What was Earth like 3 to 4 billion years ago?
Little or no oxygen, higher sulfur and methane concentrations either from volcanic activity or the primordial Venus-like atmosphere. The lack of ozone in the upper reaches of the atmosphere allowed a very high flux of ultraviolet radiation from the Sun to reach any life exposed on the Earth’s surface. Estimates of temperature are a subject of great investigation currently, with some models producing cyclical ice ages that periodically reset all but the most robust forms of life to a near extinction level. This area remains an important scientific topic for understanding both the geological, climatic and ultimately biological history of Earth.
