A team led by researchers at the University of Texas at Austin has produced what may be the strongest evidence yet that the ancient ancestors of all complex life on Earth already had the genes to use oxygen and were likely breathing it billions of years before the atmosphere had much of it. The findings, published in Nature, challenge a decades-old assumption that abundant atmospheric oxygen was a prerequisite for the rise of eukaryotes, the domain that includes animals, plants, fungi, and every organism built from cells with a nucleus. If the results hold up, they rewrite a central chapter in the story of how simple microbes gave way to the staggering diversity of life visible today.
Asgard Archaea and the Oxygen Puzzle
The study centers on Asgard archaea, a group of single-celled organisms first identified from deep-sea sediments near a hydrothermal vent field called Loki’s Castle. When a lineage dubbed Lokiarchaeota was first described in a landmark Nature report, the discovery sent ripples through evolutionary biology because these microbes carried genes for proteins previously thought to exist only in eukaryotes. Later work expanded the Asgard group to multiple phyla and reinforced the idea that eukaryotes descended from within this archaeal lineage rather than branching off beside it. That placement made Asgard archaea the closest known living relatives of the ancestor that eventually gave rise to all complex cellular life.
Yet a basic question lingered: what kind of environment did that ancestor inhabit, and did it need oxygen? The conventional narrative tied the emergence of complex cells to the Great Oxidation Event roughly 2.4 billion years ago, when photosynthetic cyanobacteria flooded the atmosphere with molecular oxygen for the first time at scale. Under that model, eukaryotic complexity could not have appeared much earlier because the energy-hungry cellular machinery of complex life demanded an oxygen-rich world. The new research directly contests that timeline by showing that the Asgard lineages most closely related to eukaryotes already possessed the genetic toolkit for aerobic metabolism, and that they thrived in environments where oxygen levels fluctuated rather than remained uniformly high.
404 New Genomes From Coastal Sediments
The research team assembled thousands of microbial genomes from ocean sediment samples collected around the world. From that dataset, they recovered 404 Asgardarchaeota genomes, including 136 belonging to a branch called Heimdallarchaeia. That branch matters because phylogenetic analyses consistently place Heimdallarchaeia as the Asgard group nearest to eukaryotes on the tree of life. The sheer scale of the sequencing effort, involving hundreds of new Asgard genomes and a protein-structure prediction workflow, gave the team enough statistical power to draw conclusions about where these organisms live and what metabolic strategies they use.
The key finding is geographic and metabolic at once. Heimdallarchaeia were disproportionately enriched in variably oxygenated coastal sediments rather than in the deep-sea, oxygen-free habitats where Asgard archaea were originally found. These coastal lineages encode complete electron transport chains and other hallmarks of aerobic respiration. Brett J. Baker from UT Austin, a co-author, described the results as a major step forward in understanding the origin of complex life. The implication is that the ancestor shared by Asgard archaea and eukaryotes was not a strictly anaerobic organism huddled around a deep-sea vent but rather a microbe capable of exploiting the small pockets of oxygen that existed in shallow coastal waters long before the Great Oxidation Event reshaped the planet’s atmosphere.
Earlier Hints and Independent Timelines
This was not the first suggestion that Asgard archaea could handle oxygen. A 2019 study in Nature Microbiology reported that Heimdallarchaeia likely occupied microoxic niches and carried genes for the oxygen-dependent kynurenine pathway, hinting that at least some Asgard lineages were already experimenting with oxygen-based metabolism. What the 2026 paper adds is scale and specificity: rather than a handful of genomes hinting at aerobic capacity, researchers now have hundreds of genomes mapping a clear ecological pattern across global ocean sediments. The difference between a suggestive signal and a statistically supported trend is significant, and it shifts the burden of proof onto those who still argue that the eukaryotic ancestor was strictly anaerobic.
