Morphology of the Brow Ridge

Certainty Style Key
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True   Likely   Speculative
Human Uniqueness Compared to "Great Apes": 
No Difference
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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.

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Timing of appearance of the difference in the Hominin Lineage as a defined date or a lineage separation event. The point in time associated with lineage separation events may change in the future as the scientific community agrees upon better time estimates. Lineage separation events are currently defined as:

  • the Last Common Ancestor (LCA) of humans and old world monkeys was 25000 thousand (25 million) years ago
  • the Last Common Ancestor (LCA) of humans and chimpanzees was 6000 thousand (6 million) years ago
  • the emergence of Homo ergaster was 2000 thousand (2 million) years ago
  • the Last Common Ancestor (LCA) of humans and neanderthals was 400 thousand years ago
  • the common ancestor of modern humans was 100 thousand years ago

Possible Appearance: 
100 Thousand Years
Probable Appearance: 
100 Thousand Years
Definite Appearance: 
100 Thousand Years
Background Information: 

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.


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