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Morphology of the Brow Ridge
Considerable variation exists between hominoid species in the morphology of the supraorbital region. Gorillas and chimpanzees (and most fossil hominins) possess a prominent supraorbital torus, or brow ridge, presenting as a continuous projecting ridge above the orbits and nose (although continuous, the torus is anatomically divisible into three regions: laterally positioned supraorbital trigones, medially positioned supercillary arches, and a midline glabellar prominence). In these species with prominent brow ridges, a supratoral sulcus is generally present as a shallow groove just posterior to the torus. Humans and orangutans lack prominent brow ridges. Brow ridges may develop as an architectural or biomechanical by-product of hafting a prognathic (projecting) face onto the low frontal bone characteristic of apes and earlier humans, such that the lack of a brow ridge in modern humans is a consequence of their having an orthognathic (vertical) face and vertical frontal (high forehead). Orangutans possess a supraorbital rim (a thin, non-projecting ridge across the orbits) rather than a torus, which may be a function of the airorynchy (backwards rotation of the face towards the neurocranium) that characterizes these apes.
Lieberman, 1995. Testing hypotheses about recent human evolution from skulls. Curr Anthropol 36:159-197. Lieberman, 2000. Ontogeny, homology, and phylogeny in the hominid craniofacial skeleton: The problem of the browridge. In O'Higgins & Cohn (eds.) Development, Growth and Evolution. London, Academic Press. pp. 85-122. Lieberman, 2008. Speculations about the selective basis for modern craniofacial form. Ev Anthropol 17:55-68.