 |archive-url=https://web.archive.org/web/20160616180402/http://jaguar.biologie.hu-berlin.de/~wolfram/pages/seminar_theoretische_biologie_2007/literatur/schaber/Eigen1978Naturwissenschaften65b.pdf |archive-date=16 June 2016 |ref=none}}</ref><ref>{{cite journal |last1=Markovitch |first1=Omer |last2=Lancet |first2=Doron |date=Summer 2012 |title=Excess Mutual Catalysis Is Required for Effective Evolvability |journal=[[Artificial Life (journal)|Artificial Life]] |volume=18 |issue=3 |pages=243–266 |doi=10.1162/artl_a_00064 |pmid=22662913 |s2cid=5236043 |doi-access=free}}</ref> a positive [[feedback]] composed of two mutual catalysts represented by a membrane site and a specific compound trapped in the vesicle. Such site/compound pairs are transmissible to the daughter vesicles, leading to the emergence of distinct [[lineage (evolution)|lineages]] of vesicles, subject to [[natural selection]].<ref>{{cite journal |last=Tessera |first=Marc |year=2011 |title=Origin of Evolution ''versus'' Origin of Life: A Shift of Paradigm |journal=[[International Journal of Molecular Sciences]] |volume=12 |issue=6 |pages=3445–3458 |doi=10.3390/ijms12063445 |pmc=3131571 |pmid=21747687 |doi-access=free}} Special Issue: "Origin of Life 2011"</ref>

A [[protocell]] is a self-organized, self-ordered, spherical collection of lipids proposed as a stepping-stone to life.<ref name="Chen 2010">{{cite journal |first1=Irene A. |last1=Chen |first2= Peter |last2=Walde |title=From Self-Assembled Vesicles to Protocells |journal=[[Cold Spring Harbor Perspectives in Biology]] |date=July 2010 |volume=2 |issue=7 |page=a002170 |doi= 10.1101/cshperspect.a002170 |pmc=2890201 |pmid=20519344}}</ref> A functional protocell has (as of 2014) not yet been achieved in a laboratory setting.<ref name="Exploring">{{cite web |title=Protocells |url=http://exploringorigins.org/protocells.html |url-status=live |archive-url=https://web.archive.org/web/20140228083459/http://exploringorigins.org/protocells.html |archive-date=28 February 2014 |access-date=September 13, 2023 |website=Exploring Life's Origins: A Virtual Exhibit |publisher=[[National Science Foundation]] |location=Arlington County, Virginia}}</ref><ref name="Chen 2006">{{cite journal |last=Chen |first=Irene A. |date=8 December 2006 |title=The Emergence of Cells During the Origin of Life |journal=[[Science (journal)|Science]] |volume=314 |issue=5805 |pages=1558–1559 |doi=10.1126/science.1137541 |pmid=17158315 |doi-access=free}}</ref><ref name="Discover 2004">{{cite magazine |last=Zimmer |first=Carl |author-link=Carl Zimmer |date=26 June 2004 |title=What Came Before DNA? |url=https://www.discovermagazine.com/planet-earth/what-came-before-dna |url-status=live |archive-url=https://web.archive.org/web/20140319001351/http://discovermagazine.com/2004/jun/cover |archive-date=19 March 2014 |journal=[[Discover (magazine)|Discover]]}}</ref> Self-assembled [[vesicle (biology and chemistry)|vesicles]] are essential components of primitive cells.<ref name="Chen 2010"/> The theory of classical irreversible thermodynamics treats self-assembly under a generalized chemical potential within the framework of [[dissipative system]]s.<ref>{{cite journal |last=Onsager |first=Lars |date=1931 |title=Reciprocal Relations in Irreversible Processes I |journal=Physical Review |volume=37 |issue=4 |page=405 |doi= 10.1103/PhysRev.37.405 |bibcode=1931PhRv...37..405O|doi-access=free}}</ref><ref>{{cite journal |last=Onsager |first=Lars |date=1931 |title=Reciprocal Relations in Irreversible Processes II |journal= Physical Review |volume=38 |issue=12 |page=2265 |doi=10.1103/PhysRev.38.2265|bibcode=1931PhRv...38.2265O |doi-access=free}}</ref><ref>{{cite book |last=Prigogine |first=Ilya |author-link=Ilya Prigogine |year=1967 |title=An Introduction to the Thermodynamics of Irreversible Processes |publisher=[[Wiley (publisher)|Wiley]] |location=New York}}</ref> The [[second law of thermodynamics]] requires that overall [[entropy]] increases, yet life is distinguished by its great degree of organization. Therefore, a boundary is needed to separate ordered [[metabolism|life processes]] from chaotic non-living matter.<ref name="SciAm 2007">{{cite journal |last=Shapiro |first=Robert |author-link=Robert Shapiro (chemist) |date=June 2007 |title=A Simpler Origin for Life |url=http://www.scientificamerican.com/article/a-simpler-origin-for-life/ |url-status=live |journal=[[Scientific American]] |volume=296 |issue=6 |pages=46–53 |bibcode=2007SciAm.296f..46S |doi=10.1038/scientificamerican0607-46 |pmid=17663224 |archive-url=https://web.archive.org/web/20150614000643/http://www.scientificamerican.com/article/a-simpler-origin-for-life/ |archive-date=14 June 2015|url-access=subscription }}</ref>

Irene Chen and [[Jack W. Szostak]] suggest that elementary protocells can give rise to cellular behaviors including primitive forms of differential reproduction, competition, and energy storage.<ref name="Chen 2006"/> Competition for membrane molecules would favor stabilized membranes, suggesting a selective advantage for cross-linked fatty acids and even modern [[phospholipid]]s.<ref name="Chen 2006"/> Such [[micro-encapsulation]] would allow for metabolism within the membrane and the exchange of small molecules, while retaining large biomolecules inside. Such a membrane is needed for a cell to create its own [[electrochemical gradient]].<ref>{{harvnb|Chang|2007}}</ref><ref name="Lane 2015"/> Fatty acid vesicles in conditions relevant to alkaline hydrothermal vents can be stabilized by isoprenoids which are synthesized by the formose reaction; the advantages and disadvantages of isoprenoids incorporated within the lipid bilayer in different microenvironments might have led to the divergence of the membranes of archaea and bacteria.<ref>{{Cite journal |last1=Jordan |first1=Sean F. |last2=Nee |first2=Eloise |last3=Lane |first3=Nick |author3-link=Nick Lane |date=2019-12-06 |title=Isoprenoids enhance the stability of fatty acid membranes at the emergence of life potentially leading to an early lipid divide |journal=Interface Focus |volume=9 |issue=6 |page=20190067 |doi=10.1098/rsfs.2019.0067 |pmc=6802135 |pmid=31641436}}</ref>

Vesicles can undergo an evolutionary process under pressure cycling conditions.<ref name = mayer2018>{{Cite journal |last1=Mayer |first1=Christian |last2=Schreiber |first2=Ulrich |last3=Dávila |first3=María J. |last4=Schmitz |first4=Oliver J. |last5=Bronja |first5=Amela |last6=Meyer |first6=Martin |last7=Klein |first7=Julia |last8=Meckelmann |first8=Sven W. |date=2018 |title=Molecular Evolution in a Peptide-Vesicle System |journal=Life |volume=8 |issue=2 |pages=16 |doi=10.3390/life8020016 |doi-access=free |pmid=29795023 |pmc=6027363 |bibcode=2018Life....8...16M}}</ref> Simulating the systemic environment in tectonic [[Fault (geology)|fault zones]] within the [[Earth's crust]], pressure cycling leads to the periodic formation of vesicles.<ref name = periodic>{{Cite journal |last1=Mayer |first1=Christian |last2=Schreiber |first2=Ulrich |last3=Dávila |first3=María J. |date=2015-06-01 |title=Periodic Vesicle Formation in Tectonic Fault Zones—an Ideal Scenario for Molecular Evolution |journal=Origins of Life and Evolution of Biospheres |volume=45 |issue=1 |pages=139–148 |doi=10.1007/s11084-015-9411-z |pmc=4457167 |pmid=25716918|bibcode=2015OLEB...45..139M}}</ref> Under the same conditions, random [[peptide]] chains are formed and selected for their ability to integrate into the vesicle membrane. A further selection of the vesicles for stability potentially leads to functional peptide structures, increasing the survival rate of the vesicles.<ref>{{Cite journal |last1=Dávila |first1=María J. |last2=Mayer |first2=Christian |date=2022 |title=Membrane Structure Obtained in an Experimental Evolution Process |journal=Life |volume=12 |issue=2 |pages=145 |doi=10.3390/life12020145 |doi-access=free |pmid=35207433 |pmc=8875328 |bibcode=2022Life...12..145D}}</ref><ref>{{Cite journal |last=Mayer |first=Christian |date=2022 |title=Spontaneous Formation of Functional Structures in Messy Environments |journal=Life |volume=12 |issue=5 |pages=720 |doi=10.3390/life12050720 |doi-access=free |pmid=35629387 |pmc=9148140 |bibcode= 2022Life...12..720M}}</ref><ref>{{Cite journal |last1=Dávila |first1=María J. |last2=Mayer |first2=Christian |date=2023 |title=Structural Phenomena in a Vesicle Membrane Obtained through an Evolution Experiment: A Study Based on MD Simulations |journal=Life |volume=13 |issue=8 |pages=1735 |doi=10.3390/life13081735 |doi-access=free |pmid=37629592 |pmc=10455627 |bibcode=2023Life...13.1735D}}</ref>

== Producing biology ==

=== Energy and entropy ===

{{further|Entropy}}

Life requires a loss of entropy, or disorder, as molecules organize themselves into living matter. At the same time, the emergence of life is associated with the formation of structures beyond a certain threshold of [[complexity]].<ref>{{Cite journal |last=Mayer |first=Christian |date=2020-01-18 |title=Life in The Context of Order and Complexity |journal=Life |volume=10 |issue=1 |pages=5 |doi=10.3390/life10010005 |doi-access=free |pmc=7175320 |pmid=31963637|bibcode=2020Life...10....5M}}</ref> The emergence of life with increasing order and complexity does not contradict the second law of thermodynamics, which states that overall entropy never decreases, since a living organism creates order in some places (e.g. its living body) at the expense of an increase of entropy elsewhere (e.g. heat and waste production).<ref name="AP-2018">{{cite book |last1=Sharov |first1=Alexei A. |last2=Gordon |first2=Richard |title=Habitability of the Universe Before Earth: Life Before Earth |chapter=Life Before Earth |series=Astrobiology Exploring Life on Earth and Beyond |chapter-url=https://www.sciencedirect.com/science/article/pii/B9780128119402000113 |date=2018 |publisher=[[Academic Press]] |pages=265–296 |doi=10.1016/B978-0-12-811940-2.00011-3 |isbn=978-0-12-811940-2 |s2cid=117048600 |access-date=30 April 2022 |archive-date=30 April 2022 |archive-url=https://web.archive.org/web/20220430161329/https://www.sciencedirect.com/science/article/pii/B9780128119402000113 |url-status=live}}</ref><ref>{{cite journal |last1=Ladyman |first1=J. |last2=Lambert |first2=J. |last3=Weisner |first3=K. B. |title=What is a Complex System? |journal=European Journal of the Philosophy of Science |year=2013 |volume=3 |pages=33–67 |doi=10.1007/s13194-012-0056-8 |s2cid=18787276}}</ref><ref>{{cite journal |last1=Esposito |first1=M. |last2=Lindenberg |first2=Katja |author2-link=Katja Lindenberg |last3=Van den Broeck |first3=C. |year=2010 |title=Entropy production as correlation between system and reservoir |journal=[[New Journal of Physics]] |volume=12 |issue=1 |page=013013 |doi=10.1088/1367-2630/12/1/013013 |arxiv=0908.1125 |bibcode=2010NJPh...12a3013E |s2cid=26657293}}</ref>

