The study, led by Philipp Radler in the laboratory of Christa Schleper at the University of Vienna and published in the journal Nature, provides the first direct observations of Asgard archaeal motility. These organisms are tiny; each cell has a volume roughly a thousand times smaller than that of a human cell. Yet the recordings show them undergoing dramatic transformations of shape on a minute-by-minute basis, extending thin appendages that can reach up to twenty times the length of the cell body, retracting them again, and using them to actively explore their surroundings. The behavior is strikingly reminiscent of the amoeboid movement seen in far more sophisticated cells, including human immune cells patrolling tissue.

Why does this matter so much? Asgard archaea occupy a central position in current models of eukaryogenesis, the process by which the first complex cells emerged roughly two billion years ago. The prevailing scientific framework holds that a eukaryotic ancestor arose from a symbiotic fusion between a bacterium and an archaeal lineage ancestral to today’s Asgard archaea. In that scenario, the bacterium eventually became the mitochondrion, the energy-producing organelle that powers virtually every complex cell on Earth. Asgard archaea are therefore not merely an interesting branch of the microbial tree; they are the closest available window into the cellular world from which our own lineage was born.

Until very recently, that window was almost entirely opaque. The first Asgard archaea were only brought into culture in 2020 and 2023, one at the JAMSTEC Institute in Japan and another in Schleper’s laboratory in Vienna, and both Japanese teams are co-authors on the new study. Before those milestones, essentially everything scientists knew about the group came from DNA sequencing of environmental samples or from electron microscopy of fixed cells. Those static images were already tantalizing: they showed a round cell body surrounded by a halo of fine, branching projections. But a fixed image cannot reveal whether a cell is extending, retracting, or moving, and so the functional significance of these elaborate shapes remained a matter of speculation.

The Vienna team closed that gap with a technically demanding approach. Asgard archaea are anaerobes, organisms that cannot tolerate oxygen, so the researchers placed the cells in an oxygen-free environment and filmed them alive under the microscope. They worked with two strains: a Lokiarchaeon cultivated in Vienna and a Heimdallarchaeon cultivated in Japan. Both organisms displayed the same remarkable repertoire of behaviors, drastically reshaping themselves every minute and deploying their thin, dynamic appendages to attach to surfaces and probe them. The crawling motion they use had not previously been described in any microbe, and its closest parallels lie in the motility of complex eukaryotic cells.

The mechanistic story behind the movement is equally significant. When the team applied actin inhibitors to the cultures, the dynamic behaviors were suppressed. Actin is one of the fundamental components of the cytoskeleton, the internal scaffolding that gives cells their shape, drives their internal transport, and powers their movement. In eukaryotes, an actin-based cytoskeleton underlies everything from muscle contraction to the crawling of immune cells through tissue. Finding that the motility of Asgard archaea likewise depends on actin suggests that a functional actin cytoskeleton, with clear parallels to the one operating in human cells, was already in place in the archaeal lineage long before the first eukaryote existed.

The international collaboration behind the study included, alongside the Japanese microbiologists, researchers from the Institute of Science and Technology Austria and the Helmholtz Centre for Infection Research in Braunschweig, Germany. Their combined expertise in cultivation, microscopy and cell biology was essential to a result that no single laboratory could have achieved alone. Culturing Asgard archaea remains extraordinarily difficult; the organisms grow slowly, in strict anaerobic conditions, and only a handful of laboratories worldwide have managed it. Every live-cell observation therefore represents years of methodological groundwork.

From an evolutionary standpoint, the discovery opens a genuinely new perspective on the deep history of cellular innovation. Complex cell motility may have far older origins than previously assumed, predating the emergence of eukaryotes themselves. If the ancestor of Asgard archaea already possessed the machinery for shape change, surface attachment and crawling, then some of the cellular capabilities once thought to be eukaryotic inventions may instead have been inherited from their archaeal partner. Such capabilities could even have played a role in the ancient symbiosis from which mitochondria later emerged, potentially facilitating the physical interactions between the archaeal host and its bacterial partner that set complex life in motion.

The findings also carry practical implications that extend well beyond evolutionary theory. Because the actin cytoskeleton governs shape changes and motility in human cells, understanding a simpler, archaeal version of the same machinery offers a comparative framework for dissecting how this fundamental system works. Insights gained from organisms that sit at the evolutionary boundary between prokaryotes and eukaryotes can illuminate which features of the eukaryotic cytoskeleton are ancient inheritances and which are later refinements. In this sense, the humble Asgard archaea, invisible to the naked eye and difficult to culture, may help explain processes that are central to human biology, including immune cell migration and cellular architecture.

Perhaps the most consequential aspect of the study is methodological. Oxygen-free live-cell microscopy of Asgard archaea now makes it possible, for the first time, to test models of the origin of complex life against empirical observation rather than inference from genomes and static images. Hypotheses about how the first eukaryotes moved, attached to surfaces, and interacted with partner cells can now be confronted with real footage of their closest living relatives in action. What has emerged from those first recordings, dynamic protrusions, actin-dependent crawling and a fluidity of form once thought to belong exclusively to complex cells, suggests that the road to eukaryotic life may have been paved with cellular behaviors far older and far more sophisticated than anyone had imagined.

