The Tentacled Microbes That Might Have Sparked Complex Life

New research sheds light on one of the biggest questions in biology: where did complex life come from? The answer may lie with Asgard archaea, a group of ancient single-celled microbes that share surprising similarities with more complex organisms. By closely examining one of these microbes, scientists have uncovered evidence that supports the idea that Asgard archaea may be direct ancestors of animals, plants, and humans.

• Asgard archaea are a recently discovered group of microbes that appear to bridge the gap between simple organisms (like bacteria and archaea) and complex life forms (like animals and plants).
• Researchers studied Lokiarchaeum ossiferum, a species of Asgard archaea, and identified cytoskeletal proteins similar to those found in eukaryotes—organisms with complex cells.
• These cytoskeletal proteins, including actin-like filaments, suggest that the cellular architecture needed for complex life may have originated in Asgard archaea.
• The findings add to growing evidence that eukaryotes, including humans, evolved from ancient Asgard archaea.

The Discovery of Asgard Archaea

Just a decade ago, scientists had no idea that Asgard archaea even existed. That changed in 2015, when researchers analyzing deep-sea sediments uncovered genetic fragments pointing to an entirely new group of microbes.

Using computational tools, they pieced together these fragments to reconstruct a full genome. Only then did they realize they had discovered a previously unknown type of archaea.

Archaea, like bacteria, are single-celled organisms. Genetically, however, the two are quite distinct, particularly in terms of their cell structures and metabolic pathways.

As researchers dug deeper, they were able to identify living organisms that matched the genetic data. These microbes were described and classified as a new subgroup of archaea, now known as Asgard archaea. Their name, inspired by the Norse mythological realm of Asgard, reflects their discovery near “Loki’s Castle,” a hydrothermal vent system in the mid-Atlantic between Norway and Svalbard.

Remarkably, Asgard archaea turned out to be more than just a new type of microbe: they may represent an evolutionary bridge between archaea and eukaryotes, the domain that includes all organisms with complex cells, like animals, plants, and fungi.

Rethinking the Tree of Life

In recent years, researchers have found growing indications of close links between Asgard archaea and eukaryotes, and that the latter may have evolved from the former. The division of all living organisms into the three domains of bacteria, archaea, and eukaryotes did not hold up to this surprising discovery.

Some researchers have since proposed regarding eukaryotes as a group within Asgard archaea. This would reduce the number of domains of life from three to two: archaea, including eukaryotes, and bacteria.

At ETH Zurich, Professor Martin Pilhofer and his team are fascinated by Asgard archaea and have examined the mysterious microbes for several years.

In an article published in Nature two years ago, the ETH researchers explored details of the cellular structure and architecture of Lokiarchaeum ossiferum. Originating in the sediments of a brackish water channel in Slovenia, this Asgard archaeon was isolated by researchers in Christa Schleper’s laboratory at the University of Vienna.

Eukaryote-Like Structures Revealed

In that study, Pilhofer and his postdoctoral researchers Jingwei Xu and Florian Wollweber demonstrated that Lokiarchaeum ossiferum possesses certain structures also typical of eukaryotes. “We found an actin protein in that species that appears very similar to the protein found in eukaryotes – and occurs in almost all Asgard archaea discovered to date,” says Pilhofer.

In the first study, the researchers combined different microscopy techniques to demonstrate that this protein – called Lokiactin – forms filamentous structures, especially in the microbes’ numerous tentacle-like protrusions. “They appear to form the skeleton for the complex cell architecture of Asgard archaea,” adds Florian Wollweber.

Tracing the Origin of Microtubules

In addition to actin filaments, eukaryotes also possess microtubules. These tube-shaped structures are the second key component of the cytoskeleton and are comprised of numerous tubulin proteins. These tiny tubes are important for transport processes within a cell and the segregation of chromosomes during cell division

The origin of these microtubules has been unclear – until now. In a newly published article in Cell, the ETH researchers discovered related structures in Asgard archaea and describe their structure. These experiments show that Asgard tubulins form very similar microtubules, albeit smaller than those in their eukaryotic relatives.

Open Questions About Function

However, only a few Lokiarchaeum cells form these microtubules. And, unlike actin, these tubulin proteins only appear in very few species of Asgard archaea.

Scientists do not yet understand why tubulins appear so rarely in Lokiarchaea, or why they are needed by cells. In eukaryotes, microtubuless are responsible for transport processes within the cell. In some cases, motor proteins “walk along” these tubes. The ETH researchers have not yet observed such motor proteins in Asgard archaea.

A Glimpse into Evolutionary History

“We have shown, however, that the tubes formed from these tubulins grow at one end. We therefore suspect that they perform similar transport functions as the microtubules in eukaryotes,” says Jingwei Xu, the co-first author of the Cell study. He produced the tubulins in a cell culture with insect cells and examined their structure.

Researchers from the fields of microbiology, biochemistry, cell biology and structural biology collaborated closely on the study. “We would never have progressed so far without this interdisciplinary approach,” emphasises Pilhofer with a degree of pride.

Cytoskeleton and the Origin of Eukaryotes

Was the cytoskeleton essential for the development of complex life? While some questions remain unanswered, the researchers are confident that the cytoskeleton was an important step in the evolution of eukaryotes.

This step could have occurred eons ago, when an Asgard archaeon entwined a bacterium with its appendages. In the course of evolution, this bacterium developed into a mitochondrion, which serves as the powerhouse of modern cells. Over time, the nucleus and other compartments evolved – and the eukaryotic cell was born.

“This remarkable cytoskeleton was probably at the beginning of this development. It could have enabled Asgard archaea to form appendages, thereby allowing them to interact with, and then seize and engulf a bacterium,” says Pilhofer.

Fishing for Asgard Archaea

Pilhofer and his colleagues now plan to turn their attention to the function of actin filaments and archaeal tubulin along with the resulting microtubules.

They also aim to identify the proteins that researchers have discovered on the surface of these microbes. Pilhofer hopes his team will be able to develop antibodies precisely tailored to these proteins. This would enable researchers to “fish” specifically for Asgard archaea in mixed microbe cultures.

“We still have a lot of unanswered questions about Asgard archaea, especially regarding their relation to eukaryotes and their unusual cell biology,” says Pilhofer. “Tracking down the secrets of these microbes is fascinating.”

References:

“Microtubules in Asgard archaea” by Florian Wollweber, Jingwei Xu, Rafael I. Ponce-Toledo, Florina Marxer, Thiago Rodrigues-Oliveira, Anja Pössnecker, Zhen-Hao Luo, Jessie James Limlingan Malit, Anastasiia Kokhanovska, Michal Wieczorek, Christa Schleper and Martin Pilhofer, 21 March 2025, Cell.
DOI: 10.1016/j.cell.2025.02.027

“Actin cytoskeleton and complex cell architecture in an Asgard archaeon” by Thiago Rodrigues-Oliveira, Florian Wollweber, Rafael I. Ponce-Toledo, Jingwei Xu, Simon K.-M. R. Rittmann, Andreas Klingl, Martin Pilhofer and Christa Schleper, 21 December 2022, Nature.
DOI: 10.1038/s41586-022-05550-y

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