Earliest known life forms#Earliest life forms

{{further|Abiogenesis|Biosphere}}

Earth is the only place in the universe known to harbor life, where it exists in myriad environments.{{cite journal |last=Graham |first=Robert W. |date=February 1990 |title=Extraterrestrial Life in the Universe |url=https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19900013148.pdf |type=NASA Technical Memorandum 102363 |access-date=2 June 2015 |place=Lewis Research Center, Cleveland, Ohio |journal=NASA|volume=90 |page=22464 |bibcode=1990STIN...9022464G }}{{cite book |last=Altermann |first=Wladyslaw |title=From Fossils to Astrobiology: Records of Life on Earth and the Search for Extraterrestrial Biosignatures |publisher=Springer |year=2009 |isbn=978-1-4020-8836-0 |editor1-last=Seckbach |editor1-first=Joseph |series=Cellular Origin, Life in Extreme Habitats and Astrobiology |volume=12 |page=xvii |chapter=From Fossils to Astrobiology — A Roadmap to Fata Morgana? |lccn=2008933212 |editor2-last=Walsh |editor2-first=Maud}} The origin of life on Earth was at least 3.5 billion years ago, possibly as early as 3.8-4.1 billion years ago. Since its emergence, life has persisted in several geological environments. The Earth's biosphere extends down to at least {{convert|10|km|mi|abbr=on|order=out}} below the seafloor,{{cite news |last=Klein |first=JoAnna |date=19 December 2018 |title=Deep Beneath Your Feet, They Live in the Octillions — The real journey to the center of the Earth has begun, and scientists are discovering subsurface microbial beings that shake up what we think we know about life. |work=The New York Times |url=https://www.nytimes.com/2018/12/19/science/subsurface-microbes.html |access-date=21 December 2018}}{{Cite journal |last1=Plümper |first1=Oliver |last2=King |first2=Helen E. |last3=Geisler |first3=Thorsten |last4=Liu |first4=Yang |last5=Pabst |first5=Sonja |last6=Savov |first6=Ivan P. |last7=Rost |first7=Detlef |last8=Zack |first8=Thomas |date=2017-04-25 |title=Subduction zone forearc serpentinites as incubators for deep microbial life |journal=Proceedings of the National Academy of Sciences |volume=114 |issue=17 |pages=4324–9 |doi=10.1073/pnas.1612147114 |doi-access=free |issn=0027-8424 |pmc=5410786 |pmid=28396389 |bibcode=2017PNAS..114.4324P }} up to {{convert|41-77|km|mi|abbr=on|order=out}}{{cite news |last=Loeb |first=Abraham |author-link=Abraham Loeb |date=4 November 2019 |title=Did Life from Earth Escape the Solar System Eons Ago? |work=Scientific American |url=https://blogs.scientificamerican.com/observations/did-life-from-earth-escape-the-solar-system-eons-ago/ |access-date=5 November 2019}}{{Cite journal |last=Smith |first=David J. |date=October 2013 |title=Microbes in the Upper Atmosphere and Unique Opportunities for Astrobiology Research |url=https://www.liebertpub.com/doi/10.1089/ast.2013.1074 |journal=Astrobiology |volume=13 |issue=10 |pages=981–990 |doi=10.1089/ast.2013.1074 |pmid=24106911 |bibcode=2013AsBio..13..981S |issn=1531-1074}} into the atmosphere,{{cite web |author=University of Georgia |date=25 August 1998 |title=First-Ever Scientific Estimate Of Total Bacteria On Earth Shows Far Greater Numbers Than Ever Known Before |url=https://www.sciencedaily.com/releases/1998/08/980825080732.htm |access-date=10 November 2014 |work=Science Daily}}{{cite web |last=Hadhazy |first=Adam |date=12 January 2015 |title=Life Might Thrive a Dozen Miles Beneath Earth's Surface |url=https://web.archive.org/web/20201102170152/https://www.astrobio.net/extreme-life/life-might-thrive-dozen-miles-beneath-earths-surface/ |work=Astrobiology Magazine }}{{cite web |last=Fox-Skelly |first=Jasmin |date=24 November 2015 |title=The Strange Beasts That Live In Solid Rock Deep Underground |url=http://www.bbc.com/earth/story/20151124-meet-the-strange-creatures-that-live-in-solid-rock-deep-underground |access-date=11 March 2017 |work=BBC online}} and includes soil, hydrothermal vents, and rock.{{cite journal |author=Suzuki, Yohey |display-authors=et al. |date=2 April 2020 |title=Deep microbial proliferation at the basalt interface in 33.5–104 million-year-old oceanic crust |journal=Communications Biology |volume=3 |issue=136 |page=136 |doi=10.1038/s42003-020-0860-1 |pmc=7118141 |pmid=32242062 |doi-access=free}}{{cite news |author=University of Tokyo |date=2 April 2020 |title=Discovery of life in solid rock deep beneath sea may inspire new search for life on Mars — Bacteria live in tiny clay-filled cracks in solid rock millions of years old |work=EurekAlert! |url=https://www.eurekalert.org/pub_releases/2020-04/uot-dol033020.php |access-date=2 April 2020}} Further, the biosphere has been found to extend at least {{convert|914.4|m|ft mi|abbr=on}} below the ice of Antarctica{{cite journal |author=Griffiths, Huw J. |display-authors=et al. |date=15 February 2021 |title=Breaking All the Rules: The First Recorded Hard Substrate Sessile Benthic Community Far Beneath an Antarctic Ice Shelf |journal=Frontiers in Marine Science |volume=8 |doi=10.3389/fmars.2021.642040 |doi-access=free|bibcode=2021FrMaS...842040G }}{{cite journal |last=Fox |first=Douglas |date=20 August 2014 |title=Lakes under the ice: Antarctica's secret garden |journal=Nature |volume=512 |issue=7514 |pages=244–6 |bibcode=2014Natur.512..244F |doi=10.1038/512244a |pmid=25143097 |doi-access=free}} and includes the deepest parts of the ocean.{{cite web |last=Choi |first=Charles Q. |date=17 March 2013 |title=Microbes Thrive in Deepest Spot on Earth |url=http://www.livescience.com/27954-microbes-mariana-trench.html |access-date=17 March 2013 |website=LiveScience}}{{cite journal |last1=Glud |first1=Ronnie |last2=Wenzhöfer |first2=Frank |last3=Middelboe |first3=Mathias |last4=Oguri |first4=Kazumasa |last5=Turnewitsch |first5=Robert |last6=Canfield |first6=Donald E. |last7=Kitazato |first7=Hiroshi |date=17 March 2013 |title=High rates of microbial carbon turnover in sediments in the deepest oceanic trench on Earth |journal=Nature Geoscience |volume=6 |issue=4 |pages=284–8 |bibcode=2013NatGe...6..284G |doi=10.1038/ngeo1773}}{{cite web |last=Oskin |first=Becky |date=14 March 2013 |title=Intraterrestrials: Life Thrives in Ocean Floor |url=http://www.livescience.com/27899-ocean-subsurface-ecosystem-found.html |access-date=17 March 2013 |website=LiveScience}}{{cite news |last=Morelle |first=Rebecca |author-link=Rebecca Morelle |date=15 December 2014 |title=Microbes discovered by deepest marine drill analysed |work=BBC News |url=https://www.bbc.com/news/science-environment-30489814 |access-date=15 December 2014}} In July 2020, marine biologists reported that aerobic microorganisms (mainly) in "quasi-suspended animation" were found in organically poor sediment {{convert|76.2|m|ft|abbr=on}} below the seafloor in the South Pacific Gyre (SPG) ("the deadest spot in the ocean").{{cite journal |author=Morono, Yuki |display-authors=et al. |date=28 July 2020 |title=Aerobic microbial life persists in oxic marine sediment as old as 101.5 million years |journal=Nature Communications |volume=11 |page=3626 |bibcode=2020NatCo..11.3626M |doi=10.1038/s41467-020-17330-1 |pmc=7387439 |pmid=32724059 |number=3626}} Microbes have been found in the Atacama Desert in Chile, one of the driest places on Earth,{{Cite journal |last=Pennisi |first=Elizabeth |date=2018-02-26 |title=Microbes found in one of Earth's most hostile places, giving hope for life on Mars |url=http://www.sciencemag.org/news/2018/02/microbes-found-one-earth-s-most-hostile-places-giving-hope-life-mars |journal=Science |doi=10.1126/science.aat4341 |issn=0036-8075}} and in deep-sea hydrothermal vent environments which can reach temperatures over 400°C.{{Cite journal |last1=Georgieva |first1=Magdalena N. |last2=Little |first2=Crispin T. S. |last3=Maslennikov |first3=Valeriy V. |last4=Glover |first4=Adrian G. |last5=Ayupova |first5=Nuriya R. |last6=Herrington |first6=Richard J. |date=2021-06-01 |title=The history of life at hydrothermal vents |journal=Earth-Science Reviews |volume=217 |pages=103602 |doi=10.1016/j.earscirev.2021.103602 |bibcode=2021ESRv..21703602G |issn=0012-8252|doi-access=free }} Microbial communities can also survive in cold permafrost conditions down to -25°C.{{Cite journal |last1=Mykytczuk |first1=Nadia C S |last2=Foote |first2=Simon J |last3=Omelon |first3=Chris R |last4=Southam |first4=Gordon |last5=Greer |first5=Charles W |last6=Whyte |first6=Lyle G |date=2013-02-07 |title=Bacterial growth at −15 °C; molecular insights from the permafrost bacterium Planococcus halocryophilus Or1 |url=http://dx.doi.org/10.1038/ismej.2013.8 |journal=The ISME Journal |volume=7 |issue=6 |pages=1211–26 |doi=10.1038/ismej.2013.8 |pmid=23389107 |pmc=3660685 |bibcode=2013ISMEJ...7.1211M |issn=1751-7362}} Under certain test conditions, life forms have been observed to survive in the vacuum of outer space.{{cite journal |last1=Dose |first1=K. |last2=Bieger-Dose |first2=A. |last3=Dillmann |first3=R. |last4=Gill |first4=M. |last5=Kerz |first5=O. |last6=Klein |first6=A. |last7=Meinert |first7=H. |last8=Nawroth |first8=T. |last9=Risi |first9=S. |last10=Stridde |first10=C. |year=1995 |title=ERA-experiment "space biochemistry" |journal=Advances in Space Research |volume=16 |issue=8 |pages=119–129 |bibcode=1995AdSpR..16h.119D |doi=10.1016/0273-1177(95)00280-R |pmid=11542696}}{{cite journal |last1=Horneck |first1=G. |author2=Eschweiler, U. |author3=Reitz, G. |author4=Wehner, J. |author5=Willimek, R. |author6=Strauch, K. |year=1995 |title=Biological responses to space: results of the experiment "Exobiological Unit" of ERA on EURECA I |journal=Adv. Space Res. |volume=16 |issue=8 |pages=105–118 |bibcode=1995AdSpR..16h.105H |doi=10.1016/0273-1177(95)00279-N |pmid=11542695}} More recently, studies conducted on the International Space Station found that bacteria could survive in outer space.{{cite journal |author=Kawaguchi, Yuko |display-authors=et al. |date=26 August 2020 |title=DNA Damage and Survival Time Course of Deinococcal Cell Pellets During 3 Years of Exposure to Outer Space |journal=Frontiers in Microbiology |volume=11 |page=2050 |doi=10.3389/fmicb.2020.02050 |pmc=7479814 |pmid=32983036 |s2cid=221300151 |doi-access=free}} In February 2023, findings of a "dark microbiome" of microbial dark matter of unfamiliar microorganisms in the Atacama Desert in Chile, a Mars-like region of planet Earth, were reported.{{cite journal |author=Azua-Bustos, Armando |display-authors=et al. |date=21 February 2023 |title=Dark microbiome and extremely low organics in Atacama fossil delta unveil Mars life detection limits |journal=Nature Communications |volume=14 |issue=808 |page=808 |doi=10.1038/s41467-023-36172-1 |pmc=9944251 |pmid=36810853|bibcode=2023NatCo..14..808A }}

