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One of the most important developments in Earth's history is the change from the anaerobic environment of the early Earth to the aerobic and highly oxidizing environment that we have today, with 21% atmospheric oxygen (O2) (1). Many geologists, atmospheric scientists, and biologists have studied this dramatic change in the redox state of Earth—the “great oxidation event” (2)—to understand when and how it took place. On page 1436 of this issue, Soo et al. report exciting new evidence toward answers to both these questions (3).