Independent molecular-clock analyses support the same broad conclusion from a completely different angle. A separate Nature study used dated gene duplications and relaxed molecular clocks to estimate when key eukaryotic innovations appeared, providing quantitative date ranges for host lineage divergence, mitochondrial acquisition, and the radiation of the last eukaryotic common ancestor. Work led by researchers at the University of Bath argues that complex cells may have arisen around 2.9 billion years ago, well before the Great Oxidation Event. Additional molecular-clock work has found that oxygen-using respiratory enzymes diversified in the Mesoarchean, reinforcing the idea that biology learned to exploit trace oxygen far earlier than geologists once assumed.
What the Debate Still Gets Wrong
Much of the popular framing around these discoveries treats the question as settled: oxygen came first, complex life followed. But the actual scientific debate is messier. Competing models of early evolution hinge on how patchy oxygen was in ancient environments, how quickly organisms could evolve the enzymes needed to use it safely, and whether the host that first acquired mitochondria was already adapted to oxygen or only became so afterward. Some researchers still favor scenarios in which an anaerobic archaeal host partnered with an oxygen-tolerant bacterial symbiont, gradually outsourcing respiration while remaining itself largely intolerant of oxygen. Others now see that picture as too simple, arguing that the host lineage was already metabolically flexible and capable of switching between oxygen-dependent and oxygen-free pathways as conditions changed.
The new Asgard data speak directly to this disagreement. By tying Heimdallarchaeia to coastal sediments with fluctuating oxygen levels, the study suggests that the closest archaeal relatives of eukaryotes were not locked into one metabolic mode. Instead, they likely occupied dynamic ecosystems where oxygen seeped into otherwise anoxic muds, creating microscale gradients. In such settings, selection would favor organisms that could cope with both presence and absence of oxygen, evolving regulatory systems and enzymes that functioned across that spectrum. That ecological backdrop makes it easier to imagine how a proto-eukaryotic cell could integrate a bacterial symbiont capable of high-efficiency respiration without being poisoned by its own metabolism.
Rewriting the Timeline of Complexity
Geological and paleontological discoveries are converging with these genomic insights to push the origin of complex traits ever earlier. Fossils from Gabon, for example, have been interpreted as evidence for centimeter-scale, coordinated organisms more than two billion years old; the work led by Abderrazak El Albani, covered in a BBC report on ancient multicellularity, suggests that organized, possibly multicellular structures appeared long before animals. While the exact biological affinities of these fossils remain debated, they underscore that complexity in form and behavior did not wait for a fully oxygenated atmosphere. Instead, life seems to have repeatedly probed the limits of size and organization whenever local conditions allowed.
Laboratory and theoretical work are also reshaping how scientists think about the prerequisites for complex cells. A 2025 study highlighted in ScienceDaily coverage of early cellular innovations argued that several advanced features, such as dynamic cytoskeletons and internal membrane systems, may have evolved in stages under low-oxygen conditions before being turbocharged by widespread respiration. Complementing this, an overview from UT Austin and collaborators, summarized in a Phys.org article on the oxygen mystery, emphasizes that the new Asgard genomes fit a growing pattern: oxygen was not a simple on/off switch for complexity but a resource that early lineages exploited opportunistically wherever it appeared.
Taken together, the genomic, fossil, and geochemical lines of evidence point toward a more nuanced narrative of life’s early evolution. Rather than a single threshold where oxygen suddenly made complexity possible, there appears to have been a long era of experimentation in patchy, low-oxygen worlds. Asgard archaea like Heimdallarchaeia, living in coastal sediments that alternated between oxic and anoxic states, may have been among the most successful experimenters. Their descendants, through a symbiotic merger with bacteria that became mitochondria, ultimately crossed an evolutionary Rubicon. The new study does not close the book on how that happened, but it narrows the possibilities and firmly embeds oxygen (scarce, fluctuating, and locally abundant) as a central character in the origin story of complex life.
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*This article was researched with the help of AI, with human editors creating the final content.