Subject of Research: Crawling motility and actin-based cytoskeleton dynamics in Asgard archaea and their implications for eukaryogenesis

Article Title: Unexpected dynamics of ancient microbes discovered

Article References: Unexpected dynamics of ancient microbes discovered. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Asgard archaea, Lokiarchaeon, Heimdallarchaeon, eukaryogenesis, actin cytoskeleton, cell motility, crawling motility, live-cell microscopy, mitochondria, evolution, University of Vienna, Nature

The fundamental concept underlying the study involves measuring the evolutionary distance from the First Eukaryotic Common Ancestor (FECA) to the Last Eukaryotic Common Ancestor (LECA), normalized by the median branch length within eukaryotes, to adjust for differing evolutionary rates. Essentially, this normalized stem length serves as a molecular clock that hypothetically times gene acquisitions from prokaryotic donors to eukaryotes. Prior research had suggested that proteins with alphaproteobacterial ancestry—mitochondria’s bacterial progenitors—exhibited significantly shorter stems compared to genes derived from archaea, thereby implying a late mitochondrial acquisition.

However, when Tobiasson and colleagues applied this framework to an expansive dataset comprising 5,850 normalized stem lengths, the patterns proved far more complex than previously appreciated. The distribution showed a sharp peak near 0.05, mirroring trends from earlier research but revealing nuanced deviations that challenge simple temporal interpretations. Notably, alphaproteobacterial stems shorter than 0.3 were longer than those of Asgard archaeal origin, while stems surpassing 0.35 reversed this relation, appearing shorter than Asgard stems on average. This bimodal and statistically robust relationship calls into question the straightforward use of stem lengths as proxies for acquisition timing.

Interestingly, genes traced back to cyanobacteria, which are known contributors to plastids but not mitochondria, exhibited similar but less pronounced patterns, suggesting a broader evolutionary phenomenon beyond mitochondrial symbiogenesis. This observation integrates well with emerging perspectives that multiple prokaryotic lineages contributed variably to early eukaryotic genetics, complicating the notion of a singular, simple lineage acquisition event.

By narrowing the scope to specific gene categories with clearly defined ancestries—ribosomal proteins of archaeal origin and oxidative phosphorylation components from alphaproteobacteria—the team discerned an amplification of these complex trends. Despite the expectation that genes acquired simultaneously during mitochondrial endosymbiosis would display uniform stem lengths, the data revealed variances spanning orders of magnitude. This inconsistency fundamentally undermines the reliability of normalized stem lengths as mere chronological markers of gene incorporation.

The investigators propose an alternative interpretation rooted in the evolutionary pressures faced by newly acquired genes. Genes inherited from Asgard archaea, hypothesized to be closely related to the protoeukaryotic host, were likely already adapted to the cellular milieu of the evolving eukaryotic cell. In contrast, genes of bacterial origin—whether from alphaproteobacteria or other sources—would have required extensive adaptation post-acquisition, effectively elongating their molecular stems. This adjustment phase introduces substantial evolutionary change that inflates inferred stem lengths, conflating acquisition age with functional integration.

Supporting this hypothesis, genes involved in genetic information processing, which one might expect to be ancient and stable, surprisingly exhibited shorter than average stem lengths, indicating a complex interplay between evolutionary conservation and functional adaptation. Meanwhile, genes associated with metabolically dynamic systems, such as oxidative phosphorylation, demonstrated dramatically extended stem lengths, consistent with prolonged adaptive evolution.

These findings necessitate a reconceptualization of stem length metrics and their interpretive power. They suggest that molecular clocks in gene trees may predominantly reflect the varying tempos of post-acquisition evolutionary adaptation rather than the chronological order of acquisition events. Thus, the evolutionary narrative of eukaryogenesis emerges as a dynamic mosaic shaped both by the timing of gene acquisitions and the diverse evolutionary pressures acting on individual genes.

This research further emphasizes the dominant role of Asgard archaea in shaping the eukaryotic lineage, lending support to theories positing Asgard archaea as the protoeukaryotic host lineage. The data contravene simplistic models of eukaryogenesis based solely on timing inferred from stem lengths and instead point to a multifactorial scenario wherein gene origin, cellular context, and functional adaptation interplay to shape the genome.

Moreover, the study highlights the limitations of phylogenetic normalization techniques when applied across deep evolutionary timescales. The extensive variability observed suggests caution in extrapolating gene tree branch lengths to infer precise historical events, especially given complex biological phenomena such as horizontal gene transfer, gene loss, and convergent evolution.

Ultimately, this work represents a significant leap in our understanding of the evolutionary dynamics underlying the origin of eukaryotes and mitochondria. It calls for refined analytical frameworks that integrate evolutionary rate heterogeneity, functional adaptation, and phylogenomic context. Future research grounded in these principles promises to unravel the intricate evolutionary history of one of life’s defining transitions.

Tobiasson et al.’s findings invite a reassessment of the molecular clocks traditionally employed in evolutionary biology, urging a more nuanced appreciation of the forces shaping gene evolution. As scientists recalibrate their tools and interpretive models, insights from this study will undoubtedly reverberate across evolutionary research, informing not only the origin of eukaryotes but broader paradigms of genome evolution.

This work exemplifies how interdisciplinary approaches blending phylogenetics, molecular evolution, and comparative genomics can challenge entrenched hypotheses and foster innovative perspectives on complex biological phenomena. In doing so, it paves the way for a more integrated and dynamic understanding of life’s earliest, most transformative episodes.

Subject of Research: Evolutionary timing and origins of core eukaryotic genes in relation to Asgard archaea and alphaproteobacteria during eukaryogenesis.

Article Title: Dominant contribution of Asgard archaea to eukaryogenesis.

Article References:
Tobiasson, V., Luo, J., Wolf, Y.I. et al. Dominant contribution of Asgard archaea to eukaryogenesis. Nature (2026). https://doi.org/10.1038/s41586-025-09960-6

Image Credits: AI Generated

DOI: https://doi.org/10.1038/s41586-025-09960-6