A team from Barcelona has re‑examined the genes shared by every eukaryote and found the first complex cells were built from multiple bacterial donations, not a single merger. By trimming down dozens of genomes to a balanced set of orthologous groups, the researchers isolated a core gene set that reveals a mosaic ancestry. Their approach also mitigates bias from over‑represented animal genomes.

The analysis confirms the long‑standing model that an Asgard archaea host engulfed an Alphaproteobacteria ancestor, which became the mitochondrion. However, roughly equal gene contributions also trace back to Planctomycetota and Myxococcota, and even giant‑virus relatives left detectable marks. Such diversity challenges any single‑origin narrative for eukaryotic complexity. These findings suggest successive waves of horizontal transfer within microbial mats, supplying metabolic tools before and after mitochondrial acquisition.

Reconstructed functions show the last common eukaryotic ancestor lived in oxygenated water, fed on other cells, and already possessed internal tracks, lysosomes, peroxisomes and the full complement of DNA‑replication machinery. Missing were dedicated cell‑cycle regulators, implying division was driven by metabolic state. These insights could refine models of organelle evolution and metabolic integration. The work reshapes how biologists view early eukaryote evolution and the prevalence of gene sharing across domains.