Multiple sources of energy were available for chemical reactions on the early Earth. Heat from [[geothermal energy|geothermal]] processes is a standard energy source for chemistry. Other examples include sunlight, lightning,<ref name="Follmann2009" /> atmospheric entries of micro-meteorites,<ref name="Bar-Nun Bar-Nun Bauer Sagan 1970">{{cite journal |last1=Bar-Nun |first1=A. |last2=Bar-Nun |first2=N. |last3=Bauer |first3=S. H. |last4=Sagan |first4=Carl |author4-link=Carl Sagan |title=Shock Synthesis of Amino Acids in Simulated Primitive Environments |journal=Science |volume=168 |issue=3930 |date=24 April 1970 |doi=10.1126/science.168.3930.470 |pages=470–473 |pmid=5436082 |bibcode=1970Sci...168..470B |s2cid=42467812}}</ref> and implosion of bubbles in sea and ocean waves.<ref name="Anbar 1968">{{cite journal |last=Anbar |first=Michael |title=Cavitation during Impact of Liquid Water on Water: Geochemical Implications |journal=[[Science (journal)|Science]] |volume=161 |issue=3848 |date=27 September 1968 |doi=10.1126/science.161.3848.1343 |pages=1343–1344 |pmid=17831346 |bibcode=1968Sci...161.1343A}}</ref> This has been confirmed by experiments<ref name="Dharmarathne Grieser 2016">{{cite journal |last1=Dharmarathne |first1=Leena |last2=Grieser |first2=Franz |title=Formation of Amino Acids on the Sonolysis of Aqueous Solutions Containing Acetic Acid, Methane, or Carbon Dioxide, in the Presence of Nitrogen Gas |journal=[[The Journal of Physical Chemistry A]] |volume=120 |issue=2 |date=7 January 2016 |doi=10.1021/acs.jpca.5b11858 |pages=191–199 |pmid=26695890 |bibcode=2016JPCA..120..191D}}</ref><ref name="Patehebieke Zhao Wang Xu 2021">{{cite journal |last1=Patehebieke |first1=Yeersen |last2=Zhao |first2=Ze-Run |last3=Wang |first3=Su |last4=Xu |first4=Hao-Xing |last5=Chen |first5=Qian-Qian |last6=Wang |first6=Xiao |title=Cavitation as a plausible driving force for the prebiotic formation of N9 purine nucleosides |journal=Cell Reports Physical Science |volume=2 |issue=3 |year=2021 |doi=10.1016/j.xcrp.2021.100375 |page=100375 |bibcode=2021CRPS....200375P |s2cid=233662126|doi-access=free}}</ref> and simulations.<ref name="Kalson Furman Zeiri 2017">{{cite journal |last1=Kalson |first1=Natan-Haim |last2=Furman |first2=David |last3=Zeiri |first3=Yehuda |title=Cavitation-Induced Synthesis of Biogenic Molecules on Primordial Earth |journal=[[ACS Central Science]] |volume=3 |issue=9 |date=11 September 2017 |doi=10.1021/acscentsci.7b00325 |pages=1041–1049 |pmid=28979946 |pmc=5620973 |s2cid=21409351}}</ref>
Unfavorable reactions can be driven by highly favorable ones, as in the case of iron-sulfur chemistry. For example, this was probably important for [[carbon fixation]].{{efn|The reactions are:
:FeS + H<sub>2</sub>S → FeS<sub>2</sub> + 2H<sup>+</sup> + 2e<sup>−</sup>
:FeS + H<sub>2</sub>S + CO<sub>2</sub> → FeS<sub>2</sub> + HCOOH}} Carbon fixation by reaction of CO<sub>2</sub> with H<sub>2</sub>S via iron-sulfur chemistry is favorable, and occurs at neutral pH and 100&nbsp;°C. Iron-sulfur surfaces, which are abundant near hydrothermal vents, can drive the production of small amounts of amino acids and other biomolecules.<ref name="Follmann2009" />

=== Chemiosmosis ===

{{Further|Chemiosmosis}}
[[File:ATP-Synthase.svg|thumb|upright|[[ATP synthase]] uses the chemiosmotic proton gradient to power ATP synthesis through [[oxidative phosphorylation]].]]
In 1961, [[Peter D. Mitchell|Peter Mitchell]] proposed [[chemiosmosis]] as a cell's primary system of energy conversion. The mechanism, now ubiquitous in living cells, powers energy conversion in micro-organisms and in the [[mitochondria]] of eukaryotes, making it a likely candidate for early life.<ref>{{cite journal |last=Muller |first=Anthonie W. J. |year=1995 |title=Were the first organisms heat engines? A new model for biogenesis and the early evolution of biological energy conversion |journal=[[Progress in Biophysics and Molecular Biology]] |volume=63 |issue=2 |pages=193–231 |doi=10.1016/0079-6107(95)00004-7 |pmid=7542789 |doi-access=free}}</ref><ref>{{cite journal |last1=Muller |first1=Anthonie W. J. |first2=Dirk |last2=Schulze-Makuch |author-link2=Dirk Schulze-Makuch |year=2006 |title=Thermal energy and the origin of life |journal=[[Origins of Life and Evolution of Biospheres]] |volume=36 |issue=2 |pages=77–189 |bibcode=2006OLEB...36..177M |doi=10.1007/s11084-005-9003-4 |pmid=16642267 |s2cid=22179552}}</ref> Mitochondria produce [[adenosine triphosphate]] (ATP), the energy currency of the cell used to drive cellular processes such as chemical syntheses. The mechanism of ATP synthesis involves a closed membrane in which the [[ATP synthase]] enzyme is embedded. The energy required to release strongly bound ATP has its origin in [[proton]]s that move across the membrane.<ref name="Junge Nelson 2015">{{cite journal |last1=Junge |first1=Wolfgang |last2=Nelson |first2=Nathan |title=ATP Synthase |journal=[[Annual Review of Biochemistry]] |volume=84 |issue=1 |date=2 June 2015 |doi=10.1146/annurev-biochem-060614-034124 |pages=631–657 |pmid=25839341|doi-access=free}}</ref> In modern cells, those proton movements are caused by the pumping of ions across the membrane, maintaining an electrochemical gradient. In the first organisms, the gradient could have been provided by the difference in chemical composition between the flow from a hydrothermal vent and the surrounding seawater,<ref name="Lane 2015">{{cite book |last=Lane |first=Nick |author-link=Nick Lane |year=2015 |title=[[The Vital Question |The Vital Question: Why Is Life The Way It Is?]] |publisher=[[Profile Books]] |isbn=978-1-78125-036-5 |pages=129–140}}</ref> or perhaps meteoric quinones that were conducive to the development of chemiosmotic energy across lipid membranes if at a terrestrial origin.<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 |pmc=7133448 |pmid=31841362 |bibcode=2020AsBio..20..429D}}</ref>

[[File:Chemiosmotic coupling mitochondrion.svg|thumb|left|upright=2|[[Chemiosmosis|Chemiosmotic]] coupling in the membranes of a [[mitochondrion]]]]
{{clear}}

=== PAH world hypothesis ===

{{main|PAH world hypothesis}}

The [[PAH world hypothesis]] is a speculative [[hypothesis]] that proposes that [[polycyclic aromatic hydrocarbon]]s (PAHs), known to be abundant in the [[universe]],<ref name="SP-20051018" /><ref name="AJ-20051010" /><ref>{{cite web |last=Allamandola |first=Louis |title=Cosmic Distribution of Chemical Complexity |url=http://amesteam.arc.nasa.gov/Research/cosmic.html |date=April 13, 2011 |work=[[NASA]] |access-date=March 3, 2014 |display-authors=etal |url-status=dead |archive-url=https://web.archive.org/web/20140227184503/http://amesteam.arc.nasa.gov/Research/cosmic.html |archive-date=February 27, 2014}}</ref> including in comets,<ref name="NASA-20150210">{{cite web |last=Clavin |first=Whitney |title=Why Comets Are Like Deep Fried Ice Cream |url=http://www.jpl.nasa.gov/news/news.php?feature=4480 |date= February 10, 2015 |work=[[NASA]] |access-date=February 10, 2015}}</ref> and assumed to be abundant in the [[Primordial soup#"Primordial soup" theory|primordial soup]] of the early [[Earth]], played a major role in the [[origin of life]] by mediating the synthesis of [[RNA]] molecules, leading into the [[RNA world]]. However, as yet, the hypothesis is untested.<ref>Platts, Simon Nicholas, [http://www.pahworld.com/ "The PAH World - Discotic polynuclear aromatic compounds as a mesophase scaffolding at the origin of life"]</ref><ref>[http://astrobiology.nasa.gov/nai/library-of-resources/annual-reports/2005/ciw/projects/project-4-prebiotic-molecular-selection-and-organization "Prebiotic Molecular Selection and Organization"] {{webarchive|url=https://web.archive.org/web/20090524050536/http://astrobiology.nasa.gov/nai/library-of-resources/annual-reports/2005/ciw/projects/project-4-prebiotic-molecular-selection-and-organization/ |date=2009-05-24}}, NASA's Astrobiology website</ref>

=== The RNA world ===

{{main|RNA world}}

[[File:Etls-2019-0024c.01.png|thumb|center|upright=2.2|The RNA world hypothesis proposes that undirected polymerisation led to the emergence of [[ribozyme]]s, and in turn to an [[RNA-dependent RNA polymerase|RNA replicase]].]]

The [[RNA world]] hypothesis describes an early Earth with self-replicating and catalytic RNA but no DNA or proteins.<ref>{{cite journal |last1=Benner |first1=S. A. |last2 = Bell |first2=E. A. |last3=Biondi |first3=E. |last4=Brasser |first4=R. |last5=Carell |first5=T. |last6=Kim |first6=H.-J. |last7=Mojzsis |first7=S. J. |last8=Omran |first8 = A. |last9=Pasek |first9=M. A. |last10=Trail |first10=D. |date=2020 |title=When Did Life Likely Emerge on Earth in an RNA-First Process? |journal=ChemSystemsChem |volume= 2 |issue=2 |doi=10.1002/syst.201900035 |doi-access=free|arxiv=1908.11327 |bibcode=2020CSysC...2...35B}}</ref> Many researchers concur that an RNA world must have preceded modern DNA-based life.<ref>{{cite journal |last1=Copley |first1=Shelley D. |last2=Smith |first2=Eric |last3=Morowitz |first3=Harold J. |author-link3= Harold J. Morowitz |date=December 2007 |title=The origin of the RNA world: Co-evolution of genes and metabolism |url=http://tuvalu.santafe.edu/~desmith/PDF_pubs/Copley_BOG.pdf |url-status=live |journal=Bioorganic Chemistry |volume=35 |issue=6 |pages=430–443 |doi=10.1016/j.bioorg.2007.08.001 |pmid=17897696 |archive-url=https://web.archive.org/web/20130905070129/http://tuvalu.santafe.edu/~desmith/PDF_pubs/Copley_BOG.pdf |archive-date=5 September 2013 |access-date=8 June 2015 |quote=The proposal that life on Earth arose from an RNA world is widely accepted. |ref=none}}</ref><ref>{{cite journal |last=Orgel |first=Leslie E. |author-link=Leslie Orgel |date=April 2003 |title = Some consequences of the RNA world hypothesis |journal=[[Origins of Life and Evolution of Biospheres]] |volume=33 |issue=2 |pages=211–218|ref=none |s2cid=32779859 |bibcode= 2003OLEB...33..211O |doi=10.1023/A:1024616317965 |pmid=12967268 |quote=It now seems very likely that our familiar DNA/RNA/protein world was preceded by an RNA world...  }}</ref><ref>{{harvnb|Robertson|Joyce|2012}}: "There is now strong evidence indicating that an RNA World did indeed exist before DNA- and protein-based life."</ref><ref>{{harvnb|Neveu|Kim|Benner|2013}}: "[The RNA world's existence] has broad support within the community today."</ref> However, RNA-based life may not have been the first to exist.<ref name="Robertson2012">{{cite journal |last1=Robertson |first1=Michael P. |last2=Joyce |first2=Gerald F. |author-link2=Gerald Joyce |date=May 2012 |title=The origins of the RNA world |journal=[[Cold Spring Harbor Perspectives in Biology]] |volume=4 |issue=5 |page=a003608 |doi=10.1101/cshperspect.a003608 |pmc=3331698 |pmid= 20739415}}</ref><ref name="Cech2012">{{cite journal |last=Cech |first=Thomas R. |author-link=Thomas Cech |date=July 2012 |title=The RNA Worlds in Context |journal= [[Cold Spring Harbor Perspectives in Biology]] |volume=4 |issue=7 |page=a006742 |doi=10.1101/cshperspect.a006742 |pmc=3385955 |pmid=21441585}}</ref> Another model echoes Darwin's "warm little pond" with cycles of wetting and drying.<ref>{{cite journal |last1=Pearce |first1=Ben K. D. |last2=Pudritz |first2=Ralph E. |last3=Semenov |first3= Dmitry A. |last4=Henning |first4=Thomas K. |date=24 October 2017 |title=Origin of the RNA world: The fate of nucleobases in warm little ponds |journal=[[PNAS]] |volume=114 |issue = 43 |pages=11327–11332 |doi=10.1073/pnas.1710339114 |pmid=28973920 |pmc=5664528 |arxiv=1710.00434 |bibcode=2017PNAS..11411327P |doi-access=free}}</ref> Some have proposed a timeline of more than 30 potential significant chemical events between pre-RNA world to near but before LUCA, just involving RNA. The timeline does not include metabolism-related events such as origins of ATP, glycolysis, the Krebs cycle, and the electron transport chain.<ref>{{cite journal |last1=Fine |first1=Jacob L. |last2=Pearlman |first2=Ronald E. |title=On the origin of life: an RNA-focused synthesis and narrative |journal=RNA |date=August 2023 |volume=29 |issue=8 |pages=1092–1094 |doi=10.1261/rna.079598.123|pmid=37142437 |pmc=10351881}}</ref>

