Erathem

In stratigraphy, paleontology, geology, and geobiology an erathem is the total stratigraphic unit deposited during a certain corresponding span of time during an era in the geologic timescale.

It can therefore be used as a chronostratigraphic unit of time which delineates a large span of years — less than an geological eon, but greater than its successively smaller and more refined subdivisions (geologic periods, epochs, and geologic ages). 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. The atmosphere was a mix of noxious and poisonous gases (Methane, Ammonia, Sulphur compounds, etc.— a so-called reducing atmosphere lacking much free oxygen which was bound up in compounds).

These simple organisms, Cyanobacteria ruled the still cooling earth for approximately a thousand million (over a billion) years and gradually transformed the atmosphere to one containing free oxygen. These changes, along with tectonic activity left chemical trails (red bed formation, etc.) and other physical clues (magnetic orientation, layer formation factors) in the rock record, and it is these changes along with the later richer fossil record which specialists use to demarcate times early in planet earth's history in various disciplines.