Evidence on human origins from haemoglobins of African apes

Molecular data have influenced views concerning human origins, first, by supporting the genealogical classification of Pan (chimpanzee) and Gorilla with Homo rather than with Pongo (orangutan) and, second, by suggesting that only a few million years separate humans and chimpanzees from their last common ancestor. Indeed, the cladistic distances in phylogenetic trees constructed from amino acid sequence data, on detecting many superimposed mutations, yielded a 'molecular-clock' divergence date between Homo and Pan of only 1-1.5 Myr BP. This date, which is even more recent than (4.2-5.3 Myr BP) calculated using phenetic distances from immunological and DNA-hybridization comparisons (Table 1), is too near the present considering the existence of 3-4 Myr-old fossils of bipedal human ancestors (and a 5.5 Myr-old jaw fragment assigned to Australopithecus). Perhaps decelerated sequence evolution occurred; alternatively, hominoid distances could have been underestimated, because chimpanzee and gorilla were represented mostly by sequences inferred from peptide amino acid compositions, as was the case for their haemoglobins. To help rectify this situation we report here the rigorously determined alpha- and beta-haemoglobin amino acid sequences not only of chimpanzee (Pan troglodytes) and Gorilla gorilla but also pygmy chimpanzee (Pan paniscus). Our findings favour the explanation of decelerated evolution and point to selection preserving perfected haemoglobin molecules.