History of life: Difference between revisions

{{Short description|Processes by which organisms evolved on Earthnone}}

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{{redirectRedirect|Prehistoric life|the book|Prehistoric Life (book){{!}}''Prehistoric Life'' (book)}}

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The '''history of life''' on [[Earth]] traces the processes by which living and [[fossil]]extinct [[organism]]s evolved, from the earliest [[Abiogenesis|emergence of life]] to the present day. Earth formed about 4.5 billion years ago (abbreviated as ''Ga'', for ''[[Year#SI prefix multipliers|gigaannum]]'') and evidence suggests that life emerged prior to 3.7 Ga.<ref name="Pearce 343–364">{{cite journal |last1=Pearce |first1=Ben K.D. |last2=Tupper |first2=Andrew S. |last3=Pudritz |first3=Ralph E. |author3-link=Ralph Pudritz |last4=Higgs |first4=Paul G. |display-authors=3 |date=March 1, 2018 |title=Constraining the Time Interval for the Origin of Life on Earth |journal=[[Astrobiology (journal)|Astrobiology]] |volume=18 |issue=3 |pages=343–364 |arxiv=1808.09460 |s2cid=4419671 |bibcode=2018AsBio..18..343P |doi=10.1089/ast.2017.1674 |issn=1531-1074 |pmid=29570409}}</ref><ref name="Rosing 674–676">{{cite journal |last=Rosing |first=Minik T. |date=January 29, 1999 |title=¹³C-Depleted Carbon Microparticles in &gt;3700-Ma Sea-Floor Sedimentary Rocks from West Greenland |journal=[[Science (journal)|Science]] |volume=283 |issue=5402 |pages=674–676 |bibcode=1999Sci...283..674R |doi=10.1126/science.283.5402.674 |issn=0036-8075 |pmid=9924024}}</ref><ref name="Ohtomo 25–28">{{cite journal |last1=Ohtomo |first1=Yoko |last2=Kakegawa |first2=Takeshi |last3=Ishida |first3=Akizumi |last4=Nagase |first4=Toshiro |last5=Rosing |first5=Minik T. |display-authors=3 |date=January 2014 |title=Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks |journal=[[Nature Geoscience]] |volume=7 |issue=1 |pages=25–28 |bibcode=2014NatGe...7...25O |doi=10.1038/ngeo2025 |issn=1752-0894}}</ref> AlthoughThe theresimilarities isamong someall evidenceknown ofpresent-day life[[species]] asindicate earlythat asthey 4.1have todiverged 4.28 Ga, it remains controversial due tothrough the possible non-biological formationprocess of the purported fossils.<ref name="Pearce 343–364"/><ref>{{cite journal |last1=Papineau |first1=Dominic |last2=De Gregorio |first2=Bradley T. |last3=Cody |first3=George D. |last4=O'Neil |first4=J. |last5=Steele |first5=A. |last6=Stroud |first6=R. M. |last7=Fogel |first7=M. L. |display-authors=3 |date=June 2011 |title=Young poorly crystalline graphite in the &gt;3.8-Gyr-old Nuvvuagittuq banded iron formation |journal=[[Nature Geoscienceevolution]] |volume=4 |issue=6 |pages=376–379 |bibcode=2011NatGe...4..376P |doi=10.1038/ngeo1155 |issn=1752-0894}}</ref><ref name="PNAS-20151124-pdf">{{cite journal |last1=Bell |first1=Elizabeth A. |last2=Boehnke |first2=Patrick |last3=Harrison |first3=T. Mark |last4=Mao |first4=Wendy L. |display-authors=3 |date=November 24, 2015 |title=Potentially biogenic carbon preserved infrom a 4.1 billion-year-old zircon |url=https://www.pnas.org/content/pnas/early/2015/10/14/1517557112.full.pdf |url-status=live |journal=[[Proceedingscommon of the National Academy of Sciencesancestor]] |volume=112 |issue=47 |pages=14518–14521 |bibcode=2015PNAS..11214518B |doi=10.1073/pnas.1517557112 |issn=0027-8424 |pmc=4664351 |pmid=26483481 |archive-url=https://web.archive.org/web/20200213002627/https://www.pnas.org/content/pnas/early/2015/10/14/1517557112.full.pdf |archive-date=2020-02-13 |access-date=2020-02-14 |doi-access=free}}</ref><ref>{{cite journal harvnb|last1=Nemchin Futuyma|first1=Alexander A. |last2=Whitehouse |first2=Martin J. |last3=Menneken |first3=Martina |last4=Geisler |first4=Thorsten |last5=Pidgeon |first5=Robert T. |last6=Wilde |first6=Simon A. |display-authors=3 |date=July 3, 2008 |title=A light carbon reservoir recorded in zircon-hosted diamond from the Jack Hills |journal=[[Nature (journal)|Nature]] |volume=454 |issue=7200 |pages=92–95 |bibcode=2008Natur.454...92N |s2cid=4415308 |doi=10.1038/nature07102 |issn=0028-0836 |pmid=185968082005}}</ref>

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

At around 1.7 Ga, [[multicellular organism]]s began to appear, with [[cellular differentiation|differentiated cell]]s performing specialised functions.<ref name="Bonner1999" /> While early organisms reproduced [[Sexualasexual reproduction|asexually]], which involves the fusionprimary method of malereproduction andfor femalethe reproductivevast cellsmajority of ([[gametemacroscopic]]s) toorganisms, createincluding aalmost all [[zygoteeukaryotes]] in(which aincludes process[[animal]]s calledand [[fertilizationplant]]s), is, in contrast to [[asexualsexual reproduction]], the primary methodfusion of reproductionmale forand thefemale vastreproductive majority of macroscopic organisms, including almost allcells ([[eukaryotesgamete]]s) (whichto includescreate [[animal]]s anda [[plantzygote]]s).<ref>{{cite journal |last1=Otto |first1=Sarah P. |author1-link=Sarah Otto |last2=Lenormand |first2=Thomas |date=April 1, 2002 |title=Resolving the paradox of sex and recombination |journal=[[Nature Reviews Genetics]] |volume=3 |issue=4 |pages=252–261 |doi=10.1038/nrg761 |s2cid=13502795 |issn=1471-0056 |pmid=11967550}}</ref> However theThe origin and [[evolution of sexual reproduction]] remain a puzzle for biologists, though it didis evolvethought fromto ahave commonevolved ancestor that wasfrom a single -celled eukaryotic speciesancestor.<ref name="Letunic I and Bork P">{{cite web |url=https://itol.embl.de/ |title=iTOL: Interactive Tree of Life |last1=Letunic |first1=Ivica |last2=Bork |first2=Peer |author2-link=Peer Bork |publisher=[[European Molecular Biology Laboratory]] |location=Heidelberg, Germany |access-date=2015-07-21 |archive-date=2022-06-10 |archive-url=https://web.archive.org/web/20220610171813/https://itol.embl.de/ |url-status=live }}

