The Sagittal keel (torus) is a thickening of bone on part or all of the midline of the frontal bone, or parietal bones where they meet along the sagittal suture, or on both bones. Sagittal keels occur in Homo erectus and occasionally Homo heidelbergensis, where they probably served as an armour against shock to the roof of the skull, and as the attachment point for the temporalis muscles. Most Homo sapiens lost them likely as part of the general trend toward thinning of the cranial bones during the Pleistocene, to make room for larger brains. However there is a very small portion of modern humans who have this, but its function and etiology are unknown. Patrick Stewart of Star Trek: The Next Generation and the martial artist Shi Yan Ming present good examples of modern humans (Homo sapiens sapiens) with this feature.
Jurmain, Kilgore, Trevathan. Introduction to Physical Anthropology. 10th Edition ISBN#0-534-64422-8