|Alternate spelling(s)||Archaean, Archæan|
J.W. Dawson, 1865
|Regional usage||Global (ICS)|
|Time scale(s) used||ICS Time Scale|
|Time span formality||Formal|
|Lower boundary definition||Defined Chronometrically|
|Lower GSSA ratified||1991|
|Upper boundary definition||Defined Chronometrically|
|Upper GSSA ratified||1991|
The Archean Eon ( // ar-KEE-ən, also spelled Archaean or Archæan) is the second of four geologic eons of Earth's history and by definition representing the time from . The Archean was preceded by the Hadean Eon and followed by the Proterozoic.
The Earth during the Archean was mostly a water world: there was continental crust, but much of it was under an ocean deeper than today's ocean. Except for some trace minerals, today's oldest continental crust dates back to the Archean. Much of the geological detail of the Archean has been destroyed by subsequent activity. The earliest known life started in the Archean. Life was simple throughout the Archean, mostly represented by shallow-water microbial mats called stromatolites, and the atmosphere lacked free oxygen.
Etymology and changes in classification
The word Archean comes from the Greek word arkhē (αρχή), meaning 'beginning, origin'. It was first used in 1872, when it meant 'of the earliest geological age'.[a] Before the Hadean Eon was recognized, the Archean spanned Earth's early history from its formation about 4,540 million years ago until 2,500 million years ago.
Instead of being based on stratigraphy, the beginning and end of the Archean Eon are defined chronometrically. The eon's lower boundary or starting point of 4 billion years ago is officially recognized by the International Commission on Stratigraphy.
When the Archean began, the Earth's heat flow was nearly three times as high as it is today, and it was still twice the current level at the transition from the Archean to the Proterozoic (2,500 Ma). The extra heat was the result of a mix of remnant heat from planetary accretion, from the formation of the metallic core, and from the decay of radioactive elements. As a result, the Earth's mantle was significantly hotter than today.
Although a few mineral grains are known to be Hadean, the oldest rock formations exposed on the surface of the Earth are Archean. Archean rocks are found in Greenland, Siberia, the Canadian Shield, Montana, Wyoming (exposed parts of the Wyoming Craton) and Minnesota (Minnesota River Valley), the Baltic Shield, the Rhodope Massif, Scotland, India, Brazil, western Australia, and southern Africa. Granitic rocks predominate throughout the crystalline remnants of the surviving Archean crust. Examples include great melt sheets and voluminous plutonic masses of granite, diorite, layered intrusions, anorthosites and monzonites known as sanukitoids. Archean rocks are often heavily metamorphized deep-water sediments, such as graywackes, mudstones, volcanic sediments, and banded iron formations. Volcanic activity was considerably higher than today, with numerous lava eruptions, including unusual types such as komatiite. Carbonate rocks are rare, indicating that the oceans were more acidic due to dissolved carbon dioxide than during the Proterozoic. Greenstone belts are typical Archean formations, consisting of alternating units of metamorphosed mafic igneous and sedimentary rocks, including Archean felsic volcanic rocks. The metamorphosed igneous rocks were derived from volcanic island arcs, while the metamorphosed sediments represent deep-sea sediments eroded from the neighboring island arcs and deposited in a forearc basin. Greenstone belts, being both types of metamorphosed rock, represent sutures between the protocontinents.: 302–303
Plate tectonics likely started vigorously in the Hadean, but slowed down in the Archean. The slowing of plate tectonics was probably due to an increase in the viscosity of the mantle due to outgassing of its water. Plate tectonics likely produced large amounts of continental crust, but the deep oceans of the Archean probably covered the continents entirely. Only at the end of the Archean did the continents likely emerge from the ocean.
Due to recycling and metamorphosis of the Archean crust, there is a lack of extensive geological evidence for specific continents. One hypothesis is that rocks that are now in India, western Australia, and southern Africa formed a continent called Ur as of 3,100 Ma. A differing conflicting hypothesis is that rocks from western Australia and southern Africa were assembled in a continent called Vaalbara as far back as 3,600 Ma. Archean rock makes up only about 8% of Earth's present-day continental crust; the rest of the Archean continents have been recycled.