RNA is central to the translation process. Small RNAs can catalyze all the chemical groups and information transfers required for life.<ref name="Cech2012" /><ref>{{cite journal |last=Yarus |first=Michael |date=April 2011 |title=Getting Past the RNA World: The Initial Darwinian Ancestor |journal=Cold Spring Harbor Perspectives in Biology |volume=3 |issue=4 |page=a003590 |doi=10.1101/cshperspect.a003590 |pmc=3062219 |pmid=20719875}}</ref> RNA both expresses and maintains genetic information in modern organisms; and the chemical components of RNA are easily synthesized under conditions that approximate the early Earth. The structure of the [[ribosome]] has been called the "smoking gun", with a central core of RNA and no amino acid side chains within 18 [[Angstrom|Å]] of the [[active site]] that catalyzes peptide bond formation.<ref>{{harvnb|Voet|Voet|2004|p=29}}</ref><ref name="Robertson2012" /><ref>{{cite journal |last1=Fox |first1=George.E. |date=9 June 2010 |title=Origin and evolution of the ribosome |journal=[[Cold Spring Harbor Perspectives in Biology]] |volume=2 |issue=9(a003483) |page=a003483 |doi=10.1101/cshperspect.a003483 |pmc=2926754 |pmid=20534711 |doi-access=free}}</ref>

The concept of the RNA world was proposed in 1962 by [[Alexander Rich]],<ref>{{cite journal |last1=Neveu |first1=Marc |last2=Kim |first2=Hyo-Joong |last3=Benner |first3=Steven A. |date=22 April 2013 |title=The 'Strong' RNA World Hypothesis: Fifty Years Old |journal=[[Astrobiology (journal)|Astrobiology]] |volume=13 |issue=4 |pages=391–403 |bibcode=2013AsBio..13..391N |doi=10.1089/ast.2012.0868 |pmid=23551238}}</ref> and the term was coined by [[Walter Gilbert]] in 1986.<ref name="Cech2012" /><ref>{{cite journal |last=Gilbert |first=Walter |author-link=Walter Gilbert |date=20 February 1986 |title=Origin of life: The RNA world |journal=Nature |volume=319 |issue=6055 |page=618 |bibcode=1986Natur.319..618G |doi=10.1038/319618a0 |doi-access=free |s2cid=8026658}}</ref> Initially it was hard to explain abiotic synthesis of the nucleotides cytosine and uracil.<ref>{{cite journal |last=Orgel |first=Leslie E. |date=October 1994 |title=The origin of life on Earth |journal=Scientific American |volume=271 |issue=4 |pages=76–83 |bibcode=1994SciAm.271d..76O |doi=10.1038/scientificamerican1094-76 |pmid=7524147}}</ref> Subsequent research has shown possible routes of synthesis; for example, formamide produces all four [[ribonucleotide]]s and other biological molecules when warmed in the presence of terrestrial minerals.<ref name="Saladino2012" /><ref name="Saladino2012b" />

[[RNA-dependent RNA polymerase|RNA replicase]] can function as both code and catalyst for further RNA replication, i.e. it can be autocatalytic. [[Jack Szostak]] has shown that certain catalytic RNAs can link smaller RNA sequences together, enabling self-replication. RNA replication systems, which include two ribozymes that catalyze each other's synthesis, had a doubling time of about one hour, and were subject to natural selection.<ref>{{cite journal |last1=Lincoln |first1=Tracey A. |last2=Joyce |first2=Gerald F. |date=27 February 2009 |title=Self-Sustained Replication of an RNA Enzyme |journal=[[Science (journal)|Science]] |volume=323 |issue=5918 |pages=1229–1232 |bibcode=2009Sci...323.1229L |doi=10.1126/science.1167856 |pmc=2652413 |pmid=19131595}}</ref><ref>{{cite journal |last=Joyce |first=Gerald F. |year=2009 |title=Evolution in an RNA world |journal=Cold Spring Harbor Perspectives in Biology |volume=74 |issue=Evolution: The Molecular Landscape |pages=17–23 |doi=10.1101/sqb.2009.74.004 |pmc=2891321 |pmid=19667013}}</ref><ref name="Robertson2012" /> If such conditions were present on early Earth, then natural selection would favor the proliferation of such [[autocatalytic set]]s, to which further functionalities could be added.<ref>{{cite web |last=Szostak |first=Jack W. |author-link=Jack W. Szostak |date=5 February 2015 |title=The Origins of Function in Biological Nucleic Acids, Proteins, and Membranes |url=http://www.hhmi.org/research/origins-cellular-life |url-status=live |archive-url=https://web.archive.org/web/20150714092225/http://www.hhmi.org/research/origins-cellular-life |archive-date=14 July 2015 |access-date=16 June 2015 |publisher=[[Howard Hughes Medical Institute]] |location=Chevy Chase, Maryland}}</ref><ref>{{cite journal |last1=Bernstein |first1=Harris |last2=Byerly |first2=Henry C. |last3=Hopf |first3=Frederick A. |last4=Michod |first4=Richard A. |last5=Vemulapalli |first5=G. Krishna |display-authors=3 |date=June 1983 |title=The Darwinian Dynamic |journal=[[The Quarterly Review of Biology]] |volume=58 |issue=2 |pages=185–207 |doi=10.1086/413216 |jstor=2828805 |s2cid=83956410}}</ref><ref>{{harvnb|Michod|1999}}</ref> Self-assembly of RNA may occur spontaneously in hydrothermal vents.<ref>{{cite arXiv |eprint=1305.5581v1 |class=q-bio.BM |first=Stan |last=Palasek |title=Primordial RNA Replication and Applications in PCR Technology |date=23 May 2013}}</ref><ref>{{cite journal |last1=Vlassov |first1=Alexander V. |last2=Kazakov |first2=Sergei A. |last3=Johnston |first3=Brian H. |last4=Landweber |first4=Laura F. |display-authors=3 |date=August 2005 |title=The RNA World on Ice: A New Scenario for the Emergence of RNA Information |journal=[[Journal of Molecular Evolution]] |volume=61 |issue=2 |pages=264–273 |bibcode=2005JMolE..61..264V |doi=10.1007/s00239-004-0362-7 |pmid=16044244 |s2cid=21096886}}</ref><ref>{{cite journal |last1=Nussinov |first1=Mark D. |last2=Otroshchenko |first2=Vladimir A. |last3=Santoli |first3=Salvatore |year=1997 |title=The emergence of the non-cellular phase of life on the fine-grained clayish particles of the early Earth's regolith |journal=[[BioSystems]] |volume=42 |issue=2–3 |pages=111–118 |doi=10.1016/S0303-2647(96)01699-1 |pmid=9184757|bibcode=1997BiSys..42..111N}}</ref> A preliminary form of tRNA could have assembled into such a replicator molecule.<ref>{{cite journal |last1=Kühnlein |first1=Alexandra |last2=Lanzmich |first2=Simon A. |last3=Brun |first3=Dieter |date=2 March 2021 |title=tRNA sequences can assemble into a replicator |journal=[[eLife]] |volume=10 |doi=10.7554/eLife.63431 |pmc=7924937 |pmid=33648631 |doi-access=free}}</ref>  When such an [[RNA]] molecule began to replicate, it may it may have been capable of the three mechanisms of Darwinian selection: [[heritability]], variation of type, and differential reproductive output. The fitness of such an RNA replicator (its per capita rate of increase) would likely have been a function of its intrinsic adaptive capabilities determined by its [[nucleic acid sequence|nucleotide sequence]], and the availability of resources.<ref name = Bernstein1983>Bernstein H, Byerly HC, Hopf FA, Michod RA, Vemulapalli GK. (1983) The Darwinian Dynamic. Quarterly Review of Biology 56, 185-187. JSTOR 2828805.</ref><ref name = Michod2006>Michod, R. E. (2006). Darwininian dynamics: evolutionary transitions in fitness and individuality. Princeton University Press.</ref>

Possible precursors to protein synthesis include the synthesis of short peptide cofactors or the self-catalysing duplication of RNA. It is likely that the ancestral ribosome was composed entirely of RNA, although some roles have since been taken over by proteins. Major remaining questions on this topic include identifying the selective force for the evolution of the ribosome and determining how the genetic code arose.<ref name="Noller2012">{{cite journal |last=Noller |first=Harry F. |author-link=Harry F. Noller |date=April 2012 |title=Evolution of protein synthesis from an RNA world. |journal=[[Cold Spring Harbor Perspectives in Biology]] |volume=4 |issue=4 |page=a003681 |doi=10.1101/cshperspect.a003681 |pmc=3312679 |pmid=20610545|bibcode=2012CSHPB...4.3681N}}</ref>

[[Eugene Koonin]] has argued that "no compelling scenarios currently exist for the origin of replication and translation, the key processes that together comprise the core of biological systems and the apparent pre-requisite of biological evolution. The RNA World concept might offer the best chance for the resolution of this conundrum but so far cannot adequately account for the emergence of an efficient RNA replicase or the translation system."<ref name="pmc1892545">{{cite journal |last=Koonin |first=Eugene V. |author-link=Eugene Koonin |date=31 May 2007 |title=The cosmological model of eternal inflation and the transition from chance to biological evolution in the history of life |journal=[[Biology Direct]] |volume=2 |page=15 |doi=10.1186/1745-6150-2-15 |pmc=1892545 |pmid=17540027 |doi-access=free}}</ref>

=== From RNA to directed protein synthesis ===
In line with the RNA world hypothesis, much of modern biology's templated protein biosynthesis is done by RNA molecules—namely tRNAs and the ribosome (consisting of both protein and rRNA components).  The most central reaction of peptide bond synthesis is understood to be carried out by base catalysis by the 23S rRNA domain V.<ref name=":1">{{Cite journal |last1=Tamura |first1=K. |last2=Alexander |first2=R. W. |date=2004-05-01 |title=Peptide synthesis through evolution |journal=Cellular and Molecular Life Sciences |volume=61 |issue=11 |pages=1317–1330 |doi=10.1007/s00018-004-3449-9 |pmid=15170510 |pmc=11138682}}</ref> Experimental evidence has demonstrated successful di- and tripeptide synthesis with a system consisting of only aminoacyl phosphate adaptors and RNA guides, which could be a possible stepping stone between an RNA world and modern protein synthesis.<ref name=":1" /><ref>{{Cite journal |last1=Tamura |first1=Koji |last2=Schimmel |first2=Paul |date=2003-07-22 |title=Peptide synthesis with a template-like RNA guide and aminoacyl phosphate adaptors |journal=Proceedings of the National Academy of Sciences |volume=100 |issue=15 |pages=8666–8669 |doi=10.1073/pnas.1432909100 |doi-access=free |pmc=166369 |pmid=12857953|bibcode=2003PNAS..100.8666T}}</ref> Aminoacylation ribozymes that can charge tRNAs with their cognate amino acids have also been selected in in vitro experimentation.<ref name=":2">{{Cite journal |last1=Pressman |first1=Abe D. |last2=Liu |first2=Ziwei |last3=Janzen |first3=Evan |last4=Blanco |first4=Celia |last5=Müller |first5=Ulrich F. |last6=Joyce |first6=Gerald F. |last7=Pascal |first7=Robert |last8=Chen |first8=Irene A. |date=2019-04-17 |title=Mapping a Systematic Ribozyme Fitness Landscape Reveals a Frustrated Evolutionary Network for Self-Aminoacylating RNA |journal=Journal of the American Chemical Society |volume=141 |issue=15 |pages=6213–6223 |doi=10.1021/jacs.8b13298 |pmc=6548421 |pmid=30912655|bibcode=2019JAChS.141.6213P}}</ref> The authors also extensively mapped fitness landscapes within their selection to find that chance emergence of active sequences was more important that sequence optimization.<ref name=":2" />

=== Early functional peptides ===
The first proteins would have had to arise without a fully-fledged system of protein biosynthesis. As discussed above, numerous mechanisms for the prebiotic synthesis of polypeptides exist. However, these random sequence peptides would not have likely had biological function. Thus, significant study has gone into exploring how early functional proteins could have arisen from random sequences. First, some evidence on hydrolysis rates shows that abiotically plausible peptides likely contained significant "nearest-neighbor" biases.<ref>{{Cite journal |last1=Tyagi |first1=Sanjay |last2=Ponnamperuma |first2=Cyril |date=1990-05-01 |title=Nonrandomness in prebiotic peptide synthesis |journal=Journal of Molecular Evolution |volume=30 |issue=5 |pages=391–399 |doi=10.1007/BF02101111 |pmid=2111852 |bibcode=1990JMolE..30..391T}}</ref> This could have had some effect on early protein sequence diversity. In other work by Anthony Keefe and Jack Szostak, [[mRNA display]] selection on a library of {{val|6e12}} 80-mers was used to search for sequences with ATP binding activity. They concluded that approximately 1 in {{val|e=11}} random sequences had ATP binding function.<ref>{{Cite journal |title=Functional proteins from a random-sequence library |doi=10.1038/35070613 |pmc=4476321 |pmid=11287961 | date=2001 | last1=Keefe | first1=Anthony D. | last2=Szostak | first2=Jack W. | journal=Nature | volume=410 | issue=6829 | pages=715–718 |bibcode=2001Natur.410..715K}}</ref> While this is a single example of functional frequency in the random sequence space, the methodology can serve as a powerful simulation tool for understanding early protein evolution.<ref>{{Cite journal |last1=Tong |first1=Cher Ling |last2=Lee |first2=Kun-Hwa |last3=Seelig |first3=Burckhard |date=June 2021 |title=De novo proteins from random sequences through in vitro evolution |journal=Current Opinion in Structural Biology |volume=68 |pages=129–134 |doi=10.1016/j.sbi.2020.12.014 |pmc=8222087 |pmid=33517151|bibcode=2021COStB..68..129T}}</ref>

