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Science

Stepping to an internal beat What caused the stepwise nature of the rise of molecular oxygen in Earth's atmosphere since it appeared in large quantities more than 2 billion years ago? Alcott et al. argue that a set of internal feedbacks involving the global phosphorus, carbon, and oxygen cycles, not individual external forces, could be responsible. Their model, which depends only on a gradual shift from reducing to oxidizing surface conditions over time, produces the same three-step pattern observed in the geological record. Science , this issue p. 1333

Stepwise Earth oxygenation is an inherent property of global biogeochemical cycling