Oxygen’s toxic rise: the Great Oxidation Event and Earth’s first mass extinction
Around 2.4 billion years ago, cyanobacteria started releasing oxygen through photosynthesis, and as sinks filled, oxygen accumulated in the oceans and air. That oxygen was poisonous to the dominant anaerobic life, sparking Earth’s first major mass extinction—the Great Oxidation Event. Evidence includes sulfur isotope patterns that indicate an oxygen-free atmosphere before 2.4 Ga and their disappearance after, and the widespread formation of banded iron formations as iron was oxidized. Oxygen’s rise also collapsed methane, possibly helping trigger the long Huronian glaciation. The fossil record is sparse and the transition was uneven and gradual, not a single moment, but it marks a pivotal shift as life adapted to breathe oxygen.
