After the Earth first formed, the temperatures were higher than they are now. Anaerobic life evolved and flourished in conditions with high methane and CO2 levels for hundreds of millions of years. Then Cyanobacteria came into being and consumed the CO2 while releasing oxygen, creating a great dying off of the anaerobics. The CO2 levels got so low that the Earth became what scientists call Snowball Earth. Eventually, however, the CO2 and oxygen levels balanced out and new life forms evolved and flourished in the pre-Cambrian Period. After this, the only times that great extinctions occurred were when CO2 levels soared due to natural catastrophes.