How old are the oldest fossils?
The metasedimentary rocks associated with the early Archaean-aged Itsaq Gneiss Complex on Akilia Island, southwestern Greenland, are reported to contain graphite microparticles that are depleted in 13C that have been interpreted to be the products of organic life (McGregor and Mason, 1977; Mojzsis and others, 1996; Nutman and others, 1996, 1997; Mojzsis and Harrison, 1999). The 13C-to-12C ratio in the graphite microparticles is essentially the same as in modern organisms (Stanley, 1986). These graphite particles were originally thought to be ~3.83-3.85 billion years in age (Nutman and others, 1997; Mojzsis and Harrison, 2002), but a recent review of the relevant geological and isotopic evidence by Kamber and others (2001) concludes that the graphite is contained in rock that is 3.65 to 3.70 billion years old.
A second occurrence of isotopically light graphite microparticles within graded beds in the Isua supracrustal belt of southwestern Greenland may provide better evidence of the earliest life on Earth, from ?3.7 billion years ago (Rosing, 1999; Lepland and others, 2005).
Other fossils reported from the Archean Eon, 4.6 to 2.5 billion years ago, include:
(after Stanley, 1986, p. 262-263, and Cooper and others, 1986, chapter 9).
The earliest life forms on Earth were simple procaryotes that could tolerate extreme environmental conditions and that reproduced asexually by cell division, similar to modern archaeobacteria. Cyanobacteria capable of photosynthesis were prevalent from 3.5 billion years, and persist today (Emiliani, 1992, chapter 19). Green algae -- the first eucaryotic organisms capable of sexual reproduction -- developed ~1.5 billion years ago, at least 2 billion years after the first procaryotes.
References and suggested reading
Cairns-Smith, A.G., 1985, Seven clues to the origin of life: Cambridge, UK, Cambridge University Press (Canto edition, 1990), 131 p., ISBN 0-521-39828-2.
Callomon, J.H., 2001, Fossils as geological clocks, in Lewis, C.L.E., and Knell, S.J., [editors], The age of the Earth -- from 4004 BC to AD 2002: The Geological Society, London, Special Publication 190, p. 237-252, ISBN 1-86239-093-2.
Cooper, J.D., Miller, R.H., and Patterson, J., 1986, A trip through time -- principles of historical geology: Columbus, Ohio, Merrill Publishing Company, 469 p., ISBN 0-675-20140-3.
Dyson, F., 1999, Origins of life [2nd edition]: Cambridge, UK, Cambridge University Press, 100 p., ISBN 0-521-62668-4.
Emiliani, C., 1992, Planet Earth -- cosmology, geology, and the evolution of life and environment: Cambridge, UK, Cambridge University Press, 717 p., ISBN 0-521-40949-7.
Lepland, A., van Zuilen, M.A., Arrhenius, G., Whitehouse, M.J., and Fedo, C.M., 2005, Questioning the evidence for Earth's earliest life -- Akilia revisited: Geology, v. 33, p. 77-79.
McGregor, V.R., and Mason, B., 1977, Petrogenesis and geochemistry of metabasaltic and metasedimentary enclaves in the Amitsoq gneisses, west Greenland: American Mineralogist, v. 62, p. 887-904.
Mojzsis, S.J., and Harrison, T.M., 1999, Geochronological studies of the oldest known marine sediments, in Ninth Annual V. M. Goldschmidt Conference: Lunar and Planetary Institute, Houston, Contribution No. 971, p. 201-202.
Mojzsis, S.J., and Harrison, T.M., 2002, Establishment of a 3.83-Ga magmatic age for the Akilia tonalite (southern West Greenland): Earth and Planetary Science Letters, v. 202, p. 563-576.
Mojzsis, S.J., Arrhenius, G., McKeegan, K.D., Harrison, T.M., Nutman, A.P., and Friend, C.R.L., 1996, Evidence for life on Earth before 3800 million years ago: Nature, v. 384, p. 55-59.
Nutman, A.P., McGregor, V.R., Friend, C.R.L., Bennett, V.C., and Kinny, P.D., 1996, The Itsaq Gneiss Complex of southern west Greenland -- the world's most extensive record of early crustal evolution (3,900-3,600 Ma): Precambrian Research, v. 78, p. 1-39.
Nutman, A.P., Mojzsis, S.J., and Friend, C.R.L., 1997, Recognition of ?3850 Ma water-lain sediments in West Greenland and their significance for the early Archaean Earth: Geochimica et Cosmochimica Acta, v. 61, p. 2475-2484.
Rosing, M.T., 1999, 13C-depleted carbon microparticles in ?3700-Ma sea-floor sedimentary rocks from West Greenland: Science, v. 283, p. 674-676.
Stanley, S.M., 1986, Earth and life through time: New York, W.H. Freeman and Company, 690 p., ISBN 0-7167-1677-1.
The information on this page was written and approved by the faculty of the Geology Department at Baylor University.