=== Phylogeny and LUCA ===

{{further|Last universal common ancestor}}

Starting with the work of [[Carl Woese]] from 1977, [[genomics]] studies have placed the last universal common ancestor (LUCA) of all modern life-forms between Bacteria and a clade formed by Archaea and [[Eukaryota]] in the phylogenetic tree of life. It lived over 4&nbsp;Gya.<ref>{{cite book |url=https://www.springer.com/life+sciences/microbiology/book/978-0-387-98771-2 |title=The ''Archaea'' and the Deeply Branching and Phototrophic ''Bacteria'' |publisher=[[Springer Science+Business Media|Springer]] |year=2001 |isbn=978-0-387-21609-6 |editor1-last=Boone |editor1-first=David R. |series=Bergey's Manual of Systematic Bacteriology |editor2-last=Castenholz |editor2-first=Richard W. |editor3-last=Garrity |editor3-first=George M. |archive-url=https://web.archive.org/web/20141225112809/http://www.springer.com/life+sciences/microbiology/book/978-0-387-98771-2 |archive-date=25 December 2014 |url-status=live}}{{page needed |date=June 2014}}</ref><ref name="Woese Fox 1977">{{cite journal |last1=Woese |first1=C. R. |last2=Fox |first2=G. E. |year=1977 |title=Phylogenetic structure of the prokaryotic domain: the primary kingdoms. |journal=[[PNAS]] |volume=7 |issue=11 |pages=5088–5090 |bibcode=1977PNAS...74.5088W |doi=10.1073/pnas.74.11.5088 |pmc=432104 |pmid=270744 |doi-access=free}}</ref> A minority of studies have placed the LUCA in Bacteria, proposing that Archaea and Eukaryota are evolutionarily derived from within Eubacteria;<ref>{{cite journal |last1=Valas |first1=R. E. |last2=Bourne |first2=P. E. |year=2011 |title=The origin of a derived superkingdom: how a gram-positive bacterium crossed the desert to become an archaeon |journal=[[Biology Direct]] |volume=6 |page=16 |doi=10.1186/1745-6150-6-16 |pmc=3056875 |pmid=21356104 |doi-access=free}}</ref> [[Thomas Cavalier-Smith]] suggested in 2006 that the phenotypically diverse bacterial phylum [[Chloroflexota]] contained the LUCA.<ref name="CS2">{{cite journal |last=Cavalier-Smith |first=Thomas |author-link=Thomas Cavalier-Smith |year=2006 |title=Rooting the tree of life by transition analyses |journal=[[Biology Direct]] |volume=1 |page=19 |doi=10.1186/1745-6150-1-19 |pmc=1586193 |pmid=16834776 |doi-access=free}}</ref>

<gallery mode="packed" heights="200px">
File:Phylogenetic tree of life LUCA.svg|[[Phylogenetic tree]] showing the [[last universal common ancestor]] (LUCA) at the root. The major clades are the [[Bacteria]] on one hand, and the [[Archaea]] and [[Eukaryota]] on the other.
</gallery>

In 2016, a set of 355 genes likely present in the LUCA was identified. A total of 6.1 million prokaryotic genes from Bacteria and Archaea were sequenced, identifying 355 protein clusters from among 286,514 protein clusters that were probably common to the LUCA. The results suggest that the LUCA was [[anaerobic organism|anaerobic]] with a Wood–Ljungdahl (reductive Acetyl-CoA) pathway, nitrogen- and carbon-fixing, thermophilic. Its [[cofactor (biochemistry)|cofactors]] suggest dependence upon an environment rich in hydrogen, carbon dioxide, iron, and [[transition metal]]s. Its genetic material was probably DNA, requiring the 4-nucleotide genetic code, messenger RNA, transfer RNA, and ribosomes to translate the code into proteins such as enzymes. LUCA likely inhabited an anaerobic hydrothermal vent setting in a geochemically active environment. It was evidently already a complex organism, and must have had precursors; it was not the first living thing.<ref name="Weiss Sousa Mrnjavac 2016">{{cite journal |last1=Weiss |first1=M. C. |last2=Sousa |first2=F. L. |last3=Mrnjavac |first3=N. |last4=Neukirchen |first4=S. |last5=Roettger |first5=M. |last6=Nelson-Sathi |first6=S. |last7=Martin |first7=W.F. |year=2016 |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]] |volume=1 |issue=9 |page=16116 |doi=10.1038/NMICROBIOL.2016.116 |pmid=27562259 |s2cid=2997255 |access-date=21 September 2022 |archive-date=29 January 2023 |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 name="Nature 2016">{{cite journal |title=Early life liked it hot |journal=Nature |volume=535 |issue=7613 |year=2016 |doi=10.1038/535468b |page=468 |s2cid=49905802|doi-access=free}}</ref> The physiology of LUCA has been in dispute.<ref>{{cite journal |last1=Gogarten |first1=Johann Peter |last2=Deamer |first2=David |author2-link=David W. Deamer |date=2016-11-25 |title=Is LUCA a thermophilic progenote? |url=https://zenodo.org/record/895471 |journal=Nature Microbiology |volume=1 |issue=12 |page=16229 |doi=10.1038/nmicrobiol.2016.229 |pmid=27886195 |s2cid=205428194 |access-date=21 September 2022 |archive-date=3 April 2020 |archive-url=https://web.archive.org/web/20200403040656/https://zenodo.org/record/895471 |url-status=live}}</ref><ref>{{cite journal |last1=Catchpole |first1=Ryan |last2=Forterre |first2=Patrick |date=2019 |title=The evolution of Reverse Gyrase suggests a non-hyperthermophilic Last Universal Common Ancestor |url=https://academic.oup.com/mbe/article/36/12/2737/5545984 |journal=Molecular Biology and Evolution |volume=36 |issue=12 |pages=2737–2747 |doi=10.1093/molbev/msz180 |pmid=31504731 |pmc=6878951 |access-date=18 September 2022 |archive-date=27 January 2023 |archive-url=https://web.archive.org/web/20230127162358/https://academic.oup.com/mbe/article/36/12/2737/5545984 |url-status=live}}</ref><ref>{{cite journal |last1=Berkemer |first1=Sarah J. |last2=McGlynn |first2=Shawn E |date=August 8, 2020 |title=A New Analysis of Archaea–Bacteria Domain Separation: Variable Phylogenetic Distance and the Tempo of Early Evolution |url=https://academic.oup.com/mbe/article/37/8/2332/5818498 |journal=Molecular Biology and Evolution |volume=37 |issue=8 |pages=2332–2340 |doi=10.1093/molbev/msaa089 |pmc=7403611 |pmid=32316034 |access-date=21 September 2022 |archive-date=27 January 2023 |archive-url=https://web.archive.org/web/20230127163913/https://academic.oup.com/mbe/article/37/8/2332/5818498 |url-status=live}}</ref> Previous research identified 60 proteins common to all life.<ref>{{cite journal | url=https://pubmed.ncbi.nlm.nih.gov/15035042/ | pmid=15035042 | date=2003 | last1=Koonin | first1=E. V. | title=Comparative genomics, minimal gene-sets and the last universal common ancestor | journal=Nature Reviews. Microbiology | volume=1 | issue=2 | pages=127–136 | doi=10.1038/nrmicro751}}</ref>

<gallery mode="packed" heights="300px">
File:LUCA systems and environment.svg|LUCA systems and environment included the [[Wood–Ljungdahl pathway]].<ref name="Weiss Sousa Mrnjavac 2016" />
</gallery>

Leslie Orgel argued that early translation machinery for the genetic code would be susceptible to [[error catastrophe]]. Geoffrey Hoffmann however showed that such machinery can be stable in function against "Orgel's paradox".<ref>{{cite journal |last=Hoffmann |first=Geoffrey W. |author-link=Geoffrey W. Hoffmann |date=25 June 1974 |title=On the origin of the genetic code and the stability of the translation apparatus |journal=[[Journal of Molecular Biology]] |volume=86 |issue=2 |pages=349–362 |doi=10.1016/0022-2836(74)90024-2 |pmid=4414916}}</ref><ref>{{cite journal |last=Orgel |first=Leslie E. |date=April 1963 |title=The Maintenance of the Accuracy of Protein Synthesis and its Relevance to Ageing |journal=[[PNAS]] |volume=49 |issue=4 |pages=517–521 |bibcode=1963PNAS...49..517O |doi=10.1073/pnas.49.4.517 |pmc=299893 |pmid=13940312 |doi-access=free}}</ref><ref>{{cite journal |last=Hoffmann |first=Geoffrey W. |date=October 1975 |title=The Stochastic Theory of the Origin of the Genetic Code |journal=[[Annual Review of Physical Chemistry]] |volume=26 |pages=123–144 |bibcode=1975ARPC...26..123H |doi=10.1146/annurev.pc.26.100175.001011}}</ref> Metabolic reactions that have also been inferred in LUCA are the incomplete [[reverse Krebs cycle]], [[gluconeogenesis]], the [[pentose phosphate pathway]], [[glycolysis]], [[reductive amination]], and [[transamination]].<ref>{{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 |volume=1863 |issue=8 |page=148597 |doi=10.1016/j.bbabio.2022.148597 |pmid=35868450 |s2cid=250707510 |doi-access=free}}</ref><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 |volume=9 |issue=1 |page=5176 |doi=10.1038/s41467-018-07220-y |pmid=30538225 |pmc=6289992 |bibcode=2018NatCo...9.5176H}}</ref>

== Suitable geological environments ==

{{further|Alternative abiogenesis scenarios}}

A variety of [[Alternative abiogenesis scenarios|geologic and environmental settings have been proposed]] for an origin of life. These theories are often in competition with one another as there are many views of prebiotic compound availability, geophysical setting, and early life characteristics. The first organism on Earth likely differed from [[Last universal common ancestor|LUCA]]. Between the first appearance of life and where all modern phylogenies began branching, an unknown amount of time passed, with unknown gene transfers, extinctions, and adaptation to environmental niches.<ref>{{Cite journal |last1=Cantine |first1=Marjorie D. |last2=Fournier |first2=Gregory P. |date=2018-03-01 |title=Environmental Adaptation from the Origin of Life to the Last Universal Common Ancestor |journal=Origins of Life and Evolution of Biospheres |volume=48 |issue=1 |pages=35–54 |doi=10.1007/s11084-017-9542-5 |pmid=28685374 |bibcode=2018OLEB...48...35C |hdl=1721.1/114219 |s2cid=254888920 |url=https://link.springer.com/article/10.1007/s11084-017-9542-5 |hdl-access=free |archive-date=31 January 2024 |archive-url=https://web.archive.org/web/20240131154929/https://link.springer.com/article/10.1007/s11084-017-9542-5 |url-status=live|url-access=subscription }}</ref> Modern phylogenies provide more genetic evidence about LUCA than about its precursors.<ref>{{Cite journal |last=Mat |first=Wai-Kin |date=May 1, 2008 |title=The genomics of LUCA |url=https://article.imrpress.com/bri/Landmark/articles/pdf/Landmark3103.pdf |journal=Frontiers in Bioscience |volume=13 |issue=14 |pages=5605–5613 |doi=10.2741/3103 |pmid=18508609 |access-date=8 December 2023 |archive-date=16 June 2022 |archive-url=https://web.archive.org/web/20220616034359/https://article.imrpress.com/bri/Landmark/articles/pdf/Landmark3103.pdf |url-status=live}}</ref>

=== Deep sea hydrothermal vents ===

==== Hot fluids ====

{{further|Hydrothermal vent}}

[[File:Champagne vent white smokers.jpg|thumb|upright=1.2|The [[earliest known life forms]] may be putative fossilized [[microorganism]]s, found in white smoker [[hydrothermal vent]] precipitates. They may have lived as early as 4.28&nbsp;Gya (billion years ago), relatively soon after the [[origin of water on Earth#History of water on Earth|formation of the oceans]] 4.41 Gya, not long after the [[Age of the Earth|formation of the Earth]] 4.54 Gya.<ref name="NAT-20170301" />]]

