The origin of eukaryotes and their cellular complexity remains one of the main enigmas in biology. It is generally accepted that eukaryotes emerged from an endosymbiosis event between an archaeal host and a bacterial symbiont. Although the identity of the host cell remains an ongoing matter of debate, a recent study has shown that it seems to be most closely related to Lokiarcheaota, a new phylum of uncultivated archaea . Lokiarchaeota not only forms a monophyletic group with Eukaryotes in phylogenetic analyses but also harbors a large amount of eukaryotic signature proteins, suggesting that the genetic repertoire of the putative archaeal ancestor was more complex than previously thought .
To be able to shed further light onto the potential archaeal ancestry of eukaryotes, we are using cultivation-independent approaches to reconstruct genomes of archaeal lineages related to Lokiarchaeota from all around the world. Preliminary results of phylogenetic and comparative genomic analyses of these novel organisms support the emergence of Eukaryotes from within the archaeal domain, i.e. from Lokiarchaeota and related lineages and reveal the presence of previously detected as well as novel eukaryotic signature proteins. Intriguingly, these proteins are involved in processes such as ubiquitin modifier systems and ESCRT- and trafficking machineries in Eukaryotes or represent major components of eukaryotic cytoskeletons. Altogether, these findings reveal fascinating novel insights into the genetic potential of the archaeal domain and the origin of eukaryotes.