The "Phoca standard": an external molecular reference for calibrating recent evolutionary divergences

Comparison of the complete mitochondrial DNA (mtDNA) of the high-Arctic ringed seal (Phoca hispida) and the sub-Arctic harbour (P. vitulina) and grey (Halichoerus grypus) seals shows that they are genetically equidistant from one another. We relate the evolutionary divergence of the three species to expanding glaciation in the Arctic Basin and establish, in conjunction with mtDNA data, a standard reference for calibration of recent divergence events among mammalian taxa. In the present study, we apply the "Phoca standard" to the dating of divergences within the hominid phylogenetic tree. After determining the relative rates of substitution over all mitochondrial protein-coding genes in the different evolutionary lineages, we estimate that humans and chimpanzees diverged from each other 6.1 Mya (95% confidence limits: 5.2-6.9 Mya). The corresponding lower-limit divergence between common chimpanzee, 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 and Pan/Homo occurred 8.4 (7.3-9.4) Mya. They also suggest that Pongo (orangutan) 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 interpretations and thus is a novel approach to the lower-limit dating of recent divergences.