Early micro-fossils may have come from a hot world of gases such as methane, ammonia, carbon dioxide, and [[hydrogen sulfide]], toxic to much current life.<ref>{{cite book |last=Brasier |first=M. D. |year=2012 |title=Secret Chambers: The Inside Story of Cells and Complex Life |publisher=Oxford University Press |page=298}}</ref> Analysis of the [[tree of life (biology)|tree of life]] places thermophilic and hyperthermophilic bacteria and archaea closest to the root, suggesting that life may have evolved in a hot environment.<ref>Ward, Peter & [[Joseph Kirschvink|Kirschvink, Joe]], op cit, p. 42</ref> The deep sea or alkaline hydrothermal vent theory posits that life began at submarine hydrothermal vents.<ref name="Colín-García 2016">{{cite journal |last1=Colín-García |first1=M. |last2=Heredia |first2=A. |last3=Cordero |first3=G. |last4=Camprubí |first4=A. |last5=Negrón-Mendoza |first5=A. |last6=Ortega-Gutiérrez |first6=F. |last7=Berald |first7=H. |last8=Ramos-Bernal |first8=S. |display-authors=3 |year=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 |journal=Boletín de la Sociedad Geológica Mexicana |volume=68 |issue=3 |pages=599–620 |url-status=live |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 |archive-date=18 August 2017 |doi=10.18268/BSGM2016v68n3a13 |doi-access=free|bibcode=2016BoSGM..68..599C }}</ref><ref name="hydrothermal vents NASA 2014">{{cite web |url=https://astrobiology.nasa.gov/articles/2014/6/24/hydrothermal-vents-could-explain-chemical-precursors-to-life/ |title=Hydrothermal Vents Could Explain Chemical Precursors to Life |last=Schirber |first=Michael |date=24 June 2014 |website=NASA Astrobiology: Life in the Universe |publisher=NASA |access-date=19 June 2015  |archive-url=https://web.archive.org/web/20141129051724/http://astrobiology.nasa.gov/articles/2014/6/24/hydrothermal-vents-could-explain-chemical-precursors-to-life/ |archive-date=29 November 2014}}</ref> [[William F. Martin|William Martin]] and [[Michael Russell (scientist)|Michael Russell]] have suggested

{{blockquote|that life evolved in structured iron monosulphide precipitates in a seepage site hydrothermal mound at a redox, pH, and temperature gradient between sulphide-rich hydrothermal fluid and iron(II)-containing waters of the Hadean ocean floor. The naturally arising, three-dimensional compartmentation observed within fossilized seepage-site metal sulphide precipitates indicates that these inorganic compartments were the precursors of cell walls and membranes found in free-living prokaryotes. The known capability of FeS and NiS to catalyze the synthesis of the acetyl-methylsulphide from carbon monoxide and methylsulphide, constituents of hydrothermal fluid, indicates that pre-biotic syntheses occurred at the inner surfaces of these metal-sulphide-walled compartments.<ref name="Martin2003">{{cite journal |last1=Martin |first1=William |author-link1=William F. Martin |last2=Russell |first2=Michael J. |date=29 January 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=59–83; discussion 83–85 |doi=10.1098/rstb.2002.1183 |pmid=12594918 |pmc=1693102}}</ref>}}

These form where hydrogen-rich fluids emerge from below the sea floor, as a result of [[serpentinite|serpentinization]] of ultra-[[mafic]] [[olivine]] with seawater and a pH interface with carbon dioxide-rich ocean water. The vents form a sustained chemical energy source derived from redox reactions, in which electron donors (molecular hydrogen) react with electron acceptors (carbon dioxide); see [[iron–sulfur world theory]]. These are [[exothermic reaction]]s.<ref name="Colín-García 2016"/>{{efn|The reactions are:<br/>
'''Reaction 1''': Fayalite + water → magnetite + aqueous silica + hydrogen
:3Fe<sub>2</sub>SiO<sub>4</sub> + 2H<sub>2</sub>O → 2Fe<sub>3</sub>O<sub>4</sub> + 3SiO<sub>2</sub> + 2H<sub>2</sub>
'''Reaction 2''': Forsterite + aqueous silica → serpentine
:3Mg<sub>2</sub>SiO<sub>4</sub> + SiO<sub>2</sub> + 4H<sub>2</sub>O → 2Mg<sub>3</sub>Si<sub>2</sub>O<sub>5</sub>(OH)<sub>4</sub>
'''Reaction 3''': Forsterite + water → serpentine + brucite
:2Mg<sub>2</sub>SiO<sub>4</sub> + 3H<sub>2</sub>O → Mg<sub>3</sub>Si<sub>2</sub>O<sub>5</sub>(OH)<sub>4</sub> + Mg(OH)<sub>2</sub>

Reaction 3 describes the hydration of olivine with water only to yield [[serpentine group|serpentine]] and Mg(OH)<sub>2</sub> ([[brucite]]). Serpentine is stable at high pH in the presence of brucite like calcium silicate hydrate, ([[calcium silicate hydrate|C-S-H]]) phases formed along with [[portlandite]] (Ca(OH)<sub>2</sub>) in hardened [[Portland cement]] paste after the hydration of [[belite]] (Ca<sub>2</sub>SiO<sub>4</sub>), the artificial calcium equivalent of forsterite.

Analogy of reaction 3 with belite hydration in ordinary Portland cement: ''Belite + water → C-S-H phase + portlandite''
:2 Ca<sub>2</sub>SiO<sub>4</sub> + 4 H<sub>2</sub>O → 3 CaO · 2 SiO<sub>2</sub> · 3 H<sub>2</sub>O + Ca(OH)<sub>2</sub>}}

==== Chemiosmotic gradient ====

{{further|Hydrothermal vent|Chemiosmosis#Emergence of chemiosmosis}}

[[File:Leaky membrane cell powered by external proton gradient.svg|thumb|upright=1.4|Proposed model of an early cell powered by external proton gradient near a deep-sea hydrothermal vent. As long as the membrane (or passive ion channels within it) is permeable to protons, the mechanism can function without ion pumps.<ref name="Lane 2015"/>]]

Russell demonstrated that alkaline vents create an abiogenic [[proton electromotive force|proton motive force]] chemiosmotic gradient,<ref name="Martin2003"/> ideal for abiogenesis. Their microscopic compartments "provide a natural means of concentrating organic molecules," composed of iron-sulfur minerals such as [[mackinawite]], endowed these mineral cells with the catalytic properties envisaged by [[Günter Wächtershäuser]].<ref name="Lane 2009">{{harvnb|Lane|2009}}</ref> This movement of ions across the membrane depends on two factors:<ref name="Lane 2015"/>

# [[Diffusion]] force caused by concentration gradient—all particles including ions diffuse from higher concentration to lower.<ref name="Lane 2015"/>
# Electrostatic force caused by electrical potential gradient—[[cation]]s like [[proton]]s H<sup>+</sup> diffuse down the electrical potential, [[anion]]s in the opposite direction.<ref name="Lane 2015"/>

These two gradients together can be expressed as an electrochemical gradient, providing energy for abiogenic synthesis. The proton motive force measures the potential energy stored as proton and voltage gradients across a membrane (differences in proton concentration and electrical potential).<ref name="Lane 2015"/>

The surfaces of mineral particles inside deep-ocean hydrothermal vents have catalytic properties similar to those of enzymes, and can create simple organic molecules, such as [[methanol]] (CH<sub>3</sub>OH) and [[formic acid|formic]], [[acetic acid|acetic]], and [[pyruvic acid]]s out of the dissolved CO<sub>2</sub> in the water, if driven by an applied voltage or by reaction with H<sub>2</sub> or H<sub>2</sub>S.<ref name="organics">{{cite press release |last=Usher |first=Oli |date=27 April 2015 |title=Chemistry of seabed's hot vents could explain emergence of life |url=https://www.ucl.ac.uk/silva/mathematical-physical-sciences/maps-news-publication/maps1526 |publisher=[[University College London]] |access-date=19 June 2015 |archive-url=https://web.archive.org/web/20150620012231/https://www.ucl.ac.uk/silva/mathematical-physical-sciences/maps-news-publication/maps1526 |archive-date=20 June 2015}}</ref><ref>{{cite journal |last1=Roldan |first1=Alberto |last2=Hollingsworth |first2=Nathan |last3=Roffey |first3=Anna |last4=Islam |first4=Husn-Ubayda |last5=Goodall |first5=Josephine B. M. |last6=Catlow |first6=C. Richard A. |author6-link=Richard Catlow |last7=Darr |first7=Jawwad A. |last8=Bras |first8=Wim |last9=Sankar |first9=Gopinathan |last10=Holt |first10=Katherine B. |last11=Hogarth |first11=Graeme |last12=de Leeuw |first12=Nora Henriette |display-authors=3 |date=May 2015 |title=Bio-inspired CO2 conversion by iron sulfide catalysts under sustainable conditions |url=http://pubs.rsc.org/en/content/articlepdf/2015/cc/c5cc02078f |journal=[[Chemical Communications]] |volume=51 |issue=35 |pages=7501–7504 |doi=10.1039/C5CC02078F |pmid=25835242 |access-date=2015-06-19 |url-status=live |archive-url=https://web.archive.org/web/20150620003943/http://pubs.rsc.org/en/content/articlepdf/2015/cc/c5cc02078f |archive-date=20 June 2015 |doi-access=free}}</ref>

Starting in 1985, researchers proposed that life arose at hydrothermal vents,<ref>{{cite journal |last1=Baross |first1=J. A. |last2=Hoffman |first2=S. E. |year= 1985 |title=Submarine hydrothermal vents and associated gradient environments as sites for the origin and evolution of life |journal=[[Origins of Life and Evolution of Biospheres]] |volume=15 |issue=4 |pages=327–345 |doi=10.1007/bf01808177 |bibcode=1985OrLi...15..327B |s2cid=4613918}}</ref><ref>{{cite journal |last1=Russell |first1=M. J. |last2= Hall |first2=A. J. |year=1997 |title=The emergence of life from iron monosulphide bubbles at a submarine hydrothermal redox and pH front |journal=[[Journal of the Geological Society]] |volume=154 |issue=3 |pages=377–402 |doi=10.1144/gsjgs.154.3.0377 |pmid=11541234 |bibcode=1997JGSoc.154..377R |s2cid=24792282}}</ref> that spontaneous chemistry in the Earth's crust driven by rock–water interactions at disequilibrium thermodynamically underpinned life's origin,<ref>{{cite journal |last1=Amend |first1=J. P. |last2= LaRowe |first2=D. E. |last3=McCollom |first3=T. M. |last4=Shock |first4=E. L. |year=2013 |title=The energetics of organic synthesis inside and outside the cell |journal= [[Philosophical Transactions of the Royal Society B]] |volume=368 |issue=1622 |page=20120255 |doi=10.1098/rstb.2012.0255 |pmid=23754809 |pmc=3685458}}</ref><ref>{{cite journal |last1=Shock |first1=E. L. |last2=Boyd |first2=E. S. |year=2015 |title=Geomicrobiology and microbial geochemistry:principles of geobiochemistry |journal=[[Elements (journal)|Elements]] |volume=11 |pages=389–394 |doi=10.2113/gselements.11.6.395}}</ref> and that the founding lineages of the archaea and bacteria were H<sub>2</sub>-dependent autotrophs that used CO<sub>2</sub> as their terminal acceptor in energy metabolism.<ref>{{cite journal |last1=Martin |first1=W. |last2=Russell |first2= M. J. |year=2007 |title=On the origin of biochemistry at an alkaline hydrothermal vent |journal=[[Philosophical Transactions of the Royal Society B]] |volume=362 |issue=1486 |pages=1887–1925 |doi=10.1098/rstb.2006.1881 |pmid=17255002 |pmc=2442388}}</ref> In 2016, Martin suggested that the LUCA "may have depended heavily on the geothermal energy of the vent to survive".<ref name="Weiss Sousa Mrnjavac 2016"/> Pores at deep sea hydrothermal vents are suggested to have been occupied by membrane-bound compartments which promoted biochemical reactions.<ref>{{cite journal |last1=Lane |first1=Nick |author1-link=Nick Lane |last2=Martin |first2=William F. |date=2012-12-21 |title=The Origin of Membrane Bioenergetics |journal=Cell |volume=151 |issue=7 |pages=1406–1416 |doi= 10.1016/j.cell.2012.11.050 |pmid=23260134 |s2cid=15028935 |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 |volume=104 |issue=22 |pages= 9346–9351 |doi=10.1073/pnas.0609592104 |pmc=1890497 |pmid=17494767 |doi-access=free|bibcode=2007PNAS..104.9346B}}</ref> Metabolic intermediates in the Krebs cycle, gluconeogenesis, amino acid bio-synthetic pathways, glycolysis, the pentose phosphate pathway, and including sugars like ribose, and lipid precursors can occur non-enzymatically at conditions relevant to deep-sea alkaline hydrothermal vents.<ref>{{Cite journal |last1=Nunes Palmeira |first1=Raquel |last2=Colnaghi |first2=Marco |last3=Harrison |first3=Stuart A. |last4=Pomiankowski |first4=Andrew |last5=Lane |first5=Nick |author5-link=Nick Lane |date=2022-11-09 |title=The limits of metabolic heredity in protocells |journal=Proceedings of the Royal Society B: Biological Sciences |volume=289 |issue=1986 |doi=10.1098/rspb.2022.1469 |pmc=9653231 |pmid=36350219}}</ref>