*{{cite journal |last1=Letunic |first1=Ivica |last2=Bork |first2=Peer |author2-link=Peer Bork |date=July 1, 2011 |title=Interactive Tree Of Life v2: online annotation and display of phylogenetic trees made easy |url=https://itol.embl.de/help/gkr201.pdf |journal=[[Nucleic Acids Research]] |volume=39 |issue=Suppl. 2 |pages=W475–W478 |doi=10.1093/nar/gkr201 |issn=0305-1048 |pmc=3125724 |pmid=21470960 |access-date=2015-07-21 |archive-date=2021-11-01 |archive-url=https://web.archive.org/web/20211101090511/https://itol.embl.de/help/gkr201.pdf |url-status=live }}</ref> [[Bilateria]], animals having a left and a right side that are mirror images of each other, appeared by 555 Ma (million years ago).<ref>{{cite journal |last1=Fedonkin |first1=Mikhail A. |author1-link=Mikhail Fedonkin |last2=Simonetta |first2=Alberto |last3=Ivantsov |first3=Andrei Yu. |date=January 1, 2007 |title=New data on ''Kimberella'', the Vendian mollusc-like organism (White Sea region, Russia): palaeoecological and evolutionary implications |url=https://www.monash.edu/__data/assets/pdf_file/0010/75673/fedonkin2.pdf |url-status=live |journal=Geological Society Special Publications |volume=286 |issue=1 |pages=157–179 |bibcode=2007GSLSP.286..157F |doi=10.1144/SP286.12 |s2cid=331187 |issn=0375-6440 |archive-url=https://web.archive.org/web/20170811081056/https://www.monash.edu/__data/assets/pdf_file/0010/75673/fedonkin2.pdf |archive-date=2017-08-11 |access-date=2020-02-18}}</ref>

TheWhile [[microorganism]]s formed the earliest [[terrestrial ecosystem]]s at least 2.7 Ga, the [[evolution of plants]] from freshwater [[green algae]] dateddates back even to about 1 billion years ago,.<ref>{{cite journal |last1=Strother |first1=Paul K. |last2=Battison |first2=Leila |last3=Brasier |first3=Martin D. |author3-link=Martin Brasier |last4=Wellman |first4=Charles H. |display-authors=3 |date=May 26, 2011 |title=Earth's earliest non-marine eukaryotes |journal=[[Nature (journal)|Nature]] |volume=473 |issue=7348 |pages=505–509 |bibcode=2011Natur.473..505S |doi=10.1038/nature09943 |s2cid=4418860 |issn=0028-0836 |pmid=21490597}}</ref> although evidence suggests that [[microorganism]]s formed the earliest [[terrestrial ecosystem]]s, at least 2.7 Ga.<ref>{{cite journal |last=Beraldi-Campesi |first=Hugo |date=February 23, 2013 |title=Early life on land and the first terrestrial ecosystems |journal=Ecological Processes |volume=2 |issue=1 |pages=1–17 |doi=10.1186/2192-1709-2-1 |doi-access=free |bibcode=2013EcoPr...2....1B |issn=2192-1709}}</ref> Microorganisms are thought to have paved the way for the inception of land plants in the [[Ordovician]] period. Land plants were so successful that they are thought to have contributed to the [[Late Devonian extinction|Late Devonian extinction event]].<ref name="AlgeoScheckler1998" /> (The long causal chain implied seems to involve (1) the success ofas early tree [[archaeopteris]] drew down CO₂ levels, leading to [[global cooling]] and lowered sea levels, (2)while their roots of archeopteris fostered soil development which ''increased'' rock weathering, and the subsequent nutrient run-offoffs which may have triggered [[algal bloom]]s resulting in [[anoxic event]]s which caused marine-life die-offs. Marine species were the primary victims of the Late Devonian extinction.)

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

The [[Permian–Triassic extinction event]] killed most complex species of its time, {{ma|252|Ma}}.<ref>{{cite web |url=https://science.nasa.gov/science-news/science-at-nasa/2002/28jan_extinction/ |url-status=live |title=The Great Dying |last=Barry |first=Patrick L. |date=January 28, 2002 |editor-last=Phillips |editor-first=Tony |work=Science@NASA |publisher=[[Marshall Space Flight Center]] |archive-url=https://web.archive.org/web/20100410015208/https://science.nasa.gov/science-news/science-at-nasa/2002/28jan_extinction/ |archive-date=2010-04-10 |access-date=2015-01-22}}</ref> During the recovery from this catastrophe, [[archosaur]]s became the most abundant land vertebrates;<ref name="TannerLucas2004">{{cite journal |last1=Tanner |first1=Lawrence H. |last2=Lucas |first2=Spencer G. |author2-link=Spencer G. Lucas |last3=Chapman |first3=Mary G. |date=March 2004 |title=Assessing the record and causes of Late Triassic extinctions |url=http://nmnaturalhistory.org/pdf_files/TJB.pdf |journal=[[Earth-Science Reviews]] |volume=65 |issue=1–2 |pages=103–139 |bibcode=2004ESRv...65..103T |doi=10.1016/S0012-8252(03)00082-5 |archive-url=https://web.archive.org/web/20071025225841/http://nmnaturalhistory.org/pdf_files/TJB.pdf |archive-date=2007-10-25 |access-date=2007-10-22}}</ref> one archosaur group, the [[dinosaur]]s, dominated the [[Jurassic]] and [[Cretaceous]] periods.<ref>{{harvnb|Benton|1997}}</ref> After the [[Cretaceous–Paleogene extinction event]] {{ma|Paleogene|Ma}} killed off the non-avian dinosaurs,<ref>{{cite journal |last1=Fastovsky |first1=David E. |last2=Sheehan |first2=Peter M. |date=March 2005 |title=The Extinction of the Dinosaurs in North America |url=https://www.geosociety.org/gsatoday/archive/15/3/pdf/i1052-5173-15-3-4.pdf |url-status=live |journal=[[Geological Society of America|GSA Today]] |volume=15 |issue=3 |pages=4–10 |doi=10.1130/1052-5173(2005)015<4:TEOTDI>2.0.CO;2 |issn=1052-5173 |archive-url=https://web.archive.org/web/20190322190338/https://www.geosociety.org/gsatoday/archive/15/3/pdf/i1052-5173-15-3-4.pdf |archive-date=2019-03-22 |access-date=2015-01-23}}</ref> mammals [[Adaptive radiation|increased rapidly in size and diversity]].<ref>{{cite news |last=Roach |first=John |date=June 20, 2007 |title=Dinosaur Extinction Spurred Rise of Modern Mammals |url=https://news.nationalgeographic.com/news/2007/06/070620-mammals-dinos.html |url-status=dead |work=National Geographic News |location=Washington, D.C. |publisher=[[National Geographic Society]] |archive-url=https://web.archive.org/web/20080511161825/https://news.nationalgeographic.com/news/2007/06/070620-mammals-dinos.html |archive-date=2008-05-11 |access-date=2020-02-21}}

