concept

The Great Oxidation Event

The turning point about 2.4 billion years ago when photosynthetic cyanobacteria first pumped out enough free oxygen to transform Earth's atmosphere from an oxygen-poor, reducing haze into the reactive, oxygen-rich mixture we breathe today. Oxygen is fiercely corrosive; its persistent presence can only be maintained by life constantly replenishing it. It is the historical proof that a biosphere can remake an entire planet's air - and the context for reading oxygen as a biosignature.