Great Oxidation Event

The Great Oxidation Event was the transformation of Earth's atmosphere from a state practically devoid of oxygen to one containing abundant free oxygen, beginning about 2.46 to 2.426 billion years ago and concluding around 2.06 billion years ago in the Paleoproterozoic era. It was driven by cyanobacteria that had evolved oxygen producing photosynthesis, releasing oxygen as a byproduct of splitting water molecules, and by the event's end atmospheric oxygen had reached roughly 10 percent of today's level. The sudden appearance of reactive free oxygen is thought to have caused a mass die off of many anaerobic microorganisms adapted to the old atmosphere, but it also opened the way for new biological diversification, eventually enabling the eukaryotic cells and aerobic metabolism that complex life depends on.