Around 2.5 billion years ago, our planet experienced what was possibly the greatest change in its history: According to the geological record, molecular oxygen suddenly went from nonexistent to becoming freely available everywhere. Evidence for the ‘great oxygenation event’ (GOE) is clearly visible, for example, in banded iron formations containing oxidized iron. The GOE, of course, is what allowed oxygen-using organisms – respirators – and ultimately ourselves, to evolve.