Or here's another perspective, Installment 2

Have you ever wondered about this?

The Great Oxygenation Event (GOE, also called the oxygen catastrophe or oxygen crisis or Great Oxidation) was the appearance of free oxygen (O₂) in Earth's atmosphere. This major environmental change happened around 2,400 million years ago.

The rising oxygen levels may have wiped out a huge portion of the Earth's inhabitants at the time. From their perspective it was a catastrophe. Cyanobacteria were essentially responsible for probably the largest extinction event in Earth's history.

http://en.wikipedia.org/wiki/Great_Oxygenation_Event