If the deep marine hydrothermal setting was the site, then abiogenesis could have happened as early as {{val|4.0|-|4.2|u=Gya}}. If life evolved in the ocean at depths of more than ten meters, it would have been shielded both from impacts and the then high levels of solar ultraviolet radiation. The available energy in hydrothermal vents is maximized at 100–150&nbsp;°C, the temperatures at which [[Hyperthermophile|hyperthermophilic]] bacteria and [[Thermoacidophile|thermoacidophilic]] [[archaea]] live.<ref name = woese>{{Cite journal |last=Woese |first=Carl R. |date=1987 |title=Bacterial evolution |journal=Microbiological Reviews |volume=51.2 (1987) |issue=2 |pages=221–271 |doi=10.1128/mr.51.2.221-271.1987 |pmid=2439888 |pmc=373105 |s2cid=734579}}</ref><ref>{{Citation |last1=Russell |first1=Michael J. |title=The onset and early evolution of life |date=2006 |work=Evolution of Early Earth's Atmosphere, Hydrosphere, and Biosphere - Constraints from Ore Deposits |publisher=Geological Society of America |last2=Hall |first2=Allan J. |doi=10.1130/2006.1198(01) |doi-broken-date=11 December 2024 |isbn=9780813711980 |url=https://pubs.geoscienceworld.org/gsa/books/book/442/chapter-abstract/3799005/The-onset-and-early-evolution-of-life?redirectedFrom=fulltext |archive-date=31 January 2024 |archive-url= https://web.archive.org/web/20240131155448/https://pubs.geoscienceworld.org/gsa/books/book/442/chapter-abstract/3799005/The-onset-and-early-evolution-of-life?redirectedFrom=fulltext |url-status=live|url-access=subscription }}</ref> Arguments against a hydrothermal origin of life state that hyperthermophily was a result of [[convergent evolution]] in bacteria and archaea, and that a [[Mesophile|mesophilic]] environment would have been more likely.<ref>{{Cite journal |last1=Boussau |first1=Bastien |last2=Blanquart |first2=Samuel |last3=Necsulea |first3=Anamaria |last4=Lartillot |first4=Nicolas |last5=Gouy |first5=Manolo |date=December 2008 |title=Parallel adaptations to high temperatures in the Archaean eon |url=https://www.nature.com/articles/nature07393 |journal=Nature |volume=456 |issue=7224 |pages=942–945 |doi=10.1038/nature07393 |pmid=19037246 |bibcode=2008Natur.456..942B |s2cid=4348746 |access-date=8 December 2023 |archive-date=16 December 2023 |archive-url=https://web.archive.org/web/20231216095135/https://www.nature.com/articles/nature07393 |url-status=live}}</ref><ref name="science.org">{{Cite journal |last1=Galtier |first1=Nicolas |last2= Tourasse |first2=Nicolas |last3=Gouy |first3=Manolo |date=1999-01-08 |title=A Nonhyperthermophilic Common Ancestor to Extant Life Forms |url=https://www.science.org/doi/10.1126/science.283.5399.220 |journal=Science |volume=283 |issue=5399 |pages=220–221 |doi=10.1126/science.283.5399.220 |pmid=9880254 |access-date=8 December 2023 |archive-date= 31 January 2024 |archive-url=https://web.archive.org/web/20240131155324/https://www.science.org/doi/10.1126/science.283.5399.220 |url-status=live|url-access=subscription }}</ref> This hypothesis, suggested in 1999 by Galtier, was proposed one year before the discovery of the Lost City Hydrothermal Field, where white-smoker hydrothermal vents average ≈45–90&nbsp;°C.<ref>{{Cite journal |last1=Kelley |first1=Deborah S. |last2=Karson |first2=Jeffrey A. |last3=Blackman |first3=Donna K. |last4=Früh-Green |first4=Gretchen L. |last5=Butterfield |first5=David A. |last6=Lilley |first6=Marvin D. |last7=Olson |first7=Eric J. |last8=Schrenk |first8=Matthew O. |last9=Roe |first9=Kevin K. |last10=Lebon |first10=Geoff T. |last11=Rivizzigno |first11=Pete |date=July 2001 |title=An off-axis hydrothermal vent field near the Mid-Atlantic Ridge at 30° N |journal=Nature |volume=412 |issue=6843 |pages=145–149 |doi=10.1038/35084000 |pmid=11449263 |bibcode=2001Natur.412..145K |s2cid=4407013 |url=https://www.nature.com/articles/35084000 |archive-date=31 January 2024 |archive-url=https://web.archive.org/web/20240131155327/https://www.nature.com/articles/35084000 |url-status=live|url-access=subscription }}</ref> <!--Moderate temperatures and alkaline seawater such as that at Lost City are now the favoured hydrothermal vent setting in contrast to acidic, high temperature (≈350&nbsp;°C) black-smokers.-->

==== Arguments against a vent setting ====

Production of prebiotic organic compounds at hydrothermal vents is estimated to be {{val|e=8|u=kg|upl=yr}}.<sup><ref name="Ehrenfreund Irvine Becker 2002">{{Cite journal |last1=Ehrenfreund |first1=P. |last2=Irvine |first2=W. |last3=Becker |first3=L. |last4=Blank |first4=J. |last5=Brucato |first5=J. R. |last6=Colangeli |first6=L. |last7=Derenne |first7=S. |last8=Despois |first8=D. |last9=Dutrey |first9=A. |last10=Fraaije |first10=H. |last11=Lazcano |first11=A. |last12=Owen |first12=T. |last13=Robert |first13=F. |display-authors=5 |date=August 2002 |title=Astrophysical and astrochemical insights into the origin of life |journal=Reports on Progress in Physics |volume=65 |issue=10 |pages=1427 |doi=10.1088/0034-4885/65/10/202 |bibcode=2002RPPh...65.1427E |s2cid=250904448}}</ref></sup> While a large amount of key prebiotic compounds, such as methane, are found at vents, they are in far lower concentrations than in a Miller-Urey Experiment environment. In the case of methane, the production rate at vents is 2–4 orders of magnitude lower than in a [[Miller–Urey experiment|Miller-Urey Experiment]] surface atmosphere.<ref name="Ehrenfreund Irvine Becker 2002" /><ref>{{cite book |last1=Chyba |first1=C.F. |chapter=Comets and Prebiotic Organic Molecules on Early Earth |title=Comets and the Origin and Evolution of Life |pages=169–206 |publisher=Springer Berlin Heidelberg |isbn=978-3-540-33086-8 |last2=Chyba |first2=C.F. |last3=Hand |first3=K.P. |series=Advances in Astrobiology and Biogeophysics |date=2006 |doi=10.1007/3-540-33088-7_6 |chapter-url=https://link.springer.com/chapter/10.1007/3-540-33088-7_6 |archive-date=31 January 2024 |archive-url=https://web.archive.org/web/20240131155457/https://link.springer.com/chapter/10.1007/3-540-33088-7_6 |url-status=live}}</ref>

Other contra-arguments include the inability to concentrate prebiotic materials, due to strong dilution by seawater. This open system cycles compounds through vent minerals, leaving little residence time to accumulate.<ref>{{cite book |last=Chatterjee |first=Sankar |title=From Stardust to First Cells |chapter=The Cradle of Life |date=2023 |work=From Stardust to First Cells: The Origin and Evolution of Early Life |pages=43–66 |editor-last=Chatterjee |editor-first=Sankar |place=Cham |publisher=Springer International Publishing |doi=10.1007/978-3-031-23397-5_6 |isbn=978-3-031-23397-5 |chapter-url=https://link.springer.com/chapter/10.1007/978-3-031-23397-5_6 |archive-date=31 January 2024 |archive-url=https://web.archive.org/web/20240131155449/https://link.springer.com/chapter/10.1007/978-3-031-23397-5_6 |url-status=live}}</ref> All modern cells rely on phosphates and potassium for nucleotide backbone and protein formation respectively, making it likely that the first life forms shared these functions. These elements were not available in high quantities in the Archaean oceans, as both primarily come from the weathering of continental rocks on land, far from vents. Submarine hydrothermal vents are not conducive to condensation reactions needed for polymerisation of macromolecules.<ref>{{cite book |last=Deamer |first=David W. |chapter=Prospects for Life on Other Planets |date=2019-02-07 |title=Assembling Life |publisher=[[Oxford University Press]] |doi=10.1093/oso/9780190646387.003.0017 |isbn=978-0-19-064638-7 |url=https://academic.oup.com/book/40676/chapter-abstract/348367627?redirectedFrom=fulltext |archive-date=31 January 2024 |archive-url=https://web.archive.org/web/20240131155333/https://academic.oup.com/book/40676/chapter-abstract/348367627?redirectedFrom=fulltext |url-status=live}}</ref><ref>{{cite journal |last1=Pearce |first1=Ben K. D. |last2=Pudritz |first2=Ralph E. |last3=Semenov |first3=Dmitry A. |last4=Henning |first4=Thomas K. |date=2017-10-02 |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 |doi=10.1073/pnas.1710339114 |pmid=28973920 |pmc=5664528 |bibcode=2017PNAS..11411327P |doi-access=free |arxiv=1710.00434}}</ref>

An older argument was that key polymers were encapsulated in vesicles after condensation, which supposedly would not happen in saltwater. However, while salinity inhibits vesicle formation from low-diversity mixtures of fatty acids,<ref name="Deamer 2021" /> vesicle formation from a broader, more realistic mix of fatty-acid and 1-alkanol species is more resilient.<ref name="Jordan 2019">{{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=2019 |title=Promotion of protocell self-assembly from mixed amphiphiles at the origin of life|url=https://www.nature.com/articles/s41559-019-1015-y |journal=Nature Ecology & Evolution |volume=3 |issue=12 |pages=1705–1714 |doi=10.1038/s41559-019-1015-y |pmid=31686020 |bibcode=2019NatEE...3.1705J|url-access=subscription }}</ref><ref name="Deamer 2021"/>

=== Surface bodies of water ===

Surface bodies of water provide environments that dry out and rewet. Wet-dry cycles concentrate prebiotic compounds and enable [[condensation reaction]]s to polymerise macromolecules. Moreover, lakes and ponds receive detrital input from weathering of continental [[apatite]]-containing rocks, the most common source of phosphates. The amount of exposed continental crust in the Hadean is unknown, but models of early ocean depths and rates of ocean island and continental crust growth make it plausible that there was exposed land.<ref>{{Cite journal |last=Korenaga |first=Jun |date=November 2021 |title=Was There Land on the Early Earth? |journal=Life |volume=11 |issue=11 |pages=1142 |doi=10.3390/life11111142 |pmc=8623345 |pmid=34833018 |bibcode=2021Life...11.1142K |doi-access=free}}</ref> Another line of evidence for a surface start to life is the requirement for [[Ultraviolet|UV]] for organism function. UV is necessary for the formation of the U+C nucleotide [[base pair]] by partial [[hydrolysis]] and nucleobase loss.<ref>{{Cite journal |last1=Powner |first1=Matthew W. |last2=Gerland |first2=Béatrice |last3=Sutherland |first3=John D. |date=May 2009 |title=Synthesis of activated pyrimidine ribonucleotides in prebiotically plausible conditions |url=https://www.nature.com/articles/nature08013 |journal=Nature |volume=459 |issue=7244 |pages=239–242 |doi=10.1038/nature08013 |pmid=19444213 |bibcode=2009Natur.459..239P |s2cid=4412117 |access-date=8 December 2023 |archive-date=12 November 2023 |archive-url=https://web.archive.org/web/20231112211237/https://www.nature.com/articles/nature08013 |url-status=live|url-access=subscription }}</ref> Simultaneously, UV can be harmful and sterilising to life, especially for simple early lifeforms with little ability to repair radiation damage. Radiation levels from a young Sun were likely greater, and, with no [[ozone layer]], harmful shortwave UV rays would reach the surface of Earth. For life to begin, a shielded environment with influx from UV-exposed sources is necessary to both benefit and protect from UV. Shielding under ice, liquid water, mineral surfaces (e.g. clay) or regolith is possible in a range of surface water settings.<ref>{{Cite journal |last1=Zahnle |first1=Kevin |last2=Arndt |first2=Nick |last3=Cockell |first3=Charles |last4=Halliday |first4=Alex |last5=Nisbet |first5=Euan |last6=Selsis |first6=Franck |last7=Sleep |first7=Norman H. |date=2007-03-01 |title=Emergence of a Habitable Planet |journal=Space Science Reviews |volume=129 |issue=1 |pages=35–78 |doi=10.1007/s11214-007-9225-z |bibcode=2007SSRv..129...35Z |s2cid=12006144 |url=https://link.springer.com/article/10.1007/s11214-007-9225-z |archive-date=31 January 2024 |archive-url=https://web.archive.org/web/20240131155452/https://link.springer.com/article/10.1007/s11214-007-9225-z |url-status=live|url-access=subscription }}</ref>

==== Hot springs ====

Most branching phylogenies are thermophilic or hyperthermophilic, making it possible that LUCA and preceding lifeforms were similarly thermophilic. Hot springs are formed from the heating of groundwater by geothermal activity. This intersection allows for influxes of material from deep penetrating waters and from surface runoff that transports eroded continental sediments. Interconnected groundwater systems create a mechanism for distribution of life to wider area.<ref name="woese"/>

