Mapping of human X-borne genes in distantly related mammals has defined a conserved region shared by the X chromosome in all three extant mammalian groups, plus a region that was recently added to the eutherian X but is still autosomal in marsupials and monotremes. Using comparative mapping of human Y-borne genes, we now directly show that the eutherian Y is also composed of a conserved and an added region which contains most of the ubiquitously expressed Y-borne genes. Little of the ancient conserved region remains, and the human Y chromosome is largely derived from the added region.