The first organisms to “breathe” oxygen—or at least use it—appeared 3.1 billion years ago, according to a new genetic analysis of dozens of families of microbes. The find is surprising because the Great Oxidation Event, which filled Earth’s atmosphere with the precious gas, didn’t occur until some 500 million years later. The new timeline suggests early oxygen producers and users didn’t immediately sweep the planet. Rather, they likely evolved in small pockets that slowly spread over hundreds of millions of years. And only when they became abundant enough did these organisms modify Earth’s environment enough to lead to the Great Oxidation Event.