Oxygenic photosynthesis appears to be necessary for an oxygen-rich atmosphere like Earth's. However, available geological and geochemical evidence suggests that at least 200 m.y., and possibly as many as 700 m.y., elapsed between the advent of oxygenic photosynthesis and the establishment of an oxygen atmosphere. The interregnum implies that at least one other necessary condition for O 2 needed to be met. Here, we argue that the second condition was the oxidation of the surface and crust to the point where free O 2 became more stable than competing reduced gases such as CH 4 , and that the cause of Earth's surface oxidation was the same cause as it is for other planets with oxidized surfaces: hydrogen escape to space. The duration of the interregnum was determined by the rate of hydrogen escape and by the size of the reduced reservoir that needed to be oxidized before O 2 became favored. We speculate that hydrogen escape determined the history of continental growth, and we are confident that hydrogen escape provided a progressive bias to biological evolution.