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A state of physical entities that utilize substances derived from outside themselves for the purposes of growth, the repair of their own structure, and the maintenance of their functional systems, and that also reproduce. The oldest known fossils are from rocks about 3300 Ma old and the Earth is estimated to be about 4600 Ma old. If it is accepted that life began on Earth (rather than being introduced from elsewhere), then it must have done so during the intervening 1300 Ma. There is disagreement on the mechanism by which this happened. The classic view is that lightning discharges in a reducing atmosphere produced prebiotic substances which became dissolved in the oceans where, protected from ultraviolet light, they were somehow assembled into the first organisms. Recently, evidence has been advanced to suggest that life may have originated in small freshwater pools in an atmosphere of carbon dioxide with traces of ammonia.

Subjects: Ecology and Conservation — Arts and Humanities.

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