Cranial variability in East African 'robust' hominis

Among Plio-Pleistocene hominins the East African 'robust' group [Australopithecus (Paranthropus) boisei sensu lato] has the largest sample. This makes it an important test case for examining within-group variability and its implications for early hominin systematics. When using the CV to test for mixed-species samples, sexual dimorphism and diachronic variation are important additional (confounding) sources of variability. After examining nineteen variables, five craniometric variables in the East African 'robust' group are identified that have low sexual dimorphism. All are characterised by CVs from a combined KNM-WT 17000 and A. (P.) boisei sample less than the CV of the bonobo. Diachronic variation is found to be an important source of variability for cranial capacity. This form of variation cannot be detected in other variables studied here, but its possible presence cannot be ignored (owing mostly to small sample sizes). It is concluded that there is insufficient evidence to refute the hypothesis that all East African 'robust' fossils belong to A. (P.) boisei (Walker and Leakey, 1988).