The Great Oxidation Event (GOE) is one of the most important events in the history of life on our planet. Before the GOE the only life forms were bacteria. They existed for 1.4 billion years with essentially no oxygen in the atmosphere. The bacteria either had not evolved to the point where they could make oxygen, or perhaps the oxygen they made was consumed by reactions with volcanic gases like methane and hydrogen sulfide. But 2.33 billion years before the present, oxygen suddenly appeared. New kinds of bacteria appeared - bacteria with nuclei and more complex machinery inside their cells. Why it took so long for the GOE to happen is a mystery. What we know about bacteria today is that they evolve quickly - like the plague spreading throughout Europe or the resistance to penicillin several years after it was first used. Another mystery is why the GOE was so abrupt once it got started. My talk identifies a feedback mechanism, where more oxygen in the atmosphere enables the bacteria to make oxygen faster. Perhaps the feedback was a mutation waiting to happen, like that due to a change in the volcanic gases. The mystery is still there.