|MadSci Network: Evolution|
The supraorbital torus, or browridge, is variably present in several species of primates, and is very prominently displayed in some apes and Pleistocene hominids. It is essentially absent in orangutans, and is very weakly developed in most early hominids, such as the australopithecines. This pattern does not appear to have any obvious relationship to some need for shade or protection from sweat (which is pronounced only in humans), and the idea that this is the role that the browridge plays is simply not accepted within physical anthropology any longer. It never was very seriously considered, in fact. A more common idea has been that the browridge serves as a boney buttress in the upper face to assist in resisting the stresses produced during chewing, but this idea is also out of favor (experimental work on macaques using strain gages to measure the strains in the supraorbital region during feeding showed that the structure simple isn't stressed very much).
The prevailing hypothesis currently is that browridge presence is related to the spatial positioning of the eye orbits and the brain case. When the orbits are positioned far forward of the frontal lobe of the brain, there is a shelf of bone covering them that probably serves to protect the orbit contents (the eyes) from blows. When the orbit is tucked in under the brain, however, there is no need for this shelf (the brain and brain case protect the orbits), and we see none. This hypothesis is consistent with measurements of orbit and brain positioning in old world monkeys, but has not been examined beyond this group.
So, eyebrows and browridges are not functionally equivalent, and the prominence of hairy eyebrows in living humans is therefore unlikely to be a solution to some problem posed by the loss of browridges. As for the adaptive significance of eyebrows, it is reasonable to hypothesize that they protect the eyes from glare and the excessive sweat modern humans produce. But this is only an hypothesis until it is tested in some way. The idea that individuals who have sparse eyebrows are in some way displaying the ancestral condition is unlikey – most traits in humans display a range of variation.
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