The age of Earth is about 4.54 billion years;{{cite journal |last=Dalrymple |first=G. Brent |author-link=Brent Dalrymple |date=2001 |title=The age of the Earth in the twentieth century: a problem (mostly) solved |journal=Special Publications, Geological Society of London |volume=190 |issue=1 |pages=205–221 |bibcode=2001GSLSP.190..205D |doi=10.1144/GSL.SP.2001.190.01.14 |s2cid=130092094}}{{cite journal |last1=Manhesa |first1=Gérard |last2=Allègre |first2=Claude J. |author-link2=Claude Allègre |last3=Dupréa |first3=Bernard |last4=Hamelin |first4=Bruno |date=May 1980 |title=Lead isotope study of basic-ultrabasic layered complexes: Speculations about the age of the earth and primitive mantle characteristics |journal=Earth and Planetary Science Letters |volume=47 |issue=3 |pages=370–382 |bibcode=1980E&PSL..47..370M |doi=10.1016/0012-821X(80)90024-2 |issn=0012-821X}} the earliest undisputed evidence of life on Earth dates from at least 3.5 billion years ago according to the stromatolite record.Multiple Sources:

• {{cite journal |author=Lepot, K. |date=October 2020 |title=Signatures of early microbial life from the Archean (4 to 2.5 Ga) eon. |journal=Earth-Science Reviews |volume=209 |page=103296 |doi=10.1016/j.earscirev.2020.103296 |ref=none |doi-access=free|bibcode=2020ESRv..20903296L |hdl=20.500.12210/62415 |hdl-access=free }}
• {{cite journal |author=Baugartner, R.J. |display-authors=et al. |date=25 September 2019 |title=Nano−porous pyrite and organic matter in 3.5-billion-year-old stromatolites record primordial life. |url=https://pubs.geoscienceworld.org/gsa/geology/article-abstract/47/11/1039/573756/Nano-porous-pyrite-and-organic-matter-in-3-5 |journal=Geology |volume=47 |pages=1039–43 |doi=10.1130/G46365.1 |access-date=13 August 2023 |ref=none |number=11|bibcode=2019Geo....47.1039B |s2cid=204258554 }}
• {{cite journal |last1=Schopf |first1=J. William |author-link1=J. William Schopf |last2=Kudryavtsev |first2=Anatoliy B. |last3=Czaja |first3=Andrew D. |last4=Tripathi |first4=Abhishek B. |date=5 October 2007 |title=Evidence of Archean life: Stromatolites and microfossils |journal=Precambrian Research |volume=158 |issue=3–4 |pages=141–155 |bibcode=2007PreR..158..141S |doi=10.1016/j.precamres.2007.04.009 |issn=0301-9268 |ref=none}}
• {{cite journal |last=Schopf |first=J. William |date=29 June 2006 |title=Fossil evidence of Archaean life |journal=Philosophical Transactions of the Royal Society B |volume=361 |issue=1470 |pages=869–885 |doi=10.1098/rstb.2006.1834 |issn=0962-8436 |pmc=1578735 |pmid=16754604 |ref=none}}
• {{cite journal |author=Allwood, A.C. |display-authors=et al. |date=8 June 2006 |title=Stromatolite reef from the Early Archaean era of Australia. |url=https://www.nature.com/articles/nature04764 |journal=Nature |volume=441 |issue=7094 |pages=714–8 |doi=10.1038/nature04764 |pmid=16760969 |bibcode=2006Natur.441..714A |s2cid=4417746 |access-date=13 August 2023 |ref=none}}
• {{cite book |last1=Raven |first1=Peter H. |url=https://archive.org/details/biologyrave00rave |title=Biology |last2=Johnson |first2=George B. |publisher=McGraw-Hill |year=2002 |isbn=978-0-07-112261-0 |edition=6th |location=Boston, MA |page=[https://archive.org/details/biologyrave00rave/page/68 68] |lccn=2001030052 |oclc=45806501 |ref=none |author-link1=Peter H. Raven |author-link2=George B. Johnson |url-access=registration}} Some computer models suggest life began as early as 4.5 billion years ago.{{cite web |author=Staff |date=20 August 2018 |title=A timescale for the origin and evolution of all of life on Earth |url=https://phys.org/news/2018-08-timescale-evolution-life-earth.html |access-date=20 August 2018 |work=Phys.org}}{{cite journal |last1=Betts |first1=Holly C. |last2=Putick |first2=Mark N. |last3=Clark |first3=James W. |last4=Williams |first4=Tom A. |last5=Donoghue |first5=Philip C.J. |last6=Pisani |first6=Davide |date=20 August 2018 |title=Integrated genomic and fossil evidence illuminates life's early evolution and eukaryote origin |journal=Nature |volume=2 |issue=10 |pages=1556–62 |doi=10.1038/s41559-018-0644-x |pmc=6152910 |pmid=30127539|bibcode=2018NatEE...2.1556B }} The oldest evidence of life is indirect in the form of isotopic fractionation processes. Microorganisms will preferentially use the lighter isotope of an atom to build biomass, as it takes less energy to break the bonds for metabolic processes.{{Cite journal |last1=Farquhar |first1=G D |last2=Ehleringer |first2=J R |last3=Hubick |first3=K T |date=June 1989 |title=Carbon Isotope Discrimination and Photosynthesis |url=https://www.annualreviews.org/doi/10.1146/annurev.pp.40.060189.002443 |journal=Annual Review of Plant Physiology and Plant Molecular Biology |language=en |volume=40 |issue=1 |pages=503–537 |doi=10.1146/annurev.pp.40.060189.002443 |issn=1040-2519}} Biologic material will often have a composition that is enriched in lighter isotopes compared to the surrounding rock it's found in. Carbon isotopes, expressed scientifically in parts per thousand difference from a standard as δ¹³C, are frequently used to detect carbon fixation by organisms and assess if purported early life evidence has biological origins. Typically, life will preferentially metabolize the isotopically light ¹²C isotope instead of the heavier ¹³C isotope. Biologic material can record this fractionation of carbon.

