Oxygen May Have Been Key to Sparking the Rise of Complex Life
The story of life’s complexity is widely understood, but one contradiction has never been fully understood. Plants, animals and fungi – known collectively as eukaryotes – likely arose when two very different microbes formed a close alliance. One of these eventually became the mitochondrion, the energy-producing structure inside our cells.
The partnership combined an oxygen-using microbe with the idea of living without it. If their habitats rarely overlapped, how did they come together? A new study in Nature suggests that the separation may have been exaggerated. Some of the microbes most closely related to our earliest ancestors appear to have been able to use oxygen.
“Most Asgard alive today were found in environments without oxygen,” explained Brett Baker, co-author of the study, in a press release. “But it turns out that those most closely related to eukaryotes live in oxygen-rich places, such as shallow coastal sediments and float in the water column, and they have many metabolic pathways that use oxygen. This suggests that our eukaryotic ancestor probably had these processes, too.”
Learn more: The “first predators” ruled a world filled with bacteria
Where the microbial ancestors of complex life lived
An expanded family tree of the archaea of Asgard.
(Image credit: University of Texas at Austin)
The microbes at the center of this study belong to a group known as the Asgardian archaea. First identified in marine sediments, they are considered the closest known relatives of eukaryotes.
Until now, most Asgards studied have been linked to low-oxygen environments. This helped shape the idea that fusion leading to complex cells occurred under low oxygen conditions.
The new research paints a broader picture. By assembling thousands of microbial genomes from marine sediment samples collected over several expeditions, the team nearly doubled the known diversity of Asgard’s archaea and constructed a more complete evolutionary tree.
When they focused on a subgroup called Heimdallarchaeia – the lineage most closely related to eukaryotes – they found genetic evidence of oxygen-based metabolism. These microbes were not strict anaerobes. They appear to have tolerated, and perhaps used, oxygen.
The role of oxygen in early evolution
The timing also corresponds to changes in Earth’s atmosphere. For much of ancient history, oxygen was scarce. About 2.4 billion years ago, levels rose in what is known as the Great Oxidation Event. Subsequent increases brought the atmosphere closer to modern conditions. Shortly after these increases, the first clear eukaryotic fossils appear in the geologic record.
If some Asgardian ancestors were already capable of using oxygen, these changes could have provided a powerful advantage. Oxygen-based metabolism generates much more energy than anaerobic pathways. Microbes that could tap into it would have had more fuel to grow, divide, and evolve greater cellular complexity.
Instead of acting as a barrier between two incompatible organisms, oxygen may have helped make their partnership successful.
Reshaping the Asgardian family tree
The results come from a massive sequencing effort. More than 13,000 microbial genomes have been assembled from approximately 15 terabytes of environmental DNA. Within this treasure trove, hundreds of new Asgard genomes have emerged.
Comparing these genomes allowed the team to construct a more detailed Asgard family tree and identify previously unknown proteins. To better understand the role of some of these proteins, they turned to AlphaFold2, an artificial intelligence tool that predicts how proteins fold into three-dimensional shapes. Because structure determines function, these predictions are important. Several proteins produced by Heimdallarchaeia closely resemble those that modern eukaryotes use for oxygen-based energy metabolism.
The origin of complex life remains one of the most important evolutionary changes in biology. These results suggest that the ancestor of complex cells may not have been confined to oxygen-free habitats. Instead, it may have been equipped to take advantage of a changing and increasingly oxygen-rich world.
Learn more: Single-celled organisms laid the foundation for complex life – here’s how
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