Not long ago, humanity knew nothing of the existence of Asgard archaea. Hidden deep beneath the ocean floor, these mysterious microbes silently thrived, eluding scientific discovery. It wasn’t until 2015 that researchers, probing sediments from the ocean’s depths, stumbled across genetic clues pointing to an entirely new domain of life. Little did they realize that these microscopic relics might hold the key to one of biology’s greatest mysteries: the origin of complex life on Earth.
Today, Asgard archaea are reshaping our understanding of how life evolved and blurring the lines that once neatly divided the tree of life. What was once a tidy model of three distinct domains—bacteria, archaea, and eukaryotes—is now being rewritten.
The Accidental Discovery That Changed Everything
The story begins not with a eureka moment in a lab, but with a painstaking analysis of fragments of genetic material. In 2015, an international team of microbiologists was examining sediments collected from hydrothermal vents deep beneath the Atlantic Ocean, near an undersea region ominously known as Loki’s Castle. These vents, spewing superheated water rich in minerals, host bizarre ecosystems powered not by sunlight but by chemosynthesis. It was here, among these alien-like habitats, that scientists uncovered the genetic fingerprints of something entirely new.
By digitally piecing together these scattered gene fragments like an ancient jigsaw puzzle, researchers assembled the genome of a microbe they dubbed Lokiarchaeota. This microbe was unlike anything previously seen. As they continued their work, they discovered a whole family of related organisms, which they collectively named the Asgard archaea, after the mythological realm of the Norse gods.
The name wasn’t just poetic flair. These microbes were found near hydrothermal vents bearing names like Loki’s Castle, Odin, and Thor, and they seemed almost divine in their significance for biology. Asgard archaea appeared to be more than just another branch of the archaeal family tree; they looked like a crucial missing link between simple microbial life and the complex, nucleus-containing cells of eukaryotes—the domain of life that includes animals, plants, fungi, and us.
What Makes Asgard Archaea So Special?
Archaea, like bacteria, are single-celled organisms that lack a true nucleus. But the Asgard archaea are different—very different. Their genes reveal an unusual array of capabilities previously thought unique to eukaryotes. These include genes associated with cell membranes, cytoskeletal elements, and intracellular trafficking, functions that are fundamental to the complex architecture of eukaryotic cells.
Among the most striking discoveries was that Asgard archaea possess genes encoding actin proteins—the same type of proteins that help form the cytoskeleton in eukaryotic cells. The cytoskeleton is the cell’s internal scaffold. It gives cells their shape, allows them to move, and facilitates the complex transportation of materials within the cell. The presence of similar proteins in Asgard archaea raised a tantalizing possibility: that these humble microbes might represent the evolutionary stepping stone between simple prokaryotic cells and the highly structured cells of eukaryotes.
But it didn’t stop there. In 2022, researchers at ETH Zurich, led by Professor Martin Pilhofer, published a groundbreaking paper describing their work on Lokiarchaeum ossiferum, a species of Asgard archaea isolated from a brackish water channel in Slovenia. What they found was nothing short of astonishing.
A Cytoskeleton from the Depths of Time
Using cutting-edge microscopy techniques, Pilhofer and his team observed filamentous structures inside Lokiarchaeum ossiferum. These filaments were formed by an actin-like protein they named Lokiactin. Just as actin forms the structural backbone of eukaryotic cells, Lokiactin appeared to build a rudimentary skeleton within these Asgard archaea.
What’s more, these filaments extended into tentacle-like protrusions—cellular extensions that looked eerily like the structures eukaryotic cells use to engulf other cells or particles in a process known as phagocytosis. This observation lent weight to a compelling hypothesis: long ago, an ancient Asgard archaeon may have used similar protrusions to grab and engulf a bacterium. Over millions of years, that bacterium could have evolved into the mitochondrion, the energy-producing powerhouse of modern eukaryotic cells. This ancient encounter would have set the stage for the evolution of all complex life.
Enter Tubulins: The Second Piece of the Puzzle
While actin had already been found in Asgard archaea, one crucial component of the eukaryotic cytoskeleton was missing: microtubules. In eukaryotes, these tube-like structures are essential for moving materials around the cell and for orchestrating cell division. They are built from proteins called tubulins.