Mulkidjanian and co-authors argue that marine environments did not provide the ionic balance and composition universally found in cells, or the ions required by essential proteins and ribozymes, especially with respect to high K<sup>+</sup>/Na<sup>+</sup> ratio, Mn<sup>2+</sup>, Zn<sup>2+</sup> and phosphate concentrations. They argue that the only environments that do this are hot springs similar to ones at Kamchatka.<ref name="Mulkidjanian Bychkov 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 |volume=109 |issue=14 |pages=E821-30 |doi=10.1073/pnas.1117774109 |pmc=3325685 |pmid=22331915 |bibcode=2012PNAS..109E.821M |doi-access=free}}</ref> Mineral deposits in these environments under an anoxic atmosphere would have suitable pH, contain precipitates of photocatalytic sulfide minerals that absorb harmful ultraviolet radiation, and have wet-dry cycles that concentrate substrate solutions enough for spontaneous formation of biopolymers<ref>{{cite journal |last1=Chandru |first1=Kuhan |last2=Guttenberg |first2=Nicholas |last3=Giri |first3=Chaitanya |last4=Hongo |first4=Yayoi |last5=Butch |first5=Christopher |last6=Mamajanov |first6=Irena |last7=Cleaves |first7=H. James |display-authors=3 |title=Simple prebiotic synthesis of high diversity dynamic combinatorial polyester libraries |journal=[[Communications Chemistry]] |date=31 May 2018 |volume=1 |issue=1 |page=30 |doi=10.1038/s42004-018-0031-1 |doi-access=free|bibcode=2018CmChe...1...30C}}</ref><ref>{{cite journal |last1=Forsythe |first1=Jay G. |last2=Yu |first2=Sheng-Sheng |last3=Mamajanov |first3=Irena |last4=Grover |first4=Martha A |author-link4=Martha Grover |last5=Krishnamurthy |first5=Ramanarayanan |last6=Fernández |first6=Facundo M. |last7=Hud |first7=Nicholas V. |display-authors=3 |date=17 August 2015 |title=Ester-Mediated Amide Bond Formation Driven by Wet–Dry Cycles: A Possible Path to Polypeptides on the Prebiotic Earth |journal=Angewandte Chemie International Edition in English |volume=54 |issue=34 |pages=9871–9875 |doi=10.1002/anie.201503792 |pmc=4678426 |pmid=26201989|bibcode=2015AngCh..54.9871F}}</ref> created both by chemical reactions in the hydrothermal environment, and by exposure to [[UV light]] during transport from vents to adjacent pools.<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 |volume=7 |issue=4 |pages=301–307 |doi=10.1038/nchem.2202 |pmc=4568310 |pmid=25803468 |bibcode=2015NatCh...7..301P}}</ref> The hypothesized pre-biotic environments are similar to hydrothermal vents, with additional components that help explain peculiarities of the LUCA.<ref name="Mulkidjanian Bychkov 2012" /><ref name="Damer 2020" />

A phylogenomic and geochemical analysis of proteins plausibly traced to the LUCA shows that the ionic composition of its intracellular fluid is identical to that of hot springs. The LUCA likely was dependent upon synthesized organic matter for its growth.<ref name="Mulkidjanian Bychkov 2012"/> Experiments show that RNA-like polymers can be synthesized in wet-dry cycling and UV light exposure. These polymers were encapsulated in vesicles after condensation.<ref name="Deamer 2021">{{cite journal |last=Deamer |first=David |author-link=David W. Deamer |date=10 February 2021 |title=Where Did Life Begin? Testing Ideas in Prebiotic Analogue Conditions |journal=[[Life (journal)|Life]] |volume=11 |issue=2 |page=134 |doi=10.3390/life11020134 |pmid=33578711 |pmc=7916457 |bibcode=2021Life...11..134D |doi-access=free}}</ref> Potential sources of organics at hot springs might have been transport by interplanetary dust particles, extraterrestrial projectiles, or atmospheric or geochemical synthesis. Hot springs could have been abundant in volcanic landmasses during the Hadean.<ref name="Damer 2020"/>

==== Temperate surface bodies of water ====

A [[Mesophile|mesophilic]] start in surface bodies of waters hypothesis has evolved from Darwin's concept of a '[[warm little pond]]' and the [[Primordial soup|Oparin-Haldane hypothesis]]. Freshwater bodies under temperate climates can accumulate prebiotic materials while providing suitable environmental conditions conducive to simple life forms. The Archaean climate is uncertain. Atmospheric reconstructions from geochemical proxies and models suggest that sufficient greenhouse gases were present to maintain surface temperatures between 0–40&nbsp;°C. If so, the temperature was suitable for life could begin.<ref>{{Cite journal |last1=Catling |first1=David C. |last2=Zahnle |first2=Kevin J. |date=2020-02-28 |title=The Archean atmosphere |journal=Science Advances |volume=6 |issue=9 |pages=eaax1420 |doi=10.1126/sciadv.aax1420 |pmc=7043912 |pmid=32133393|bibcode=2020SciA....6.1420C}}</ref>

Strong evidence for mesophily from biomolecular studies includes Galtier's [[GC-content|G+C]] nucleotide thermometer. G+C are more abundant in thermophiles due to the added stability of an additional hydrogen bond not present between A+T nucleotides. [[Ribosomal RNA|rRNA]] sequencing on a diverse range of modern lifeforms shows that [[Last universal common ancestor|LUCA]]'s reconstructed G+C content was likely representative of moderate temperatures.<ref name="science.org"/>

It is possible that the diversity of thermophiles today is a product of convergent evolution and horizontal gene transfer rather than an inherited trait from LUCA.<ref>{{Cite journal |last1=Miller |first1=Stanley L. |last2=Lazcano |first2=Antonio |date=December 1995 |title=The origin of life?did it occur at high temperatures? |journal=Journal of Molecular Evolution |volume=41 |issue=6 |pages=689–692 |doi=10.1007/bf00173146 |pmid=11539558 |bibcode=1995JMolE..41..689M |s2cid=25141419 |hdl=2060/19980211388 |url=https://link.springer.com/article/10.1007/BF00173146 |hdl-access=free |archive-date=31 January 2024 |archive-url=https://web.archive.org/web/20240131155843/https://link.springer.com/article/10.1007/BF00173146 |url-status=live|url-access=subscription }}</ref> The [[reverse gyrase]] [[topoisomerase]] is found exclusively in thermophiles and hyperthermophiles as it allows for coiling of DNA.<ref>{{Cite journal |last1=Forterre |first1=Patrick |last2=Bergerat |first2=Agnes |last3=Lopex-Garcia |first3=Purificacion |date=May 1996 |title=The unique DNA topology and DNA topoisomerases of hyperthermophilic archaea |journal=FEMS Microbiology Reviews |volume=18 |issue=2–3 |pages=237–248 |doi=10.1111/j.1574-6976.1996.tb00240.x |pmid=8639331 |s2cid=6001830|doi-access=free}}</ref> This enzyme requires the complex molecule [[ATP synthase|ATP]] to function. If an origin of life is hypothesised to involve a simple organism that had not yet evolved a membrane, let alone ATP, this would make the existence of reverse gyrase improbable. Moreover, phylogenetic studies show that reverse gyrase had an archaeal origin, and transferred to bacteria by horizontal gene transfer, implying it was not present in the LUCA.<ref>{{Cite journal |last1=Brochier-Armanet |first1=Céline |last2=Forterre |first2=Patrick |date=May 2006 |title=Widespread distribution of archaeal reverse gyrase in thermophilic bacteria suggests a complex history of vertical inheritance and lateral gene transfers |journal=Archaea |volume=2 |issue=2 |pages=83–93 |doi=10.1155/2006/582916 |pmid=17350929 |pmc=2686386 |doi-access=free}}</ref>

==== Icy surface bodies of water ====

Cold-start theories presuppose large ice-covered regions. Stellar evolution models predict that the Sun's luminosity was ≈25% weaker than it is today. Fuelner states that although this significant decrease in solar energy would have formed an icy planet, there is strong evidence for the presence of liquid water, possibly driven by a greenhouse effect. This would mean an early Earth with both liquid oceans and icy poles.<ref>{{Cite journal |last=Feulner |first=Georg |date=June 2012 |title=The faint young Sun problem |url=https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2011RG000375 |journal=Reviews of Geophysics |volume=50 |issue=2 |doi=10.1029/2011RG000375 |arxiv=1204.4449 |bibcode=2012RvGeo..50.2006F |s2cid=119248267 |access-date=8 December 2023 |archive-date=8 December 2023 |archive-url=https://web.archive.org/web/20231208060325/https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2011RG000375 |url-status=live}}</ref>

Ice melts that form from ice sheets or glacier melts create freshwater pools, another niche capable of wet-dry cycles. While surface pools would be exposed to intense UV radiation, bodies of water within and under ice would be shielded, while remaining connected to exposed areas through ice cracks. Impact melting would allow freshwater and meteoritic input, creating prebiotic components.<ref>{{Cite journal |last1=Bada |first1=J. L. |last2=Bigham |first2=C. |last3=Miller |first3=S. L. |date=1994-02-15 |title=Impact melting of frozen oceans on the early Earth: Implications for the origin of life |journal=Proceedings of the National Academy of Sciences |volume=91 |issue=4 |pages=1248–1250 |doi=10.1073/pnas.91.4.1248 |pmc=43134 |pmid=11539550 |bibcode=1994PNAS...91.1248B |doi-access=free}}</ref> Near-seawater levels of sodium chloride destabilize fatty acid membrane self-assembly, making freshwater settings appealing for early membranous life.<ref>{{Cite journal |last1=Monnard |first1=Pierre-Alain |last2=Apel |first2=Charles L. |last3=Kanavarioti |first3=Anastassia |last4=Deamer |first4=David W. |date=June 2002 |title=Influence of Ionic Inorganic Solutes on Self-Assembly and Polymerization Processes Related to Early Forms of Life: Implications for a Prebiotic Aqueous Medium |url=https://www.liebertpub.com/doi/10.1089/15311070260192237 |journal=Astrobiology |volume=2 |issue=2 |pages=139–152 |doi=10.1089/15311070260192237 |pmid=12469365 |bibcode=2002AsBio...2..139M |access-date=8 December 2023 |archive-date=31 January 2024 |archive-url=https://web.archive.org/web/20240131155030/https://www.liebertpub.com/doi/10.1089/15311070260192237 |url-status=live|url-access=subscription }}</ref>

Icy environments would trade the faster reaction rates that occur in warm environments for increased stability and accumulation of larger polymers.<ref>{{Cite journal |last1 = Attwater |first1=James |last2=Wochner |first2=Aniela |last3=Holliger |first3=Philipp |date=December 2013 |title=In-ice evolution of RNA polymerase ribozyme activity |journal = Nature Chemistry |volume=5 |issue=12 |pages=1011–1018 |doi=10.1038/nchem.1781 |pmid=24256864 |pmc=3920166|bibcode=2013NatCh...5.1011A}}</ref> Experiments simulating Europa-like conditions of ≈20&nbsp;°C have synthesised amino acids and adenine, showing that Miller-Urey type syntheses can occur at low temperatures.<ref name="jeffreybada" /> In an [[RNA world]], the ribozyme would have had even more functions than in a later DNA-RNA-protein-world. For RNA to function, it must be able to fold, a process hindered by temperatures above 30&nbsp;°C. While RNA folding in [[Psychrophile|psychrophilic]] organisms is slower, so is hydrolysis, so folding is more successful. Shorter nucleotides would not suffer from higher temperatures.<ref>{{Cite journal |last1=Moulton |first1=Vincent |last2= Gardner |first2=Paul P. |last3=Pointon |first3=Robert F. |last4=Creamer |first4=Lawrence K. |last5=Jameson |first5=Geoffrey B. |last6=Penny |first6=David |date=2000-10-01 |title=RNA Folding Argues Against a Hot-Start Origin of Life |journal=Journal of Molecular Evolution |volume=51 |issue=4 |pages=416–421 |doi=10.1007/s002390010104 |pmid= 11040293 |bibcode=2000JMolE..51..416M |s2cid=20787323 |url=https://link.springer.com/article/10.1007/s002390010104 |archive-date=31 January 2024 |archive-url=https://web.archive.org/web/20240131155950/https://link.springer.com/article/10.1007/s002390010104 |url-status=live|url-access=subscription }}</ref><ref>{{Cite journal |last1=Zemora |first1=Georgeta |last2=Waldsich |first2=Christina |date=November 2010 |title=RNA folding in living cells |journal=RNA Biology |volume=7 |issue=6 |pages=634–641 |doi=10.4161/rna.7.6.13554 |pmid=21045541 |pmc=3073324}}</ref>

