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R. Buick (Earth and Space Sciences/Astrobiology Program, U. Washington)
Earth's earliest fossil record is the only one we have of primitive life anywhere in the universe. As Earth may have had a similar early environmental, and perhaps biological, history to other rocky planets, this record provides our only model for astrobiological exploration elsewhere. There is, however, a paucity of data from the oldest and most interesting part of this record because of the rarity of ancient rocks due to constant recycling on our tectonically active planet, metamorphic and deformational destruction of rocks, and poor rock exposure on deeply weathered old continents.
The best evidence of primitive life potentially comes from microfossils, the preserved bodily relics of microbes. However, the oldest known examples are all highly controversial because of doubts about their biological origin and ancient age. Stromatolites, sedimentary structures constructed by microbes, provide tangible evidence of early life but yield less information than actual microfossils. Carbon isotopes constitute perhaps the most robust evidence of primordial biological processes, because they are relatively immune to later geological perturbation. Perhaps the most informative evidence on early life comes from sulfur isotopes and hydrocarbon molecules that allow determination of particular types of early metabolism, as well as inferences regarding evolutionary relationships.
From this range of data, we can conclude that life was probably present on Earth by 3.8 billion years ago, that microbes with modern types of metabolism existed as early as 3.45 Byr ago, and that our lineage of life, the eukaryotes, has a history as ancient as 2.7 Byr.
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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.