File:Quartz-pebble metaconglomerate (Jack Hills Quartzite, Archean, 2.65 to 3.05 Ga; Jack Hills, Western Australia) 1 (26668804034).jpg in metaconglomerates from the Jack Hills in Australia show carbon isotopic evidence for early life.]]

The oldest disputed geochemical evidence of life is isotopically light graphite inside a single zircon grain from the Jack Hills in Western Australia.{{Cite web |last=Netburn |first=Deborah |date=2015-10-31 |title=Tiny zircons suggest life on Earth started earlier than we thought, UCLA researchers say |url=https://www.latimes.com/science/la-sci-oldest-rocks-20151031-story.html |access-date=2023-12-04 |website=Los Angeles Times |language=en-US}} The graphite showed a δ¹³C signature consistent with biogenic carbon on Earth. Other early evidence of life is found in rocks both from the Akilia Sequence{{Cite journal |last1=Mojzsis |first1=S. J. |last2=Arrhenius |first2=G. |last3=McKeegan |first3=K. D. |last4=Harrison |first4=T. M. |last5=Nutman |first5=A. P. |last6=Friend |first6=C. R. L. |date=1996-11-07 |title=Evidence for life on Earth before 3,800 million years ago |url=https://www.nature.com/articles/384055a0 |journal=Nature |language=en |volume=384 |issue=6604 |pages=55–59 |doi=10.1038/384055a0 |pmid=8900275 |bibcode=1996Natur.384...55M |hdl=2060/19980037618 |s2cid=4342620 |issn=0028-0836|hdl-access=free }} and the Isua Supracrustal Belt (ISB) in Greenland.{{Cite journal |last1=Hassenkam |first1=T. |last2=Rosing |first2=M. T. |date=2017-11-02 |title=3.7 billion year old biogenic remains |journal=Communicative & Integrative Biology |language=en |volume=10 |issue=5–6 |pages=e1380759 |doi=10.1080/19420889.2017.1380759 |issn=1942-0889 |pmc=5731516 |pmid=29260796}} These 3.7 Ga metasedimentary rocks also contain graphite or graphite inclusions with carbon isotope signatures that suggest biological fractionation.

The primary issue with isotopic evidence of life is that abiotic processes can fractionate isotopes and produce similar signatures to biotic processes.{{Cite journal |last1=van Zuilen |first1=Mark A. |last2=Lepland |first2=Aivo |last3=Arrhenius |first3=Gustaf |date=2002-08-08 |title=Reassessing the evidence for the earliest traces of life |url=http://dx.doi.org/10.1038/nature00934 |journal=Nature |volume=418 |issue=6898 |pages=627–630 |doi=10.1038/nature00934 |pmid=12167858 |bibcode=2002Natur.418..627V |s2cid=62804341 |issn=0028-0836}} Reassessment of the Akilia graphite show that metamorphism, Fischer-Tropsch mechanisms in hydrothermal environments, and volcanic processes may be responsible for enrichment lighter carbon isotopes.{{Cite journal |last1=Papineau |first1=Dominic |last2=De Gregorio |first2=Bradley T. |last3=Stroud |first3=Rhonda M. |last4=Steele |first4=Andrew |last5=Pecoits |first5=Ernesto |last6=Konhauser |first6=Kurt |last7=Wang |first7=Jianhua |last8=Fogel |first8=Marilyn L. |date=October 2010 |title=Ancient graphite in the Eoarchean quartz-pyroxene rocks from Akilia in southern West Greenland II: Isotopic and chemical compositions and comparison with Paleoproterozoic banded iron formations |url=http://dx.doi.org/10.1016/j.gca.2010.07.002 |journal=Geochimica et Cosmochimica Acta |volume=74 |issue=20 |pages=5884–5905 |doi=10.1016/j.gca.2010.07.002 |bibcode=2010GeCoA..74.5884P |issn=0016-7037}}{{Cite journal |last1=MCCOLLOM |first1=T |last2=SEEWALD |first2=J |date=2006-03-15 |title=Carbon isotope composition of organic compounds produced by abiotic synthesis under hydrothermal conditions |url=http://dx.doi.org/10.1016/j.epsl.2006.01.027 |journal=Earth and Planetary Science Letters |volume=243 |issue=1–2 |pages=74–84 |doi=10.1016/j.epsl.2006.01.027 |bibcode=2006E&PSL.243...74M |hdl=1912/878 |issn=0012-821X|hdl-access=free }}{{Cite journal |last1=Lepland |first1=Aivo |last2=van Zuilen |first2=Mark A. |last3=Arrhenius |first3=Gustaf |last4=Whitehouse |first4=Martin J. |last5=Fedo |first5=Christopher M. |date=2005 |title=Questioning the evidence for Earth's earliest life—Akilia revisited |url=http://dx.doi.org/10.1130/g20890.1 |journal=Geology |volume=33 |issue=1 |pages=77 |doi=10.1130/g20890.1 |bibcode=2005Geo....33...77L |issn=0091-7613}} The ISB rocks that contain the graphite may have experienced a change in composition from hot fluids, i.e. metasomatism, thus the graphite may have been formed by abiotic chemical reactions. However, the ISB's graphite is generally more accepted as biologic in origin after further spectral analysis.

Metasedimentary rocks from the 3.5 Ga Dresser Formation, which experienced less metamorphism than the sequences in Greenland, contain better preserved geochemical evidence.{{Citation |last1=Van Kranendonk |first1=Martin J. |title=Depositional Setting of the Fossiliferous, c.3480 Ma Dresser Formation, Pilbara Craton |date=2019 |url=http://dx.doi.org/10.1016/b978-0-444-63901-1.00040-x |work=Earth's Oldest Rocks |pages=985–1006 |access-date=2023-11-16 |publisher=Elsevier |last2=Djokic |first2=Tara |last3=Poole |first3=Greg |last4=Tadbiri |first4=Sahand |last5=Steller |first5=Luke |last6=Baumgartner |first6=Raphael|doi=10.1016/b978-0-444-63901-1.00040-x |isbn=978-0-444-63901-1 |s2cid=133958822 }} Carbon isotopes as well as sulfur isotopes found in barite, which are fractionated by microbial metabolisms during sulfate reduction,{{Cite journal |last1=Sim |first1=Min Sub |last2=Woo |first2=Dong Kyun |last3=Kim |first3=Bokyung |last4=Jeong |first4=Hyeonjeong |last5=Joo |first5=Young Ji |last6=Hong |first6=Yeon Woo |last7=Choi |first7=Jy Young |date=2023-03-15 |title=What Controls the Sulfur Isotope Fractionation during Dissimilatory Sulfate Reduction? |journal=ACS Environmental Au |language=en |volume=3 |issue=2 |pages=76–86 |doi=10.1021/acsenvironau.2c00059 |issn=2694-2518 |pmc=10125365 |pmid=37102088|bibcode=2023ACSEA...3...76S }} are consistent with biological processes.{{Cite journal |last1=Ueno |first1=Yuichiro |last2=Yamada |first2=Keita |last3=Yoshida |first3=Naohiro |last4=Maruyama |first4=Shigenori |last5=Isozaki |first5=Yukio |date=March 2006 |title=Evidence from fluid inclusions for microbial methanogenesis in the early Archaean era |url=http://dx.doi.org/10.1038/nature04584 |journal=Nature |volume=440 |issue=7083 |pages=516–9 |doi=10.1038/nature04584 |pmid=16554816 |bibcode=2006Natur.440..516U |s2cid=4423306 |issn=0028-0836}}{{Cite journal |last1=Wacey |first1=David |last2=Noffke |first2=Nora |last3=Cliff |first3=John |last4=Barley |first4=Mark E. |last5=Farquhar |first5=James |date=September 2015 |title=Micro-scale quadruple sulfur isotope analysis of pyrite from the ∼3480Ma Dresser Formation: New insights into sulfur cycling on the early Earth |url=http://dx.doi.org/10.1016/j.precamres.2014.12.012 |journal=Precambrian Research |volume=258 |pages=24–35 |doi=10.1016/j.precamres.2014.12.012 |bibcode=2015PreR..258...24W |issn=0301-9268}} However, the Dresser formation was deposited in an active volcanic and hydrothermal environment, and abiotic processes could still be responsible for these fractionations.{{Cite journal |last1=Lollar |first1=Barbara Sherwood |last2=McCollom |first2=Thomas M. |date=December 2006 |title=Biosignatures and abiotic constraints on early life |url=http://dx.doi.org/10.1038/nature05499 |journal=Nature |volume=444 |issue=7121 |pages=E18; discussion E18-9 |doi=10.1038/nature05499 |pmid=17167427 |issn=0028-0836}} Many of these findings are supplemented by direct evidence, typically by the presence of microfossils, however.

