Earth Had Oxygen Much Earlier Than Thought

[Posteritatis]

5. None at all, for some kinds of life

The weird thing about oxygen, when you get down to it, is that it's really this horrible all-destroying poison of staggering power. It eats iron - eats iron! - makes things catch fire, and generally reacts to just about anything. The difference between oxygen and, say, elemental fluorine is one of degree rather than kind in that way. It's terrific for us, obviously, but it's jarring at times to sit there and think that, in order to live, we need a steady supply of a gas that turns old cars into piles of red powder.

The original oxygenation of the planet was a disaster to life that was present on it at the time; it would have nearly sterilized the planet (at least as far as their microscopic little minds would have been concerned), and would certainly have wrought a mass extinction against most anaerobic life.