Eon in planet Earth's geological record lasting from the origin of the Earth to ~2.5 billion years ago.
It is during the Archaean eon that life first evolved, particularly in terms of prokaryotic cells. Included among these prokaryotes were cyanobacteria, which are the first as well as only organisms that generate molecular oxygen in the course of photosynthesis.
It is this accumulation of oxygen in the atmosphere and its geological consequences that serves to define the end of the Archaean eon.
The Archaean Eon is followed by the Proterozoic eon and with the Proterozoic eon defines what previously was known as the precambrian.
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