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New evidence indicates that before 2400 million years ago, Earth's atmosphere contained no more than trace amounts of oxygen. In his Perspective, Wiechert reviews the latest evidence reported by Farquhar et al., who have analyzed the sulfur isotope composition of ancient inclusions in diamond. These inclusions were transported into the mantle by subduction and reached Earth's surface again by explosive volcanism. Further insights into conditions on early Earth are provided by Habicht et al., whose experiments suggest that the sulfate concentrations in the ocean were much lower than today, with important implications for the composition of the atmosphere.