The origin of the eukaryotic cell, with its complex structural features, marks one of the prime hallmarks in the evolution of life on Earth. Still, the events that led to the emergence of cellular complexity in eukaryotic cells are a matter of hot debate. Current data supports scenarios in which an archaeal host cell and an alphaproteobacterial (mitochondrial) endosymbiont merged, triggering the emergence of the first eukaryotic cell. The host cell was recently shown to share a common ancestry with the Asgard archaea, an archaeal superphylum uniquely displaying a number of eukaryotic features. However, important details about the nature of the archaeal host and the presumed syntrophic interaction between this host and the alleged mitochondrial endosymbiont remain elusive, limiting our current understanding about the early stages of eukaryogenesis. In the current proposal, I aim to gain new insights in the emergence of eukaryotic cellular complexity by focusing on the newly discovered Asgard archaea. First, I will acquire and screen numerous environmental samples and employ state-of-the-art genome-resolved metagenomics approaches to lay out the diversity of the Asgard superphylum at the genomic level. By performing in-depth phylogenomic analyses of the newly generated genomic data, I will pinpoint the position of eukaryotes in the Tree of Life, and reconstruct the evolutionary history of eukaryotic genome content using advanced ancestral genome reconstruction algorithms. Next, I will develop in situ culturing and innovative cell extraction techniques that allow me to determine metabolic and syntrophic properties of Asgard archaeal cells, and to study their cells in ultra-high resolution. Altogether, the proposed research will provide novel insights into the genomic and cell-biological identity and nature of our closest prokaryotic relatives, and reveal, in more detail than ever before, the events that led to the emergence of complex cell types on Earth.