The discovery of life on another world would be a monumental moment in the history of human civilization. The most likely way we will, one day, find alien life will be discovering chemical markers of life in the atmosphere of planets orbiting distant stars. One of these markers is oxygen, which currently makes up a little over 20 percent of the atmosphere of our own world. However, our own world held onto relatively little oxygen until 2.4 billion years ago, when oxygen-producing cyanobacteria filled the atmosphere with the gas, leading to the first widespread extinction — the Great Oxidation event.…

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