[madhadron]

Oxygen absorbing isn't necessary, of course. Life on earth got along without it for a while until those darned cyanobacteria starting poisoning everyone else.

[hansbo]

Eukaryote life did not appear until after the Oxygen Catastrophe though.

[madhadron]

That's true, but how much earlier might it have occurred if the oxygen catastrophe had not occurred and caused a mass extinction? Hard to say...