This variation in height could have also characterized human species in the past. A recently published pelvis from the site of Gona in Ethiopia (Simpson et al., 2008) dates to 0.9–1.4 mya and shows a very wide maximum width between the iliac crests. At the same time, the size of the acetabulum indicates a very small body, like the australopithecines and paranthropines. Although this specimen has been attributed to H. erectus, its chronology, geographic location, and some aspects of its anatomy are also consistent with an assignment to Paranthropus boisei (Ruff, 2009a). If it represents H. ergaster/erectus, it would have to be a very small-bodied population, considerably smaller than living pygmies.

The size of the brain is not without importance in hominid taxonomy, and it has been extensively used. In principal component analyses of neurocranial variables, the first factor is always size (by far explaining the most variation) and this is strongly correlated with brain volume. Standardizing the raw values for size does not solve the problem because there is a tight relationship between size and shape. That is, most of the differences in the neurocranial architecture of fossils attributed to Homo are simply related to the size of the brain. Other features used in taxonomy are related to bone thickness, cranial superstructures (e.g., tori), or more or less subtle features of the temporal bone.

However, this is not always the case. Neanderthals and modern humans show similar cranial capacities but differ markedly in neurocranial morphology. The brain of Homo sapiens seems to follow a very different pattern from that of other species (Bruner et al., 2003). At the same time, some of the H. erectus fossils from Ngandong (Java) have cranial capacities similar to that of Cranium 5 from the Sima de los Huesos but the neurocranial anatomy is very different.

Consideration of the neurocranium, then, should complement the study of the postcranial morphotype. When brain size is related to body size using allometric equations (to eliminate the size factor), an encephalization curve is obtained that can be used for systematics (Arsuaga and Martínez, 2001). Compared with the australopithecines, H. habilis shows an increase in encephalization because the brain size increases whereas the body size does not. The shift to the subsequent cranial and postcranial combined morphotype (which starts with H. ergaster/erectus) involves an important increase in both brain size and body size, but the increase in encephalization is small. However, this may still indicate an advance in cognitive abilities. Comparison between chimpanzees and gorillas indicates that closely related species may show large differences in body size but little difference in brain mass and intelligence. In contrast, the cranial capacity in H. ergaster/erectus is much larger than that of the australopithecines.