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. But was it indeed a 'great event' in the sense that the change was radical and sudden, or were the organisms alive at the time already using free oxygen, just at lower levels?