The origins and molecular evolution of sperm

Summary Sperm is a nearly universal cell type among animals, yet its evolutionary origins remain unclear. Using comparative proteomics and phylogenomics across 32 species, we reconstruct the 700+ million year evolutionary history of sperm to define the Last Universal Common Sperm (LUCS), a conserved core of 301 gene families enriched in motility and energy metabolism. We found that most of the LUCS toolkit was developed in filozoan unicellular ancestors approximately 400 Ma before animals, revealing sperm as a reconfigured legacy from our unicellular past rather than an invention of multicellularity. Sperm shows a within-cell evolutionary gradient, both spatial and temporal, where ancient proteins dominate the tail, while younger innovations concentrate in the head. The oldest sperm components are disproportionately associated with human infertility, establishing an empirical bridge between evolutionary depth and clinical relevance. Together, our findings reveal how evolutionary history is inscribed within a single cell and can guide clinical insights.