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Understanding how complex eukaryotic cells emerged from their prokaryotic ancestors is a fundamental challenge in biology. The widely discussed endosymbiotic theory posits that the last eukaryotic common ancestor was derived from a host cell that had taken up an alphaproteobacteria-related symbiont that later became the mitochondrion 11 , 12 , 13 , 14 . Although the identity of the putative host cell in this theory has long been enigmatic, recent metagenomic and phylogenetic analyses indicate that eukaryotes probably evolved from a lineage closely related to members of the archaeal group Asgard archaea 2 , 6 , 7 , 15 , 16 , 17 , which has now been formally classified as the phylum Promethearchaeota within the kingdom Promethearchaeati 3 . This relationship is further reflected in the presence of abundant eukaryotic signature proteins (ESPs) in their genomes 15 , 16 , 18 .…