The Archaean eon lasted from 4.0bya to 2.5bya.  During this time the atmosphere had little oxygen and the sun was only between 70% and 75% as bright as it is today.  The first single-celled life appeared during this eon which produced the oxygen in the atmosphere and the oldest fossils are found in this time period.
Archean stromatolites 1280

Precambrian Stromatolites

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