Oxygen is everywhere on Earth today. But that hasn’t always been the case. Scientists think oxygen only became a lasting part of the atmosphere about 2.3 billion years ago during the Great Oxidation Event (GOE), a turning point that ultimately shaped the rise of oxygen-using life.

Now, new research from MIT points to an even earlier chapter in this story. The team suggests that some ancient organisms may have learned to use oxygen hundreds of millions of years before the GOE. If correct, the evidence could rank among the earliest signs of aerobic respiration ever identified.