Koalas, papionines and humans: Understanding variation and diversity in human evolution

The number of species that occurred in hominin evolution is still debated. Positions of various researchers range from a single lineage through just a few species to well over 10 species and several genera. The debate can be resolved only when a common standard of intraspecific variation is applied to the now abundant hominin fossil record. We use here multivariate measures of craniometric variation (11 dimensions of neuro- and splanchnocranium) - multivariate coefficient of variation (MCV) and average multivariate deviation (d) - calculated for an undisputed single arboreal mammalian species, the koala (Phascolarctos cinereus), a set of six species of Papio, modern humans and a sample of all hominin fossil crania dated to the period 1.5 to 2.0 Ma who may contain as many as three genera and several species. These measures describe variation within two single known species (a marsupial and modern human) in a sample of multiple species (Papio) and in a sample of unknown and debated speciosity, hominins (dated 1.5-2 Ma). The MCV and (d) values are: Papio 19.2(56.8), koala 10.6(35.6), hominins 11.8(27.9) modern humans 5.4(20.1). Differences between Papio and both single known species are significant. Hominin variation is significantly less than Papio, not different from koalas, and larger than in modern humans. It should be, however, noted that hominin specimens cover over 500 ka time span and thus must include chronological variation. This indicates that Plio-Pleistocene hominins were not as speciose as most commonly accepted in the literature.