Pilhofer’s team set out to find them—and they did. In a 2024 study published in Cell, the researchers identified tubulin proteins in Asgard archaea. Remarkably, these tubulins could assemble into microtubule-like structures, albeit smaller and simpler than those found in eukaryotes. They were able to grow at one end, a key feature shared with their eukaryotic cousins, and possibly hinting at a primitive transport system inside these ancient microbes.
However, not all Asgard archaea produced tubulins, and even in species that did, only a few cells displayed microtubule structures. Why? That’s still a mystery. Pilhofer’s team suspects these tubules could have served similar transport roles as in eukaryotic cells, but they have yet to observe the presence of motor proteins—the molecular machines that walk along microtubules to ferry cargo.
Rewriting the Tree of Life
These findings don’t just fill in a few gaps in evolutionary history—they shake the whole structure. For decades, life on Earth has been classified into three primary domains: bacteria, archaea, and eukaryotes. But if eukaryotes evolved from within the Asgard archaea, they may not represent a separate domain after all.
Some scientists now argue for a two-domain model, with life divided into bacteria and archaea, the latter including eukaryotes. If true, this reclassification profoundly changes our understanding of life’s history, emphasizing the deep evolutionary connections between ancient microbes and all complex life forms.
The Quest to Understand Asgard Archaea
Despite these breakthroughs, there’s still much to learn about Asgard archaea. Growing them in the lab has proven notoriously difficult. Only a handful of species have been successfully cultured, and even then, they grow extremely slowly—sometimes taking months to double in number. This makes them difficult subjects for detailed study.
Nonetheless, Pilhofer and his colleagues are pushing ahead. One of their goals is to develop antibodies specifically designed to latch onto proteins found on the surface of Asgard archaea. With these molecular tools, they hope to “fish” for these microbes in complex microbial communities, making it easier to isolate and study them.
Why It Matters: Understanding Our Origins
Why should we care about a bunch of microbes lurking deep in the ocean? Because understanding Asgard archaea might help answer one of the most profound questions in science: How did complex life arise?
The prevailing theory is that long ago, an Asgard archaeon and a bacterium formed an intimate partnership—an event known as endosymbiosis. Over time, the bacterium became the mitochondrion, and the Asgard archaeon transformed into the first eukaryotic cell. From that humble beginning came all animals, plants, fungi, and other complex organisms.
But for endosymbiosis to occur, the Asgard archaeon needed a certain level of cellular complexity. The presence of a cytoskeleton—composed of Lokiactin and tubulins—may have been the crucial innovation that made this ancient union possible. Without it, the Asgard archaeon might never have been able to engulf its bacterial partner, and complex life as we know it might never have evolved.
Looking Forward: The Next Frontier
As research continues, scientists are eager to uncover even more secrets locked within Asgard archaea. Pilhofer’s team plans to study how the cytoskeleton operates within these cells and whether motor proteins eventually emerge as pieces of the puzzle. They’re also investigating other unusual proteins that might reveal new insights into the earliest days of complex life.
And they’re not alone. Around the world, teams of microbiologists, geneticists, biochemists, and cell biologists are joining forces to explore these enigmatic microbes. The discovery of Asgard archaea has sparked a scientific gold rush in the deep oceans and the muddy sediments of ancient waterways.
Conclusion: Rediscovering Life’s Ancient Threads
In a sense, Asgard archaea are like living fossils, offering us a glimpse into a time billions of years ago when life was poised to make the leap from simplicity to complexity. They challenge us to rethink what we know about the origins of life and remind us that even the most ordinary-seeming microbes can hold extraordinary secrets.
As Pilhofer puts it, “We still have a lot of unanswered questions about Asgard archaea, especially regarding their relation to eukaryotes and their unusual cell biology. Tracking down the secrets of these microbes is fascinating.”
Indeed, these tiny architects of complexity may ultimately reveal how we came to be—and they’re proving that even the smallest organisms can have the biggest stories to tell.
Reference: Florian Wollweber et al, Microtubules in Asgard archaea, Cell (2025). DOI: 10.1016/j.cell.2025.02.027. www.cell.com/cell/fulltext/S0092-8674(25)00254-5