Proterozoic

Clasically, the boundary between the Proterozoic and the Paleozoic was set at the base of the Cambrian period when the first fossils of animals known as trilobites and archeocyathids appeared. In the second half of the 20th century, a number of fossil forms have been found in Precambrian rocks, but the boundary of the Proterozoic has remained fixed at the base of the Cambrian -- currently thought to be around 545Million years before the present.

Proterozoic consists of 5 geologic periods called, from oldest to youngest:

• Hadean
• Archean[?]
• Paleoproterozoic
• Mesoproterozoic[?]
• Neoproterozooic[?].

The Hadean is thought to occupy the time from 4500Ma to 3800Ma. Conditions in that time span are subject to considerable debate. The debate extends into the early Archean[?], however it is clear that by roughly 3600Ma temperatures were stable at near modern levels, continental crusts were in place, oceans of liquid water existed, and simple lifeforms either had appeared or would appear shortly. Conditions of that sort existed throughout most of the remaining Proterozoic. The major well identified events were the transition to an Oxygenated atmosphere that probably occurred in the Mesoproterozoic[?] and several glaciations the most severe of which took place in the late Neoproterozoic.