Photosynthetic bacteria poisoned the atmosphere with oxygen and nearly ended early life

Around 2.4 billion years ago, cyanobacteria that had been producing oxygen for hundreds of millions of years finally overwhelmed the chemical sinks that absorbed it, flooding the atmosphere with a gas toxic to most life at the time. The Great Oxidation Event wiped out huge swaths of anaerobic organisms and, combined with methane loss, likely triggered a long global ice age. It also set the stage for the oxygen-breathing life that eventually included us.

— Heinrich D. Holland, Great Oxidation Event — Philosophical Transactions of the Royal Society B, 2006