By 3,500 million years ago (Mya) simple life had developed on earth (the oldest known microbial fossils in Australia are dated to this figure).
[3] The atmosphere was a mix of noxious and poisonous gases (methane, ammonia, sulfur compounds, etc.
These simple organisms, cyanobacteria ruled the still cooling earth for approximately a billion years[3] and gradually transformed the atmosphere to one containing free oxygen.
While they are subdivisions of eonothems and are themselves subdivided into systems, dating experts prefer the finer resolution of smaller spans of time when evaluating strata.
Similarly, the Proterozoic eonothem is divided youngest to oldest into the Neoproterozoic, Mesoproterozoic and Paleoproterozoic erathems, and the Archean eon and eonothem are divided similarly into the Neoarchean, Mesoarchean, Paleoarchean and the Eoarchean, for which a lower (oldest) limit is undefined.