Evolutionary insights into global patterns of human cranial diversity: population history, climatic and dietary effects.

The study of cranial variation has a long, and somewhat difficult, history within anthropology. Much of this difficulty is rooted in the historical use of craniometric data to justify essentialist typological racial classification schemes. In the post-war era of the "New Physical Anthropology" (sensu Washburn, 1951), anthropologists began to analyse human variation in an explicitly populationist and evolutionary philosophical and analytical framework. However, even within recent decades, substantially different approaches have been employed; some advocate a focus on the analysis of individual traits or clines, while others are explicitly adaptationist, with a focus on natural selection as the preeminent force of phenotypic diversification. In recent years, a series of studies have analysed craniometric data in an explicitly quantitative genetic framework, which emphasises the importance of neutral forces such as migration, gene flow and genetic drift in creating global patterns of phenotypic diversity. This approach has revealed that global patterns of cranial variation can largely be explained on the basis of neutral theory. Therefore, human cranial data can be productively employed as a proxy for neutral genetic data in archaeological contexts. Moreover, there is a growing recognition that regions of the cranium differ in the extent to which they fit a neutral model of microevolutionary expectation, allowing for a more detailed assessment of patterns of adaptation and phenotypic plasticity within the human skull. Taking an historical perspective, the current state of knowledge regarding patterns of cranial adaptation in response to climatic and dietary effects is reviewed. Further insights will be gained by better incorporating the study of cranial and postcranial variation, as well as understanding the impact of neutral versus non-neutral evolution in creating among-species diversity patterns in primates more generally. However, this will most effectively be achieved when comparative anatomy studies are situated within an explicitly quantitative genetic evolutionary framework.