Emergent Archaen Eon: 4000 - 2500 mya

Features of the Archaen Eon: 4000 - 2500 mya:

2) The first life forms arise around 3500 mya: cyanbacteria - blue-green algae - fossil bacteria discovered in rocks, in the form of fossil stromatolites, the oldest macroscopic evidence for life.

(the 2nd, or middle, eon of Precambrian times)

1) In the Earth's primordial atmosphere, precursors of organic chemicals lead to amino acid and RNA synthesis, 

3) These early prokaryotic cells that lack internal structures such as mitochondria and a nuclear membrane, possess chromosomal DNA, which can replicate itself.

4) Oxygen, as a cyanobacteria waste-product, saturates the oceans as iron oxide, and seeps into the atmosphere, UV radiation eventually converting the atmospheric oxygen to a life-promoting ozone shield.