POPULAR MECHANICS - Earth was a vastly different planet 2.4 billion years ago. Shrouded in a haze of carbon dioxide, methane, and water vapor, its atmosphere was more similar to that of Saturn’s moon Titan than to the nitrogen-and-oxygen mix that dominates today. Any atmospheric oxygen would react with methane and with rock at the surface, locking it into the crust and leaving it unavailable to early life-forms.
Nascent Earth was also atmospherically unstable. Asteroid impacts released immense amounts of hydrogen and methane, and that methane reacted with sunlight and trace amounts of oxygen, slowly oxidizing to carbon monoxide and carbon dioxide over millions of years—a mix still lethal for most living things to breathe. As impacts grew less frequent, the climate cooled, and the carbon dioxide that remained was eventually drawn down into the crust, leaving a frozen surface. At some point, however, oxygen began to accumulate in the atmosphere. How that oxygen managed to keep the planet habitable has remained a mystery, but now, researchers from the University of California, Riverside, and the University of Leeds have found evidence for how it emerged and endured.