Activity, climate, and postcranial robusticity: implications for modern human origins and scenarios of adaptive change.

Postcranial robusticity--the massiveness of the skeleton--figures prominently in the debate over the origin of modern humans. Anthropologists use postcranial robusticity to infer the activity levels of prehistoric populations, and changes in robusticity are often used to support scenarios of adaptive change. These scenarios explain differences in morphology as the result of a change in lifestyle (habitual activity). One common scenario posits that early modern humans were more gracile than Neandertals because the modern humans' complex culture required less physical exertion. However, lifestyle is only one of many influences on morphology. Climate has clear correlations with physique and skeletal proportions. Analysis of recent humans that differ in terms of lifestyle and climatic adaptations reveals that limb bone robusticity varies with climate as much as or more than with lifestyle. Many of the differences in robusticity between Neandertals and early modern humans appear to be related to climatic adaptations. The results support the single-recent origin model of modern human origins. The differences in robusticity between Neandertals and early modern humans suggest that population replacement rather than local evolution best explains the emergence of modern humans in Europe. Both climatic adaptations (primarily body proportions) and lifestyle should be considered in analyses of robusticity.