Researchers at The University of Texas at Austin have discovered that some Asgard archaea, close relatives of complex life's ancestors, can tolerate and use oxygen. This finding resolves a long-standing puzzle about how oxygen-dependent and oxygen-avoiding microbes formed the partnership that led to eukaryotes. The evidence, published in Nature, suggests complex life emerged in oxygenated environments after the Great Oxidation Event.
For decades, scientists have theorized that complex life, including plants, animals, and fungi, arose from a merger between two distinct microbes: an archaeon and a bacterium. A key challenge was reconciling the oxygen needs of these partners, as one was thought to require oxygen while the other avoided it. Now, a study led by Brett Baker, an associate professor of marine science and integrative biology at The University of Texas at Austin, provides evidence that the archaeal partner could handle oxygen.
The research focused on Asgard archaea, microbes considered evolutionary cousins to the ancestors of eukaryotes. While many Asgards inhabit oxygen-poor settings like deep-sea vents, the team found that those most closely related to eukaryotes thrive in oxygenated areas, such as shallow coastal sediments and the water column. "Most Asgards alive today have been found in environments without oxygen," Baker explained. "But it turns out that the ones most closely related to eukaryotes live in places with oxygen... and they have a lot of metabolic pathways that use oxygen. That suggests that our eukaryotic ancestor likely had these processes, too."
This discovery aligns with Earth's geological history. More than 1.7 billion years ago, oxygen levels were low until the Great Oxidation Event caused a sharp rise. Shortly after, around a few hundred thousand years later, the first eukaryotic microfossils appear in the record. "Oxygen appeared in the environment, and Asgards adapted to that," Baker said. "They found an energetic advantage to using oxygen, and then they evolved into eukaryotes."
The prevailing model posits that eukaryotes formed when an Asgard archaeon symbiotically engulfed an alphaproteobacterium, which evolved into mitochondria for energy production. To support this, the researchers sequenced over 13,000 microbial genomes from marine sediments, nearly doubling known Asgard diversity. They identified groups like Heimdallarchaeia as particularly close to eukaryotes.
Using AI tool AlphaFold2, the team analyzed protein structures, revealing similarities between Heimdallarchaeia enzymes and those in eukaryotes for oxygen-based metabolism. Co-author Kathryn Appler, a postdoctoral researcher at the Institut Pasteur, noted: "These Asgard archaea are often missed by low-coverage sequencing... The massive sequencing effort... enabled us to see patterns that were not visible prior to this genomic expansion."
The study expands understanding of how oxygen facilitated the rise of complex life, though it does not speculate on further evolutionary steps.