By the Neoarchean, plate tectonic activity may have been similar to that of the modern Earth, although there was a significantly greater occurrence of slab detachment resulting from a hotter mantle, rheologically weaker plates, and increased tensile stresses on subducting plates due to their crustal material metamorphosing from basalt into eclogite as they sank. There are well-preserved sedimentary basins, and evidence of volcanic arcs, intracontinental rifts, continent-continent collisions and widespread globe-spanning orogenic events suggesting the assembly and destruction of one and perhaps several supercontinents. Evidence from banded iron formations, chert beds, chemical sediments and pillow basalts demonstrates that liquid water was prevalent and deep oceanic basins already existed.
Asteroid impacts were frequent in the early Archean. Evidence from spherule layers suggests that impacts continued into the later Archean, at an average rate of about one impactor with a diameter greater than 10 kilometers (6 mi) every 15 million years. This is about the size of the Chicxulub impactor. These impacts would have been an important oxygen sink and would have caused drastic fluctuations of atmospheric oxygen levels.
The Archean atmosphere is thought to have nearly lacked free oxygen; oxygen levels were less than 0.001% of their present atmospheric level, with some analyses suggesting they were as low as 0.00001% of modern levels. However, transient episodes of heightened oxygen concentrations are known from this eon around 2,980-2,960 Ma, 2,700 Ma, and 2,501 Ma. The pulses of increased oxygenation at 2,700 and 2,501 Ma have both been considered by some as potential start points of the Great Oxygenation Event, which most scholars consider to have begun in the Palaeoproterozoic. Furthermore, oases of relatively high oxygen levels existed in some nearshore shallow marine settings by the Mesoarchean. The ocean was broadly reducing and lacked any persistent redoxcline, a water layer between oxygenated and anoxic layers characterised by a strong redox gradient, that would become a feature in later, more oxic oceans. Despite the lack of free oxygen, the rate of organic carbon burial appears to have been roughly the same as in the present. Due to extremely low oxygen levels, sulphate was rare in the Archean ocean, and sulphides were produced primarily through reduction of organically sourced sulphite or through mineralisation of compounds containing reduced sulphur. The Archean ocean was enriched in heavier oxygen isotopes relative to the modern ocean, though δ18O values decreased to ones comparable to those of modern oceans over the course of the later part of the eon as a result of increased continental weathering.
Astronomers think that the Sun had about 75–80 percent of the present luminosity, yet temperatures on Earth appear to have been near modern levels only 500 million years after Earth's formation (the faint young Sun paradox). The presence of liquid water is evidenced by certain highly deformed gneisses produced by metamorphism of sedimentary protoliths. The moderate temperatures may reflect the presence of greater amounts of greenhouse gases than later in the Earth's history. Alternatively, Earth's albedo may have been lower at the time, due to less land area and cloud cover.
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 identifiable fossils consist of stromatolites, which are microbial mats formed in shallow water by cyanobacteria. The earliest stromatolites are found in 3.48 billion-year-old sandstone discovered in Western Australia. Stromatolites are found throughout the Archean and become common late in the Archean.: 307 Cyanobacteria were instrumental in creating free oxygen in the atmosphere.
Further evidence for early life is found in 3.47 billion-year-old baryte, in the Warrawoona Group of Western Australia. This mineral shows sulfur fractionation of as much as 21.1%, which is evidence of sulfate-reducing bacteria that metabolize sulfur-32 more readily than sulfur-34.
Evidence of life in the Late Hadean is more controversial. In 2015, biogenic carbon was detected in zircons dated to 4.1 billion years ago, but this evidence is preliminary and needs validation.
Earth was very hostile to life before 4.2–4.3 Ga 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.
Life in the Archean was limited to simple single-celled organisms (lacking nuclei), called prokaryotes. In addition to the domain Bacteria, microfossils of the domain Archaea have also been identified. There are no known eukaryotic fossils from the earliest Archean, though they might have evolved during the Archean without leaving any.: 306, 323 Fossil steranes, indicative of eukaryotes, have been reported from Archean strata but were shown to derive from contamination with younger organic matter. No fossil evidence has been discovered for ultramicroscopic intracellular replicators such as viruses.
