Editing Archean (section)

==Early life==
{{Main|Earliest known life forms}}

{{For|details on how life got started|Abiogenesis}}
The processes that gave rise to life on Earth are not completely understood, but there is substantial evidence that life came into existence either near the end of the Hadean Eon  or early in the Archean Eon. 

The earliest evidence for life on Earth is [[graphite]] of [[Biogenic substance|biogenic]] origin found in 3.7&nbsp;billion–year-old [[metasedimentary rock]]s discovered in [[Kitaa|Western Greenland]].<ref name=NG-20131208>{{cite journal |vauthors=Ohtomo Y, Kakegawa T, Ishida A, Nagase T, Rosing MT |date=8 December 2013 |title=Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks |journal=Nature Geoscience |doi=10.1038/ngeo2025 |bibcode=2014NatGe...7...25O |volume=7 |issue=1 |pages=25–28}}</ref>

[[File:Lake Thetis-Stromatolites-LaRuth.jpg|thumb|right|[[Lithification|Lithified]] [[stromatolite]]s on the shores of [[Lake Thetis]], [[Western Australia]]. Archean stromatolites are the first direct fossil traces of life on Earth.]]

The earliest identifiable fossils consist of [[stromatolite]]s, which are [[microbial mats]] formed in shallow water by [[cyanobacteria]]. The earliest stromatolites are found in 3.48&nbsp;billion-year-old [[sandstone]] discovered in [[Western Australia]].<ref name=AP-20131113>{{cite news |last=Borenstein |first=Seth |agency=[[AP News]] |date=13 November 2013 |title=Oldest fossil found: Meet your microbial mom |url=http://apnews.excite.com/article/20131113/DAA1VSC01.html |access-date=15 November 2013}}</ref><ref name=AST-20131108>{{cite journal |vauthors=[[Nora Noffke|Noffke N]], Christian D, Wacey D, Hazen RM |date=December 2013 |title=Microbially induced sedimentary structures recording an ancient ecosystem in the ca.&nbsp;3.48&nbsp;billion-year-old Dresser Formation, Pilbara, Western Australia |journal=Astrobiology |pmid=24205812 |pmc=3870916 |doi=10.1089/ast.2013.1030 |bibcode=2013AsBio..13.1103N |volume=13 |issue=12 |pages=1103–1124}}</ref> Stromatolites are found throughout the Archean<ref name=PiP>{{cite journal |last=Garwood |first=Russell J. |year=2012 |title=Patterns In Palaeontology: The first 3&nbsp;billion years of evolution |journal=Palaeontology Online |volume=2 |issue=11 |pages=1–14 |url=http://www.palaeontologyonline.com/articles/2012/patterns-in-palaeontology-the-first-3-billion-years-of-evolution/ |access-date=June 25, 2015 |df=dmy-all}}</ref> and become common late in the Archean.<ref name=Stanley1999/>{{rp|page=307}} Cyanobacteria were instrumental in creating free oxygen in the atmosphere.{{citation needed|date=March 2023|reason=There was a citation here but it turned out to a page that was using Wikipedia's Great Oxidation Event article as source; so not a good source.}}

Further evidence for early life is found in 3.47&nbsp;billion-year-old [[baryte]], in the [[Warrawoona Group]] of Western Australia. This mineral shows sulfur [[fractionation]] of as much as 21.1%,<ref>{{cite journal |vauthors=Shen Y, Buick R, Canfield DE |date=March 2001 |title=Isotopic evidence for microbial sulphate reduction in the early Archaean era |journal=Nature |pmid=11242044 |doi=10.1038/35065071 |bibcode=2001Natur.410...77S |s2cid=25375808 |volume=410 |issue=6824 |pages=77–81}}</ref> which is evidence of [[sulfate-reducing bacteria]] that metabolize [[Isotopes of sulfur|sulfur-32]] more readily than sulfur-34.<ref>{{cite journal |vauthors=Seal RR |year=2006 |title=Sulfur isotope geochemistry of sulfide minerals |journal=Reviews in Mineralogy and Geochemistry |doi=10.2138/rmg.2006.61.12 |bibcode=2006RvMG...61..633S |volume=61 |issue=1 |pages=633–677 |url=http://digitalcommons.unl.edu/cgi/viewcontent.cgi?article=1354&context=usgsstaffpub}}</ref>

