THE PHOCA STANDARD - AN EXTERNAL MOLECULAR REFERENCE FOR CALIBRATING RECENT EVOLUTIONARY DIVERGENCES

Comparison of the complete mitochondrial DNA (mtDNA) of the high-Arcti c ringed seal (Phoca hispida) and the sub-Arctic harbour (P. vitulina) and grey (Halichoerus grypus) seals shows that they are genetically e quidistant from one another. We relate the evolutionary divergence of the three species to expanding glaciation in the Arctic Basin and esta blish, in conjunction with mtDNA data, a standard reference for calibr ation of recent divergence events among mammalian taxa. In the present study, we apply the ''Phoca standard'' to the dating of divergences w ithin the hominid phylogenetic tree. After determining the relative ra tes of substitution over all mitochondrial protein-coding genes in the different evolutionary lineages, we estimate that humans and chimpanz ees diverged from each other 6.1 Mya (95% confidence limits: 5.2-6.9 M ya). The corresponding lower-limit divergence between common chimpanze e, Pan troglodytes, and pygmy chimpanzee, P. paniscus, occurred 3 (2.4 -3.6) Mya, and the primary split within the P. troglodytes complex 1.6 (1.3-2.0) Mya. The analyses suggest that the split between Gorilla an d Pan/Homo occurred 8.4 (7.3-9.4) Mya. They also suggest that Pongo (o rangutan) and the lineage leading to gorillas, chimpanzees, and humans diverged 18.1 (16.5-19.6) Mya. The present analysis is independent of the hominid paleontological record and inferential morphological inte rpretations and thus is a novel approach to the lower-limit dating of recent divergences.