|note7=Earliest indisputable [[multicellular organism]]<ref name="PTRS-2015">{{cite journal |last=Erwin |first=Douglas H. |author-link=Douglas Erwin |date=December 19, 2015 |title=Early metazoan life: divergence, environment and ecology |journal=[[Philosophical Transactions of the Royal Society B]] |volume=370 |issue=1684 |doi=10.1098/rstb.2015.0036 |issn=0962-8436 |pmc=4650120 |pmid=26554036 |id=Article 20150036}}</ref><ref name="El Albani2010">{{cite journal |last1=El Albani |first1=Abderrazak |author1-link=Abderrazak El Albani |last2=Bengtson |first2=Stefan |last3=Canfield |first3=Donald E. |author3-link=Donald Canfield |last4=Bekker |first4=Andrey |last5=Macchiarelli |first5=Reberto |last6=Mazurier |first6=Arnaud |last7=Hammarlund |first7=Emma U. |last8=Boulvais |first8=Philippe |last9=Dupuy |first9=Jean-Jacques |display-authors=3 |date=July 1, 2010 |title=Large colonial organisms with coordinated growth in oxygenated environments 2.1 Gyr ago |journal=[[Nature (journal)|Nature]] |volume=466 |issue=7302 |pages=100–104 |bibcode=2010Natur.466..100A |doi=10.1038/nature09166 |s2cid=4331375 |issn=0028-0836 |pmid=20596019}}</ref><ref>{{cite journal |url=https://www.cambridge.org/core/journals/paleobiology/article/bangiomorpha-pubescens-n-gen-n-sp-implications-for-the-evolution-of-sex-multicellularity-and-the-mesoproterozoicneoproterozoic-radiation-of-eukaryotes/E61F0C87E1F2CD5A622AEF57ADCC97EF |doi=10.1666/0094-8373(2000)026<0386:BPNGNS>2.0.CO;2 |title=Bangiomorpha pubescensn. Gen., n. Sp.: Implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes |year=2000 |last1=Butterfield |first1=Nicholas J. |journal=Paleobiology |volume=26 |issue=3 |pages=386–404 |bibcode=2000Pbio...26..386B |s2cid=36648568 |access-date=2022-11-09 |archive-date=2020-11-12 |archive-url=https://web.archive.org/web/20201112014759/https://www.cambridge.org/core/journals/paleobiology/article/bangiomorpha-pubescens-n-gen-n-sp-implications-for-the-evolution-of-sex-multicellularity-and-the-mesoproterozoicneoproterozoic-radiation-of-eukaryotes/E61F0C87E1F2CD5A622AEF57ADCC97EF |url-status=live }}</ref><ref>{{cite journal |last1=Butterfield |first1=Nicholas J. |title=Bangiomorpha pubescensn. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes |journal=Paleobiology |date=September 2000 |volume=26 |issue=3 |pages=386–404 |doi=10.1666/0094-8373(2000)026<0386:BPNGNS>2.0.CO;2 |bibcode=2000Pbio...26..386B |s2cid=36648568 |url=https://www.researchgate.net/publication/278693786 |access-date=22 December 2022 |language=en}}</ref>

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

Until 2001, the oldest rocks found on Earth were about 3.8 billion years old,<ref>{{cite news |last=Thompson |first=Andrea |date=September 25, 2008 |title=Oldest Rocks on Earth Found |url=https://www.livescience.com/2896-oldest-rocks-earth.html |work=[[Live Science]] |publisher=[[Imaginova]] |location=Watsonville, CA |access-date=2015-01-23 |archive-date=2012-06-12 |archive-url=https://web.archive.org/web/20120612013656/https://www.livescience.com/2896-oldest-rocks-earth.html |url-status=live }}</ref><ref name="ageOfEarth" /> leading scientists to estimate that the Earth's surface had been molten until then. Accordingly, they named this part of Earth's history the [[Hadean]].<ref name="Cohen2000">{{cite journal |last1=Cohen |first1=Barbara A. |author1-link=Barbara Cohen (scientist) |last2=Swindle |first2=Timothy D. |last3=Kring |first3=David A. |date=December 1, 2000 |title=Support for the Lunar Cataclysm Hypothesis from Lunar Meteorite Impact Melt Ages |journal=[[Science (journal)|Science]] |volume=290 |issue=5497 |pages=1754–1756 |bibcode=2000Sci...290.1754C |doi=10.1126/science.290.5497.1754 |issn=0036-8075 |pmid=11099411}}</ref> However, analysis of [[zircon]]s formed 4.4 Ga indicates that Earth's crust solidified about 100 million years after the planet's formation and that the planet quickly acquired oceans and an [[atmosphere]], which may have been capable of supporting life.<ref>{{cite press release |author=<!--Staff writer(s); no by-line.--> |title=Early Earth Likely Had Continents And Was Habitable |url=http://www.colorado.edu/news/releases/2005/11/17/early-earth-likely-had-continents-and-was-habitable-says-new-study |url-status=dead |location=Boulder, CO |publisher=[[University of Colorado]] |date=November 17, 2005 |access-date=2015-01-23 |archive-url=https://web.archive.org/web/20150124005824/http://www.colorado.edu/news/releases/2005/11/17/early-earth-likely-had-continents-and-was-habitable-says-new-study |archive-date=2015-01-24}}

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

The earliest identified organisms were minute and relatively featureless, and their fossils looklooked like small rods that are very difficult to tell apart from structures that arise through abiotic physical processes. The oldest undisputed evidence of life on Earth, interpreted as fossilized bacteria, dates to 3 Ga.<ref name="BrasierMcLoughlinEtAl2006" /> Other finds in rocks dated to about 3.5 Ga have been interpreted as bacteria,<ref>{{cite journal |last=Schopf |first=J. William |author-link=J. William Schopf |date=April 30, 1993 |title=Microfossils of the Early Archean Apex Chert: New Evidence of the Antiquity of Life |journal=[[Science (journal)|Science]] |volume=260 |issue=5108 |pages=640–646 |bibcode=1993Sci...260..640S |doi=10.1126/science.260.5108.640 |s2cid=2109914 |issn=0036-8075 |pmid=11539831}}

*{{cite journal |last1=Altermann |first1=Wladyslaw |last2=Kazmierczak |first2=Józef |date=November 2003 |title=Archean microfossils: a reappraisal of early life on Earth |journal=Research in Microbiology |volume=154 |issue=9 |pages=611–617 |doi=10.1016/j.resmic.2003.08.006 |issn=0923-2508 |pmid=14596897|doi-access=free }}</ref> with [[Geochemistry|geochemical]] evidence also seeming to show the presence of life 3.8 Ga.<ref>{{cite journal |last1=Mojzsis |first1=Stephen J. |last2=Arrhenius |first2=Gustaf |last3=McKeegan |first3=Kevin D. |last4=Harrison |first4=T. Mark |last5=Nutman |first5=Allen P. |last6=Friend |first6=Clark R. L. |display-authors=3 |date=November 7, 1996 |title=Evidence for life on Earth before 3,800 million years ago |journal=[[Nature (journal)|Nature]] |volume=384 |issue=6604 |pages=55–59 |bibcode=1996Natur.384...55M |doi=10.1038/384055a0 |pmid=8900275 |issn=0028-0836 |hdl=2060/19980037618 |s2cid=4342620 |hdl-access=free}}</ref> However, these analyses were closely scrutinized, and non-biological processes were found which could produce all of the "signatures of life" that had been reported.<ref name="GrotzingerRothman1996">{{cite journal |last1=Grotzinger |first1=John P. |author1-link=John P. Grotzinger |last2=Rothman |first2=Daniel H. |date=October 3, 1996 |title=An abiotic model for stromatolite morphogenesis |journal=[[Nature (journal)|Nature]] |volume=383 |issue=6599 |pages=423–425 |bibcode=1996Natur.383..423G |doi=10.1038/383423a0 |s2cid=4325802 |issn=0028-0836}}</ref><ref>{{cite journal |last1=Fedo |first1=Christopher M. |last2=Whitehouse |first2=Martin J. |date=May 24, 2002 |title=Metasomatic Origin of Quartz-Pyroxene Rock, Akilia, Greenland, and Implications for Earth's Earliest Life |journal=[[Science (journal)|Science]] |volume=296 |issue=5572 |pages=1448–1452 |bibcode=2002Sci...296.1448F |doi=10.1126/science.1070336 |s2cid=10367088 |issn=0036-8075 |pmid=12029129}}