=== Inside the continental crust ===

An alternative geological environment has been proposed by the geologist Ulrich Schreiber and the physical chemist Christian Mayer: the [[continental crust]].<ref>{{Cite journal |last1=Schreiber |first1=Ulrich |last2=Locker-Grütjen |first2=Oliver |last3=Mayer |first3=Christian |date=2012 |title=Hypothesis: Origin of Life in Deep-Reaching Tectonic Faults |url=https://link.springer.com/10.1007/s11084-012-9267-4 |journal=Origins of Life and Evolution of Biospheres |volume=42 |issue=1 |pages=47–54 |doi=10.1007/s11084-012-9267-4 |pmid=22373604 |bibcode=2012OLEB...42...47S|url-access=subscription }}</ref> [[Fault (geology)|Tectonic fault]] zones could present a stable and well-protected environment for long-term prebiotic evolution. Inside these systems of cracks and cavities, water and carbon dioxide present the bulk solvents. Their phase state would depend on the local temperature and pressure conditions and could vary between liquid, gaseous and [[Supercritical fluid|supercritical]]. When forming two separate phases (e.g., liquid water and supercritical carbon dioxide in depths of little more than 1&nbsp;km), the system provides optimal conditions for [[Phase-transfer catalyst|phase transfer reactions]]. Concurrently, the contents of the tectonic fault zones are being supplied by a multitude of inorganic educts (e.g., carbon monoxide, hydrogen, ammonia, hydrogen cyanide, nitrogen, and even phosphate from dissolved apatite) and simple organic molecules formed by hydrothermal chemistry (e.g. amino acids, long-chain amines, fatty acids, long-chain aldehydes).<ref>{{Cite journal |last1=Schreiber |first1=Ulrich |last2=Mayer |first2=Christian |last3=Schmitz |first3=Oliver J. |last4=Rosendahl |first4=Pia |last5=Bronja |first5=Amela |last6=Greule |first6=Markus |last7=Keppler |first7=Frank |last8=Mulder |first8=Ines |last9=Sattler |first9=Tobias |last10=Schöler |first10=Heinz F. |date=2017-06-14 |editor-last=Stüeken |editor-first=Eva Elisabeth |title=Organic compounds in fluid inclusions of Archean quartz—Analogues of prebiotic chemistry on early Earth |journal=PLOS ONE |volume=12 |issue=6 |pages=e0177570 |doi=10.1371/journal.pone.0177570 |doi-access=free |pmc=5470662 |pmid=28614348|bibcode=2017PLoSO..1277570S}}</ref><ref>{{Cite journal |last1=Großmann |first1=Yildiz |last2=Schreiber |first2=Ulrich |last3=Mayer |first3=Christian |last4=Schmitz |first4=Oliver J. |date=2022-06-21 |title=Aliphatic Aldehydes in the Earth's Crust—Remains of Prebiotic Chemistry? |journal=Life |volume=12 |issue=7 |pages=925 |doi=10.3390/life12070925 |doi-access=free |pmc=9319801 |pmid=35888015|bibcode=2022Life...12..925G}}</ref>

An especially interesting section of the tectonic fault zones is located at a depth of approximately 1000 m. For the carbon dioxide part of the bulk solvent, it provides temperature and pressure conditions near the [[phase transition]] point between the [[Supercritical fluid|supercritical]] and the gaseous state. This leads to a natural accumulation zone for [[Lipophilicity|lipophilic organic molecules]] that dissolve well in [[Supercritical carbon dioxide|supercritical CO<sub>2</sub>]], but not in its gaseous state, leading to their local precipitation.<ref>{{Cite journal |last1=Mayer |first1=Christian |last2=Schreiber |first2=Ulrich |last3=Dávila |first3=María |date=2017-01-07 |title=Selection of Prebiotic Molecules in Amphiphilic Environments |journal=Life |volume=7 |issue=1 |pages=3 |doi=10.3390/life7010003 |doi-access=free |pmc=5370403 |pmid=28067845|bibcode=2017Life....7....3M}}</ref> Periodic pressure variations such as caused by [[Geyser|geyser activity]] or [[Tidal force|tidal influences]] result in periodic phase transitions, keeping the local reaction environment in a constant [[Non-equilibrium thermodynamics|non-equilibrium state]]. In presence of [[Amphiphile|amphiphilic compounds]] (such as the long chain amines and fatty acids mentioned above), subsequent generations of vesicles are being formed<ref name="periodic" /> that are constantly and efficiently being selected for their stability.<ref name="mayer2018"/>

== Homochirality ==

{{main|Homochirality}}

[[File:Glutamic-acid-from-xtal-view-2-3D-bs-17.png|thumb|Many biomolecules, such as [[glutamic acid|L-glutamic acid]], are [[chirality|asymmetric]], and occur in living systems in only one of the two possible forms, in the case of [[amino acid]]s the left-handed form. Prebiotic chemistry would produce both forms, creating a puzzle for abiogenesis researchers.<ref name="Plasson2007" />]]

Homochirality is the geometric uniformity of materials composed of [[chirality|chiral]] (non-mirror-symmetric) units. Living organisms use molecules that have the same chirality (handedness): with almost no exceptions,<ref>{{harvnb|Chaichian|Rojas|Tureanu|2014|pp=353–364}}</ref> amino acids are left-handed while nucleotides and [[carbohydrate|sugars]] are right-handed. Chiral molecules can be synthesized, but in the absence of a chiral source or a chiral catalyst, they are formed in a 50/50 (racemic) mixture of both [[enantiomer|forms]]. Known mechanisms for the production of non-racemic mixtures from racemic starting materials include: asymmetric physical laws, such as the [[electroweak interaction]]; asymmetric environments, such as those caused by [[circular polarization|circularly polarized]] light, [[quartz|quartz crystals]], or the Earth's rotation, [[statistical fluctuations]] during racemic synthesis,<ref name="Plasson2007">{{cite journal |last1=Plasson |first1=Raphaël |last2=Kondepudi |first2=Dilip K. |last3=Bersini |first3=Hugues |last4=Commeyras |first4=Auguste |last5=Asakura |first5=Kouichi |display-authors=3 |date=August 2007 |title=Emergence of homochirality in far-from-equilibrium systems: Mechanisms and role in prebiotic chemistry |journal=[[Chirality (journal)|Chirality]] |volume=19 |issue=8 |pages=589–600 |doi=10.1002/chir.20440 |pmid=17559107}} "Special Issue: Proceedings from the Eighteenth International Symposium on Chirality (ISCD-18), Busan, Korea, 2006"</ref> and [[spontaneous symmetry breaking]].<ref name="jafarpour2017">{{cite journal |last1=Jafarpour |first1=Farshid |last2=Biancalani |first2=Tommaso |last3=Goldenfeld |first3=Nigel |author-link3=Nigel Goldenfeld |year=2017 |title=Noise-induced symmetry breaking far from equilibrium and the emergence of biological homochirality |url=http://dspace.mit.edu/bitstream/1721.1/109170/1/PhysRevE.95.032407.pdf |journal=[[Physical Review E]] |volume=95 |issue=3 |page=032407 |bibcode=2017PhRvE..95c2407J |doi=10.1103/PhysRevE.95.032407 |pmid=28415353 |doi-access=free |access-date=29 August 2019 |archive-date=2 April 2023 |archive-url=https://web.archive.org/web/20230402201812/http://dspace.mit.edu/bitstream/handle/1721.1/109170/PhysRevE.95.032407.pdf;jsessionid=14B762FE82E5B78CD32F99AEE6E1A5F5?sequence=1 |url-status=live}}</ref><ref name="jafarpour2015">{{cite journal |last1=Jafarpour |first1=Farshid |last2=Biancalani |first2=Tommaso |last3=Goldenfeld |first3=Nigel |author-link3=Nigel Goldenfeld |year=2015 |title=Noise-induced mechanism for biological homochirality of early life self-replicators |journal=[[Physical Review Letters]] |volume=115 |issue=15 |page=158101 |arxiv=1507.00044 |bibcode=2015PhRvL.115o8101J |doi=10.1103/PhysRevLett.115.158101 |pmid=26550754 |s2cid=9775791}}</ref><ref name="frank1953">{{cite journal |last1=Frank |first1=F. C. |year=1953 |title=On spontaneous asymmetric synthesis |journal=[[Biochimica et Biophysica Acta]] |volume=11 |issue=4 |pages=459–463 |doi=10.1016/0006-3002(53)90082-1 |pmid=13105666}}</ref>

Once established, chirality would be selected for.<ref>{{cite journal |last=Clark |first=Stuart |author-link=Stuart Clark (author) |date=July–August 1999 |title=Polarized Starlight and the Handedness of Life |journal=[[American Scientist]] |volume=87 |issue=4 |page=336 |bibcode=1999AmSci..87..336C |doi=10.1511/1999.30.336 |s2cid=221585816}}</ref> A small bias ([[enantiomeric excess]]) in the population can be amplified into a large one by [[autocatalysis#Asymmetric autocatalysis|asymmetric autocatalysis]], such as in the [[Soai reaction]].<ref>{{cite journal |last1=Shibata |first1=Takanori |last2=Morioka |first2=Hiroshi |last3=Hayase |first3=Tadakatsu |last4=Choji |first4=Kaori |last5=Soai |first5=Kenso |author5-link=Kensō Soai |display-authors=3 |date=17 January 1996 |title=Highly Enantioselective Catalytic Asymmetric Automultiplication of Chiral Pyrimidyl Alcohol |journal=[[Journal of the American Chemical Society]] |volume=118 |issue=2 |pages=471–472 |doi=10.1021/ja953066g|bibcode=1996JAChS.118..471S}}</ref> In asymmetric autocatalysis, the catalyst is a chiral molecule, which means that a chiral molecule is catalyzing its own production. An initial enantiomeric excess, such as can be produced by polarized light, then allows the more abundant enantiomer to outcompete the other.<ref name="Soai2001">{{cite journal |last1=Soai |first1=Kenso |last2=Sato |first2=Itaru |last3=Shibata |first3=Takanori |year=2001 |title=Asymmetric autocatalysis and the origin of chiral homogeneity in organic compounds |journal=The Chemical Record |volume=1 |issue=4 |pages=321–332 |doi=10.1002/tcr.1017 |pmid=11893072}}</ref>

Homochirality may have started in outer space: on the [[Murchison meteorite]], the left-handed amino acid [[alanine|L-alanine]] is more than twice as frequent as its right-handed D form, and [[glutamic acid|L-glutamic acid]] is more than three times as abundant as its D counterpart.<ref>{{harvnb|Hazen|2005|p=184}}</ref><ref name="Meierhenrich">{{cite book |last1=Meierhenrich |first1=Uwe |title=Amino acids and the asymmetry of life caught in the act of formation |date=2008 |publisher=Springer |isbn=978-3-540-76886-9 |location=Berlin |pages=76–79}}</ref> Amino acids from meteorites show a left-handed bias, whereas sugars show a predominantly right-handed bias: this is the same preference found in living organisms, suggesting an abiogenic origin of these compounds.<ref name="StarStuff">{{cite journal |last=Mullen |first=Leslie |title=Building Life from Star-Stuff |journal=[[Astrobiology Magazine]] |date=5 September 2005 |url=http://www.astrobio.net/news-exclusive/building-life-from-star-stuff/ |url-status=usurped |archive-url=https://web.archive.org/web/20150714084344/http://www.astrobio.net/news-exclusive/building-life-from-star-stuff/ |archive-date=14 July 2015}}</ref>

In a 2010 experiment by Robert Root-Bernstein, "two D-RNA-oligonucleotides having inverse base sequences (D-CGUA and D-AUGC) and their corresponding L-RNA-oligonucleotides (L-CGUA and L-AUGC) were synthesized and their affinity determined for Gly and eleven pairs of L- and D-amino acids". The results suggest that homochirality, including codon directionality, might have "emerged as a function of the origin of the genetic code".<ref>{{Cite journal |last=Root-Bernstein |first=Robert |date=23 June 2010 |title=Experimental Test of L- and D-Amino Acid Binding to L- and D-Codons Suggests that Homochirality and Codon Directionality Emerged with the Genetic Code |journal=Symmetry |volume=2 |issue=2 |pages=1180–1200 |doi=10.3390/sym2021180 |bibcode=2010Symm....2.1180R |doi-access=free}}</ref>

== See also ==
* [[Alternative abiogenesis scenarios]]
* [[Autopoiesis]]
* {{Annotated link|Formamide-based prebiotic chemistry}}
* {{Annotated link|Proto-metabolism}} 
* {{Annotated link|GADV-protein world hypothesis}}
* {{Annotated link|Genetic recombination}}
* {{Annotated link|Shadow biosphere}}
* [[Manganese metallic nodules]]

== Notes ==

{{notelist}}

== References ==

{{reflist}}

== Sources ==

{{Refbegin|colwidth=30em}}
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{{Refend}}

== External links ==

{{Library resources box}}

* [https://www.pnas.org/doi/10.1073/pnas.2105383118 Making headway with the mysteries of life's origins] – Adam Mann ([[PNAS]]; 14 April 2021)
* [https://exploringorigins.org/ Exploring Life's Origins] {{Webarchive|url=https://web.archive.org/web/20230408001731/https://exploringorigins.org/ |date=8 April 2023}} a virtual exhibit at the [[Museum of Science (Boston)]]
* [https://www.earthfacts.com/evolution-and-life/howlifebeganearth/ How life began on Earth] – Marcia Malory (Earth Facts; 2015)
* [https://www.bbc.co.uk/programmes/p004y29f The Origins of Life] – [[Richard Dawkins]] et al. ([[BBC Radio]]; 2004)
* [https://www.hawking.org.uk/in-words/lectures/life-in-the-universe Life in the Universe] – Essay by [[Stephen Hawking]] (1996)

{{Origin of life}}
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