[ Again from that Series ] How did oxygen come to Earth's atmosphere? About 3 billion years ago there was no oxygen in Earth's atmosphere. Micro organisms that lived in that time were evolved to live without oxygen. Then some type of bacteria called cyanobacteria emerged that could photosynthesize. They consumed CO2 and produced oxygen as a waste product for hundreds of millions of years. Cyanobacteria are also believed to be the reason for series of ice ages in Earth's early history. Back then methane was the main greenhouse gas in the atmosphere but the surplus oxygen produced by cyanobacteria reacted with methane replacing it with less stronger (I don't know if this is the right adjective!) greenhouse gases. This reduced Earth's temperature to freezing levels. Imagine a giant snowball the size of the entire planet Earth!