- Abiogenesis – Natural process by which life arises from non-living matter
- Cosmic Calendar – Method to visualize the chronology of the universe
- Earliest known life forms – Putative fossilized microorganisms found near hydrothermal vents
- Geologic time scale – System that relates geologic strata to time
- Geological history of oxygen – Timeline of the development of free oxygen in the Earth's seas and atmosphere
- History of Earth – Development of planet Earth from its formation to the present day
- Precambrian – History of Earth 4600–539 million years ago
- Timeline of natural history – universe events since the Big Bang 13.8 billion years ago
- Archean felsic volcanic rocks – Felsic volcanic rocks formed in the Archean Eon
- The name Archean was coined by American geologist James Dwight Dana (1813–1895). The Pre-Cambrian eon had been believed to be without life (azoic); however, because fossils had been found in deposits that had been judged to belong to the Azoic age, "... I propose to use for the Azoic era and its rocks the general term Archæn (or Arche'an), from the Greek άρχαιος, pertaining to the beginning.": 253
- Plumb, K. A. (1 June 1991). "New Precambrian time scale". Episodes. 14 (2): 139–140. doi:10.18814/epiiugs/1991/v14i2/005. Retrieved 16 January 2023.
- Harper, Douglas. "Archaean". Online Etymology Dictionary.
- Dana JD (1872). "Green Mountain geology. On the quartzite". American Journal of Science and Arts. 3rd series. 3 (16): 250–257.
- "International Chronostratigraphic Chart v.2013/01" (PDF). International Commission on Stratigraphy. January 2013. Retrieved 6 April 2013.
- Galer, Stephen J. G.; Mezger, Klaus (1 December 1998). "Metamorphism, denudation and sea level in the Archean and cooling of the Earth". Precambrian Research. 92 (4): 389–412. doi:10.1016/S0301-9268(98)00083-7. Retrieved 24 November 2022.
- Dostal J (2008). "Igneous Rock Associations 10. Komatiites". Geoscience Canada. 35 (1).
- Cooper JD, Miller RH, Patterson J (1986). A Trip Through Time: Principles of historical geology. Columbus: Merrill Publishing Company. p. 180. ISBN 978-0675201407.
- Stanley, Steven M. (1999). Earth System History. New York: W.H. Freeman and Company. ISBN 978-0716728825.
- Korenaga, J (2021). "Was There Land on the Early Earth?". Life. 11 (11): 1142. doi:10.3390/life11111142. PMC 8623345. PMID 34833018.
- Korenaga, J (2021). "Hadean geodynamics and the nature of early continental crust". Precambrian Res. 359: 106178. Bibcode:2021PreR..359j6178K. doi:10.1016/j.precamres.2021.106178. S2CID 233441822.
- Bada, JL; Korenaga, J (2018). "Exposed areas above sea level on Earth >3.5 Gyr ago: Implications for prebiotic and primitive biotic chemistry". Life. 8 (4): 55. doi:10.3390/life8040055. PMC 6316429. PMID 30400350.
- Bindeman, IN; Zakharov, DO; Palandri, J.; Greber, ND; et al. (2018). "Rapid emergence of subaerial landmasses and onset of a modern hydrologic cycle 2.5 billion years ago". Nature. 557 (7706): 545–548. Bibcode:2018Natur.557..545B. doi:10.1038/s41586-018-0131-1. PMID 29795252. S2CID 43921922.
- Rogers JJ (1996). "A history of continents in the past three billion years". Journal of Geology. 104 (1): 91–107. Bibcode:1996JG....104...91R. doi:10.1086/629803. JSTOR 30068065. S2CID 128776432.
- Cheney ES (1996). "Sequence stratigraphy and plate tectonic significance of the Transvaal succession of southern Africa and its equivalent in Western Australia". Precambrian Research. 79 (1–2): 3–24. Bibcode:1996PreR...79....3C. doi:10.1016/0301-9268(95)00085-2.
- Marty, Bernard; Dauphas, Nicolas (15 February 2003). "The nitrogen record of crust–mantle interaction and mantle convection from Archean to Present". Earth and Planetary Science Letters. 206 (3–4): 397–410. doi:10.1016/S0012-821X(02)01108-1. Retrieved 16 November 2022.
- Halla, Jaana; Van Hunen, Jeroen; Heilimo, Esa; Hölttä, Pentti (October 2009). "Geochemical and numerical constraints on Neoarchean plate tectonics". Precambrian Research. 174 (1–2): 155–162. doi:10.1016/j.precamres.2009.07.008. Retrieved 12 November 2022.
- Borgeat, Xavier; Tackley, Paul J. (12 July 2022). "Hadean/Eoarchean tectonics and mantle mixing induced by impacts: a three-dimensional study". Progress in Earth and Planetary Science. 9. doi:10.1186/s40645-022-00497-0. S2CID 243973728. Retrieved 16 November 2022.
