The Ontogeny of Basicranial Flexion in Children of African and European Ancestry

This study examined ontogenetic changes in the cranial base angle in individuals between the ages of 2 and 25 years old. Also, variation in the cranial base angle between males and females, and between blacks and whites was examined. This study was initially conceived as an examination of the spectrum of human variation in the growth and development of the basicranium, as well as its possible correlation to language development. This study was designed to replicate Lieberman and McCarthy’s 1999 examination of the processes of basicranial flexion, with additional consideration of variation by sex and by race. To that end, this study assessed a sample of 39 individuals, composed of 10 black males, 10 black females, 10 white males, and 9 white females. Individuals were drawn from the Krogman Growth Study, a mixed longitudinal and cross-sectional dataset housed at the Penn Museum. A total of 7 cranial base angles were measured, of which 5 were borrowed from Lieberman and McCarthy (designated CBA 1-5), and 2 from Zuckerman (1955) (designated Z1-2), to more thoroughly capture changes in spatial relationships between cranial bones. Results largely indicated that no significant increase or decrease in cranial base angle occurs after the cessation of brain growth at age 2. However, the mean values of 5 out of the 7 cranial base angles were shown to be statistically significantly different by sex, and 3 out of 7 angles revealed statistically significant difference by race. An examination of the Z1-2 angles against CBA 1-5 using regression indicated that, although Zuckerman’s angles did not capture any new variation by sex compared to CBA 1-5, they did reveal an additional spatial relationship which varied by race. Therefore, results confirm Lieberman and McCarthy’s assessment that cranial base flexion does not change significantly past the age of two. In addition, results of this study indicate that cranial base angle is also dimorphic by race, a factor which Lieberman and McCarthy did not assess. No conclusions could be drawn as to the relationship of CBA 5 to language development.