[adrianN]

It took about a billion years of photosynthesis on earth before all the ferrous iron dissolved in the oceans was oxidized and atmospheric oxygen concentration began to take off.

[gus_massa]

... and it probably killed most of the then current bacteria/archea, because they were adapted to an atmosphere without oxygen https://en.wikipedia.org/wiki/Great_Oxidation_Event

[prox]

Fascinating