Early Life May Have Been Oxygen-Ready Before Endosymbiosis, Recent Research Suggests

A new study challenges conventional wisdom about the origins of complex life, suggesting that the ability to utilize oxygen may have predated the endosymbiotic event that led to the development of mitochondria. The research, published in Nature in February 2026, indicates that archaeal ancestors of eukaryotes possessed the metabolic machinery for oxygen-based energy production before incorporating bacteria into their cells.

Challenging the Traditional Timeline

Traditionally, scientists believed that the efficient use of oxygen – a crucial step towards complex life – only emerged after a significant increase in atmospheric oxygen levels. This increase coincided with an endosymbiotic event where an early archaeal cell engulfed a bacterium, eventually leading to the formation of the mitochondrion, the powerhouse of the cell. Although, the question of how this process occurred in an oxygen-poor environment remained a puzzle.

Reconstructing Ancient Metabolism

Researchers, including Valerie De Anda from the Department of Functional and Evolutionary Ecology at the University of Vienna, reconstructed the metabolism of early Asgard archaea – microorganisms considered the closest known relatives to eukaryotes. The team analyzed extensive metagenomic datasets from marine sediments, encompassing over 13,000 reconstructed microbial genomes, including approximately 400 Asgard genomes. They utilized a method called MEBS analysis, developed by De Anda, to infer the biochemical capabilities of microorganisms from their genomic data. Protein structure predictions were also used to confirm functionality. University of Vienna researchers contributed significantly to this metabolic reconstruction.

Evidence of Oxygen Utilization in Asgard Archaea

The analysis revealed that Asgard archaea closely related to eukaryotes inhabit oxygen-rich environments, such as shallow coastal sediments and open water. These archaea possess numerous metabolic pathways that utilize oxygen, including genes for key components of the respiratory chain (like complex IV), heme biosynthesis, and mechanisms to detoxify reactive oxygen species. “The genetic signatures and reconstructed metabolic profiles show that the archaeal ancestor of complex cells already had the prerequisites for oxygen-based energy metabolism,” explains Valerie De Anda.

Implications for Eukaryotic Origins

These findings suggest that the emergence of efficient energy metabolism occurred earlier in evolution than previously thought – before the symbiotic merger with a bacterial partner. The study proposes that this evolutionary step should be viewed as a transition between organisms with already similar metabolic processes, rather than a sudden energetic leap. The molecular data aligns with geological evidence regarding the rise of atmospheric oxygen, suggesting that early microorganisms adapted to this environmental change and contributed to the subsequent diversification of complex life forms.

Endosymbiosis and the Evolution of Life

Endosymbiotic relationships have played a crucial role in shaping the evolution of biological diversity. Research on endosymbiosis continues to provide insights into how these interactions evolve and shape life on Earth. Megan Sørensen, a microbiologist at CeMESS at the University of Vienna, has been awarded an ERC Starting Grant to further investigate the evolution of endosymbiosis, focusing on partnerships between protists and examining the transition from endosymbiont to organelle.

Key Takeaways

• The ability to use oxygen may have existed in archaeal ancestors of eukaryotes before endosymbiosis.
• Analysis of over 13,000 microbial genomes revealed genes for aerobic energy metabolism in Asgard archaea.
• The emergence of eukaryotes may have been a transition between organisms with similar metabolic processes, rather than a sudden shift.