Evidence of life in the Late Hadean is more controversial. In 2015, biogenic carbon was detected in [[zircon]]s dated to 4.1&nbsp;billion years ago, but this evidence is preliminary and needs validation.<ref name=AP-20151019>{{cite news |last=Borenstein |first=Seth |agency=[[Associated Press]] |date=19 October 2015 |title=Hints of life on what was thought to be desolate early Earth |website=[[Excite (web portal)|Excite]] |publisher=[[Mindspark Interactive Network]] |location=Yonkers, NY |url=http://apnews.excite.com/article/20151019/us-sci--earliest_life-a400435d0d.html |access-date=2015-10-20 |df=dmy-all}}</ref><ref name=PNAS-20151014-pdf>{{cite journal |vauthors=Bell EA, Boehnke P, Harrison TM, [[Wendy Mao|Mao WL]] |date=November 2015 |title=Potentially biogenic carbon preserved in a 4.1&nbsp;billion-year-old zircon |edition=Early, published online before print |journal=Proceedings of the National Academy of Sciences of the United States of America |pmid=26483481 |pmc=4664351 |doi=10.1073/pnas.1517557112 |doi-access=free |bibcode=2015PNAS..11214518B |volume=112 |issue=47 |pages=14518–14521}}</ref>

Earth was very hostile to life before 4,300 to 4,200 Ma, and the conclusion is that before the Archean Eon, life as we know it would have been challenged by these environmental conditions. While life could have arisen before the Archean, the conditions necessary to sustain life could not have occurred until the Archean Eon.<ref>{{cite journal |last=Nisbet |first=Euan |year=1980 |title=Archaean stromatolites and the search for the earliest life |journal=Nature |bibcode=1980Natur.284..395N |doi=10.1038/284395a0 |s2cid=4262249 |volume=284 |issue=5755 |pages=395–396}}</ref>

Life in the Archean was limited to simple single-celled organisms (lacking nuclei), called [[prokaryote]]s. In addition to the domain [[Bacteria]], microfossils of the domain [[Archaea]] have also been identified. There are no known [[eukaryote|eukaryotic]] fossils from the earliest Archean, though they might have evolved during the Archean without leaving any.<ref name=Stanley1999/>{{rp|pages=306,323}} Fossil [[sterane]]s, indicative of eukaryotes, have been reported from Archean strata but were shown to derive from contamination with younger organic matter.<ref>{{cite journal |vauthors=French KL, Hallmann C, Hope JM, Schoon PL, Zumberge JA, Hoshino Y, Peters CA, George SC, Love GD, Brocks JJ, Buick R, Summons RE |date=May 2015 |title=Reappraisal of hydrocarbon biomarkers in Archean rocks |journal=Proceedings of the National Academy of Sciences of the United States of America |pmid=25918387 |pmc=4434754 |doi=10.1073/pnas.1419563112 |doi-access=free |bibcode=2015PNAS..112.5915F |volume=112 |issue=19 |pages=5915–5920}}</ref> No fossil evidence has been discovered for [[ultramicroscopic]] [[intracellular]] replicators such as [[virus]]es.

Fossilized microbes from terrestrial microbial mats show that life was already established on land 3.22&nbsp;billion years ago.<ref>{{cite journal |last1=Homann |first1=Martin |last2=Sansjofre |first2=Pierre |last3=Van Zuilen |first3=Mark |last4=Heubeck |first4=Christoph |last5=Gong |first5=Jian |last6=Killingsworth |first6=Bryan |last7=Foster |first7=Ian S. |last8=Airo |first8=Alessandro |last9=Van Kranendonk |first9=Martin J. |last10=Ader |first10=Magali |last11=Lalonde |first11=Stefan V. |date=23 July 2018 |title=Microbial life and biogeochemical cycling on land 3,220 million years ago |journal=[[Nature Geoscience]] |doi=10.1038/s41561-018-0190-9 |s2cid=134935568 |volume=11 |issue=9 |pages=665–671 |bibcode=2018NatGe..11..665H |url=https://www.nature.com/articles/s41561-018-0190-9?error=cookies_not_supported&code=de9a5c60-153f-4b56-a4d4-e0ae6ad9222d |access-date=14 January 2023}}</ref><ref>{{cite web |last=Woo |first=Marcus |date=30 July 2018 |title=Oldest Evidence for life on land unearthed in South Africa |website=Live Science |url=https://www.livescience.com/63199-oldest-life-on-land.html}}</ref>

''[[Thuchomyces]]'', a Mesoarchean-Neoarchean fossil from South Africa, may be the first evidence of macroscopic land life, however it is likely a microbial mat due to a lack of eukaryotic features.<ref>{{cite journal |last1=Lücking |first1=Robert |last2=Nelsen |first2=Matthew P. |title=Ediacarans, Protolichens, and Lichen-Derived Penicillium |journal=Transformative Paleobotany |date=2018 |pages=551–590 |doi=10.1016/B978-0-12-813012-4.00023-1}}</ref>