Bold claim: oxygen-loving ancestors helped unlock life as we know it, and new research nudges us closer to understanding this pivotal switch. Scientists widely agree that the birth of complex life—the split that led to plants, animals, and fungi (collectively, eukaryotes)—stemmed from a deep partnership between two very different microbes. Yet a long-standing puzzle lingered: how did these two meet if one partner required oxygen while the other supposedly thrived without it? A study from The University of Texas at Austin offers fresh evidence that may resolve this paradox. In the journal Nature, researchers examine Asgard archaea, a group of microbes considered among the closest relatives to the ancestors of complex life. Although most observed Asgard archaea inhabit oxygen-poor settings like deep-sea environments, the latest work shows that some members can tolerate oxygen—or even rely on it. This finding reinforces the traditional view that complex life likely emerged in an oceanic or coastal world where oxygen was present, aligning with the broader timeline of Earth’s early atmosphere.
Brett Baker, associate professor of marine science and integrative biology at UT, explains: "Most Asgards today live in oxygen-free environments. Yet the most closely related lineages to eukaryotes are found in oxygenated habitats—shallow coastal sediments and organisms floating in the water column—and they feature metabolic pathways that use oxygen. That implies our eukaryotic forebears had these same oxygen-using processes." This aligns with a broader narrative in which oxygen availability acts as a catalyst for metabolic complexity.
The Great Oxidation Event and the dawn of eukaryotes
Baker’s group investigates Asgard archaeal genomes to map new branches within the lineage and to uncover how these microbes generate energy. Their findings fit with what geologists and paleontologists infer about Earth’s early atmosphere: more than 1.7 billion years ago, atmospheric oxygen was scarce, but it surged during the Great Oxidation Event, eventually reaching levels similar to today. Within a few hundred thousand years of that surge, the earliest microfossils attributed to eukaryotes appear in the fossil record, suggesting a potential link between rising oxygen and the origin of complex life.
"The fact that some Asgards—our ancestors—could exploit oxygen dovetails nicely with this scenario," Baker notes. "As oxygen became available, Asgards adapted, gaining an energetic edge by using it, and this likely set the stage for the evolution of eukaryotes."
Symbiosis and the origin of mitochondria
The prevailing theory holds that eukaryotes arose when an Asgard archaeon entered into a symbiotic relationship with an alphaproteobacterium. Over time, the two organisms integrated into a single cell, with the alphaproteobacterium evolving into mitochondria—the cellular powerhouses responsible for energy production.
In the current study, researchers broadened the known genetic diversity of Asgard archaea, identifying groups—such as Heimdallarchaeia—that are especially close to eukaryotes but less common today. Co-author Kathryn Appler, a postdoctoral researcher at the Institut Pasteur, notes that these archaea are often missed by low-coverage sequencing. The team’s expansive sequencing effort and the combination of sequence and structural analyses revealed patterns that earlier work could not detect.
A massive genome sequencing effort
The project has its roots in Appler’s Ph.D. work at UT’s Marine Science Institute in 2019, when she extracted DNA from marine sediments. By coordinating samples from multiple expeditions and analyzing roughly 15 terabytes of environmental DNA, the UT team and collaborators assembled more than 13,000 new microbial genomes. The result is a dramatically expanded map of Asgard genomic diversity, nearly doubling what was previously known. By comparing these genomes, the researchers refined the Asgard tree of life and uncovered dozens of previously unrecognized protein families, effectively doubling the catalog of enzymatic classes observed in these microbes.
AI-assisted insights into oxygen metabolism
To probe how Heimdallarchaeia might handle energy production and oxygen metabolism, the researchers turned to protein structure prediction. They compared Heimdallarchaeia proteins with their eukaryotic counterparts using AlphaFold2, an AI system that forecasts three-dimensional protein structures. Because a protein’s shape governs its function, this approach provided critical clues about potential capabilities.
The analysis revealed several Heimdallarchaeia proteins that closely resemble those used by modern eukaryotes for oxygen-based, efficient energy production. The structural similarity lends additional support to the idea that the ancestors of complex life were already equipped to utilize oxygen.
Contributors and funding
The study involved former UT researchers Xianzhe Gong, Pedro Leão, Marguerite Langwig, and Valerie De Anda, along with collaborators James Lingford, Chris Greening, Kassiani Panagiotou, and Thijs Ettema. Institutions represented include Shandong University, Radboud University, the University of Wisconsin–Madison, the University of Vienna, Monash University, and Wageningen University. Funding came from the Gordon and Betty Moore Foundation, the Simons Foundation, the National Natural Science Foundation of China, and the National Health and Medical Research Council of Australia.
Discussion prompts
- If early oxygen use helped drive the evolution of energy metabolism, what other environmental shifts might have accelerated the rise of complex life?
- Do these findings change how you view the tie between oxygen levels and the emergence of eukaryotes? Why or why not?
- How might future discoveries about Asgard diversity influence our understanding of the eukaryotic origin story?