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

{{PhylomapA|size=300px|caption=[[Phylogenetic tree|Evolutionary tree]] showing the divergence of modern species from their common ancestor in the center.<ref name="Ciccarelli">{{cite journal |last1=Ciccarelli |first1=Francesca D. |last2=Doerks |first2=Tobias |last3=von Mering |first3=Christian |last4=Creevey |first4=Christopher J. |last5=Snel |first5=Berend |last6=Bork |first6=Peer |author6-link=Peer Bork |display-authors=3 |date=March 3, 2006 |title=Toward Automatic Reconstruction of a Highly Resolved Tree of Life |url=https://bioinformatics.bio.uu.nl/pdf/Ciccarelli.s06-311.pdf |url-status=dead |journal=[[Science (journal)|Science]] |volume=311 |issue=5765 |pages=1283–1287 |bibcode=2006Sci...311.1283C |doi=10.1126/science.1123061 |pmid=16513982 |citeseerx=10.1.1.381.9514 |s2cid=1615592 |archive-url=https://web.archive.org/web/20070824132444/https://bioinformatics.bio.uu.nl/pdf/Ciccarelli.s06-311.pdf |archive-date=2007-08-24}}</ref> The three [[Domain (biology)|domains]] are colored, with [[bacteria]] blue, [[archaea]] green, and [[eukaryote]]s red.}}

According to a different scenario<ref name="Kandler-1994">{{Cite book |last=Kandler |first=Otto |url=https://books.google.com/books?id=lOqdAwAAQBAJ&q=Bengtson+%22Early+Life+on+Earth%22++Nobel+Symposium+84 |title=Early Life on Earth. Nobel Symposium 84 |publisher=Columbia U.P. |year=1994 |isbn=978-02310808800-231-08088-0 |editor-last=Bengtson |editor-first=Stefan |location=New York |pages=152–160 |chapter=The early diversification of life |author-link=Otto Kandler |access-date=2023-03-19 |archive-date=2023-04-08 |archive-url=https://web.archive.org/web/20230408093724/https://books.google.com/books?id=lOqdAwAAQBAJ&q=Bengtson+%22Early+Life+on+Earth%22++Nobel+Symposium+84 |url-status=live }}</ref><ref name="Kandler-1995">{{Cite journal |last=Kandler |first=Otto |author-link=Otto Kandler |date=1995 |title=Cell Wall Biochemistry in Archaea and its Phylogenetic Implications |url=https://www.semanticscholar.org/paper/Cell-wall-biochemistry-in-Archaea-and-its-Kandler/7729cf2cc863edba0d73038ae0a28316ddae31a1 |journal=Journal of Biological Physics |volume=20 |issue=1–4 |pages=165–169 |doi=10.1007/BF00700433 |s2cid=83906865 |via= |access-date=2023-01-25 |archive-date=2023-01-25 |archive-url=https://web.archive.org/web/20230125214308/https://www.semanticscholar.org/paper/Cell-wall-biochemistry-in-Archaea-and-its-Kandler/7729cf2cc863edba0d73038ae0a28316ddae31a1 |url-status=live }}</ref><ref name="Kandler-1998">{{Cite book |last=Kandler |first=Otto |title=Thermophiles: The keys to molecular evolution and the origin of life? |publisher=Taylor and Francis Ltd. |year=1998 |isbn=978-0-203-48420-3 |editor-last=Wiegel |editor-first=Jürgen |location=London |pages=19–31 |chapter=The early diversification of life and the origin of the three domains: A proposal |author-link=Otto Kandler |editor-last2=Adams |editor-first2=Michael W.W. |chapter-url=https://books.google.com/books?id=FtSzl4iastsC |access-date=2023-01-29 |archive-date=2023-02-25 |archive-url=https://web.archive.org/web/20230225201522/https://books.google.com/books?id=FtSzl4iastsC |url-status=live }}</ref> a single last universal ancestor, e.g. a "first cell" or a first individual precursor cell has never existed. Instead, the early biochemical evolution of life<ref name="Wächtershäuser-2000" /> led to diversification through the development of a multiphenotypical population of [[pre-cell]]s from which the precursor cells ([[protocell]]s) of the three [[Domain (biology)|domains of life]]<ref name="Woese-1990">{{Cite journal |last1=Woese |first1=Carl R. |author-link=Carl Woese |last2=Kandler |first2=Otto |author-link2=Otto Kandler |last3=Wheelis |first3=Mark |date=1990 |title=Towards a natural system of organisms: proposal for the domains Archaea, Bacteria, and Eucarya |journal=Proc. Natl. Acad. Sci. USA |volume=87 |issue=12 |pages=4576–4579 |bibcode=1990PNAS...87.4576W |doi=10.1073/pnas.87.12.4576 |pmc=54159 |pmid=2112744 |doi-access=free}}</ref> emerged. Thus, the formation of cells was a successive process. See {{slink||Metabolism first: Pre-cells, successive cellularisation}}, below.

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

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

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

In this scenario, the biochemical evolution of life<ref name="Wächtershäuser-2000" /> led to diversification through the development of a multiphenotypical population of [[Prepre-cell|pre-cells]]s,<ref name="Kandler-1994" /><ref name="Kandler-1995" /><ref name="Kandler-1998" /> i.e. evolving entities of primordial life with different characteristics and wide-spread [[horizontal gene transfer]].