- Marchi, S.; Drabon, N.; Schulz, T.; Schaefer, L.; Nesvorny, D.; Bottke, W. F.; Koeberl, C.; Lyons, T. (November 2021). "Delayed and variable late Archaean atmospheric oxidation due to high collision rates on Earth". Nature Geoscience. 14 (11): 827–831. Bibcode:2021NatGe..14..827M. doi:10.1038/s41561-021-00835-9. S2CID 239055744.
- Trainer, Melissa G.; Pavlov, Alexander A.; DeWitt, H. Langley; Jimenez, Jose L.; McKay, Christopher P.; Toon, Owen B.; Tolbert, Margaret A. (28 November 2006). "Organic haze on Titan and the early Earth". Proceedings of the National Academy of Sciences. 103 (48): 18035–18042. doi:10.1073/pnas.0608561103. ISSN 0027-8424. PMC 1838702. PMID 17101962.
- Pavlov, A. A.; Kasting, J. F. (5 July 2004). "Mass-Independent Fractionation of Sulfur Isotopes in Archean Sediments: Strong Evidence for an Anoxic Archean Atmosphere". Astrobiology. 2 (1): 27–41. doi:10.1089/153110702753621321. PMID 12449853. Retrieved 12 November 2022.
- Zhang, Shuichang; Wang, Xiaomei; Wang, Huajian; Bjerrum, Christian J.; Hammarlund, Emma U.; Costa, M. Mafalda; Connelly, James N.; Zhang, Baomin; Su, Jin; Canfield, Donald Eugene (4 January 2016). "Sufficient oxygen for animal respiration 1,400 million years ago". Proceedings of the National Academy of Sciences. 113 (7): 1731–1736. Bibcode:2016PNAS..113.1731Z. doi:10.1073/pnas.1523449113. PMC 4763753. PMID 26729865.
- Laakso, T. A.; Schrag, D. P. (5 April 2017). "A theory of atmospheric oxygen". Geobiology. 15 (3): 366–384. doi:10.1111/gbi.12230. PMID 28378894. S2CID 22594748. Retrieved 12 November 2022.
- Crowe, Sean A.; Døssing, Lasse N.; Beukes, Nicolas J.; Bau, Michael; Kruger, Stephanus J.; Frei, Robert; Canfield, Donald Eugene (25 September 2013). "Atmospheric oxygenation three billion years ago". Nature. 501 (7468): 535–538. doi:10.1038/nature12426. PMID 24067713. S2CID 4464710. Retrieved 12 November 2022.
- Large, Ross R.; Hazen, Robert M.; Morrison, Shaunna M.; Gregory, Dan D.; Steadman, Jeffrey A.; Mukherjee, Indrani (May 2022). "Evidence that the GOE was a prolonged event with a peak around 1900 Ma". Geosystems and Geoenvironment. 1 (2). doi:10.1016/j.geogeo.2022.100036. Retrieved 7 January 2023.
- Anbar, Ariel D.; Duan, Yun; Lyons, Timothy W.; Arnold, Gail N.; Kendall, Brian; Creaser, Robert A.; Kaufman, Alan J.; Gordon, Gwyneth W.; Scott, Clinton; Garvin, Jessica; Buick, Roger (28 September 2007). "A Whiff of Oxygen Before the Great Oxidation Event?". Science. 317 (5846): 1903–1906. doi:10.1126/science.1140325. PMID 17901330. S2CID 25260892. Retrieved 12 November 2022.
- Reinhard, Christopher T.; Raiswell, Robert; Scott, Clinton; Anbar, Ariel D.; Lyons, Timothy W. (30 October 2009). "A Late Archean Sulfidic Sea Stimulated by Early Oxidative Weathering of the Continents". Science. 326 (5953): 713–716. doi:10.1126/science.1176711. PMID 19900929. S2CID 25369788. Retrieved 12 November 2022.
- Warke, Matthew R.; Di Rocco, Tommaso; Zerkle, Aubrey L.; Lepland, Aivo; Prave, Anthony R.; Martin, Adam P.; Ueno, Yuichiro; Condon, Daniel J.; Claire, Mark W. (16 June 2020). "The Great Oxidation Event preceded a Paleoproterozoic "snowball Earth"". Proceedings of the National Academy of Sciences. 117 (24): 13314–13320. doi:10.1073/pnas.2003090117. ISSN 0027-8424. PMC 7306805. PMID 32482849. Retrieved 7 January 2023.
