The origin of the eukaryotic cell represents an enigmatic yet dramatically incomplete evolutionary puzzle. The few pieces of this puzzle that we have managed to identify thus far indicate that the eukaryotic cell most probably emerged via a fusion of an archaeal and an alphaproteobacterial cell. Yet, beyond this, scientific debate is mainly engulfed by speculation, which, to a large extent, is fed by our poor understanding of the identity of these bacterial and archaeal fusion partners. In the current research proposal, I aim to identify contemporary relatives of the bacterial and archaeal lineages that once founded the eukaryotic cell using novel single-cell genomics and phylogenomics approaches. Till this end, environmental samples that are enriched for the target microbial lineages will be collected and analyzed at the single cell level. After target cells have been identified, their genomic material will be amplified, and subjected to next-generation sequencing analysis. Subsequent state-of-the-art phylogenomic analyses will elucidate the taxonomic affiliation of these target cells in relation to other archaeal and bacterial lineages, as well as to eukaryotes. The proposed strategy will identify hundreds of novel prokaryotic lineages, some of which representing close contemporary relatives of the ‘parental’ lineages that founded the eukaryotic cell. The genomic exploration of these lineages will add the taxonomic resolution that is needed to start solving the evolutionary puzzle of the emergence of the eukaryotic cell.