Oxygen-breathing microbes may have sparked all complex life

The missing piece of the puzzle

For decades, scientists have puzzled over a fundamental contradiction: if complex life arose when two radically different microbes merged, how did they ever meet when one needed oxygen to survive and the other supposedly couldn't tolerate it? A team at the University of Texas at Austin has found an answer in an unexpected place — in ancient microbes that shouldn't have been able to breathe oxygen at all.

Researchers studying Asgard archaea, a group of microorganisms discovered only in the last decade, have found that some members can actually tolerate or even use oxygen. The discovery reframes our understanding of how the first complex cells might have emerged. More than 1.7 billion years ago, oxygen levels in the atmosphere spiked dramatically — the Great Oxidation Event. Within a few hundred thousand years, the earliest fossils of eukaryotes (cells with a nucleus, the building blocks of all plants, animals, and fungi) appear in the geological record. The timing is too precise to be coincidence.

"Most Asgards alive today have been found in oxygen-free environments," explains Brett Baker, an associate professor at UT. "But the ones most closely related to eukaryotes live in places with oxygen, like shallow coastal sediments, and they have lots of metabolic pathways that use oxygen. That suggests our eukaryotic ancestor likely had these processes too."

From ancient partnership to the cells we know

The prevailing theory holds that eukaryotes arose through an unlikely merger: an Asgard archaeon engulfed an alphaproteobacterium (a type of bacteria). Rather than being destroyed, the bacterium survived inside its host and evolved into the mitochondria — the energy-producing powerhouse found in nearly every complex cell today. This wasn't a one-time accident. It was a partnership that worked so well it became the foundation for all multicellular life.

In this study, researchers significantly expanded what we know about Asgard genetic diversity, identifying specific groups like Heimdallarchaeia that sit closer to eukaryotes on the evolutionary tree than any other known organism. Using an AI system called AlphaFold2, the team analyzed proteins in these archaea involved in energy production and oxygen metabolism. The results showed these proteins closely resemble the oxygen-using machinery found in eukaryotic cells — further evidence that the ancestors of every plant, animal, and fungus were adapted to an oxygen-rich world.

"These Asgard archaea are often missed by standard sequencing methods," says postdoctoral researcher Kathryn Appler. "The massive sequencing effort and layering of different analytical methods enabled us to see patterns that were invisible before."

This isn't just academic archaeology. It's a reminder that life's complexity emerged not from a single innovation, but from ancient organisms finding advantage in new conditions — oxygen in the air, a partnership between cells, energy efficiency. The story of how we got here is one of adaptation and collaboration written in microbial DNA.