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The story of how complex life arose on Earth has taken a fascinating turn. For decades, scientists have grappled with a central puzzle: how did the two very different types of microbes that ultimately joined forces to create all plants, animals, and fungi – known collectively as eukaryotes – even encounter each other, given their seemingly incompatible environmental needs? New research from The University of Texas at Austin suggests the answer lies in the surprising oxygen tolerance of ancient microbes called Asgard archaea, bolstering the long-held theory that complex life evolved in an oxygen-rich environment.
The prevailing scientific explanation for the emergence of eukaryotes centers on a symbiotic relationship between an Asgard archaeon and an alphaproteobacterium. This partnership eventually led to the development of mitochondria, the energy-producing structures within eukaryotic cells. But a key question remained: how could these organisms have come together if one thrived in oxygen-free spaces while the other required oxygen to survive? Researchers publishing in the journal Nature now believe they’ve found a crucial piece of the puzzle.
Asgard Archaea and the Oxygen Paradox
Most Asgard archaea currently known to science inhabit oxygen-deprived environments like the deep sea. However, the latest study reveals that certain Asgard groups, particularly those most closely related to eukaryotes, actually live in oxygenated environments – such as shallow coastal sediments and open water – and possess metabolic pathways that utilize oxygen. This discovery significantly strengthens the hypothesis that the ancestors of complex life were capable of thriving in the presence of oxygen.
“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.”
This finding aligns with geological and paleontological reconstructions of Earth’s early atmosphere. More than 1.7 billion years ago, oxygen levels were extremely low. Then, during a period known as the Great Oxidation Event, oxygen concentrations rose dramatically, eventually reaching levels comparable to those of today. Remarkably, the earliest known microfossils of eukaryotes appear in the fossil record within a few hundred thousand years of this oxygen increase, suggesting a strong link between the two events.
A Massive Genomic Effort
The research involved an extensive genomic analysis, beginning with DNA extracted from marine sediments off the coast of Uruguay in December 2025. The UT team and collaborators assembled over 13,000 new microbial genomes, analyzing roughly 15 terabytes of environmental DNA. This massive dataset nearly doubled the known genomic diversity of Asgard archaea, revealing previously unknown protein groups and enzymatic classes within these microbes.
Researchers identified specific groups, including Heimdallarchaeia, as being particularly close relatives of eukaryotes. Kathryn Appler, a postdoctoral researcher at the Institut Pasteur in Paris, France, explained that these Asgard archaea are often overlooked due to low-coverage sequencing. “The massive sequencing effort and layering of sequence and structural methods enabled us to witness patterns that were not visible prior to this genomic expansion.”
AI-Powered Protein Analysis
To further investigate the oxygen-utilizing capabilities of Heimdallarchaeia, the team employed artificial intelligence. They used AlphaFold2, an AI system, to predict the three-dimensional structures of proteins within these microbes. By comparing these structures to those of eukaryotic proteins involved in energy production and oxygen metabolism, researchers found striking similarities. This structural resemblance provides additional evidence that the ancestors of complex life were already adapted to using oxygen for efficient energy production.
The research involved contributions from scientists at multiple institutions, including Shandong University in China, Radboud University in the Netherlands, the University of Wisconsin-Madison, the University of Vienna, Monash University in Australia, and Wageningen University in the Netherlands.
This discovery doesn’t just fill a gap in our understanding of evolutionary history; it provides a more complete picture of the conditions that allowed for the emergence of the complex life forms that populate our planet today. As research continues, scientists will undoubtedly uncover further details about the intricate processes that shaped the evolution of life on Earth.
What comes next for this line of inquiry? Researchers will continue to explore the genomes of Asgard archaea, seeking to identify additional clues about the metabolic capabilities and environmental adaptations of these ancient microbes. Further investigation into the symbiotic relationship between Asgard archaea and alphaproteobacteria will also be crucial for unraveling the full story of eukaryotic origins.
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