A team led by Brett Baker at the University of Texas at Austin has found that some Asgard archaea, the ancient microbial group most closely related to all complex life on Earth, carried the molecular machinery to use oxygen long before scientists assumed they could. The findings, published in Nature, challenge a decades-old model that placed the origin of oxygen-based metabolism squarely on the shoulders of a bacterial partner that merged with an archaeal host. If the Asgard host already had oxygen-handling tools of its own, the standard story of how plants, animals, fungi and every other complex organism came to exist needs a significant revision.
Hundreds of New Genomes Expand the Asgard Family Tree
The scale of the new dataset is what makes the claim credible. Baker’s team assembled 404 Asgardarchaeota metagenome-assembled genomes from marine sediments around the world, including 136 that belong to Heimdallarchaeia, the Asgard subgroup considered the closest living relatives of eukaryotes. That effort nearly doubled the known genomic diversity of the entire Asgard superphylum, bringing the total catalog to 869 metagenome-assembled genomes. Much of the underlying sequence information comes from large marine microbiome projects, with data sets such as PRJNA743900, PRJNA692327 and PRJNA1112871 providing raw reads from diverse sediment cores and water-column samples. The raw sequence data are deposited across multiple public repositories, allowing independent verification of what was sequenced and where the samples came from, and can be accessed using standard tools linked through an NCBI account.
Most known Asgards live in deep-sea or other oxygen-poor environments. But the new study shows that some members of this group live in settings where oxygen was present, according to ScienceDaily. That geographic spread matters because it allowed the researchers to compare genomes from oxygenated coastal sediments against those from anoxic deep-sea sites, revealing metabolic differences that would have been invisible in a smaller or less varied sample. By mapping the distribution of gene families across this expanded tree, the team could infer which traits were ancestral, which were gained later in specific lineages, and how environmental oxygen shaped the evolution of Asgard metabolism over time.
Oxygen Machinery Already Built Into the Ancestor
When the team reconstructed the metabolic pathways encoded in those genomes, a striking pattern emerged. Several Heimdallarchaeia lineages carry genes for complex IV terminal oxidases and for haem biosynthesis, both of which are signatures of organisms that can run aerobic respiration. Until now, the textbook explanation held that the archaeal ancestor of eukaryotes was strictly anaerobic and gained the ability to use oxygen only after engulfing an alpha-proteobacterium, the precursor to mitochondria. Finding those same aerobic tools already present in the Asgard lineage closest to eukaryotes complicates that narrative considerably, suggesting that at least some oxygen-handling capacity evolved on the archaeal side before the symbiosis that produced mitochondria.
A separate phylogenetic study supports the idea that these oxygen-related genes were not recent horizontal acquisitions. Analysis of terminal oxidase evolution in Asgard archaea points to long-term vertical inheritance, meaning the genes were passed down from parent to offspring over vast stretches of time rather than picked up from unrelated bacteria. A not-yet-peer-reviewed preprint examining oxygen-adaptive plasticity in Asgard archaea reaches a similar conclusion, tracing terminal oxidase and globin genes through close relatives of eukaryotes. However, the sources present a tension: according to that same preprint, the common ancestor of all Asgard archaea likely had a strictly anaerobic lifestyle, while the Nature paper argues that key hallmarks for aerobic respiration may have been present in the Asgard–eukaryotic ancestor. The difference hinges on exactly when and in which branch oxygen tolerance first appeared, and whether the relevant enzymes were already configured for efficient respiration or initially served more modest detoxification roles.
Rewriting the Timeline of Complex Life
Baker and his colleagues argue that the first steps on the path to eukaryotes took place on early Earth, when it lacked oxygen, and that the Asgard lineage closest to us later ramped up its oxygen-based metabolism as the atmosphere changed. That timing lines up with a well-known geological event: the Great Oxidation Event roughly 2.4 billion years ago, when photosynthetic cyanobacteria flooded the atmosphere with molecular oxygen for the first time. Within a few hundred thousand years after that shift, the first known microfossils of eukaryotes appeared in the fossil record. The proximity is suggestive: if the archaeal host already had rudimentary oxygen-handling capacity, the Great Oxidation Event could have been the selective pressure that made full aerobic metabolism, and eventually mitochondria, advantageous, turning a previously toxic gas into an energy bonanza.
“The fact that some of the Asgards, which are our ancestors, were able to use oxygen fits in with this very well,” Baker said, as reported by Phys.org. His lab now proposes an updated Heimdallarchaeia-centric model of eukaryogenesis in which both hydrogen production and aerobic respiration played roles in the merger that produced the first complex cells. In this view, the bacterial partner that became mitochondria still contributed efficient oxidative phosphorylation, but it joined an archaeal host that already knew how to cope with oxygen and may have used it for limited energy generation. That scenario softens the sharp divide between “anaerobic archaeon” and “aerobic bacterium,” replacing it with a more gradual metabolic partnership.
Debates Over Ancestral Lifestyles and Future Tests
The new model does not go unchallenged. An independent analysis in another Nature study emphasizes that many Asgard lineages still appear specialized for low-oxygen or anoxic niches, and that the earliest stages of eukaryogenesis likely unfolded in environments where hydrogen-based syntrophy, rather than oxygen respiration, dominated energy exchanges. From that perspective, oxygen-tolerant Heimdallarchaeia might represent a later offshoot that capitalized on rising oxygen levels, rather than a direct stand-in for the actual host that merged with the mitochondrial ancestor. Reconciling these pictures will require more precise phylogenetic placement of oxygen-related genes and better constraints on when key enzyme families arose relative to major shifts in Earth’s atmosphere.
Researchers are now looking for ways to test these ideas beyond comparative genomics. One priority is to culture additional Asgard strains, or at least enrich them in laboratory microcosms, under controlled oxygen gradients to see how their predicted pathways behave in real cells. Another is to refine molecular clock analyses that date the emergence of terminal oxidases, globins and haem biosynthesis genes, tying those timelines more tightly to geological markers of oxygenation. As more Asgard genomes are sequenced and added to public projects, and as tools for handling ultra-low-biomass sediment samples improve, scientists expect the picture of how oxygen shaped the rise of complex life to sharpen. For now, the growing evidence that some Asgard archaea carried oxygen machinery of their own forces a rethinking of one of biology’s central origin stories, turning what once seemed like a simple handoff from bacteria into a much more intertwined evolutionary partnership.
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*This article was researched with the help of AI, with human editors creating the final content.
