Living Asgard archaea crawl using actin machinery, illuminating eukaryotic origins

Under a microscope in Vienna, researchers watched something that shouldn't exist—or at least, something that shouldn't exist in the way it does. Tiny microorganisms called Asgard archaea were stretching out thin, finger-like protrusions, pulling themselves across a glass surface with the kind of controlled, purposeful movement you'd expect from an amoeba or a white blood cell. But these organisms are prokaryotes, single-celled creatures without a nucleus. They belong to a lineage that split off from bacteria billions of years ago. What they were doing—crawling with sophisticated cellular machinery—suggested that the roots of complex life run deeper than anyone had directly observed before.

Philipp Radler and his team at the University of Vienna had solved a technical problem that had stymied researchers for years. Asgard archaea are finicky organisms. They grow slowly, often depend on other microbes to survive, and they need oxygen-free environments. The first living cultures had only recently become available. But Radler developed a microscopy system that could keep these anaerobic cells alive while recording them in real time. What emerged from those recordings was the first live view of behavior that had previously existed only as static images from electron microscopy and as genetic sequences in databases.

The two species they studied—Candidatus Lokiarchaeum ossiferum from Vienna and Candidatus Margulisarchaeum peptidophilum from Japan—moved in ways that revealed something fundamental about how cells work. The protrusions of L. ossiferum grew at about 1.5 micrometers per minute, extending as far as 15 micrometers before snapping back, sometimes retracting five times faster than they had grown. The second species moved even faster, its protrusions extending at roughly 4.8 micrometers per minute. A typical cell carried about five of these structures at once, constantly reshaping itself. The cells weren't building new membrane continuously; instead, they were redistributing the membrane they already had, pulling it from one protrusion to extend another. It was a tightly choreographed dance of biological economy.

These protrusions did more than change shape. They moved the entire organism. About half of the L. ossiferum cells observed actually crawled across the surface, with a median speed of 1.6 micrometers per minute. The second species moved faster still, at roughly 3.4 micrometers per minute. Some cells appeared to attach the tip of a forward protrusion to the surface and then shorten it, pulling the cell body along like a grappling hook. Others glided more smoothly. Most wandered without apparent direction, a behavior that might help them explore the tiny spaces between sediment particles where they could find nutrients or other microbes to partner with.

The mechanism behind this movement pointed to something unexpected. Running through the bodies and protrusions of these cells were filaments made of Lokiactin, an archaeal protein that resembles actin, the fundamental building block of the cytoskeleton in eukaryotic cells. When researchers exposed the cells to swinholide A, a compound that disrupts eukaryotic actin, the effect was dramatic. At concentrations as low as 50 nanograms per milliliter, protrusion growth slowed dramatically. At higher concentrations, the cells stopped crawling altogether. The protrusions lost their stiffness and began drifting passively, buffeted by random molecular motion rather than controlled by the cell. The team also found proteins related to gelsolins and profilins—regulatory proteins that eukaryotic cells use to control actin assembly and disassembly—interacting with Lokiactin in these archaea. What emerged was not just a protein that looked like actin, but a dynamically regulated system capable of reshaping membranes and moving entire cells.

This matters because Asgard archaea are the closest known prokaryotic relatives of eukaryotes. Current models of how complex life arose involve an ancient Asgard-related archaeon entering into a close association with a bacterium, which eventually became the mitochondrion. The question that has haunted evolutionary biology is what that ancestral cell actually looked like and what capabilities it possessed. Previous studies had already revealed that Asgard archaea carry genes resembling those used by eukaryotic cells for membrane remodeling, protein trafficking, and cytoskeletal structure. A 2023 study had shown an elaborate actin cytoskeleton in L. ossiferum. Research published in 2025 found that Asgard archaea can produce microtubule-like structures from relatives of tubulin proteins. But these were all static observations, reconstructions from genes and frozen images. What Radler's team provided was behavior—the ability to watch these cells stretch, attach, pull, and crawl under a microscope.

This does not mean modern Asgard archaea are unchanged descendants of our ancestors. They have experienced roughly two billion years of their own evolution since the split. Competing models still disagree about the precise mechanics of how the archaeal and bacterial partners combined during the emergence of eukaryotes. But what researchers now possess is something they previously lacked: living cells in which evolutionary ideas can be experimentally tested. Rather than reconstructing the dawn of complex life entirely from genes and fossils, scientists can now manipulate these organisms, watch them respond, and begin to understand how the cellular machinery that defines modern life might have first emerged.