The origin and cellular complexity of eukaryotes represent a major enigma in biology. Current data support scenarios in which an archaeal host cell and an alphaproteobacterial (mitochondrial) endosymbiont merged together, resulting in the first eukaryotic cell. The host cell is related to Lokiarchaeota, an archaeal phylum with many eukaryotic features. The emergence of the structural complexity that characterizes eukaryotic cells remains unclear. Here we describe the ‘Asgard’ superphylum, a group of uncultivated archaea that, as well as Lokiarchaeota, includes Thor-, Odin- and Heimdallarchaeota. Asgard archaea affiliate with eukaryotes in phylogenomic analyses, and their genomes are enriched for proteins formerly considered specific to eukaryotes. Notably, thorarchaeal genomes encode several homologues of eukaryotic membrane-trafficking machinery components, including Sec23/24 and TRAPP domains. Furthermore, we identify thorarchaeal proteins with similar features to eukaryotic coat proteins involved in vesicle biogenesis. Our results expand the known repertoire of ‘eukaryote-specific’ proteins in Archaea, indicating that the archaeal host cell already contained many key components that govern eukaryotic cellular complexity. This work describes the Asgard superphylum, an assemblage of diverse archaea that comprises Odinarchaeota, Heimdallarchaeota, Lokiarchaeota and Thorarchaeota, offering insights into the earliest days of eukaryotic cells and their complex features. Although the origin of eukaryotic cells from prokaryotic ancestors remains an enigma, it has become clear that the root of eukaryotes lies among a group of prokaryotes known as archaea. The recent identification of newly described archaea belonging to the Asgard superphylum, including Lokiarchaeota and Thorarchaeota, revealed a group of prokaryotes containing many proteins and genetic sequences that are otherwise found only in eukaryotes. Thijs Ettema and colleagues extend the search for eukaryotic roots by describing further additions to the Asgard superphylum: the Odinarchaeota and Heimdallarchaeota. The new Asgard genomes encode homologues of several components of eukaryotic membrane-trafficking machineries, suggesting that the archaeal ancestor of eukaryotes was well equipped to evolve the complex cellular features that are characteristic of eukaryotic cells.

Asgard archaea illuminate the origin of eukaryotic cellular complexity