For years, scientists have agreed on a broad explanation for how complex life first appeared on Earth, yet one critical question remained unanswered. Plants, animals, and fungi, collectively known as eukaryotes, are thought to have emerged when two very different microbes joined forces. One depended on oxygen to survive, while the other was believed to live only in oxygen-free environments. What puzzled researchers was how these two organisms could have encountered each other in the first place.

A new study from The University of Texas at Austin, published today (February 18) in the journal Nature, offers a compelling answer. Researchers focused on a group of microbes called Asgard archaea, widely considered close relatives of the ancestors of complex life. Although most known Asgard archaea inhabit deep-sea, oxygen-free settings, the team discovered that some members of this group can tolerate or even use oxygen. This finding strengthens the theory that complex life evolved in oxygen-rich conditions.

“Most Asgards alive today have been found in environments without oxygen,” explained Brett Baker an associate professor of marine science and integrative biology at UT. “But it turns out that the ones most closely related to eukaryotes live in places with oxygen, such as shallow coastal sediments and floating in the water column, and they have a lot of metabolic pathways that use oxygen. That suggests that our eukaryotic ancestor likely had these processes, too.”