A groundbreaking discovery has shed new light on the long-standing mystery surrounding the origin of complex life on Earth. For decades, scientists have theorized that all complex organisms—including plants, animals, and fungi—descended from an ancient symbiotic event involving two fundamentally different microorganisms. These microorganisms, one aerobic and the other anaerobic, somehow merged to give rise to eukaryotes, cells with intricate internal structures. The critical question that has puzzled researchers was: how could two microbes with such opposing oxygen requirements have come into such intimate contact in the primordial world?
Recent research spearheaded by Brett Baker and his team at The University of Texas at Austin presents compelling evidence resolving this paradox. Their study, published in Nature, suggests that a group of ancient microbes known as Asgard archaea, widely regarded as close relatives of eukaryotes, possess the unexpected ability to utilize or at least tolerate oxygen. This revelation realigns the evolutionary timeline with the environmental conditions of early Earth, indicating that the emergence of complex life likely occurred in oxygenated niches rather than strictly anoxic habitats as previously assumed.
Historically, Asgard archaea have been primarily recovered from deep-sea sediments and oxygen-poor environments. However, Baker’s research reveals that the subsets of Asgard archaea most genetically akin to eukaryotes thrive in oxygenated coastal sediments and water columns. These oxygen-exposed habitats enable them to engage metabolic pathways dependent on oxygen—metabolic functions that would have conferred significant bioenergetic advantages. Such capacities suggest that the ancestral archaeal host which eventually developed into the eukaryotic lineage had already evolved adaptations for aerobic metabolism.
This discovery is strongly supported by the geochemical record of Earth’s atmosphere. Around 1.7 billion years ago, our planet experienced the Great Oxidation Event, a dramatic increase in atmospheric oxygen levels. Shortly after this event, fossilized evidence of eukaryotic life begins to appear, implying a connection between oxygen availability and the advent of cellular complexity. Baker points out that these Asgard archaea likely capitalized on the newfound availability of oxygen, evolving highly efficient oxidative pathways that set the stage for the subsequent evolution of eukaryotic cells.
The new research delves deeply into the genomics of Asgard archaea, vastly expanding their known diversity by doubling the number of recovered genomic sequences. The project involved extensive metagenomic analyses of marine sediments collected during multiple scientific expeditions, involving the processing of a massive dataset of environmental DNA spanning approximately 15 terabytes. This unprecedented dataset allowed the team to reconstruct a comprehensive phylogenetic tree of Asgard archaea, uncovering previously uncharacterized lineages and enzymatic classes, effectively doubling the known enzymatic diversity in this clade.
One of the most intriguing lineages highlighted in the study is Heimdallarchaeia, a group closely related to the ancestor of all eukaryotes but increasingly rare in modern samples. Researchers utilized cutting-edge artificial intelligence-driven protein modeling, particularly AlphaFold2, to predict the three-dimensional conformations of proteins produced by Heimdallarchaeia. The structural analyses revealed striking similarities between these archaeal proteins and those found in eukaryotes that mediate aerobic respiration and energy metabolism, providing molecular evidence that these archaea had functional aerobic systems before the rise of complex life forms.
The implications of these findings extend beyond evolutionary biology to reshape our understanding of cellular bioenergetics. Aerobic respiration yields significantly more energy compared to anaerobic processes, thus providing a potent selective advantage for early eukaryotic ancestors. This metabolic innovation may have been crucial for the development of cellular complexity, enabling increased biosynthetic capabilities and facilitating the evolution of organelles such as mitochondria. Indeed, the study supports the scenario in which the symbiosis between an oxygen-using Asgard archaeon and an alphaproteobacterium led to the emergence of mitochondria, the powerhouse of eukaryotic cells.
This research also highlights the methodological advancements that propelled these discoveries. By employing high-coverage sequencing techniques and integrating multi-layered sequence and structural data analysis, the researchers overcame the limitations of low-coverage metagenomic surveys that often failed to detect rare or hard-to-culture archaea. The comprehensive genomic sampling strategy allowed for a more nuanced understanding of Asgard archaeal diversity and evolution, moving the field closer to resolving the intricate web of life’s origins.
Collaborative efforts played a pivotal role in this scientific milestone. In addition to the UT Austin team, notable contributions came from researchers in China, France, Australia, and Europe. These partnerships brought in expertise ranging from genomics to evolutionary biology, structural bioinformatics, and marine science, creating a multidisciplinary framework essential for tackling such complex biological questions.
This research not only enriches our evolutionary narrative but also opens new avenues for exploring bioenergetic evolution. Understanding how ancestral archaea adapted to oxygen-rich environments provides critical insights into the metabolic and genetic innovations that underpinned the rise of eukaryotic life—a defining event in Earth’s history that set the stage for the biodiversity we see today.
As the team continues to examine the depth of Asgard archaeal metabolism, future studies may further unravel the nuances of early symbiotic relationships and cellular complexity. These findings underscore the dynamic nature of microbial evolution and the profound influence ancient microorganisms exerted on shaping life on Earth.
In parallel, the expansion of metagenomic datasets and application of AI-driven protein structure predictions promise to revolutionize the exploration of microbial dark matter, those elusive microbial lineages that remain largely uncharacterized. Such technologies redefine our capacity to peer into the molecular underpinnings of evolution, metabolism, and environmental adaptation.
Ultimately, Brett Baker and colleagues have paved the way for a more detailed and accurate picture of the archaeal-eukaryotic transition. The identification of oxygen-metabolizing lineages within Asgard archaea, combined with molecular structural evidence, positions oxygen as a crucial factor in the evolutionary saga. This work challenges previous assumptions and sets a new benchmark for evolutionary biology, highlighting the intricate interplay between environment, metabolism, and genomic innovation in the origin of complex life.
Subject of Research: Not applicable
Article Title: Oxygen metabolism in descendants of the archaeal-eukaryotic ancestor
News Publication Date: 18-Feb-2026
Web References:
https://doi.org/10.1038/s41586-026-10128-z
References:
Baker, B. et al. (2026). Oxygen metabolism in descendants of the archaeal-eukaryotic ancestor. Nature. DOI: 10.1038/s41586-026-10128-z
Image Credits: Brett Baker
Keywords: Evolution, Microbial evolution, History of life, Phylogenetic analysis, Phylogenetic trees, Eukaryotes, Archaea, Metagenomics, Genetics, Genomics