Fossils are direct evidence of life. In the search for the earliest life, fossils are often supplemented by geochemical evidence. The fossil record does not extend as far back as the geochemical record due to metamorphic processes that erase fossils from geologic units.

{{main|Stromatolite}}

Stromatolites are laminated sedimentary structures created by photosynthetic organisms as they establish a microbial mat on a sediment surface. An important distinction for biogenicity is their convex-up structures and wavy laminations, which are typical of microbial communities who build preferentially toward the sun.{{Cite journal |last1=Buick |first1=Roger |last2=Dunlop |first2=J.S.R. |last3=Groves |first3=D.I. |date=January 1981 |title=Stromatolite recognition in ancient rocks: an appraisal of irregularly laminated structures in an Early Archaean chert-barite unit from North Pole, Western Australia |url=http://dx.doi.org/10.1080/03115518108566999 |journal=Alcheringa: An Australasian Journal of Palaeontology |volume=5 |issue=3 |pages=161–181 |doi=10.1080/03115518108566999 |bibcode=1981Alch....5..161B |issn=0311-5518}} A disputed report of stromatolites is from the 3.7 Ga Isua metasediments that show convex-up, conical, and domical morphologies.{{Cite journal |last1=Nutman |first1=Allen P. |last2=Bennett |first2=Vickie C. |last3=Friend |first3=Clark R. L. |last4=Van Kranendonk |first4=Martin J. |last5=Chivas |first5=Allan R. |date=2016-08-31 |title=Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures |url=http://dx.doi.org/10.1038/nature19355 |journal=Nature |volume=537 |issue=7621 |pages=535–8 |doi=10.1038/nature19355 |pmid=27580034 |bibcode=2016Natur.537..535N |s2cid=205250494 |issn=0028-0836}}{{cite news |last=Wade |first=Nicholas |date=31 August 2016 |title=World's Oldest Fossils Found in Greenland |work=The New York Times |url=https://www.nytimes.com/2016/09/01/science/oldest-fossils-on-earth.html |access-date=31 August 2016}} Further mineralogical analysis disagrees with the initial findings of internal convex-up laminae, a critical criterion for stromatolite identification, suggesting that the structures may be deformation features (i.e. boudins) caused by extensional tectonics in the Isua Supracrustal Belt.{{Cite journal |last1=Zawaski |first1=Mike J. |last2=Kelly |first2=Nigel M. |last3=Orlandini |first3=Omero Felipe |last4=Nichols |first4=Claire I. O. |last5=Allwood |first5=Abigail C. |last6=Mojzsis |first6=Stephen J. |date=2020-09-01 |title=Reappraisal of purported ca. 3.7 Ga stromatolites from the Isua Supracrustal Belt (West Greenland) from detailed chemical and structural analysis |url=https://www.sciencedirect.com/science/article/pii/S0012821X20303538 |journal=Earth and Planetary Science Letters |volume=545 |pages=116409 |doi=10.1016/j.epsl.2020.116409 |bibcode=2020E&PSL.54516409Z |s2cid=225256458 |issn=0012-821X}}{{cite web |last=Wei-Haas |first=Maya |date=17 October 2018 |title='World's oldest fossils' may just be pretty rocks — Analysis of 3.7-billion-year-old outcrops has reignited controversy over when life on Earth began. |url=https://www.nationalgeographic.com/science/2018/10/news-oldest-stromatolite-fossilized-life-rocks-greenland/ |access-date=19 October 2018 |work=National Geographic}}

File:Stromatolite - National Museum of Nature and Science, Tokyo - DSC07686.JPG fossil showing convex-up structures.]]

The earliest direct evidence of life are stromatolites found in 3.48 billion-year-old chert in the Dresser formation of the Pilbara Craton in Western Australia. Several features in these fossils are difficult to explain with abiotic processes, for example, the thickening of laminae over flexure crests that is expected from more sunlight.{{Cite journal |last1=Walter |first1=M. R. |last2=Buick |first2=R. |last3=Dunlop |first3=J. S. R. |date=April 1980 |title=Stromatolites 3,400–3,500 Myr old from the North Pole area, Western Australia |url=http://dx.doi.org/10.1038/284443a0 |journal=Nature |volume=284 |issue=5755 |pages=443–5 |doi=10.1038/284443a0 |bibcode=1980Natur.284..443W |s2cid=4256480}} Sulfur isotopes from barite veins in the stromatolites also favor a biologic origin.{{Cite journal |last1=Philippot |first1=Pascal |last2=Van Zuilen |first2=Mark |last3=Lepot |first3=Kevin |last4=Thomazo |first4=Christophe |last5=Farquhar |first5=James |last6=Van Kranendonk |first6=Martin J. |date=2007-09-14 |title=Early Archaean Microorganisms Preferred Elemental Sulfur, Not Sulfate |url=http://dx.doi.org/10.1126/science.1145861 |journal=Science |volume=317 |issue=5844 |pages=1534–7 |doi=10.1126/science.1145861 |pmid=17872441 |bibcode=2007Sci...317.1534P |s2cid=41254565}} However, while most scientists accept their biogenicity, abiotic explanations for these fossils cannot be fully discarded due to their hydrothermal depositional environment and debated geochemical evidence.

Most archean stromatolites older than 3.0 Ga are found in Australia or South Africa. Stratiform stromatolites from the Pilbara Craton have been identified in the 3.47 Ga Mount Ada Basalt.{{Cite journal |last1=Awramik |first1=S.M. |last2=Schopf |first2=J.W. |last3=Walter |first3=M.R. |date=June 1983 |title=Filamentous fossil bacteria from the Archean of Western Australia |url=https://linkinghub.elsevier.com/retrieve/pii/0301926883900815 |journal=Precambrian Research |language=en |volume=20 |issue=2–4 |pages=357–374 |doi=10.1016/0301-9268(83)90081-5|bibcode=1983PreR...20..357A }} Barberton, South Africa hosts stratiform stromatolites in the 3.46 Hooggenoeg, 3.42 Kromberg and 3.33 Ga Mendon Formations of the Onverwacht Group.{{Citation |last1=Hickman-Lewis |first1=Keyron |title=Traces of Early Life From the Barberton Greenstone Belt, South Africa |date=2019 |url=http://dx.doi.org/10.1016/b978-0-444-63901-1.00042-3 |work=Earth's Oldest Rocks |pages=1029–58 |access-date=2023-11-21 |publisher=Elsevier |last2=Westall |first2=Frances |last3=Cavalazzi |first3=Barbara|doi=10.1016/b978-0-444-63901-1.00042-3 |isbn=978-0-444-63901-1 |s2cid=134488803 }}{{Citation |last=Hofmann |first=H. J. |title=Archean Stromatolites as Microbial Archives |date=2000 |url=https://doi.org/10.1007/978-3-662-04036-2_34 |work=Microbial Sediments |pages=315–327 |editor-last=Riding |editor-first=Robert E. |access-date=2023-11-22 |place=Berlin, Heidelberg |publisher=Springer |language=en |doi=10.1007/978-3-662-04036-2_34 |isbn=978-3-662-04036-2 |editor2-last=Awramik |editor2-first=Stanley M.}} The 3.43 Ga Strelley Pool Formation in Western Australia hosts stromatolites that demonstrate vertical and horizontal changes that may demonstrate microbial communities responding to transient environmental conditions.{{Cite journal |last1=Allwood |first1=Abigail C. |last2=Grotzinger |first2=John P. |last3=Knoll |first3=Andrew H. |last4=Burch |first4=Ian W. |last5=Anderson |first5=Mark S. |last6=Coleman |first6=Max L. |last7=Kanik |first7=Isik |date=2009-06-16 |title=Controls on development and diversity of Early Archean stromatolites |journal=Proceedings of the National Academy of Sciences |volume=106 |issue=24 |pages=9548–55 |doi=10.1073/pnas.0903323106 |doi-access=free |pmc=2700989 |pmid=19515817|bibcode=2009PNAS..106.9548A }} Thus, it is likely anoxygenic or oxygenic photosynthesis has been occurring since at least 3.43 Ga Strelley Pool Formation.{{Cite journal |last1=Duda |first1=Jan-Peter |last2=Kranendonk |first2=Martin J. Van |last3=Thiel |first3=Volker |last4=Ionescu |first4=Danny |last5=Strauss |first5=Harald |last6=Schäfer |first6=Nadine |last7=Reitner |first7=Joachim |date=2016-01-25 |title=A Rare Glimpse of Paleoarchean Life: Geobiology of an Exceptionally Preserved Microbial Mat Facies from the 3.4 Ga Strelley Pool Formation, Western Australia |journal=PLOS ONE |language=en |volume=11 |issue=1 |pages=e0147629 |doi=10.1371/journal.pone.0147629 |doi-access=free |issn=1932-6203 |pmc=4726515 |pmid=26807732 |bibcode=2016PLoSO..1147629D }}

