One day, a new guy shows up. Or several million new guys, because they're bacteria and they come in large groups. These new guys, the cyanobacteria, are pretty different. They go through this whole process of photosynthesis and end up producing oxygen, and it's kind of weird. You're not a fan. In fact, the stuff is toxic to you.
|More like toxygen, amirite? Guys? Hehe. Classic.|
Luckily for you, dissolved iron and organic matter ends up capturing most of the oxygen, so you continue on your merry way for another 200 million years or so. At that point, the oxygen sinks become fully saturated. That means oxygen starts becoming a part of the atmosphere. For you and all your friends, that's a bad thing. A very, very bad thing.
The "anaerobic" part of "obligate anaerobic" means "without air/oxygen." The "obligate" part means oxygen will straight murder you. Which is exactly what happened. The same event that started Earth's atmosphere down the path toward being breathable also caused one of the largest mass extinctions of all time. It was essentially an apocalypse. The only survivors were the culprits -- who passed on the trait of photosynthesis to future generations -- and a few varieties of anaerobic bacteria that could tolerate the new atmosphere.
|Oxygen was basically the Cthluhu of the obligate anaerobic world.|
To get an idea of just how big the Oxygen Catastrophe was (and yes, that is an official name for it), just imagine the same thing happening today. Instead of oxygen, imagine it's an element that humans can't breathe. One that might cause dangerous fluctuations in the living conditions on Earth for the majority of its species. Something like, I don't know, carbon dioxide.
|Well this isn't totally comforting.|