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

Catalytic mineral particles and transition metal sulfides at these environments are capable of catalyzing organic compounds.<ref>{{Cite web |title=Origin of life: Chemistry of seabed's hot vents could explain emergence of life |url=https://www.sciencedaily.com/releases/2015/04/150427101635.htm |access-date=2022-10-07 |website=ScienceDaily |language=en |archive-date=2022-10-07 |archive-url=https://web.archive.org/web/20221007223205/https://www.sciencedaily.com/releases/2015/04/150427101635.htm |url-status=live }}</ref> Scientists simulated laboratory conditions that were identical to white smokers and successfully oligomerized RNA, measured to be 4 units long.<ref>{{Cite journal |last1=Burcar |first1=Bradley T. |last2=Barge |first2=Laura M. |last3=Trail |first3=Dustin |last4=Watson |first4=E. Bruce |last5=Russell |first5=Michael J. |last6=McGown |first6=Linda B. |date=2015-07-01 |title=RNA Oligomerization in Laboratory Analogues of Alkaline Hydrothermal Vent Systems |url=https://www.liebertpub.com/doi/abs/10.1089/ast.2014.1280 |journal=Astrobiology |volume=15 |issue=7 |pages=509–522 |doi=10.1089/ast.2014.1280 |pmid=26154881 |bibcode=2015AsBio..15..509B |issn=1531-1074 |access-date=2022-10-07 |archive-date=2020-06-03 |archive-url=https://web.archive.org/web/20200603131804/https://www.liebertpub.com/doi/abs/10.1089/ast.2014.1280 |url-status=live }}</ref> Long chain fatty acids can be synthesized via [[Fischer–Tropsch process|Fischer-Tropsch synthesis]].<ref name=":0Harrison-2022">{{Cite journal |last1=Harrison |first1=Stuart A. |last2=Palmeira |first2=Raquel Nunes |last3=Halpern |first3=Aaron |last4=Lane |first4=Nick |date=2022-11-01 |title=A biophysical basis for the emergence of the genetic code in protocells |url=https://www.sciencedirect.com/science/article/pii/S0005272822000664 |journal=Biochimica et Biophysica Acta (BBA) - Bioenergetics |language=en |volume=1863 |issue=8 |pagespage=148597 |doi=10.1016/j.bbabio.2022.148597 |pmid=35868450 |s2cid=250707510 |issn=0005-2728|doi-access=free }}</ref> Another experiment that replicated conditions also similar white smokers, with long chain fatty acids present resulted in the assembly of vesicles.<ref>{{Cite journal |last1=Jordan |first1=Sean F. |last2=Rammu |first2=Hanadi |last3=Zheludev |first3=Ivan N. |last4=Hartley |first4=Andrew M. |last5=Maréchal |first5=Amandine |last6=Lane |first6=Nick |date=November 4, 2019 |title=Promotion of protocell self-assembly from mixed amphiphiles at the origin of life |url=https://eprints.bbk.ac.uk/id/eprint/29841/1/Jordan%20Lane%20Nat%20Ecol%20Evol%20accepted%20MS%202019.pdf |journal=Nature Ecology & Evolution |language=en |volume=3 |issue=12 |pages=1705–1714 |doi=10.1038/s41559-019-1015-y |pmid=31686020 |bibcode=2019NatEE...3.1705J |s2cid=207891212 |issn=2397-334X |access-date=October 10, 2022 |archive-date=October 10, 2022 |archive-url=https://web.archive.org/web/20221010214545/https://eprints.bbk.ac.uk/id/eprint/29841/1/Jordan%20Lane%20Nat%20Ecol%20Evol%20accepted%20MS%202019.pdf |url-status=live }}</ref> Exergonic reactions at hydrothermal vents are suggested to have been a source of free energy that promoted chemical reactions, synthesis of organic molecules, and are inducive to chemical gradients.<ref>{{Cite web |last1=Colín-García |first1=María |last2=Heredia |first2=Alejandro |last3=Cordero |first3=Guadalupe |last4=Camprubí |first4=Antoni |last5=Negrón-Mendoza |first5=Alicia |last6=Ortega-Gutiérrez |first6=Fernando |last7=Beraldi |first7=Hugo |last8=Ramos-Bernal |first8=Sergio |date=2016 |title=Hydrothermal vents and prebiotic chemistry: a review |url=http://boletinsgm.igeolcu.unam.mx/bsgm/index.php/component/content/article/309-sitio/articulos/cuarta-epoca/6803/1620-6803-13-colin |access-date=2022-10-07 |website=Boletín de la Sociedad Geológica Mexicana |language=en-gb |archive-date=2017-08-18 |archive-url=https://web.archive.org/web/20170818175803/http://boletinsgm.igeolcu.unam.mx/bsgm/index.php/component/content/article/309-sitio/articulos/cuarta-epoca/6803/1620-6803-13-colin |url-status=live }}</ref> In small rock pore systems, membranous structures between alkaline seawater and the acidic ocean would be conducive to natural proton gradients.<ref>{{Cite journal |last1=Lane |first1=Nick |last2=Allen |first2=John F. |last3=Martin |first3=William |date=2010-01-27 |title=How did LUCA make a living? Chemiosmosis in the origin of life |url=https://onlinelibrary.wiley.com/doi/abs/10.1002/bies.200900131 |journal=BioEssays |language=en |volume=32 |issue=4 |pages=271–280 |doi=10.1002/bies.200900131 |pmid=20108228 |access-date=2022-10-24 |archive-date=2022-10-24 |archive-url=https://web.archive.org/web/20221024030805/https://onlinelibrary.wiley.com/doi/abs/10.1002/bies.200900131 |url-status=live }}</ref> Nucleobase synthesis could occur by following universally conserved biochemical pathways by using metal ions as catalysts.<ref name=":0Harrison-2022" /> RNA molecules of 22 bases can be polymerized in alkaline hydrothermal vent pores. Thin pores are shown to only accumulate long polynucleotides whereas thick pores accumulate both short and long polynucleotides. Small mineral cavities or mineral gels could have been a compartment for abiogenic processes.<ref>{{Cite journal |last1=Westall |first1=F. |last2=Hickman-Lewis |first2=K. |last3=Hinman |first3=N. |last4=Gautret |first4=P. |last5=Campbell |first5=K.a. |last6=Bréhéret |first6=J.g. |last7=Foucher |first7=F. |last8=Hubert |first8=A. |last9=Sorieul |first9=S. |last10=Dass |first10=A.v. |last11=Kee |first11=T.p. |last12=Georgelin |first12=T. |last13=Brack |first13=A. |date=2018-03-01 |title=A Hydrothermal-Sedimentary Context for the Origin of Life |journal=Astrobiology |volume=18 |issue=3 |pages=259–293 |doi=10.1089/ast.2017.1680 |issn=1531-1074 |pmc=5867533 |pmid=29489386 |bibcode=2018AsBio..18..259W}}</ref><ref>{{Cite journal |last1=Lane |first1=Nick |last2=Martin |first2=William F. |date=2012-12-21 |title=The Origin of Membrane Bioenergetics |journal=Cell |language=en |volume=151 |issue=7 |pages=1406–1416 |doi=10.1016/j.cell.2012.11.050 |pmid=23260134 |s2cid=15028935 |issn=0092-8674 |doi-access=free}}</ref><ref>{{Cite journal |last1=Baaske |first1=Philipp |last2=Weinert |first2=Franz M. |last3=Duhr |first3=Stefan |last4=Lemke |first4=Kono H. |last5=Russell |first5=Michael J. |last6=Braun |first6=Dieter |date=2007-05-29 |title=Extreme accumulation of nucleotides in simulated hydrothermal pore systems |journal=Proceedings of the National Academy of Sciences |language=en |volume=104 |issue=22 |pages=9346–9351 |doi=10.1073/pnas.0609592104 |issn=0027-8424 |pmc=1890497 |pmid=17494767 |doi-access=free }}</ref> A genomic analysis supports this hypothesis as they found 355 genes that likely traced to [[Last universal common ancestor|LUCA]] upon 6.1 million sequenced prokaryotic genes. They reconstruct LUCA as a thermophilic anaerobe with a Wood-Ljungdahl pathway, implying an origin of life at white smokers. LUCA would also have exhibited other biochemical pathways such as [[gluconeogenesis]], [[Reverse Krebs cycle|reverse incomplete Krebs cycle]], [[glycolysis]], and the [[pentose phosphate pathway]], including biochemical reactions such as [[reductive amination]] and [[transamination]].<ref>{{Cite journal |last1=Weiss |first1=Madeline C. |last2=Sousa |first2=Filipa L. |last3=Mrnjavac |first3=Natalia |last4=Neukirchen |first4=Sinje |last5=Roettger |first5=Mayo |last6=Nelson-Sathi |first6=Shijulal |last7=Martin |first7=William F. |date=2016-07-25 |title=The physiology and habitat of the last universal common ancestor |url=https://www.almendron.com/tribuna/wp-content/uploads/2019/10/the-physiology-and-habitat-of-the-last-universal-common-ancestor.pdf |journal=Nature Microbiology |language=en |volume=1 |issue=9 |page=16116 |doi=10.1038/nmicrobiol.2016.116 |pmid=27562259 |s2cid=2997255 |issn=2058-5276 |access-date=2022-10-24 |archive-date=2023-01-29 |archive-url=https://web.archive.org/web/20230129185028/https://www.almendron.com/tribuna/wp-content/uploads/2019/10/the-physiology-and-habitat-of-the-last-universal-common-ancestor.pdf |url-status=live }}</ref><ref>{{Cite journal |last1=Weiss |first1=Madeline C. |last2=Preiner |first2=Martina |last3=Xavier |first3=Joana C. |last4=Zimorski |first4=Verena |last5=Martin |first5=William F. |date=2018-08-16 |title=The last universal common ancestor between ancient Earth chemistry and the onset of genetics |journal=PLOS Genetics |language=en |volume=14 |issue=8 |pages=e1007518 |doi=10.1371/journal.pgen.1007518 |issn=1553-7404 |pmc=6095482 |pmid=30114187 |doi-access=free }}</ref><ref name=":0Harrison-2022" /><ref>{{Cite journal |last1=Harrison |first1=Stuart A. |last2=Lane |first2=Nick |date=2018-12-12 |title=Life as a guide to prebiotic nucleotide synthesis |journal=Nature Communications |language=en |volume=9 |issue=1 |pagespage=5176 |doi=10.1038/s41467-018-07220-y |pmid=30538225 |issn=2041-1723|pmc=6289992 |bibcode=2018NatCo...9.5176H }}</ref>

