Is all complex life on Earth related to a concept from Norse mythology? Kind of

Scientists are one step closer to understanding the origins of complex life on Earth after shedding new light on the mystery of our microbial ancestors. The key, they suspect, may lie in how simple microbes that lived billions of years ago adapted to the presence of oxygen.

Humans, like all plants, fungi and animals on Earth, are eukaryotes – organisms with cells that have a nucleus containing clearly defined DNA and other structures such as mitochondria, organelles that provide energy to cells by converting nutrients into energy.

Between 2.4 and 2.1 billion years ago, oxygen levels in Earth’s atmosphere increased significantly, known as the Great Oxidation Event. A few hundred thousand years after the event, the first identifiable traces of eukaryotes, preserved as microfossils, appeared on our planet, suggesting that oxygen has long been a crucial ingredient for the evolution of complex life.

Many scientists believe that eukaryotes arose from the combination of two types of microbes.

But, curiously, one of these microbes, known as Asgard archaea, has only been found in oxygen-deprived environments, such as hydrothermal vents on the ocean floor, although it appears to share complex similarities with eukaryotes.

Researchers wondered how the Asgard even interbred with other microbes that needed oxygen to survive to create eukaryotes if they existed in such different environments.

But a new survey of Asgard’s genomes has revealed previously unknown lineages of microbes found in shallow coastal sediments, some of which appear tolerant and use oxygen, according to a study published Feb. 18 in the journal Nature.

“The fact that some Asgard, who are our ancestors, were able to use oxygen fits very well into this context,” Brett Baker, study co-author and associate professor of marine sciences and integrative biology at the University of Texas at Austin, said in a statement. “Oxygen appeared in the environment and the Asgard adapted to that. They found an energy advantage in using oxygen, and then they evolved into eukaryotes.”

Understanding the role of Asgard in the development of complex life could help solve the biggest mystery of how exactly microbes evolved into eukaryotes — and why we are all here, Baker said.

A microbe with mythological roots

Asgard archaea, named for the celestial home of Norse gods such as Odin and Thor, is a superphylum, or a group descended from a common ancestor.

A single phylum within this group was first discovered in 2015 near an underwater volcano in the North Atlantic Ocean, known as Loki’s Castle due to its resemblance to the horned helmet worn by the Marvel Comics character – who also happens to be a god in Norse mythology. The microbe has been dubbed Lokiarchaeota.

Other phyla of Asgard microbes are also named after gods from Norse mythology.

Compared to microbes in other superphyla, Asgards appear to be closely related to eukaryotes and contain genes seen only in complex life.

“They’ve been hailed as sort of a missing link in the evolution of life, from single-celled microbial life to complex life like plants and animals,” Baker told CNN.

By examining samples from a wide range of environments, researchers are discovering more and more types of Asgard microbes, such as Heimdallarchaeia, named for the guardian of Asgard.

In 2023, Baker and colleagues discovered that eukaryotes appear to be most closely related to the Heimdall group of Asgard microbes, which have high-energy metabolic pathways. The results support the idea that animals and other life forms must get the most energy from breathing oxygen and reinforce the theory that an increase in oxygen on Earth correlates with the emergence of complex life, Baker said.

The next step was to understand what energy generation processes could take place in different types of Asgard microbes based on their genes.

To study this question, Baker and colleagues performed large-scale DNA sequencing of samples collected from deep-sea hydrothermal vents as well as shallow coastal areas. The team was able to bring together hundreds of previously unknown genomes and construct a tree of life for Asgard microbes by comparing genetic similarities and differences between microbes in the superphylum.

Previously unknown groups of proteins in microbes were discovered during the assembly of the Asgard family tree, allowing Baker and his colleagues to compare the proteins with those used by eukaryotes to generate energy and metabolize oxygen. An artificial intelligence model helped the team identify how proteins can fold into different structures, which corresponds to how they work.

Several proteins produced by Heimdall microbes are similar to eukaryotic proteins that process oxygen to efficiently generate energy, suggesting that at least some ancient Asgard may have been tolerant of oxygen.

Discover an ancient ancestor

At first, scientists thought that the microbial ancestor of complex life was a simple cell living in oxygen-free environments.

This cell, they theorized, adapted to use oxygen after combining with a bacteria, ultimately giving rise to the presence of mitochondria in our cells.

“Eukaryotes almost always rely on mitochondria to burn the hydrocarbons in oxygen and do all the amazing things we do,” Buzz Baum, a cell biologist and group leader of the Baum Lab at the Medical Research Council Laboratory of Molecular Biology in Cambridge, England, wrote in an email. Baum was not involved in the new study.

But the new findings suggest that the Asgard may have already adapted to processing oxygen before combining with bacteria. This tolerance would have placed the Asgard in oxygenated environments and could have helped facilitate their fusion with bacteria, according to the study.

“The transition to complex life did not require innovating oxygen metabolism from scratch: the building blocks were already there,” Burak Avci, assistant professor of microbiology at Aarhaus University in Denmark, wrote in an email. Avci was not involved in the new research.

“However, it is important to recognize that we are looking at modern representatives of an ancient event that occurred billions of years ago,” he said. “There is a significant evolutionary lag, and the actual encounters of this event may have involved different metabolic strategies in the formation of the first eukaryotic cell.”

The study authors also noted that more evidence is needed to biologically confirm the study’s genetic predictions, especially when it comes to determining the exact abilities of ancient Asgard from nearly 2 billion years ago.

Modern Asgard have likely changed and adapted to use oxygen, Baum noted.

The study adds to a growing body of evidence that eukaryotic cells originated in oxygen-containing coastal environments, said Daniel Brady Mills, a postdoctoral researcher at the Institute of Molecular Evolution at the University of Düsseldorf in Germany. Mills, who was not involved in the research, said he hopes the study will inspire others to grow their own specimens in the lab to test whether or not Asgard microbes can use oxygen.

Baker hopes scientists will take an important step in the next five to 10 years: observing the evolution of lab-grown Asgard microbes as they transform into eukaryotic cells, a process known as eukaryogenesis.

“There’s no reason to think this only happened once, 2 billion years ago,” Baker said.

Future studies should also measure the amount of oxygen in environments where Asgard are present and identify microbes within the superphylum that can thrive with only small amounts of oxygen, Baum said.

The Asgard are humanity’s closest living relatives from an ancient event, meaning they hold clues to our origins, Baum said. So it is crucial to determine when they started using oxygen.

“If you look around, our planet is dominated by eukaryotes,” Baker said. “Understanding how they formed represents a huge transition in the evolution of life on Earth. The fact that we found oxygen in our close ancestors, the Asgard, fits this puzzle very well.”

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