The Great Oxygenation Event

For over a billion years, anaerobic organisms dominated Earth, thriving in an atmosphere with almost no free oxygen. Cyanobacteria had been photosynthesizing for hundreds of millions of years, splitting water molecules and releasing oxygen as a byproduct. For most of that time, the oxygen was absorbed by dissolved iron in the oceans and by reducing gases in the atmosphere — chemical sinks that soaked up the oxygen as fast as it was produced. When those sinks were exhausted, free oxygen began accumulating. Oxygen is corrosive; it rips electrons from organic molecules. For the anaerobes adapted to the pre-oxygenated world, it was poison. The Great Oxygenation Event drove most anaerobic life into peripheral niches — deep ocean sediments, anoxic soils — where their descendants survive today, diminished and marginal. From the perspective of what came after, the event was the most generative moment in the history of life. Aerobic metabolism is roughly eighteen times more efficient than