Eukaryotic Cells Built by Multiple Bacteria and Giant Viruses, Not One

Computational archaeology rewrites eukaryotic origin

Researchers led by Toni Gabaldón at IRB Barcelona used supercomputer analysis of genomic data spanning all known eukaryotic biodiversity to reconstruct the protein repertoire of LECA (last eukaryotic common ancestor), which existed roughly two billion years ago. The team traced each protein family back to its microbial or viral source by comparing against tens of thousands of bacterial, archaeal, and viral genomes.

The analysis identified multiple waves of horizontal gene transfer. Planctomycetota bacteria—which have unusual internal compartments—contributed earlier signals. Myxococcota, known for metabolic functions including lipid and membrane processes, appeared in a second wave closer in time to mitochondrial acquisition. Most unexpectedly, large-scale gene transfer from Nucleocytoviricota (giant viruses with genomes far larger than typical viruses) also left a genetic fingerprint in early eukaryotes. The researchers propose these viruses functioned as vehicles for genetic exchange in dense microbial ecosystems such as mats or biofilms.

The team deliberately adopted a conservative approach, retaining only evolutionary signals as strong as those already accepted for the archaeal host and mitochondrial bacterium. The work took over five years and required processing vast genomic sequences through complex mathematical models to detect signals otherwise invisible in aggregate data.

The single-encounter narrative breaks down

For four decades, the standard explanation held that eukaryotic complexity emerged when an archaeon engulfed a bacterium, acquiring the mitochondrion. This study does not reject the mitochondrion's central role but places it within a longer, messier collaboration among multiple microbial actors.

This matters because it fundamentally shifts how we ask questions about cellular assembly. Instead of a binary acquisition event, the evidence points to a multi-generational flux of genes moving among coexisting microorganisms in shared environments. Viruses appear as active agents of genetic transfer, not just pathogens. The implication is that eukaryotic complexity emerged through gradual accumulation of metabolic and structural capabilities, not a sudden architectural change.

For fields attempting to engineer synthetic cells or understand the limits of genetic modularity, this changes the baseline: we now know which genes are truly foundational and which arrived as add-ons in specific ecological contexts millions of years ago. That distinction matters when predicting whether a given gene combination will be stable or prone to rejection.

How to use this finding

If you maintain or build eukaryotic pathway databases, cross-reference your annotations against the LECA proteome and ancestral signal map published in this paper. The researchers have made explicit which protein families trace to Myxococcota, Planctomycetota, and other donors. Use those attributions to flag which genes in your analysis are true core machinery versus later acquisitions. In phylogenetic alignment work, this helps separate genuine homology from convergent evolution.

For synthetic biology teams designing minimal eukaryotic systems, this work provides a map of which functions cluster together evolutionarily and which do not. If you are attempting to build a stripped-down eukaryotic cell, prioritize the archaeal and early bacterial contributions; downstream genes are more likely to be context-dependent or redundant.