By Ashley Strickland, CNN
(CNN) — 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.
Researchers have questioned how Asgards even crossed paths with other microbes that required oxygen for survival to create eukaryotes if they existed in such different environments.
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, according to a study published February 18 in the journal Nature.
“The fact that some of the Asgards, which are our ancestors, were able to use oxygen fits in with this very well,” study coauthor Brett Baker, associate professor of marine science and integrative biology at the University of Texas at Austin, said in a statement. “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 sort of the 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 broad range of environments, researchers are increasingly finding more types of Asgard microbes, such as Heimdallarchaeia, named for the guardian of Asgard.
In 2023, Baker and his colleagues found that 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