Scientists Crack Origin of All Complex Life: Ancient Asgard Microbes Used Oxygen, Solving Evolution's Deepest Puzzle

Scientists at the University of Texas at Austin have cracked one of biology's most vexing puzzles — how complex life first came to exist on Earth — by discovering that an ancient lineage of microbes called Asgard archaea can metabolize oxygen, a finding that may explain the unlikely union of two vastly different life forms that eventually gave rise to every plant, animal, and fungus on the planet.

The research, published in the journal Nature, involved assembling more than 13,000 new microbial genomes from marine sediments — nearly doubling the known genomic diversity of Asgard archaea — and using cutting-edge artificial intelligence to compare protein structures between the ancient microbes and modern eukaryotic cells. The analysis revealed that certain Asgard archaea, particularly those belonging to a group called Heimdallarchaeia, possess oxygen-metabolizing pathways remarkably similar to those found in eukaryotes. Just as importantly, these particular archaea are found in oxygen-rich environments like shallow coastal sediments, not in the deep, oxygen-free zones where Asgard archaea were previously assumed to live exclusively.

The discovery resolves a long-standing evolutionary contradiction. Scientists have known for years that all complex life — eukaryotes — emerged when an Asgard archaeon formed a symbiotic relationship with an alphaproteobacterium, which eventually became the mitochondrion, the power generator of every complex cell. The problem was that eukaryotes require oxygen for efficient metabolism, while Asgard archaea were believed to be strictly anaerobic — obligate oxygen-avoiders. How could a union that gave rise to oxygen-dependent life have formed between two organisms with such incompatible metabolic needs?

'Oxygen appeared in the environment, and Asgards adapted to that,' said Brett Baker, the study's lead author and an associate professor of marine science and integrative biology at UT Austin. 'They found an energetic advantage to using oxygen, and then they evolved into eukaryotes.' The timing aligns remarkably with Earth's geological record. The Great Oxidation Event — when atmospheric oxygen levels spiked dramatically — occurred approximately 1.7 billion years ago. Within a few hundred thousand years of that event, the first eukaryotic microfossils appear in the fossil record. The new research suggests that surge of oxygen catalyzed the archaea's metabolic transformation, creating the conditions for the most consequential symbiosis in the history of life.

The team gathered environmental DNA from marine sediments across a range of sites and used the AlphaFold2 artificial intelligence system to compare the three-dimensional protein structures produced by Heimdallarchaeia with the proteins eukaryotes use for oxygen-based energy metabolism. The structural similarities were striking. 'The ones most closely related to eukaryotes live in places with oxygen, and they have a lot of metabolic pathways that use oxygen,' Baker said. The research does not settle every question about eukaryotic origins — the exact details of how the original merger occurred remain subject to intense scientific debate — but it removes what many biologists considered the most puzzling obstacle to the prevailing theory.