One theory traces the [[Abiogenesis|origins of life]] to the abundant carbonate-rich lakes which would have dotted the [[early Earth]]. [[Phosphate]] would have been an essential cornerstone to the origin of life since it is a critical component of [[nucleotide]]s, [[phospholipid]]s, and [[Adenosineadenosine triphosphate|Adenosine Triphosphate]].<ref name="Schwartz-2006">{{Cite journal |last=Schwartz |first=Alan W |date=2006-09-07 |title=Phosphorus in prebiotic chemistry |url=http://dx.doi.org/10.1098/rstb.2006.1901 |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |volume=361 |issue=1474 |pages=1743–1749 |doi=10.1098/rstb.2006.1901 |pmid=17008215 |pmc=1664685 |issn=0962-8436 |access-date=2023-02-26 |archive-date=2023-07-16 |archive-url=https://web.archive.org/web/20230716080401/https://royalsocietypublishing.org/doi/10.1098/rstb.2006.1901 |url-status=live }}</ref> Phosphate is often depleted in natural environments due to its uptake by microbes and its affinity for [[calcium]] ions. In a process called ‘'[[apatite]] precipitation’precipitation', free phosphate ions react with the calcium ions abundant in water to precipitate out of solution as apatite minerals.<ref name="Schwartz-2006" /> When attempting to simulate prebiotic [[phosphorylation]], scientists have only found success when using phosphorus levels far above modern day natural concentrations.<ref name="Toner-2020" />

This problem of low phosphate is solved in [[carbonate]]-rich environments. When in the presence of carbonate, calcium readily reacts to form [[calcium carbonate]] instead of apatite minerals.<ref name="Nathan-1981">{{Cite journal |last1=Nathan |first1=Yaacov |last2=Sass |first2=Eytan |date=November 1981 |title=Stability relations of apatites and calcium carbonates |url=http://dx.doi.org/10.1016/0009-2541(81)90075-9 |journal=Chemical Geology |volume=34 |issue=1–2 |pages=103–111 |doi=10.1016/0009-2541(81)90075-9 |bibcode=1981ChGeo..34..103N |issn=0009-2541 |access-date=2023-02-26 |archive-date=2023-07-16 |archive-url=https://web.archive.org/web/20230716080410/https://www.sciencedirect.com/science/article/abs/pii/0009254181900759?via%3Dihub |url-status=live }}</ref> With the free calcium ions removed from [[Solution (chemistry)|solution]], phosphate ions are no longer precipitated from solution.<ref name="Nathan-1981" /> This is specifically seen in lakes with no inflow, since no new calcium is introduced into the water body.<ref name="Toner-2020" /> After all of the calcium is sequestered into calcium carbonate ([[calcite]]), phosphate concentrations are able to increase to levels necessary for facilitating [[biomolecule]] creation.<ref>{{Cite journal |last=Gulbrandsen |first=R. A. |date=1969-06-01 |title=Physical and chemical factors in the formation of marine apatite |url=http://dx.doi.org/10.2113/gsecongeo.64.4.365 |journal=Economic Geology |volume=64 |issue=4 |pages=365–382 |doi=10.2113/gsecongeo.64.4.365 |bibcode=1969EcGeo..64..365G |issn=1554-0774 |access-date=2023-02-26 |archive-date=2023-07-16 |archive-url=https://web.archive.org/web/20230716080402/http://mr.crossref.org/iPage?doi=10.2113%2Fgsecongeo.64.4.365 |url-status=live }}</ref>

Though carbonate-rich lakes have [[Base (chemistry)|alkaline]] chemistry in modern times, models suggest that carbonate lakes had a pH low enough for prebiotic synthesis when placed in the acidifying context of Earth’sEarth's early [[Early atmosphere|carbon dioxide rich atmosphere]].<ref name="Toner-2020" /> Rainwater rich in [[carbonic acid]] [[Weathering|weathered]] the rock on the surface of the Earth at rates far greater than today.<ref>{{Cite journal |last1=Zahnle |first1=K. |last2=Schaefer |first2=L. |last3=Fegley |first3=B. |date=2010-06-23 |title=Earth's Earliest Atmospheres |url=http://dx.doi.org/10.1101/cshperspect.a004895 |journal=Cold Spring Harbor Perspectives in Biology |volume=2 |issue=10 |pages=a004895 |doi=10.1101/cshperspect.a004895 |pmid=20573713 |pmc=2944365 |issn=1943-0264 |access-date=2023-02-26 |archive-date=2023-07-16 |archive-url=https://web.archive.org/web/20230716080401/https://cshperspectives.cshlp.org/content/2/10/a004895 |url-status=live }}</ref> With high phosphate influx, no phosphate precipitation, and no [[Microorganism|microbial]] usage of phosphate at this time, models show phosphate reached concentrations approximately 100 times greater than they are today.<ref name="Toner-2020" /> Modeled [[pH]] and phosphate levels of early Earth carbonate-rich lakes nearly match the conditions used in current laboratory experiments on the origin of life.<ref name="Toner-2020" />

Similar to the process predicted by [[Geothermal springs|geothermal]] [https://www.liebertpub.com/doi/10.1089/ast.2019.2045 hot spring hypotheses], changing lake levels and wave action deposited phosphorus-rich brine onto dry shore and marginal pools.<ref name="Damer-2020-2">{{Cite journal |last1=Damer |first1=Bruce |last2=Deamer |first2=David |date=2020-04-01 |title=The Hot Spring Hypothesis for an Origin of Life |url=http://dx.doi.org/10.1089/ast.2019.2045 |journal=Astrobiology |volume=20 |issue=4 |pages=429–452 |doi=10.1089/ast.2019.2045 |pmid=31841362 |pmc=7133448 |bibcode=2020AsBio..20..429D |issn=1531-1074 |access-date=2023-02-26 |archive-date=2023-07-16 |archive-url=https://web.archive.org/web/20230716080927/https://www.liebertpub.com/doi/10.1089/ast.2019.2045 |url-status=live }}</ref> This drying of the solution promotes [[polymerization]] reactions and removes enough water to promote phosphorylation, a process integral to biological energy storage and transfer.<ref name="Toner-2020" /><ref name="Damer-2020-2"/><ref>{{Cite journal |last1=Ross |first1=David |last2=Deamer |first2=David |date=2016-07-26 |title=Dry/Wet Cycling and the Thermodynamics and Kinetics of Prebiotic Polymer Synthesis |journal=Life |volume=6 |issue=3 |pagespage=28 |doi=10.3390/life6030028 |pmid=27472365 |pmc=5041004 |bibcode=2016Life....6...28R |issn=2075-1729 |doi-access=free}}</ref> When washed away by further precipitation and wave action, researchers concludeconcluded these newly formed biomolecules may have washed back into the lake - allowing the first prebiotic syntheses on Earth to occur.<ref name="Toner-2020" />

[[Stromatolite]]s are stubby pillars built as microorganisms in mats slowly migrate upwards to avoid being smothered by sediment deposited on them by water.<ref name="KrumbeinBrehmEtAl2003" /> There has been vigorous debate about the validity of alleged stromatolite fossils from before 3 Ga,<ref>{{cite journal |author=<!--Staff writer(s); no by-line.--> |date=June 8, 2006 |title=Biodiversity rocks |url=https://www.nature.com/nature/journal/v441/n7094/edsumm/e060608-01.html |url-status=dead |journal=[[Nature (journal)|Nature]] |type=Editor's summary |volume=441 |issue=7094 |issn=0028-0836 |archive-url=https://web.archive.org/web/20060619105759/https://www.nature.com/nature/journal/v441/n7094/edsumm/e060608-01.html |archive-date=2006-06-19 |access-date=2020-03-03}}

