Human figure added for perspective.

What I am about to tell you isn't exactly breaking news, but it's new to me. I've learned that about 2.4 billion years ago, free oxygen created by a group of tiny photosynthetic organisms called cyanobacteria caused the extinction of untold numbers of species that had evolved to live in the hitherto oxygen-free atmosphere of the planet Earth. Every air-breathing species that exists or has ever existed since then does so thanks to this catastrophic die-off, which has come to be called the Great Oxygen Event, although it seems unlikely the organisms involved actually referred to it as such.