The geologic record shows evidence for early life in two ways: 1) carbon isotopes, and 2) fossil stromatolites. The isotope evidence significantly predates the fossils.
One difficulty in identifying the earliest evidence of life is that most of the rocks from this period have been destroyed by erosion and plate tectonics. Geologists find rocks of this antiquity in only a few places in the world, such as Western Australia and Greenland.
Carbon occurs in three varieties: 12C, 13C, and 14C. Normal carbon has 6 protons and 6 neutrons, and is called 12C. However, carbon may have one or two extra neutrons, creating 13C or 14C. Atoms with the same number of protons, but different numbers of neutrons, are called different isotopes of the same element.
When organisms ingest carbon, they preferentially use 12C over 13C. (14C is radioactive, and thus won’t remain over a long time period.) Carbon with a high ratio of 12C compared to 13C is therefore an indicator of living processes. Carbon enriched in 12C has been identified in rocks from Greenland dated at 3.85 billions of years ago (Ga or Gyr). This is the earliest evidence for life on Earth.
Stromatolites are structures created by photosynthetic bacteria (prokaryotes). These rounded, bulbous shapes are usually less than 1 meter across and contain fine layering.
Photosynthetic bacteria live on thin mats in shallow water. As they become covered in clay and particles, these bacteria move upwards toward the light they require. This movement leaves behind dead layers, much in the same way that trees leave behind inner wood as they grow. Stromatolites are recognizable because of the “tree ring” structure that records the movement of bacteria.
The earliest known fossils of stromatolites are dated at 3.45 Ga, and come from Western Australia. Ironically, the highly-saline bays of Western Australia are one of the few places in the world today where living stromatolites exist.
- 4.54 Ga: Earth forms
- 4.4 Ga: continental crust present (Wilde et al., 2001)
- 4.3 Ga: liquid water present at surface (Mojzsis et al., 2001)
- 3.85 Ga: isotopically “light” carbon shows the earliest chemical signature of life (Nutman et al., 1997; Mojzsis et al., 1996)
- 3.45 Ga: stromatolites, Western Australia photosynthetic prokaryotes
- 2.7 Ga: steranes, a chemical signature of eukaryotes
- Mojzsis, S.J., Harrison, T.M., and Pidgeon, R.T., 2001. “Oxygen-isotopes evidence from ancient zircons for liquid water at the Earth’s surface 4,300 Myr ago.” Nature, 11 January 2001, v. 409, no. 6817, p. 178-81.
- Mojzsis, S.J., Arrehenius G., McKeegan, K.D., Harrison, T.M., Nutman, A.P., and Friend, C.R., 1996. “Evidence for life on Earth before 3,800 million years ago.” Nature, 7 November 1996, v. 384, no. 6604, p. 55-59.
- Nutman, A.P., Mojzsis, S.J., and Friend, C.R., 1997. “Recognition of > or = 3850 Ma water-laden sediments in West Greenland and their significance for the early Archaean Earth.” Geochimica et Cosmochimica Acta, v. 61, no. 12, p. 2475-2484.
- Wilde, S.A., Valley, J.W., Peck, W.H., and Graham, C.M., 2001. “Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago.” Nature, 11 January 2001, v. 409, no. 6817, p. 175-80.