{{Further information|Microfossil}}

Claims of the earliest life using fossilized microorganisms (microfossils) are from hydrothermal vent precipitates from an ancient sea-bed in the Nuvvuagittuq Belt of Quebec, Canada. These may be as old as 4.28 billion years, which would make it the oldest evidence of life on Earth, suggesting "an almost instantaneous emergence of life" after ocean formation 4.41 billion years ago.{{cite journal |author=Dodd, Matthew S. |author2=Papineau, Dominic |author3=Grenne, Tor |author4=slack, John F. |author5=Rittner, Martin |author6=Pirajno, Franco |author7=O'Neil, Jonathan |author8=Little, Crispin T. S. |date=2 March 2017 |title=Evidence for early life in Earth's oldest hydrothermal vent precipitates |url=http://eprints.whiterose.ac.uk/112179/1/ppnature21377_Dodd_for%20Symplectic.pdf |journal=Nature |volume=543 |issue=7643 |pages=60–64 |bibcode=2017Natur.543...60D |doi=10.1038/nature21377 |pmid=28252057 |s2cid=2420384 |doi-access=free}}{{Cite news |date=2017-03-01 |title=Earliest evidence of life on Earth 'found' |language=en-GB |work=BBC News |url=https://www.bbc.com/news/science-environment-39117523 |access-date=2023-12-04}} These findings may be better explained by abiotic processes: for example, silica-rich waters,{{Cite journal |last1=García-Ruiz |first1=Juan Manuel |last2=Nakouzi |first2=Elias |last3=Kotopoulou |first3=Electra |last4=Tamborrino |first4=Leonardo |last5=Steinbock |first5=Oliver |date=2017-03-03 |title=Biomimetic mineral self-organization from silica-rich spring waters |journal=Science Advances |language=en |volume=3 |issue=3 |pages=e1602285 |doi=10.1126/sciadv.1602285 |issn=2375-2548 |pmc=5357132 |pmid=28345049|bibcode=2017SciA....3E2285G }} "chemical gardens,"{{Cite journal |last=McMahon |first=Sean |date=2019-12-04 |title=Earth's earliest and deepest purported fossils may be iron-mineralized chemical gardens |journal=Proceedings of the Royal Society B: Biological Sciences |language=en |volume=286 |issue=1916 |pages=20192410 |doi=10.1098/rspb.2019.2410 |issn=0962-8452 |pmc=6939263 |pmid=31771469}} circulating hydrothermal fluids,{{Cite journal |last1=Johannessen |first1=Karen C. |last2=McLoughlin |first2=Nicola |last3=Vullum |first3=Per Erik |last4=Thorseth |first4=Ingunn H. |date=January 2020 |title=On the biogenicity of Fe-oxyhydroxide filaments in silicified low-temperature hydrothermal deposits: Implications for the identification of Fe-oxidizing bacteria in the rock record |url=https://onlinelibrary.wiley.com/doi/10.1111/gbi.12363 |journal=Geobiology |language=en |volume=18 |issue=1 |pages=31–53 |doi=10.1111/gbi.12363 |pmid=31532578 |bibcode=2020Gbio...18...31J |issn=1472-4677|hdl=11250/2632364 |hdl-access=free }} and volcanic ejecta{{Cite journal |last1=Wacey |first1=David |last2=Saunders |first2=Martin |last3=Kong |first3=Charlie |date=April 2018 |title=Remarkably preserved tephra from the 3430 Ma Strelley Pool Formation, Western Australia: Implications for the interpretation of Precambrian microfossils |url=http://dx.doi.org/10.1016/j.epsl.2018.01.021 |journal=Earth and Planetary Science Letters |volume=487 |pages=33–43 |doi=10.1016/j.epsl.2018.01.021 |bibcode=2018E&PSL.487...33W}} can produce morphologies similar to those presented in Nuvvuagittuq.

File:Halobacteria.jpg (prokaryotic microbes) were first found in extreme environments, such as hydrothermal vents.]]

The 3.48 Ga Dresser formation hosts microfossils of prokaryotic filaments in silica veins, the earliest fossil evidence of life on Earth,{{Cite journal |last1=Ueno |first1=Yuichiro |last2=Isozaki |first2=Yukio |last3=Yurimoto |first3=Hisayoshi |last4=Maruyama |first4=Shigenori |date=March 2001 |title=Carbon Isotopic Signatures of Individual Archean Microfossils(?) from Western Australia |url=https://www.tandfonline.com/doi/full/10.1080/00206810109465008 |journal=International Geology Review |language=en |volume=43 |issue=3 |pages=196–212 |doi=10.1080/00206810109465008 |bibcode=2001IGRv...43..196U |s2cid=129302699 |issn=0020-6814}} but their origins may be volcanic.{{Cite journal |last1=Wacey |first1=David |last2=Noffke |first2=Nora |last3=Saunders |first3=Martin |last4=Guagliardo |first4=Paul |last5=Pyle |first5=David M. |date=May 2018 |title=Volcanogenic Pseudo-Fossils from the ∼3.48 Ga Dresser Formation, Pilbara, Western Australia |journal=Astrobiology |volume=18 |issue=5 |pages=539–555 |doi=10.1089/ast.2017.1734 |issn=1531-1074 |pmc=5963881 |pmid=29461869|bibcode=2018AsBio..18..539W }} 3.465-billion-year-old Australian Apex chert rocks may once have contained microorganisms,{{cite web |last=Tyrell |first=Kelly April |date=18 December 2017 |title=Oldest fossils ever found show life on Earth began before 3.5 billion years ago |url=https://news.wisc.edu/oldest-fossils-ever-found-show-life-on-earth-began-before-3-5-billion-years-ago/ |access-date=18 December 2017 |website=University of Wisconsin–Madison}}{{cite journal |last1=Schopf |first1=J. William |last2=Kitajima |first2=Kouki |last3=Spicuzza |first3=Michael J. |last4=Kudryavtsev |first4=Anatolly B. |last5=Valley |first5=John W. |date=2017 |title=SIMS analyses of the oldest known assemblage of microfossils document their taxon-correlated carbon isotope compositions |journal=PNAS |volume=115 |issue=1 |pages=53–58 |bibcode=2018PNAS..115...53S |doi=10.1073/pnas.1718063115 |pmc=5776830 |pmid=29255053 |doi-access=free}} although the validity of these findings has been contested.{{Cite journal |last1=Brasier |first1=Martin D. |last2=Green |first2=Owen R. |last3=Lindsay |first3=John F. |last4=McLoughlin |first4=Nicola |last5=Steele |first5=Andrew |last6=Stoakes |first6=Cris |date=2005-10-21 |title=Critical testing of Earth's oldest putative fossil assemblage from the ~3.5Ga Apex chert, Chinaman Creek, Western Australia |url=https://www.sciencedirect.com/science/article/pii/S0301926805000926 |journal=Precambrian Research |volume=140 |issue=1 |pages=55–102 |doi=10.1016/j.precamres.2005.06.008 |bibcode=2005PreR..140...55B |issn=0301-9268}}{{Cite journal |last1=Pinti |first1=Daniele L. |last2=Mineau |first2=Raymond |last3=Clement |first3=Valentin |date=2009-08-02 |title=Hydrothermal alteration and microfossil artefacts of the 3,465-million-year-old Apex chert |url=http://dx.doi.org/10.1038/ngeo601 |journal=Nature Geoscience |volume=2 |issue=9 |pages=640–3 |doi=10.1038/ngeo601 |bibcode=2009NatGe...2..640P |issn=1752-0894}} "Putative filamentous microfossils," possibly of methanogens and/or methanotrophs that lived about 3.42-billion-year-old in "a paleo-subseafloor hydrothermal vein system of the Barberton greenstone belt, have been identified in South Africa." A diverse set of microfossil morphologies have been found in the 3.43 Ga Strelley Pool Formation including spheroid, lenticular, and film-like microstructures.{{Cite journal |last1=Sugitani |first1=K. |last2=Mimura |first2=K. |last3=Takeuchi |first3=M. |last4=Lepot |first4=K. |last5=Ito |first5=S. |last6=Javaux |first6=E. J. |date=November 2015 |title=Early evolution of large micro-organisms with cytological complexity revealed by microanalyses of 3.4 Ga organic-walled microfossils |url=https://onlinelibrary.wiley.com/doi/10.1111/gbi.12148 |journal=Geobiology |language=en |volume=13 |issue=6 |pages=507–521 |doi=10.1111/gbi.12148 |pmid=26073280 |bibcode=2015Gbio...13..507S |s2cid=1215306 |issn=1472-4677}} Their biogenicity are strengthened by their observed chemical preservation.{{Cite journal |last1=Alleon |first1=J. |last2=Bernard |first2=S. |last3=Le Guillou |first3=C. |last4=Beyssac |first4=O. |last5=Sugitani |first5=K. |last6=Robert |first6=F. |date=August 2018 |title=Chemical nature of the 3.4 Ga Strelley Pool microfossils |url=http://www.geochemicalperspectivesletters.org/article1817 |journal=Geochemical Perspectives Letters |volume=7 |issue=7 |pages=37–42 |doi=10.7185/geochemlet.1817|s2cid=59402752 |doi-access=free |hdl=20.500.12210/9169 |hdl-access=free }} The early lithification of these structures allowed important chemical tracers, such as the carbon-to-nitrogen ratio, to be retained at levels higher than is typical in older, metamorphosed rock units.

