The mechanisms that drive the evolution of high taxonomic groups of bacteria and archaea are largely unknown. Here, Nelson-Sathi et al. examined 267,568 protein-coding genes from 134 archaeal genomes and compared them with their homologues from 1,847 reference bacterial genomes. Notably, about one-third of the archaeal genes had bacterial homologues; phylogenetic analysis of these genes suggested that they were acquired by archaea from bacteria. A phylogenetic tree based on these imported genes closely mirrored a reference tree comprising archaea-specific genes from 13 higher taxa, suggesting that these gene acquisitions from bacteria drove higher taxa evolution in archaea. Methanogenic archaea were the main recipients of bacterial genes, and most imported genes are associated with metabolic functions, suggesting that the acquisition of new metabolic pathways from bacteria was one of the key drivers of the evolution of high archaeal taxa.