(Why) was the earth's atmosphere so much more oxygenated way back when?

[ultrafilter]

In the discussion of the Titanoboa fossils that were recently found, it came up that larger creatures were feasible back then because the earth had a lot more oxygen in its atmosphere. Is that the case, and if so, what happened?

[Cider_Depot]

Originally there was 0% atmospheric oxygen. Then came the cyanobacteria that made it. Then came plants, of course, famous for their oxygen producing capabilities. Then this, from Wikipedia:

This modern atmosphere has a composition which is enforced by oceanic blue-green algae as well as geological processes. O2 does not remain naturally free in an atmosphere but tends to be consumed by inorganic chemical reactions, and by animals, bacteria, and even land plants at night. CO2 tends to be produced by respiration and decomposition and oxidation of organic matter. Due to this, O2 would vanish within a few million years by chemical reactions, and CO2 dissolves in water and would be gone in millennia if not replaced. Both are maintained by biological productivity and geological forces seemingly working hand-in-hand to maintain reasonably steady levels over millions of years.

[Fubaya]

It happened, they call it the Great Oxidation Event. I’m not sure if there is really a solid answer for where all the oxygen went, but I think you’d be safe to say the GOE was a spike which was probably absorbed by other gases and geological processes.