{{Further information|Biosignature}}

Biomarkers are compounds of biologic origin found in the geologic record that can be linked to past life.{{Citation |last=Condie |first=Kent C. |title=9. The biosphere |date=2022-01-01 |url=https://www.sciencedirect.com/science/article/pii/B9780128199145000032 |work=Earth as an Evolving Planetary System |edition=F4th |pages=269–303 |editor-last=Condie |editor-first=Kent C. |access-date=2023-11-28 |publisher=Academic Press |doi=10.1016/b978-0-12-819914-5.00003-2 |isbn=978-0-12-819914-5|s2cid=262021891 }} Although they aren't preserved until the late Archean, they are important indicators of early photosynthetic life. Lipids are particularly useful biomarkers because they can survive for long periods of geologic time and reconstruct past environments.{{Cite journal |last1=Finkel |first1=Pablo L. |last2=Carrizo |first2=Daniel |last3=Parro |first3=Victor |last4=Sánchez-García |first4=Laura |date=May 2023 |title=An Overview of Lipid Biomarkers in Terrestrial Extreme Environments with Relevance for Mars Exploration |journal=Astrobiology |volume=23 |issue=5 |pages=563–604 |doi=10.1089/ast.2022.0083 |issn=1531-1074 |pmc=10150655 |pmid=36880883|bibcode=2023AsBio..23..563F }}

File:Common lipid types.svgs are commonly used in geologic studies to find evidence of oxygenic photosynthesis.]]

Fossilized lipids were reported from 2.7 Ga laminated shales from the Pilbara Craton{{Cite journal |last1=Brocks |first1=Jochen J. |last2=Logan |first2=Graham A. |last3=Buick |first3=Roger |last4=Summons |first4=Roger E. |date=1999-08-13 |title=Archean Molecular Fossils and the Early Rise of Eukaryotes |url=https://www.science.org/doi/10.1126/science.285.5430.1033 |journal=Science |language=en |volume=285 |issue=5430 |pages=1033–6 |doi=10.1126/science.285.5430.1033 |pmid=10446042 |bibcode=1999Sci...285.1033B |issn=0036-8075}} and the 2.67 Ga Kaapvaal craton in South Africa.{{Cite journal |last1=Waldbauer |first1=Jacob R. |last2=Sherman |first2=Laura S. |last3=Sumner |first3=Dawn Y. |author-link3=Dawn Sumner|last4=Summons |first4=Roger E. |date=2009-03-01 |title=Late Archean molecular fossils from the Transvaal Supergroup record the antiquity of microbial diversity and aerobiosis |url=https://www.sciencedirect.com/science/article/pii/S0301926808002507 |journal=Precambrian Research |series=Initial investigations of a Neoarchean shelf margin–basin transition (Transvaal Supergroup, South Africa) |volume=169 |issue=1 |pages=28–47 |doi=10.1016/j.precamres.2008.10.011 |bibcode=2009PreR..169...28W |issn=0301-9268}} However, the age of these biomarkers and whether their deposition was synchronous with their host rocks were debated,{{Cite journal |last1=Rasmussen |first1=Birger |last2=Fletcher |first2=Ian R. |last3=Brocks |first3=Jochen J. |last4=Kilburn |first4=Matt R. |date=October 2008 |title=Reassessing the first appearance of eukaryotes and cyanobacteria |url=https://www.nature.com/articles/nature07381 |journal=Nature |language=en |volume=455 |issue=7216 |pages=1101–4 |doi=10.1038/nature07381 |pmid=18948954 |bibcode=2008Natur.455.1101R |s2cid=4372071 |issn=1476-4687}} and further work showed that the lipids were contaminants.{{Cite journal |last1=French |first1=Katherine L. |last2=Hallmann |first2=Christian |last3=Hope |first3=Janet M. |last4=Schoon |first4=Petra L. |last5=Zumberge |first5=J. Alex |last6=Hoshino |first6=Yosuke |last7=Peters |first7=Carl A. |last8=George |first8=Simon C. |last9=Love |first9=Gordon D. |last10=Brocks |first10=Jochen J. |last11=Buick |first11=Roger |last12=Summons |first12=Roger E. |display-authors=5 |date=2015-04-27 |title=Reappraisal of hydrocarbon biomarkers in Archean rocks |journal=Proceedings of the National Academy of Sciences |volume=112 |issue=19 |pages=5915–20 |doi=10.1073/pnas.1419563112 |doi-access=free |pmid=25918387 |pmc=4434754 |bibcode=2015PNAS..112.5915F |issn=0027-8424}} The oldest "clearly indigenous"{{Cite journal |last1=Vinnichenko |first1=Galina |last2=Jarrett |first2=Amber J. M. |last3=Hope |first3=Janet M. |last4=Brocks |first4=Jochen J. |date=September 2020 |title=Discovery of the oldest known biomarkers provides evidence for phototrophic bacteria in the 1.73 Ga Wollogorang Formation, Australia |url=https://onlinelibrary.wiley.com/doi/10.1111/gbi.12390 |journal=Geobiology |volume=18 |issue=5 |pages=544–559 |doi=10.1111/gbi.12390 |pmid=32216165 |bibcode=2020Gbio...18..544V |s2cid=214680085 |issn=1472-4677}} biomarkers are from the 1.64 Ga Barney Creek Formation in the McArthur Basin in Northern Australia,{{Cite journal |last1=Summons |first1=Roger E |last2=Powell |first2=Trevor G |last3=Boreham |first3=Christopher J |date=1988-07-01 |title=Petroleum geology and geochemistry of the Middle Proterozoic McArthur Basin, Northern Australia: III. Composition of extractable hydrocarbons |url=https://dx.doi.org/10.1016/0016-7037%2888%2990001-4 |journal=Geochimica et Cosmochimica Acta |volume=52 |issue=7 |pages=1747–63 |doi=10.1016/0016-7037(88)90001-4 |bibcode=1988GeCoA..52.1747S |issn=0016-7037}}{{Cite journal |last1=Brocks |first1=Jochen J. |last2=Love |first2=Gordon D. |last3=Summons |first3=Roger E. |last4=Knoll |first4=Andrew H. |last5=Logan |first5=Graham A. |last6=Bowden |first6=Stephen A. |date=October 2005 |title=Biomarker evidence for green and purple sulphur bacteria in a stratified Palaeoproterozoic sea |url=https://www.nature.com/articles/nature04068 |journal=Nature |language=en |volume=437 |issue=7060 |pages=866–870 |doi=10.1038/nature04068 |pmid=16208367 |bibcode=2005Natur.437..866B |s2cid=4427285 |issn=1476-4687}} but hydrocarbons from the 1.73 Ga Wollogorang Formation in the same basin have also been detected.

