Sphenoid shortening and the evolution of modern human cranial shape.

Crania of 'anatomically modern' Homo sapiens from the Holocene and Upper Pleistocene epochs differ from those of other Homo taxa, including Neanderthals, by only a few features. These include a globular braincase, a vertical forehead, a dimunitive browridge, a canine fossa and a pronounced chin. Humans are also unique among mammals in lacking facial projection: the face of the adult H. sapiens lies almost entirely beneath the anterior cranial fossa, whereas the face in all other adult mammals, including Neanderthals, projects to some extent in front of the braincase. Here I use radiographs and computed tomography to show that many of these unique human features stem partly from a single, ontogenetically early reduction in the length of the sphenoid, the central bone of the cranial base from which the face grows forward. Sphenoid reduction, through its effects on facial projection and cranial shape, may account for the apparently rapid evolution of modern human cranial form, and suggests that Neanderthals and other archaic Homo should be excluded from H. sapiens.