From humans and houseplants to mushrooms on pizza and the yeast that helps the dough rise, every complex organism on Earth may share the same microscopic ancestor. That ancestor was likely an Asgard archaeon, a tiny microbe that lived more than two billion years ago in ancient oceans and hot springs.
In 2023, an international team led by researchers at The University of Texas at Austin and collaborators in Europe used hundreds of microbial genomes to redraw part of the tree of life. Their work suggests that all known eukaryotes, the group that includes animals, plants, fungi, and many microbes with complex cells, trace back to a single Asgardian lineage.
What are Asgard archaea?
Archaea are single-celled organisms that often live in extreme environments and look a lot like bacteria under a microscope, even though they belong to a different branch of life. Asgard archaea are a special group within this domain, first found in deep-sea sediments and later in hot springs and other hidden corners of the planet.
Eukaryotes are organisms whose cells contain a membrane wrapped nucleus and tiny internal compartments that act like specialized rooms inside a house.
The new work shows that eukaryotes form a tightly-nested branch inside the Asgard group, which means our own cells sit inside the broader Asgard family rather than on a separate limb of the tree of life.
Tracing complex life back to Hodarchaeales
To chase down this ancestor, the team compared more than 180 archaeal genomes, including dozens of newly-reconstructed Asgard genomes pulled from deep-sea mud and other sediment samples. They focused on sets of genes shared between archaea and eukaryotes and used them like fingerprints to work out who is most closely related to whom.
Biologist Valerie De Anda described this approach as “a time machine that uses the genetic blueprints of modern microbes to reconstruct the past.”
Those comparisons pointed to an order of Asgard archaea called Hodarchaeales, or Hods, which live in marine sediments and seem to prefer cooler environments. Members of this group carry many so-called eukaryotic signature proteins, molecules once thought to exist only in complex cells, which makes them strong candidates for our closest microbial cousins.
A separate Nature study in 2025 suggested that eukaryotes might instead sit as sisters to the wider Heimdallarchaeia group, yet both lines of research still place complex life firmly inside the Asgard branch.
The project was coordinated by Thijs Ettema of Wageningen University, working with scientists around the world. Coauthor Brett Baker, an associate professor of integrative biology and marine science, said researchers are “starting to see the transition from what biologists think is an archaeon to this organism Hodarchaeales that is more like a eukaryote.”
In everyday terms, they are watching the deep roots of complex life come into focus, one genome at a time.
Why this ancient microbe matters today
Reconstructing this Asgardian ancestor helps explain how simple microbes could slowly gain the tools needed for bigger, more complicated cells. By mapping which genes Asgard archaea share with us, scientists can see which parts of eukaryotic complexity were already in place long before the first animals, forests, or fungi appeared.
At the end of the day, it offers a roadmap for how chemistry in ancient mud became the biology that now fills your neighborhood and your own body.
Other research is filling in more pieces of the story. A 2024 study in the journal Nature Communications cataloged antiviral defense systems across thousands of archaeal genomes and found that Asgard archaea carry many of the same molecular tools that underlie innate immunity in modern eukaryotes, including viperin and argonaute-like proteins that help cells fight viruses.
That work suggests our immune defenses did not appear out of nowhere but were built on mechanisms that Asgard microbes were already using to survive ancient infections.
So when your body fights off a cold or a plant on your windowsill shrugs off a pathogen, you are seeing echoes of strategies that may have started in these long gone Asgardians. The story is still evolving and scientists caution that details of exactly which Asgard branch sat right next to the first eukaryote are still being debated.
The main study was published in Nature.
