Humans aren't the only species that has caused mass extinctions (although we are well ahead of the rest). The Great Oxygen Catastrophe, sometimes called the Oxygen Holocaust, is one of the largest extinction events ever to hit the planet. It wasn't caused by some great master race or cataclysmic asteroid impact, but by a humble bacteria called Cyanobacteria. Around 2.3 billion years ago, the Earth was mainly inhabited by colonies of anaerobic bacteria, then cyanobacteria crashed the party and started photosynthesising all over the place, polluting the atmosphere with a horrible, deadly gas: Oxygen. As the oxygen accumulated in the atmosphere, it basically wiped out most life on Earth. Not only that, but it reacted with greenhouse gasses in the atmosphere and plunged the entire planet into an ice age, turning it into a Snowball Earth. Eventually, aerobic (oxygen breathing) life evolved, including us. It's kinda weird to think that we owe our entire existence to a deadly, polluting bacteria.
