Microscopic air bubbles trapped inside rock salt offer the oldest whiff of an oxygen-rich atmosphere. Image: Chris Lecuyer via Science News

Buried inside ancient grains of rock salt, a team of geologists has discovered traces of a breathable, animal-friendly atmosphere. If confirmed, the finding will push back the rise of oxygen on Earth hundreds of millions of years, raising new questions about the evolution of complex life both here and beyond our solar system.

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Earth may be our cozy blue marble today, but hop in a time machine and travel back two billion years, and you’d soon asphyxiate from a lack of oxygen. While scientists have found traces of oxygen on Earth dating back more than three billion years, it took eons of microbial activity before the air became anything close to what we’d consider breathable.

According to models and indirect geochemical evidence, atmospheric oxygen levels rose sharply toward the end of the Neoproterozoic era some 600 million years ago, closely coinciding with the appearance of marine animals in the fossil record. But a new study published in the journal Geology calls that date—and its tight link to the emergence of multicellular life—into question.

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“Scientists have long debated which came first: higher life or O2,” Uwe Brand, a geologist at Brock University and co-author on the study told Gizmodo. “With our finding, we can put that debate to rest.”

By analyzing air bubbles trapped inside 815-million-year-old grains of halite, or rock salt, Brand and his co-authors have now produced the oldest direct measurements of an oxygen-rich atmosphere. The ancient air samples have oxygen contents ranging from 10.3 to 13.1 percent, more than five times higher than what scientists had previously estimated for the mid Neoproterozoic. (Our modern atmosphere is approximately 21 percent oxygen.)

“I think our results will take people by surprise,” study co-author Nigel Blamey told Science News. “We came out of left field, and I think some people are going to embrace it, and other people are going to be very skeptical. But the data is what the data is.”

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The scientists are continuing to study additional halite grains from earlier and later chapters of Earth’s history, and they hope to have a more detailed chronology of the rise of oxygen soon. If their initial discovery holds up, it’d imply a substantial gap between the emergence of oxygen and the rise of animals during the Cambrian explosion, suggesting that some other preconditions had to be met before multicellular life took over.

And that would have profound implications for the discovery of complex life beyond our solar system. With Earth as our only example to go on, one might assume that complex life always follows an oxygen-rich atmosphere. If it turns out that’s not the case, then biospheres like our own and beings like ourselves may be less cosmically common than we hope.

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[University of Aberdeen News via Science News]

This article has been updated to reflect additional comments from the researchers.