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

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

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

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

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

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

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

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

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

[[Bacteria]] also exchange [[DNA]] by [[bacterial conjugation]], the benefits of which include resistance to [[antibiotic resistance|antibiotics]] and other [[toxin]]s, and the ability to utilize new [[metabolite]]s.<ref>{{cite book |last1=Holmes |first1=Randall K. |last2=Jobling |first2=Michael G. |year=1996 |chapter=Genetics |chapter-url=https://www.ncbi.nlm.nih.gov/books/NBK7908/ |editor-last=Baron |editor-first=Samuel |title=Medical Microbiology |edition=4th |location=Galveston, TX |publisher=[[University of Texas Medical Branch]] |at=Exchange of Genetic Information |isbn=0-9631172-1-1 |lccn=95050499 |oclc=33838234 |pmid=21413277}}</ref> However, conjugation is not a means of reproduction and is not limited to members of the same species, and there are cases where bacteria transfer DNA to plants and animals.<ref name="Christie, P. J. 2001 294–305"/> Nevertheless, it may be an example of the "selfish genetic element" hypothesis, as it transfers DNA by means of such a "selfish gene,", the [[F-plasmid]].<ref name="Judson2002"/>

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

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

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

The controversial [[Francevillian biota]] fossils, dated to 2.1 Ga, are the earliest known fossil organisms that are clearly multicellular, if they are indeed fossils.<ref name="El Albani2010"/> They may have had differentiated cells.<ref name="Dickey2010">{{cite magazine |last=Dickey |first=Gwyneth |date=July 31, 2010 |title=Evidence for earlier multicellular life |url=http://www.sciencenewsdigital.org/sciencenews/20100731?pg=19#pg19 |format=PNG |magazine=[[Science News]] |volume=178 |issue=3 |page=17 |doi=10.1002/scin.5591780322 |issn=0036-8423 |access-date=2015-02-02}}</ref> Another early multicellular fossil, ''[[Qingshania]]'', dated to 1.7 Ga, appears to consist of virtually identical cells. The red algae called ''[[Bangiomorpha]]'', dated at 1.2 Ga, is the earliest known organism that certainly has differentiated, specialized cells, and is also the oldest known sexually reproducing organism.<ref name="Butterfield2000">{{cite journal |last=Butterfield |first=Nicholas J. |date=Summer 2000 |title=''Bangiomorpha pubescens'' n. gen., n. sp.: implications for the evolution of sex, multicellularity, and the Mesoproterozoic/Neoproterozoic radiation of eukaryotes |url=http://paleobiol.geoscienceworld.org/content/26/3/386.abstract |journal=[[Paleobiology (journal)|Paleobiology]] |volume=26 |issue=3 |pages=386–404 |doi=10.1666/0094-8373(2000)026<0386:BPNGNS>2.0.CO;2 |bibcode=2000Pbio...26..386B |s2cid=36648568 |issn=0094-8373 |access-date=2015-02-01 |archive-date=2016-10-23 |archive-url=https://web.archive.org/web/20161023233131/http://paleobiol.geoscienceworld.org/content/26/3/386.abstract |url-status=live }}</ref> The 1.43 billion-year-old fossils interpreted as fungi appear to have been multicellular with differentiated cells.<ref name="Butterfield2005" /> The "string of beads" organism ''[[Horodyskia]]'', found in rocks dated from 1.5 Ga to 900 Ma, may have been an early metazoan;<ref name="Fedonkin2003" /> however, it has also been interpreted as a colonial [[foraminifera]]n.<ref name="DongEtAl2008">{{cite journal |author1=Lin Dong |author2=Shuhai Xiao |author3=Bing Shen |author4=Chuanming Zhou |display-authors=3 |date=January 1, 2008 |title=Silicified ''Horodyskia'' and ''Palaeopascichnus'' from upper Ediacaran cherts in South China: tentative phylogenetic interpretation and implications for evolutionary stasis |journal=[[Journal of the Geological Society]] |volume=165 |issue=1 |pages=367–378 |bibcode=2008JGSoc.165..367D |doi=10.1144/0016-76492007-074 |s2cid=129309037 |issn=0016-7649}}</ref>

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

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

The [[small shelly fauna]] are a very mixed collection of fossils found between the Late Ediacaran and [[Cambrian Series 3|Middle Cambrian]] periods. The earliest, ''[[Cloudinid|Cloudina]]'', shows signs of successful defense against predation and may indicate the start of an [[evolutionary arms race]]. Some tiny Early Cambrian shells almost certainly belonged to molluscs, while the owners of some "armor plates,", ''[[Halkieriid|Halkieria]]'' and ''[[Microdictyon]]'', were eventually identified when more complete specimens were found in Cambrian [[lagerstätte]]n that preserved soft-bodied animals.<ref name="Bengtson2004">{{harvnb|Bengtson|2004|pp=67–78}}</ref>

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

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

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

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

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

Before the colonization of land there was no [[soil]], a combination of mineral particles and decomposed [[organic matter]]. Land surfaces were either bare rock or shifting sand produced by [[weathering]]. Water and dissolved nutrients would have drained away very quickly.<ref name="Shear2000" /> In the [[Sub-Cambrian peneplain]] in Sweden, for example, maximum depth of [[kaolin]]itization by [[Neoproterozoic]] [[weathering]] is about 5 m, while nearby kaolin deposits developed in the [[Mesozoic]] are [[Sub-Mesozoic hilly peneplains|much thicker]].<ref name=Lidmar-Bergstrometal2013>{{cite journal |last1=Lidmar-Bergström |first1=Karna |last2=Bonow |first2=Johan M. |last3=Japsen |first3=Peter |author-link=Karna Lidmar-Bergström |date=January 2013 |title=Stratigraphic Landscape Analysis and geomorphological paradigms: Scandinavia as an example of Phanerozoic uplift and subsidence |journal=[[Global and Planetary Change]] |volume=100 |pages=153–171 |doi=10.1016/j.gloplacha.2012.10.015 |bibcode=2013GPC...100..153L |issn=0921-8181}}</ref> It has been argued that in the late Neoproterozoic [[sheet erosion|sheet wash]] was a dominant process of erosion of surface material due to the [[Evolutionary history of plants|lack of plants]] on land.<ref name=Karna1993>{{cite journal |last1=Lidmar-Bergström |first1=Karna |author1-link=Karna Lidmar-Bergström |date=December 1993 |title=Denudation surfaces and tectonics in the southernmost part of the Baltic Shield |journal=[[Precambrian Research]] |volume=64 |issue=1–4 |pages=337–345 |doi=10.1016/0301-9268(93)90086-H |bibcode=1993PreR...64..337L |issn=0301-9268}}</ref>

In aquatic algae, almost all cells are capable of photosynthesis and are nearly independent. Life on land requiredrequires plants to become internally more complex and specialized: photosynthesis is most efficient at the top; roots extract water and nutrients from the ground; and the intermediate parts support and transport.<ref name="CowenHistLifeEd3P120To122" /><ref name="KenrickCrane1997" />

