To sum up - because I taught this to a kid yesterday - for a period, basically all life photosynthesized and oxygen was toxic to most living things. There were large growths of photosynthetic Cyanobacteria that pumped the air full of oxygen. Living things at the time (like, bacteria and microbes, nowhere near vertebrates) couldn’t handle all the O2 and died.

Iron had been taking in most of the oxygen, but it got all rusted up. There’s beautiful geological evidence of this.

This pic sexy as fuck: