The earliest identified organisms were minute and relatively featureless, and their fossils look like small rods, which are very difficult to tell apart from structures that arise through abiotic physical processes. The oldest undisputed evidence of life on Earth, interpreted as fossilized bacteria, dates to 3 Billion years ago. Other finds in rocks dated to about 3.5 billion years ago have been interpreted as bacteria, with geochemical evidence also seeming to show the presence of life 3.8 billion years ago.

However these analyses were closely scrutinized, and non-biological processes were found which could produce all of the “signatures of life” that had been reported. While this does not prove that the structures found had a non-biological origin, they cannot be taken as clear evidence for the presence of life. Geochemical signatures from rocks deposited 3.4 billion years ago have been interpreted as evidence for life, although these statements have not been thoroughly examined by critics.