- Luo, Genming; Ono, Shuhei; Beukes, Nicolas J.; Wang, David T.; Xie, Shucheng; Summons, Roger E. (6 May 2016). "Rapid oxygenation of Earth's atmosphere 2.33 billion years ago". Science Advances. 2 (5): e1600134. doi:10.1126/sciadv.1600134. ISSN 2375-2548. PMC 4928975. PMID 27386544. Retrieved 7 January 2023.
- Poulton, Simon W.; Bekker, Andrey; Cumming, Vivien M.; Zerkle, Aubrey L.; Canfield, Donald E.; Johnston, David T. (April 2021). "A 200-million-year delay in permanent atmospheric oxygenation". Nature. 592 (7853): 232–236. doi:10.1038/s41586-021-03393-7. ISSN 1476-4687. Retrieved 7 January 2023.
- Gumsley, Ashley P.; Chamberlain, Kevin R.; Bleeker, Wouter; Söderlund, Ulf; De Kock, Michiel O.; Larsson, Emilie R.; Bekker, Andrey (6 February 2017). "Timing and tempo of the Great Oxidation Event". Proceedings of the National Academy of Sciences of the United States of America. 114 (8): 1811–1816. doi:10.1073/pnas.1608824114. ISSN 0027-8424. PMC 5338422. PMID 28167763. Retrieved 7 January 2023.
- Eickmann, Benjamin; Hofmann, Axel; Wille, Martin; Bui, Thi Hao; Wing, Boswell A.; Schoenberg, Ronny (15 January 2018). "Isotopic evidence for oxygenated Mesoarchaean shallow oceans". Nature Geoscience. 11 (2): 133–138. doi:10.1038/s41561-017-0036-x. S2CID 135023426. Retrieved 25 December 2022.
- Zhou, Hang; Zhou, Wenxiao; Wei, Yunxu; Chi Fru, Ernest; Huang, Bo; Fu, Dong; Li, Haiquan; Tan, Mantang (December 2022). "Mesoarchean banded iron-formation from the northern Yangtze Craton, South China and its geological and paleoenvironmental implications". Precambrian Research. 383: 106905. doi:10.1016/j.precamres.2022.106905. Retrieved 17 December 2022.
- Fischer, W. W.; Schroeder, S.; Lacassie, J. P.; Beukes, N. J.; Goldberg, T.; Strauss, H.; Horstmann, U. E.; Schrag, D. P.; Knoll, A. H. (March 2009). "Isotopic constraints on the Late Archean carbon cycle from the Transvaal Supergroup along the western margin of the Kaapvaal Craton, South Africa". Precambrian Research. 169 (1–4): 15–27. doi:10.1016/j.precamres.2008.10.010. Retrieved 24 November 2022.
- Fakhraee, Mojtaba; Katsev, Sergei (7 October 2019). "Organic sulfur was integral to the Archean sulfur cycle". Nature Communications. 10 (1): 4556. doi:10.1038/s41467-019-12396-y. PMC 6779745. PMID 31591394.
- Johnson, Benjamin W.; Wing, Boswell A. (2 March 2020). "Limited Archaean continental emergence reflected in an early Archaean 18O-enriched ocean". Nature Geoscience. 13: 243–248. doi:10.1038/s41561-020-0538-9. Retrieved 7 January 2023.
- Dauphas, Nicolas; Kasting, James Fraser (1 June 2011). "Low pCO2 in the pore water, not in the Archean air". Nature. 474 (7349): E2-3, discussion E4-5. doi:10.1038/nature09960. PMID 21637211. S2CID 205224575. Retrieved 12 November 2022.
- Walker, James C. G. (November 1982). "Climatic factors on the Archean earth". Palaeogeography, Palaeoclimatology, Palaeoecology. 40 (1–3): 1–11. doi:10.1016/0031-0182(82)90082-7. hdl:2027.42/23810. Retrieved 12 November 2022.
- Walker, James C.G. (June 1985). "Carbon dioxide on the early earth" (PDF). Origins of Life and Evolution of the Biosphere. 16 (2): 117–127. Bibcode:1985OrLi...16..117W. doi:10.1007/BF01809466. hdl:2027.42/43349. PMID 11542014. S2CID 206804461. Retrieved 30 January 2010.