Other indigenous biomarkers can be dated to the Mesoproterozoic era (1.6-1.0 Ga). The 1.4 Ga Hongshuizhuang Formation in the North China Craton contains hydrocarbons in shales that were likely sourced from prokaryotes.{{Cite journal |last1=Luo |first1=Qingyong |last2=George |first2=Simon C. |last3=Xu |first3=Yaohui |last4=Zhong |first4=Ningning |date=2016-09-01 |title=Organic geochemical characteristics of the Mesoproterozoic Hongshuizhuang Formation from northern China: Implications for thermal maturity and biological sources |url=https://www.sciencedirect.com/science/article/pii/S0146638016300420 |journal=Organic Geochemistry |volume=99 |pages=23–37 |doi=10.1016/j.orggeochem.2016.05.004 |bibcode=2016OrGeo..99...23L}} Biomarkers were found in siltstones from the 1.38 Ga Roper Group of the McArthur Basin.{{Cite journal |last1=Jarrett |first1=Amber J. M. |last2=Cox |first2=Grant M. |last3=Brocks |first3=Jochen J. |last4=Grosjean |first4=Emmanuelle |last5=Boreham |first5=Chris J. |last6=Edwards |first6=Dianne S. |date=July 2019 |title=Microbial assemblage and palaeoenvironmental reconstruction of the 1.38 Ga Velkerri Formation, McArthur Basin, northern Australia |journal=Geobiology |volume=17 |issue=4 |pages=360–380 |doi=10.1111/gbi.12331 |pmc=6618112 |pmid=30734481 |bibcode=2019Gbio...17..360J }} Hydrocarbons possibly derived from bacteria and algae were reported in 1.37 Ga Xiamaling Formation of the NCC.{{Cite journal |last1=Luo |first1=Genming |last2=Hallmann |first2=Christian |last3=Xie |first3=Shucheng |last4=Ruan |first4=Xiaoyan |last5=Summons |first5=Roger E. |date=2015-02-15 |title=Comparative microbial diversity and redox environments of black shale and stromatolite facies in the Mesoproterozoic Xiamaling Formation |url=https://www.sciencedirect.com/science/article/pii/S0016703714007431 |journal=Geochimica et Cosmochimica Acta |volume=151 |pages=150–167 |doi=10.1016/j.gca.2014.12.022 |bibcode=2015GeCoA.151..150L}} The 1.1 Ga Atar/El Mreïti Group in the Taoudeni Basin, Mauritania show indigenous biomarkers in black shales.{{Cite journal |last1=Blumenberg |first1=Martin |last2=Thiel |first2=Volker |last3=Riegel |first3=Walter |last4=Kah |first4=Linda C. |last5=Reitner |first5=Joachim |date=2012-02-01 |title=Biomarkers of black shales formed by microbial mats, Late Mesoproterozoic (1.1Ga) Taoudeni Basin, Mauritania |url=https://www.sciencedirect.com/science/article/pii/S0301926811002506 |journal=Precambrian Research |volume=196-197 |pages=113–127 |doi=10.1016/j.precamres.2011.11.010 |bibcode=2012PreR..196..113B}}

{{Main|Last universal common ancestor}}

By comparing the genomes of modern organisms (in the domains Bacteria and Archaea), it is evident that there was a last universal common ancestor (LUCA). Another term for the LUCA is the cenancestor and can be viewed as a population of organisms rather than a single entity.{{Cite journal |last1=Ragan |first1=Mark A |last2=McInerney |first2=James O |last3=Lake |first3=James A |date=2009-08-12 |title=The network of life: genome beginnings and evolution |journal=Philosophical Transactions of the Royal Society B: Biological Sciences |language=en |volume=364 |issue=1527 |pages=2169–2175 |doi=10.1098/rstb.2009.0046 |pmid=19571237 |pmc=2874017 }} LUCA is not thought to be the first life on Earth, but rather the only type of organism of its time to still have living descendants. In 2016, M. C. Weiss and colleagues proposed a minimal set of genes that each occurred in at least two groups of Bacteria and two groups of Archaea. They argued that such a distribution of genes would be unlikely to arise by horizontal gene transfer, and so any such genes must have derived from the LUCA.{{cite journal |last1=Weiss |first1=M. C. |last2=Sousa |first2=F. L. |last3=Mrnjavac |first3=N. |last4=Neukirchen |first4=S. |last5=Roettger |first5=M. |last6=Nelson-Sathi |first6=S. |last7=Martin |first7=W. F. |s2cid=2997255 |year=2016 |title=The physiology and habitat of the last universal common ancestor |journal=Nature Microbiology |volume=1 |issue=9 |page=16116 |doi=10.1038/nmicrobiol.2016.116 |pmid=27562259}} A molecular clock model suggests that the LUCA may have lived 4.477—4.519 billion years ago, within the Hadean eon.

{{further|Abiogenesis|RNA world}}

Model Hadean-like geothermal microenvironments were demonstrated to have the potential to support the synthesis and replication of RNA and thus possibly the evolution of primitive life.{{cite journal |vauthors=Salditt A, Karr L, Salibi E, Le Vay K, Braun D, Mutschler H |title=Ribozyme-mediated RNA synthesis and replication in a model Hadean microenvironment |journal=Nat Commun |volume=14 |issue=1 |pages=1495 |date=March 2023 |pmid=36932102 |pmc=10023712 |doi=10.1038/s41467-023-37206-4 |bibcode=2023NatCo..14.1495S }} Porous rock systems, comprising heated air-water interfaces, were shown to facilitate ribozyme catalyzed RNA replication of sense and antisense strands and then subsequent strand-dissociation. This enabled combined synthesis, release and folding of active ribozymes.

File:Porous chondriteIDP.jpg speculates that life on Earth may have come from biological matter carried by space dust{{cite journal |last=Berera |first=Arjun |title=Space dust collisions as a planetary escape mechanism |journal=Astrobiology |volume=17 |issue=12 |pages=1274–82 |date=6 November 2017 |arxiv=1711.01895 |bibcode=2017AsBio..17.1274B |doi=10.1089/ast.2017.1662 |pmid=29148823 |s2cid=126012488 }} or meteorites.{{cite journal |author=Chan, Queenie H. S. |display-authors=etal |date=10 January 2018 |title=Organic matter in extraterrestrial water-bearing salt crystals |journal=Science Advances |volume=4 |pages=eaao3521 |doi=10.1126/sciadv.aao3521 |pmc=5770164 |pmid=29349297 |number=1, eaao3521|bibcode=2018SciA....4.3521C }}]]

While current geochemical evidence dates the origin of life to possibly as early as 4.1 Ga, and fossil evidence shows life at 3.5 Ga, some researchers speculate that life may have started nearly 4.5 billion years ago. According to biologist Stephen Blair Hedges, "If life arose relatively quickly on Earth ... then it could be common in the universe."{{cite news |last=Borenstein |first=Seth |date=19 October 2015 |title=Hints of life on what was thought to be desolate early Earth |newspaper=Associated Press |url=https://apnews.com/e6be2537b4cd46ffb9c0585bae2b2e51 |access-date=9 October 2018}}{{cite news |last=Schouten |first=Lucy |date=20 October 2015 |title=When did life first emerge on Earth? Maybe a lot earlier than we thought |work=The Christian Science Monitor |publisher=Christian Science Publishing Society |location=Boston, Massachusetts |url=https://www.csmonitor.com/Science/2015/1020/When-did-life-first-emerge-on-Earth-Maybe-a-lot-earlier-than-we-thought |access-date=9 October 2018 |archive-url=https://web.archive.org/web/20160322214217/http://www.csmonitor.com/Science/2015/1020/When-did-life-first-emerge-on-Earth-Maybe-a-lot-earlier-than-we-thought |archive-date=22 March 2016 |issn=0882-7729}}{{cite web |last=Johnston |first=Ian |date=2 October 2017 |title=Life first emerged in 'warm little ponds' almost as old as the Earth itself — Charles Darwin's famous idea backed by new scientific study |url=https://www.independent.co.uk/news/science/origins-life-ponds-organisms-earth-age-study-a7978906.html |access-date=2 October 2017 |work=The Independent}} The possibility that terrestrial life forms may have been seeded from outer space has been considered.{{cite journal |author=Steele, Edward J. |display-authors=et al. |date=1 August 2018 |title=Cause of Cambrian Explosion — Terrestrial or Cosmic? |journal=Progress in Biophysics and Molecular Biology |volume=136 |pages=3–23 |doi=10.1016/j.pbiomolbio.2018.03.004 |pmid=29544820 |s2cid=4486796 |doi-access=free|hdl=1885/143614 |hdl-access=free }}{{cite news |last=McRae |first=Mike |date=28 December 2021 |title=A Weird Paper Tests The Limits of Science by Claiming Octopuses Came From Space |work=ScienceAlert |url=https://www.sciencealert.com/a-weird-paper-tests-the-limits-of-science-by-claiming-octopuses-came-from-space |access-date=29 December 2021}} In January 2018, a study found that 4.5 billion-year-old meteorites found on Earth contained liquid water along with prebiotic complex organic substances that may be ingredients for life.

