The supraorbital ridge, or brow ridge, refer to a bony ridge located above the eye sockets of all primates. In Homo sapiens sapiens (modern man) the eyebrows are located on their lower margin.
Other terms in use are:
- supraorbital arch
- supraorbital torus
- superciliary ridge
- arcus superciliaris (Latin, meaning "superciliary arch")
- supraorbital margin and the margin of the orbit
The size of these ridges varies also between different species of Primate, either living or fossil. The closest living relatives of man, the Great Apes, have relatively pronounced supraorbital ridges, while in modern humans it is relatively reduced. The fossil record indicates that the supraorbital ridge in early homo was reduced as the cranial vault grew and became positioned vertically, above the face.
Some paleoanthropologists distinguish between torus and ridge. In anatomy, a torus is a projecting shelf of bone. Fossil hominids, in this theory, have the torus, but modern humans only have the ridge.
The brow ridge is a thick piece of bone on top of the eyes. Its purpose is to reinforce the weaker bones of the face in much the same way that the chin of modern humans was developed to reinforce their comparatively thin mandibles. This was necessary in pongids and early hominids because of the tremendous strain put on the cranium by their tremendous chewing apparatuses, which is best demonstrated by any of the members of the genus Paranthropus
. The brow ridge was one of the last traits to be lost in the path to modern humans, and only disappeared with the development of the modern pronounced frontal lobe. This is one of the most salient differences between Homo sapiens sapiens
and Homo sapiens neanderthalensis
The name for this theory is the Bio-mechanical model for brow ridge formation
The folk-myth that the size of the ridges is a mark of the degree of development of rationality has no basis in fact. There is no link between the size of the ridges and any other anatomical trait, including intelligence, in modern humans.
In modern humans
Forensic anthropologist Caroline Wilkenson says that Australoids have the largest brow ridges "with moderate to large supraorbital arches
". Caucasoids have the second largest brow ridges with "moderate supraorbital ridges
". Negroids have the third largest brow ridges with an "undulating supraorbital ridge
". Mongoloids are "absent browridges
", so they have the smallest brow ridges.