- Pavlov AA, Kasting JF, Brown LL, Rages KA, Freedman R (May 2000). "Greenhouse warming by CH4 in the atmosphere of early Earth". Journal of Geophysical Research. 105 (E5): 11981–11990. Bibcode:2000JGR...10511981P. doi:10.1029/1999JE001134. PMID 11543544.
- Rosing MT, Bird DK, Sleep NH, Bjerrum CJ (April 2010). "No climate paradox under the faint early Sun". Nature. 464 (7289): 744–747. Bibcode:2010Natur.464..744R. doi:10.1038/nature08955. PMID 20360739. S2CID 205220182.
- Ohtomo Y, Kakegawa T, Ishida A, Nagase T, Rosing MT (8 December 2013). "Evidence for biogenic graphite in early Archaean Isua metasedimentary rocks". Nature Geoscience. 7 (1): 25–28. Bibcode:2014NatGe...7...25O. doi:10.1038/ngeo2025.
- Borenstein, Seth (13 November 2013). "Oldest fossil found: Meet your microbial mom". AP News. Retrieved 15 November 2013.
- Noffke N, Christian D, Wacey D, Hazen RM (December 2013). "Microbially induced sedimentary structures recording an ancient ecosystem in the ca. 3.48 billion-year-old Dresser Formation, Pilbara, Western Australia". Astrobiology. 13 (12): 1103–1124. Bibcode:2013AsBio..13.1103N. doi:10.1089/ast.2013.1030. PMC 3870916. PMID 24205812.
- Garwood, Russell J. (2012). "Patterns In Palaeontology: The first 3 billion years of evolution". Palaeontology Online. 2 (11): 1–14. Retrieved 25 June 2015.
- "Early life: Oxygen enters the atmosphere". BBC. Retrieved 20 September 2012.
- Shen Y, Buick R, Canfield DE (March 2001). "Isotopic evidence for microbial sulphate reduction in the early Archaean era". Nature. 410 (6824): 77–81. Bibcode:2001Natur.410...77S. doi:10.1038/35065071. PMID 11242044. S2CID 25375808.
- Seal RR (2006). "Sulfur isotope geochemistry of sulfide minerals". Reviews in Mineralogy and Geochemistry. 61 (1): 633–677. Bibcode:2006RvMG...61..633S. doi:10.2138/rmg.2006.61.12.
- Borenstein S (19 October 2015). "Hints of life on what was thought to be desolate early Earth". Excite. Yonkers, NY: Mindspark Interactive Network. Associated Press. Retrieved 20 October 2015.
- Bell EA, Boehnke P, Harrison TM, Mao WL (November 2015). "Potentially biogenic carbon preserved in a 4.1 billion-year-old zircon". Proceedings of the National Academy of Sciences of the United States of America (Early, published online before print ed.). 112 (47): 14518–14521. Bibcode:2015PNAS..11214518B. doi:10.1073/pnas.1517557112. PMC 4664351. PMID 26483481.
- Nisbet, Euan (1980). "Archaean stromatolites and the search for the earliest life". Nature. 284 (5755): 395–396. Bibcode:1980Natur.284..395N. doi:10.1038/284395a0. S2CID 4262249.
- French KL, Hallmann C, Hope JM, Schoon PL, Zumberge JA, Hoshino Y, Peters CA, George SC, Love GD, Brocks JJ, Buick R, Summons RE (May 2015). "Reappraisal of hydrocarbon biomarkers in Archean rocks". Proceedings of the National Academy of Sciences of the United States of America. 112 (19): 5915–5920. Bibcode:2015PNAS..112.5915F. doi:10.1073/pnas.1419563112. PMC 4434754. PMID 25918387.
- Homann, Martin; Sansjofre, Pierre; Van Zuilen, Mark; Heubeck, Christoph; Gong, Jian; Killingsworth, Bryan; Foster, Ian S.; Airo, Alessandro; Van Kranendonk, Martin J.; Ader, Magali; Lalonde, Stefan V. (23 July 2018). "Microbial life and biogeochemical cycling on land 3,220 million years ago". Nature Geoscience. 11: 665–671. doi:10.1038/s41561-018-0190-9. Retrieved 14 January 2023.
- Woo, Marcus (30 July 2018). "Oldest Evidence for life on land unearthed in South Africa". livescience.com.