Great Oxygenation Event (2.4 billion years ago)

The Great Oxygenation Event is the single deadliest proportional extinction in Earth’s history, wiping out over 99% of anaerobic life—a kill rate 10x higher than the average extinction event. When cyanobacteria began mass-producing oxygen 2.4 billion years ago, they poisoned the dominant methane-producing archaea, triggering what scientists call the "Oxygen Catastrophe." This waste product reshaped the planet’s atmosphere, leaving only 0.1% of existing species able to survive. As a result, this event outkills #10 by eliminating nearly 100% of the anaerobic biosphere, making it far more devastating than the Eocene-Oligocene transition.