Analysis of conserved herpesvirus genes shows that human herpesvirus 8 (HHV8) belongs to a clade of primate herpesviruses within the gamma2 sublineage. Overall, this clade has developed by cospeciation with host species. Seventeen genes of HHV8 and its nearest relatives lack simple homologues in other herpesviruses, and eight of these have known mammalian homologues, presumed to be the sources of the HHV8 versions. Comparative analyses yield further insights into the source or time of acquisition for several of these genes. All were acquired by the HHV8 lineage in the very distant past. Two unusual processes are manifest in the contemporary evolution of HHV8. First, the K1 gene is uniquely undergoing a process of extensive and positively selected substitution, of unknown significance. Secondly, the K15 gene has been substituted in some lineages by a distant homologue, presumably by recombination with an unknown herpesvirus. While available data are otherwise compatible with a cospeciational relationship between HHV8 and the human species, they do not exclude relatively recent transfer to humans from another primate host.