Hydrothermal vents have long been hypothesized to be the grounds from which life originated. The properties of ancient hydrothermal vents, such as the geochemistry, pressure, and temperatures, have the potential to create organic molecules from inorganic molecules.{{Cite journal |last1=Martin |first1=William |last2=Baross |first2=John |last3=Kelley |first3=Deborah |last4=Russell |first4=Michael J. |date=November 2008 |title=Hydrothermal vents and the origin of life |url=https://www.nature.com/articles/nrmicro1991 |journal=Nature Reviews Microbiology |language=en |volume=6 |issue=11 |pages=805–814 |doi=10.1038/nrmicro1991 |pmid=18820700 |bibcode=2008NRvM....6..805M |issn=1740-1534}} In experiments performed by NASA, it was shown that the organic compounds formate and methane could be created from inorganics in the conditions of ancient hydrothermal vents.{{Cite web |date=2020-04-15 |title=Simulating Early Ocean Vents Shows Life's Building Blocks Form Under Pressure - NASA |url=https://www.nasa.gov/science-research/earth-science/simulating-early-ocean-vents-shows-lifes-building-blocks-form-under-pressure/ |access-date=2025-05-01 |language=en-US}} The production of organic molecules could have led to the formation of more complex organic molecules, such as amino acids that can eventually form RNA or DNA.

Charles Darwin is well-known for his theory of evolution via natural selection. His theory for the origin of life was a “warm little pond” that harbored necessary elements for the creation of life such as “ammonia and phosphoric salts, lights, heat, electricity … so that a protein compound was chemically formed ready to undergo still more complex changes.”{{Cite journal |last=Schopf |first=J. William |date=2024-10-21 |title=Pioneers of Origin of Life Studies-Darwin, Oparin, Haldane, Miller, Oró-And the Oldest Known Records of Life |journal=Life (Basel, Switzerland) |volume=14 |issue=10 |pages=1345 |doi=10.3390/life14101345 |doi-access=free |issn=2075-1729 |pmc=11509469 |pmid=39459645|bibcode=2024Life...14.1345S }} However, he mentioned that such an environment today would likely have been destroyed faster than it would take to form life. With this, Darwin’s ideas are generally regarded as the spontaneous generation hypothesis.{{Citation needed|date=May 2025|reason=Since when did Charles Darwin's "Warm Little Pond" became a full on Theory, rather than just a mere musing (or a hypothesis at best) in his letter to a friend? Also, "Spontaneous Generation" refers to an entirely different and separate hypothesis, which is in no way relevant or related to Darwin's letter. Also, also: what's described here sounds a lot more like one of the more modern hypotheses of Abiogenesis (which, in my opinion, by the way, it seems to me that whoever made this section conflates "Abiogenesis" with "Spontaneous Generation"); IIRC, this is a lot more than what Darwin actually wrote/described.}}

In 1924, Aleksander Oparin suggested that the early atmosphere on Earth was full of reducing components such as ammonia, methane, water vapor, and hydrogen gas.{{Cite journal |last=Schopf |first=J. William |date=2024-10-21 |title=Pioneers of Origin of Life Studies-Darwin, Oparin, Haldane, Miller, Oró-And the Oldest Known Records of Life |journal=Life (Basel, Switzerland) |volume=14 |issue=10 |pages=1345 |doi=10.3390/life14101345 |doi-access=free |issn=2075-1729 |pmc=11509469 |pmid=39459645|bibcode=2024Life...14.1345S }} This was proposed after atmospheric methane was discovered on other planets. Later in 1929, J.B.S. Haldane published an article that proposed the same conditions for early life on Earth as Oparin suggested. Their hypothesis was later supported by the Miller-Urey experiment.

File:Miller-Urey experiment-en.svg

At the University of Chicago in 1953, a graduate student named Stanley Miller carried out an experiment under his professor, Harold Urey.{{Cite web |date=2025-03-05 |title=Miller-Urey experiment {{!}} Description, Purpose, Results, & Facts {{!}} Britannica |url=https://www.britannica.com/science/Miller-Urey-experiment |access-date=2025-05-01 |website=www.britannica.com |language=en}} The method would allow for reducing gases to simulate the atmosphere early on Earth and a “spark” to simulate lightning. There was a reflux apparatus that would heat water and mix into the atmosphere where it would then cool and run into the “primordial ocean.” The gases that were used to mimic the reducing atmosphere were methane, ammonia, water vapor, and hydrogen gas. Within a day of allowing the apparatus to run, the experiment yielded a “brown sludge” which was later tested and found to include the following amino acids: glycine, alanine, aspartic acid, and aminobutyric acid. In the following years, many scientists attempted to replicate the results of the experiment and is now known as a fundamental approach to the study of “abiogenesis”. The Miller-Urey experiment was able to simulate the early conditions of Earth’s atmosphere and produced essential amino acids that likely contributed to the production of life.{{Cite web |date=2025-03-05 |title=Miller-Urey experiment {{!}} Description, Purpose, Results, & Facts {{!}} Britannica |url=https://www.britannica.com/science/Miller-Urey-experiment |access-date=2025-05-01 |website=www.britannica.com |language=en}}

Cairns-Smith first introduced this hypothesis in 1966, where they proposed that any crystallization process is likely to involve a basic biological evolution.{{Cite journal |last1=Kloprogge |first1=Jacob Teunis Theo |last2=Hartman |first2=Hyman |date=2022-02-09 |title=Clays and the Origin of Life: The Experiments |journal=Life (Basel, Switzerland) |volume=12 |issue=2 |pages=259 |doi=10.3390/life12020259 |doi-access=free |issn=2075-1729 |pmc=8880559 |pmid=35207546|bibcode=2022Life...12..259K }} Hartman then added on to this hypothesis by proposing in 1975 that metabolism could have developed from a simple environment such as clays. Clays have the ability to synthesize monomers such as amino acids, nucleotides, and other building blocks and polymerize them to create macromolecules. This makes it possible for nucleic acids like RNA or DNA to be created from clay, and cells could further evolve from there.

{{collapsetop|Earliest known life forms|expand=yes}}

{{Life timeline}}

File:Stromatolites in Sharkbay.jpg|Stromatolites may have been made by microbes moving upward to avoid being smothered by sediment.{{cite journal |last=Allwood |first=Abigail C. |title=Evidence of life in Earth's oldest rocks |date=22 September 2016 |journal=Nature |volume=537 |issue=7621 |pages=500–1 |doi=10.1038/nature19429 |pmid=27580031 |s2cid=205250633 }}

File:Stromatolites.jpg|Stromatolites left behind by cyanobacteria are one of the oldest fossils of life on Earth.

File:Cyanobacterial-algal mat.jpg|The cyanobacterial-algal mat, salty lake on the White Sea seaside

File:PIA24374-Mars-SearchForLife-LakeSaldaTurkey-20210127.jpg|Microbialites in the Lake Salda rocks of Turkey

File:Runzelmarken.jpg|Wrinkled Kinneyia-type sedimentary structures formed beneath cohesive microbial mats in peritidal zones.{{cite journal|author=Porada H. |author2=Ghergut J. |author3=Bouougri El H. |year=2008 |title=Kinneyia-Type Wrinkle Structures – Critical Review And Model Of Formation. |journal=PALAIOS |volume=23 |pages=65–77 |doi=10.2110/palo.2006.p06-095r |issue=2|bibcode=2008Palai..23...65P |s2cid=128464944 }}

File:Kinneyia Grimsby Silurian Niagara Gorge.jpg|Kinneyia-like structure in the Grimsby Formation (Silurian) exposed in Niagara Gorge, NY

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{{reflist|colwidth=30em}}

• [https://www.biolib.cz/en/taxon/id14772 Vitae] (BioLib)
• [https://web.archive.org/web/20140715055239/http://taxonomicon.taxonomy.nl/TaxonTree.aspx?id=1&src=0 Biota] (Taxonomicon)
• [https://web.archive.org/web/20051222163318/http://sn2000.taxonomy.nl/Main/Classification/1.htm Life] (Systema Naturae 2000)
• Wikispecies — a free directory of life
• [https://www.hawking.org.uk/in-words/lectures/life-in-the-universe Life in the Universe] — Stephen Hawking (1996)
• {{YouTube|CFDPLkddRqE|Video (24:32): "Migration of Life in the Universe"}} — Gary Ruvkun, 2019.

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