The oceans were full of life, but it was simple single-cellular life. On the surface of the water were mats of blue-green algae. These unassuming critters are the deadliest killers in the history of life. Blue-green algae, or cyanobacteria, were the first photosynthesizers. They breathed in carbon dioxide and breathed out oxygen. Oxygen is a volatile gas; it causes iron to rust (oxidation) and wood to burn (vigorous oxidation). When cyanobacteria first appeared, the oxygen they breathed out was toxic to nearly all other forms of life. The resulting extinction is called the oxygen catastrophe.
