The question of robusticity and the relationship between cranial size and shape inHomo sapiens

Abstract

Although cranial gracility is generally considered to be a characteristic feature of modern skulls, a quantification of the degree of development of cranial superstructures discloses varying levels of robusticity in certain recent and sub-recent populations. In order to account for these structures, biomechanical interpretations in terms of masticatory stress and the effects of structural constraints have been put forward. This paper examines the multivariate relationships between robusticity and cranial dimensions in modern humans through canonical correlation and principal components analyses. These analyses reveal a very significant association between robusticity and cranial size, by which the larger the size of the skull, the greater the development of the cranial superstructures. In this context, size is biased towards maximum cranial length, various measures of upper facial breadth, basion–nasion length and palato–dental size. A statistically less important relationship between robusticity and cranial shape reveals that there are two main circumstances in which a modern skull will develop superstructures, in either very large crania, independent of the breadth of the vault, or in very narrow ones with large teeth. The cranial superstructures are not only strongly influenced by certain cranial dimensions, but are inter-correlated in their expression. These morphological features should not be treated as independent phylogenetic traits. The majority of recent crania are relatively small and gracile, clearly the result of a common temporal trend in the evolution of modern human cranial diversity. Although not unique in their level of robusticity among recent human groups, Australian aboriginals are the only population to show a relationship between robusticity and cranial breadth. It is suggested that the total morphological pattern observed in Australia does not reflect a plesio-morphic condition, but the specific history of occupation of that continent favouring size reduction with the maintenance of robusticity. Finally, it is shown that most early modern fossils are comparatively large and robust. It is suggested that the definition of a modern cranial form should be disassociated from the concept of gracility, only observed in some recent human populations.