Homo erectus and Middle Pleistocene hominins: Brain size, skull form, and species recognition

G. Philip Rightmire

Source - http://www.sciencedirect.com/science/article/pii/S0047248413001358

Journal of Human Evolution

Abstract

Hominins that differ from Homo erectus, the Neanderthals, and recent humans are known from Middle Pleistocene localities across the Old World. The taxonomic status of these populations has been clouded by controversy. Perhaps the most critical problem has been an incomplete understanding of variation in skull form. Here, both H. erectus and later mid-Pleistocene hominins are the focus of an investigation aimed at clarifying the relationships among brain volume, basicranial dimensions, neurocranial shape, and certain facial characters. Brain size in H. erectus averages about 950 cm3, while in a series of Middle Pleistocene crania from Africa and Europe, volume is about 1230 cm3. If encephalization is the primary mechanism operating in the mid-Pleistocene, then diverse aspects of cranial form cannot all be treated as independent variables. Correlation is utilized to examine the associations among measurements for more than 30 H. erectus crania that are reasonably well preserved. A similar approach is used with the Middle Pleistocene sample. Patterns of covariation are compared in order to assess integration. Next, factor analysis is applied to the H. erectus specimens in an attempt to identify modules, tightly integrated traits that can evolve independently. Studies of the variation within H. erectus are followed by direct comparisons with the Middle Pleistocene population. Discriminant functions facilitate the description of intergroup differences. Traits that vary independently from brain volume include anterior frontal broadening, lateral expansion of the parietal vault, elevation of the lambda–inion chord, and rounding of the sagittal contour of the occipital. This finding helps to resolve the problem of species recognition. Neurocranial proportions as well as characters from the cranial base and face can be incorporated into a differential diagnosis for the mid-Pleistocene sample. Evidence presented here supports arguments for speciation in the Middle Pleistocene.

Figure 1. Crania of Homo erectus and recent H. sapiens (images approximately to scale). (a) KNM-ER 3733 from Koobi Fora has a capacity of 848 cm3. The vault is relatively long and low, with an angulated occipital, prominent browridges, and well developed supramastoid crests. (b) A recent human skull is much more voluminous and globular in shape, with a high frontal and rounded occipital. There is no supraorbital torus, and cranial cresting is less noticeable.

Figure 2. Crania of Homo erectus and an MP hom (images approximately to scale). In both individuals, the frontal contour is flattened, while the face is massive and set forward from the anterior part of the braincase. (a) Sangiran 17 from Java has a capacity of 1004 cm3. The parietal is relatively short. The occipital is strongly flexed, with a long nuchal plane. (b) Broken Hill has a capacity of 1280 cm3, a higher vault, and a longer bregma–lambda arc. The occipital upper scale is expanded, relative to the nuchal plane below.