* They removed more carbon dioxide from the atmosphere, reducing the [[greenhouse effect]] and thus causing an [[ice age]] in the [[Carboniferous]] period.<ref name="AlgeoScheckler1998">{{cite journal |last1=Algeo |first1=Thomas J. |last2=Scheckler |first2=Stephen E. |date=January 29, 1998 |title=Terrestrial-marine teleconnections in the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic events |journal=[[Philosophical Transactions of the Royal Society B]] |volume=353 |issue=1365 |pages=113–130 |doi=10.1098/rstb.1998.0195 |issn=0962-8436 |pmc=1692181}}</ref> This did not repeat in later ecosystems, since the carbon dioxide "locked up" in wood was returned to the atmosphere by decomposition of dead wood, but the earliest fossil evidence of fungi that can decompose wood also comes from the Late Devonian.<ref>{{cite journal |last1=Taylor |first1=Thomas N. |last2=Osborn |first2=Jeffrey M. |date=February 1996 |title=The importance of fungi in shaping the paleoecosystem |journal=[[Review of Palaeobotany and Palynology]] |volume=90 |issue=3–4 |pages=249–262 |doi=10.1016/0034-6667(95)00086-0 |bibcode=1996RPaPa..90..249T |issn=0034-6667}}</ref>

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

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

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

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

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

The first flowering plants appeared around 130 Ma.<ref>{{cite encyclopedia |encyclopedia=[[Encyclopædia Britannica Online]] |title=evolution: plant timeline |url=https://www.britannica.com/EBchecked/media/24/Significant-events-in-plant-evolution |url-status=dead |year=1996 |publisher=[[Encyclopædia Britannica (company)|Encyclopædia Britannica, Inc.]] |isbn=9780691185507978-0-691-18550-7 |oclc=42796406 |archive-url=https://web.archive.org/web/20150327154926/https://www.britannica.com/EBchecked/media/24/Significant-events-in-plant-evolution |archive-date=2015-03-27 |access-date=2020-10-23}}</ref> The 250,000 to 400,000 species of flowering plants outnumber all other ground plants combined, and are the dominant vegetation in most terrestrial ecosystems. There is fossil evidence that flowering plants diversified rapidly in the Early Cretaceous, from {{ma|130|90|Ma|Cretaceous}},<ref name="CraneFriisPedersen2000">[[#Gee 2000|Crane, Friis and Pedersen 2000]], "The Origin and Early Diversification of Angiosperms," pp. [https://books.google.com/books?id=ZJe_Dmdbm-QC&pg=PA233 233–250] {{Webarchive|url=https://web.archive.org/web/20230716080907/https://books.google.com/books?id=ZJe_Dmdbm-QC&pg=PA233 |date=2023-07-16 }}</ref><ref name="Crepet2000">{{cite journal |last=Crepet |first=William L. |date=November 21, 2000 |title=Progress in understanding angiosperm history, success, and relationships: Darwin's abominably 'perplexing phenomenon' |journal=[[Proceedings of the National Academy of Sciences of the United States of America|Proc. Natl. Acad. Sci. U.S.A.]] |volume=97 |issue=24 |pages=12939–12941 |bibcode=2000PNAS...9712939C |doi=10.1073/pnas.97.24.12939 |issn=0027-8424 |pmc=34068 |pmid=11087846 |doi-access=free}}</ref> and that their rise was associated with that of [[pollination|pollinating]] insects.<ref name="Crepet2000" /> Among modern flowering plants ''[[Magnoliaceae|Magnolia]]'' are thought to be close to the common ancestor of the group.<ref name="CraneFriisPedersen2000" /> However, paleontologists have not succeeded in identifying the earliest stages in the evolution of flowering plants.<ref name="CraneFriisPedersen2000" /><ref name="Crepet2000" />

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

The fossil record appears to show that the gaps between mass extinctions are becoming longer and that the average and background rates of extinction are decreasing. Both of these phenomena could be explained in one or more ways:<ref name="MacLeod2001">{{cite web |url=http://www.firstscience.com/SITE/ARTICLES/macleod.asp |url-status=live |title=Extinction! |last=MacLeod |first=Norman |year=1999 |website=FirstScience.com |archive-url=https://web.archive.org/web/20000819004853/http://www.firstscience.com/site/articles/macleod.asp |archive-date=2000-08-19 |access-date=2020-12-25}}</ref>

* {{cite book |last=Arrhenius |first=Svante |author-link=Svante Arrhenius |year=1980 |orig-yeardate=Arrhenius paper originally published 1903 |chapter=The Propagation of Life in Space |editor-last=Goldsmith |editor-first=Donald |title=The Quest for Extraterrestrial Life: A Book of Readings |others=Foreword by Sir [[Fred Hoyle]] |location=Mill Valley, CA |publisher=University Science Books |bibcode=1980qel..book...32A |isbn=978-0-935702-02-6 |lccn=79057423 |oclc=7121102}}

* {{cite book |last=Benton |first=Michael J. |year=2005 |orig-yeardate=Originally published 2003 |title=When Life Nearly Died: The Greatest Mass Extinction of All Time |edition=1st paperback |location=London |publisher=[[Thames & Hudson]] |isbn=978-0-500-28573-2 |lccn=2002109744 |oclc=62145244}}

* {{cite book |last1=Bernstein |first1=Harris |last2=Bernstein |first2=Carol |last3=Michod |first3=Richard E. |year=2012 |chapter=DNA Repair as the Primary Adaptive Function of Sex in Bacteria and Eukaryotes |chapter-url=https://www.novapublishers.com/catalog/product_info.php?products_id=31918 |editor1-last=Kimura |editor1-first=Sakura |editor2-last=Shimizu |editor2-first=Sora |title=DNA Repair: New Research |location=Hauppauge, NY |publisher=[[Nova Science Publishers]] |isbn=978-1-62100-808-8 |lccn=2011038504 |oclc=828424701 |access-date=2013-07-21 |archive-date=2013-10-29 |archive-url=https://web.archive.org/web/20131029202307/https://www.novapublishers.com/catalog/product_info.php?products_id=31918 |url-status=dead }}

* {{cite book |last1=Birdsell |first1=John A. |last2=Wills |first2=Christopher |author2-link=Christopher Wills |year=2003 |chapter=The Evolutionary Origin and Maintenance of Sexual Recombination: A Review of Contemporary Models |editor1-last=MacIntyre |editor1-first=Ross J. |editor2-last=Clegg |editor2-first=Michael T. |title=Evolutionary Biology |series=Evolutionary Biology |volume=33 |location=New York |publisher=[[Springer Science+Business Media]] |isbn=978-1-4419-3385-0 |oclc=751583918}}

* {{cite book |last1=Labandeira |first1=Conrad C. |last2=Eble |first2=Gunther J. |date=1999 |chapter=The Fossil Record of Insect Diversity and Disparity |chapter-url=http://www.santafe.edu/media/workingpapers/00-08-044.pdf |editor1-last=Anderson |editor1-first=John M. |editor2-last=Thackeray |editor2-first=John Francis |editor3-last=Van Wyk |editor3-first=B. |editor4-last=De Wit |editor4-first=M. |title=Towards Gondwana Alive: Promoting biodiversity & stemming the Sixth Extinction |location=Pretoria |publisher=Gondwana Alive Society |isbn=978-1-919795-43-0 |lccn=2001385090 |oclc=44822625 |display-editors=2 |access-date=2015-02-17 |archive-date=2016-04-12 |archive-url=https://web.archive.org/web/20160412065128/http://www.santafe.edu/media/workingpapers/00-08-044.pdf |url-status=dead }} "Preview booklet for 'Gondwana alive : biodiversity and the evolving terrestrial biosphere', book planned for September 2000, and associated projects."

* {{cite book |last1=Smith |first1=John Maynard |author1-link=John Maynard Smith |last2=Szathmáry |first2=Eörs |author2-link=Eörs Szathmáry |year=1997 |orig-yeardate=1995; Oxford: [[W. H. Freeman and Company|W. H. Freeman/Spektrum]] |title=The Major Transitions in Evolution |location=Oxford; New York |publisher=Oxford University Press |isbn=978-0-19-850294-4 |lccn=94026965 |oclc=715217397 |title-link=The Major Transitions in Evolution |ref=none}}