The Great Oxygen Massacre

Talk about the ultimate biological betrayal! About 2.5 billion years ago, cyanobacteria started photosynthesizing and pumping oxygen into the atmosphere like it was nobody's business. Meanwhile, anaerobic microorganisms who were just vibing in their oxygen-free paradise were like "WTF dude?!"

This oxygen apocalypse (literally called the Great Oxygenation Event) wiped out most anaerobic life forms in what was essentially the first and most devastating mass extinction on Earth. Imagine showing up to a party and changing the atmosphere so drastically that 99% of the guests die. Power move, cyanobacteria. Savage.

Now we oxygen-breathers get to exist because these microscopic rebels decided to completely terraform the planet. Thanks for the air, you tiny blue-green assassins!