Wednesday, September 6, 2017
After the Earth first formed, for hundreds of millions of years the temperatures were higher then than they are now despite the sun being only 70% as bright as it is currently. Anaerobic life evolved and flourished in conditions with high methane and CO2 levels during this time.
Cyanobacteria then came into being and consumed CO2 while releasing oxygen as a by product; the oxygen oxidized the methane into more CO2; this was then consumed releasing even more oxygen, and so forth and so on until the anaerobic bacteria died off. Meanwhile, the CO2 levels had been driven so low that the Earth became what scientists call Snowball Earth because of the crippled greenhouse effect.
Eventually though, the CO2 and oxygen levels balanced out and new life forms evolved and flourished in the pre-Cambrian Period.
After this, the great extinctions that occurred were associated with increased CO2 levels because of sustained volcanic activity over eons of time promulgated by such events as huge asteroid strikes and tectonic plate actions. Example: Read "K-T & Permian Extinction Events' Massive Lava Floods"