Sagittal Keel

The Sagittal Keel (torus) is a thickening of bone on part or all of the midline of the frontal, or parietals where they meet along the sagittal suture, or on both bones. Sagittal keels occur in Homo ergaster, 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 we do not know why they do. Patrick Stewart of Star Trek is a good example of a modern human (Homo sapiens) that has this feature.

References

"Introduction to Physical Anthropology" Jurmain, Kilgore, Trevathan 10th Edition ISBN#0-534-64422-8