In a groundbreaking study that reshapes our understanding of early eukaryotic evolution, researchers have rigorously analyzed the ancestral stem lengths of core eukaryotic genes to gain fresh insights into the timing and origins of mitochondrial acquisition. This approach revisits and extends the methodology first proposed by Pittis and Gabaldon, who postulated that the relative lengths of gene stems in phylogenetic trees could reveal when key genes were integrated into the protoeukaryotic lineage. The controversy over whether mitochondria appeared early or late in eukaryogenesis is invigorated by this comprehensive reassessment.
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
