The origin of eukaryotic cells remains a contentious issue in modern biology. Recent studies have supported the emergence of eukaryotes from within the archaeal domain, but the nature of the putative archaeal ancestor is still debated. This study introduces 'Lokiarchaeota', a novel archaeal phylum that forms a monophyletic group with eukaryotes in phylogenomic analyses. The Lokiarchaeum genome encodes an expanded repertoire of eukaryotic signature proteins (ESPs), suggesting sophisticated membrane remodeling capabilities. The presence of these proteins indicates that many components of eukaryotic complexity were already present in the archaeal ancestor, providing a rich genomic "starter kit" for the increase in cellular and genomic complexity characteristic of eukaryotes. The Lokiarchaeum genome also contains a dynamic actin cytoskeleton and a primordial ESCRT complex, suggesting that the archaeal ancestor of eukaryotes had advanced membrane remodeling and vesicular trafficking capabilities. These findings provide strong support for the hypothesis that eukaryotes evolved from a bona fide archaeon and shed light on the evolutionary transition from prokaryotes to eukaryotes.The origin of eukaryotic cells remains a contentious issue in modern biology. Recent studies have supported the emergence of eukaryotes from within the archaeal domain, but the nature of the putative archaeal ancestor is still debated. This study introduces 'Lokiarchaeota', a novel archaeal phylum that forms a monophyletic group with eukaryotes in phylogenomic analyses. The Lokiarchaeum genome encodes an expanded repertoire of eukaryotic signature proteins (ESPs), suggesting sophisticated membrane remodeling capabilities. The presence of these proteins indicates that many components of eukaryotic complexity were already present in the archaeal ancestor, providing a rich genomic "starter kit" for the increase in cellular and genomic complexity characteristic of eukaryotes. The Lokiarchaeum genome also contains a dynamic actin cytoskeleton and a primordial ESCRT complex, suggesting that the archaeal ancestor of eukaryotes had advanced membrane remodeling and vesicular trafficking capabilities. These findings provide strong support for the hypothesis that eukaryotes evolved from a bona fide archaeon and shed light on the evolutionary transition from prokaryotes to eukaryotes.