Take a deep breath. About 20 percent of the air that just moved through your mouth or nostrils is oxygen—the gas much of life on Earth needs to survive. If you had taken that breath about 1.87 billion years ago, however, you would have croaked.
Until recently, little was known about oxygen’s abundance in the atmosphere back then, when microbes were the only life on the planet. Now geologists doing fieldwork in northern Canada have confirmed for the first time that oxygen was extremely scarce.
The fact that life flourished amid such low oxygen levels presents a problem for scientists hunting for extraterrestrial life. The presence of the gas in the atmosphere of a planet is considered a telltale sign that it could harbor life, explains Noah Planavsky, a biogeochemist at Yale University and a co-author of the new study, published in July in the Proceedings of the National Academy of Sciences USA. But if environments with extremely low oxygen concentrations can still support life, space telescopes designed to detect an abundance of the gas may never find such life. “Even [if such planets are] teeming with complex life, they may appear—from a remote detectability point of view—as dead planets,” Planavsky says.
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