The Sagittal keel is a thickening of bone on part or all of the midline of the frontal

Frontal bone

The frontal bone is a bone in the human skull that resembles a cockleshell in form, and consists of two portions:* a vertical portion, the squama frontalis, corresponding with the region of the forehead....

, or parietal

Parietal bone

The parietal bones are bones in the human skull which, when joined together, form the sides and roof of the cranium. Each bone is roughly quadrilateral in form, and has two surfaces, four borders, and four angles. It is named from the Latin pariet-, wall....

s where they meet along the sagittal suture

Sagittal suture

The sagittal suture is a dense, fibrous connective tissue joint between the two parietal bones of the skull. The term is derived from the Latin word Sagitta, meaning "arrow". The derivation of this term may be demonstrated by observing how the sagittal suture is notched posteriorly, like an arrow,...

, or on both bones. Sagittal keels occur in Homo erectus

Homo erectus

Homo erectus is an extinct species of hominid that lived from the end of the Pliocene epoch to the later Pleistocene, about . The species originated in Africa and spread as far as India, China and Java. There is still disagreement on the subject of the classification, ancestry, and progeny of H...

and occasionally Homo heidelbergensis

Homo heidelbergensis

Homo heidelbergensis is an extinct species of the genus Homo which may be the direct ancestor of both Homo neanderthalensis in Europe and Homo sapiens. The best evidence found for these hominins date between 600,000 and 400,000 years ago. H...

, 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

Pleistocene

The Pleistocene is the epoch from 2,588,000 to 11,700 years BP that spans the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....

, 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

Patrick Stewart

Sir Patrick Hewes Stewart, OBE is an English film, television and stage actor, who has had a distinguished career in theatre and television for around half a century...

 of Star Trek

Star Trek

Star Trek is an American science fiction entertainment franchise created by Gene Roddenberry. The core of Star Trek is its six television series: The Original Series, The Animated Series, The Next Generation, Deep Space Nine, Voyager, and Enterprise...

and the martial artist Shi Yan Ming

Shi Yan Ming

Shi Yan Ming is a 34th generation Shaolin warrior monk, teacher and actor, best known as the founder of USA Shaolin Temple...

 present good examples of modern humans (Homo sapiens sapiens) with this feature.

References

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