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Eyebrows, the strip of thick, delicate hairs following the brow ridge above the eye, are an absolute difference between humans and non-human primates. Simply put, humans have little facial hair coverage, which makes the strip of hair on the brow ridge remarkable, particularly as it exists across sexes. Other monkeys do have increased musculature around the brow region, indicating increased brow control over other mammals, and some (notably some species of guenons) have differing coloration on the hair on their brow, both which may functionally mimic human eyebrows (Emery 2000). Primates seemed to have developed strong brows and musculature, but human development isolated the eyebrows themselves, losing most other facial hair. It appears to be unknown when eyebrows themselves developed.
The most common explanation, dating from the Greek physician Herophilos, is that eyebrows prevent sweat and debris from coming into the eye, something which is necessary in humans as there is no other hair covering the face, nor an adequately protruding brow. However, other explanations are multitude. Another common explanation for the human uniqueness of eyebrows is to communicate emotion. Desmond Morris, among many others (including Charles Darwin) posits that eyebrows evolved to convey a wide range of emotion from surprise to sadness (Morris 2008). The face, particularly gazing at another’s eyes, is a particularly salient emotional feature in humans, and eyebrows offer a way to communicate complex emotional states, in conjunction with other features. Other animals, including well documented cases of both old and new world monkeys and other mammals, have been observed to manipulate their brows as part of their communicative displays (Andrews 1964), including brow raising as a threat, and brow lowering as a greeting. The human eyebrow, with its contrasting hair, accentuates the movement of the brow, making movement more salient (such would also be the case of the guenons orange brow). Eyebrows also draw attention to the eye, a key expresser of emotion and communication, and may help with gaze-following (Emery 2000).
Eyebrows have also been hypothesized to be key to facial recognition. A significant drop in facial recognition is seen when eyebrows are removed [one study found a drop from people recognizing 60% of intact celebrity faces to 40% of those with removed eyebrows (Sadrô et al 2003)]. This effect was found to be stronger than when eyes were removed from the picture, and eyebrows remained intact. As humans have had to deal with progressively larger social groups, any cue to identifying individuals is helpful, and eyebrows seem to be particularly salient.
Overall, it appears that eyebrows are uniquely human, having much to do with human’s general hairlessness and perhaps an extension of other primates’ strong brow musculature. In humans, they serve a variety of functions, ranging from the physically practical (blocking sweat) to the social (recognition, emotional expression).
The Naked Man: A Study of the Male Body, , London, p.288, (2008)
The role of eyebrows in face recognition., , Perception, 2003, Volume 32, Issue 3, p.285-93, (2003)
The eyes have it: the neuroethology, function and evolution of social gaze., , Neurosci Biobehav Rev, 2000 Aug, Volume 24, Issue 6, p.581-604, (2000)
The Displays of the Primates, , Evolutionary and Genetic Biology of Primates, p.227 - 309, (1964)