The origin of complex life on Earth, long explained by a single symbiotic event between an archaeon and a bacterium, has been recast as a far more intricate and gradual process involving a wider cast of microorganisms, including giant viruses. A study published on 10 June 2026 in Nature, led by researchers at the Barcelona Supercomputing Center (BSC-CNS) and the Institute for Research in Biomedicine (IRB Barcelona), used over five years of computational work on the MareNostrum supercomputer to reconstruct the genetic makeup of the Last Eukaryotic Common Ancestor (LECA).

A more crowded evolutionary stage

For decades, the dominant theory, pioneered by biologist Lynn Margulis in 1967, held that complex eukaryotic cells arose when an ancient archaeon engulfed a bacterium, which then became the mitochondrion, the cell's energy-producing organelle. The new research does not discard this endosymbiotic event but places it within a longer, more complex narrative. The team's analysis of modern genomes suggests that before and alongside this merger, other microorganisms contributed essential genes.

“For a long time, we have explained the origin of complex cells as a story with two main protagonists: an archaeon and the bacterium that gave rise to the mitochondrion. Our study suggests that this narrative is incomplete and that there were more actors on the stage, including other bacterial groups and giant viruses that may have facilitated genetic exchange.”

The role of bacteria and giant viruses

The study identifies specific bacterial groups, such as myxococcota and planctomycetota, as likely contributors. The former provided genetic material for building internal cell compartments, while the latter helped develop the cell's structural complexity. Giant viruses also appear to have played a role, acting as vehicles for horizontal gene transfer among these ancient microbes, shuffling genetic material in a natural laboratory of evolutionary experimentation.

Molecular archaeology with a supercomputer

Without physical fossils from two billion years ago, the team practiced a form of molecular archaeology. They compared genomic data from modern eukaryotes, bacteria, archaea, and viruses, tracing the ancestry of gene families to determine their origins. This required the immense computing power of the MareNostrum supercomputer to sift through a vast diversity of organisms and detect ancient evolutionary signals.

A gradual paradigm shift

The findings support a shift toward a more gradual model of eukaryotic evolution. Rather than a sudden leap, the emergence of the complex cell was likely a multi-million-year process of genetic mixing and integration. The work refines the understanding of what the researchers call the greatest evolutionary discontinuity in the history of life, providing a more complete picture of how animals, plants, and fungi ultimately came to be.