Scientists are one step closer to understanding the origins of complex life on Earth after shedding new light on a mystery about 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 clearly defined DNA-containing nucleus and other structures such as mitochondria, organelles that provide cells with power by converting nutrients into energy.

Between 2.4 billion and 2.1 billion years ago, oxygen levels dramatically increased in Earth’s atmosphere, 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 evolved from the combination of two types of microbes.

But in a puzzling twist, one of the microbes, known as Asgard archaea, has only been found in oxygen-deprived environments such as hydrothermal vents on the ocean floor — despite appearing to share complex similarities with eukaryotes.

But a new investigation of Asgard genomes has revealed previously unknown lineages of the microbes in shallow coastal sediments, some of which appear tolerant of and use oxygen.

The fact that some of the Asgards, which are our ancestors, were able to use oxygen fits in with this very well. Oxygen appeared in the environment, and Asgards adapted to that. They found an energetic advantage to using oxygen, and then they evolved into eukaryotes.

Understanding the role of Asgards in the development of complex life could help solve the bigger mystery of how exactly microbes evolved into eukaryotes — and why we’re 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 that evolved 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 was dubbed Lokiarchaeota. Other phyla of Asgard microbes have also been named after gods from Norse mythology.

When compared with microbes in other superphyla, Asgards appear to be closely related to eukaryotes and contain genes only seen in complex life. They were hailed as the missing link in the evolution of life, from single-celled microbial life to complex life like plants and animals.

By examining samples from a broad range of environments, researchers are increasingly finding more types of Asgard microbes, such as Heimdallarchaeia, named for the guardian of Asgard.

Eukaryotes appear most closely related to the Heimdall group of Asgard microbes, which have high-energy metabolic pathways. The findings supported the idea that animals and other life forms must get the most energy from breathing oxygen and bolstered the theory that a rise in oxygen on Earth correlated with the appearance of complex life, Baker said.

The next step was to understand what energy-generating processes could unfold in different types of Asgard microbes based on their genes.

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

Asgard microbes were found in mud samples taken from the Guaymas Basin in the Gulf of California.

Brett Baker/National Science Foundation

Previously unknown protein groups in the microbes were uncovered while assembling the Asgard family tree, allowing comparison to the proteins with those used by eukaryotes to generate energy and metabolize oxygen. An artificial intelligence model helped identify how the proteins can fold into different structures, which correlates with how they function.

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

Uncovering an ancient ancestor

At first, scientists thought the ancient microbial ancestor of complex life was a simple cell, dwelling in oxygen-free environments. That cell, they theorized, adapted to use oxygen after combining with a bacterium, eventually giving rise to the presence of mitochondria in our cells. Eukaryotes almost always rely on mitochondria to burn hydrocarbons in oxygen to do all the amazing things we do.

But the new findings suggest that Asgards might have already adapted to process oxygen before combining with bacteria. This tolerance would have placed Asgards in oxygenated environments and might have helped facilitate their merger with bacteria.

The research submersible Alvin was used to search the deep ocean for Asgard microbes.

The transition to complex life didn’t require innovating oxygen metabolism from scratch — the building blocks were already there. However, it is important to recognize that we are examining modern-day representatives of an ancient event that occurred billions of years ago. There is a significant evolutionary time gap, and the actual encounters of this event may have involved different metabolic strategies in forming the first eukaryotic cell.”

More evidence is needed to biologically confirm the genetic predictions in the study, especially when trying to determine the exact capabilities of ancient Asgards from almost 2 billion years ago. Modern Asgards have likely changed and adapted to use oxygen.

The study adds to a growing body of evidence that eukaryotic cells originated in oxygen-containing coastal environments. Others need to culture their own specimens in lab settings to test whether or not Asgard microbes can use oxygen. It is hoped that scientists will reach a milestone within the next five to 10 years: observing the evolution of lab-grown Asgard microbes as they turn into eukaryotic cells, a process known as eukaryogenesis. There’s no reason to think that this only happened once 2 billion years ago.

Future studies should also measure the amount of oxygen in the environments where Asgards are present and identify microbes within the superphylum that can grow with only small amounts of oxygen. Asgards are humanity’s closest living relatives from an ancient event, which means they hold clues to our origins, so it’s crucial to determine when they began using oxygen.

Our planet is dominated by eukaryotes. Understanding how they formed is a huge transition in the evolution of life on Earth. The fact that we found oxygen in our close ancestors, in Asgards fits that puzzle really well.