When Dr. Toni Gabaldón's team fed billions of years of cellular evolution into the MareNostrum supercomputers at Barcelona's research center, they found something unexpected: the origin story of complex life was far more crowded than science had long believed. A landmark study published in Nature reshapes our understanding of how eukaryotic cells—the sophisticated, compartmentalized cells that make up animals, plants, fungi, and every cell in the human body—first emerged on Earth roughly two billion years ago.
For decades, biologists told a spare narrative: an ancient microbe called an archaeon merged with a bacterium, which transformed into the mitochondrion, and that partnership alone unlocked the door to cellular complexity. It's a compelling story of two protagonists changing the world. But Gabaldón and his team at IRB Barcelona and the Barcelona Supercomputing Center have revealed a messier, richer reality. The emergence of complex cells was not a simple merger but a prolonged collaboration involving multiple bacterial groups, giant viruses, and complex genetic exchanges that unfolded over millions of years in ancient microbial communities.
The researchers approached the problem like computational archaeologists, excavating the fossil record hidden inside genomes. They reconstructed the genetic and protein inventory of LECA—the Last Eukaryotic Common Ancestor—by analyzing tens of thousands of bacterial, archaeal, and viral genomes. After more than five years of painstaking analysis, they identified evolutionary signals that were previously invisible. "We are trying to reconstruct a story that took place billions of years ago and for which we have no direct fossils," explains Moisès Bernabeu, a lead author on the study. "That is why we have been very conservative: we only kept the most robust evolutionary signals."
Beyond the mitochondrion's ancestor, the team detected two particularly significant bacterial signals. Myxococcota bacteria appear to have contributed metabolic capabilities related to lipids and membranes. Planctomycetota, which are structurally unusual for bacteria because they possess internal compartments, may have come aboard even earlier. This wasn't a sudden takeover but a gradual process: the evidence suggests Planctomycetota left their mark first, while Myxococcota and mitochondrial ancestors arrived at closer intervals. These ancient cells likely thrived in microbial mats—dense, layered communities of different microorganisms living under varying chemical conditions—where genetic exchange between neighbors was routine and transformative.
Perhaps most strikingly, the research reveals that giant viruses, particularly Nucleocytoviricota, played an unexpected role in this early cellular assembly. These viruses, which carry genomes far larger than typical viruses and infect single-celled eukaryotes, appear to have acted as vehicles for transferring genes between unrelated organisms. This finding challenges the traditional view of viruses as merely parasitic invaders; instead, they emerge as facilitators of evolutionary innovation.
The implications ripple across biology. Eukaryotic complexity did not spring from a single heroic merger but evolved through an intricate network of collaborations—a reminder that life's greatest breakthroughs often come not from solitary genius but from unexpected partnerships. This reframing holds lessons for how we understand evolution itself: complex systems arise not through simple, sudden transitions but through long, collaborative processes that leave their signatures written deep in the DNA of every living thing.