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.

Planavsky and his team tested rocks for concentrations of the element cerium, which serves as a proxy for ancient oxygen levels. Oxygen binds to cerium in seawater and removes it, leaving less cerium behind to be deposited in sedimentary rock. The measured cerium levels correspond to oxygen concentrations of about 0.1 percent of present atmospheric levels, the team reported.

Such hard data, Planavsky says, should help inform the construction of the next generation of telescopes designed to hunt for life on other worlds. Those currently in the works—such as NASA’s James Webb Space Telescope (JWST)—cannot detect oxygen at such low concentrations, says Edward Schwieterman, an astrobiologist at the University of California, Riverside, who was not involved in the work.

Future space telescope missions may be better able to detect low oxygen concentrations. For now, researchers scanning the night sky for E.T. should not hold their breath.