If the eyes are the windows to the soul, the eyebrows must surely be the frames, shaping the face and giving definition to the eyes and forehead. Many women spend a great deal of time maintaining the appearance of the eyebrows, via plucking and shaping, cosmetics, or even surgical implants. Historically, fashionable trends revolved around thickness, hue, and the shape of the arch; and there is no doubt that eyebrows have always contributed significantly to the perception of facial attractiveness.
However, compared with other primates, modern humans are relatively hairless, so the persistence of the eyebrow over time is curious.1 Do eyebrows merely represent evolutionary vestiges, or do they serve a greater, or more functional, purpose? Indeed, as one of the most powerful and versatile features of the human face, the eyebrow informs the perception of beauty and plays a critical role in sexual dimorphism, facial recognition, and non-verbal communication.
Muscular Anatomy of the Brow
The musculature of the brow has received intense study with the advent of brow treatments with neuromodulators and fillers. Three musclesâ€”the corrugator supercilii, procerus, and depressor superciliiâ€”work together to cause the head of the brow to rotate medially and descend in the frown.2 The frontalis, the primary elevator muscle, raises the forehead and eyebrows medially and can elevate the eyelid as high as 5 mm at maximal action.3 (Figure 1).
From the age of 25 in women and 45 in men, there is a progressive resorption of bone and fat in the periocular and perioral regions.4 For the first time in history, brow position can be affected by voluntary medical injection treatments,5 a trend that is summarized by the 4900.1% increase in injectable treatments seen from 1997 to 2012.6 (Figures 2a and 2b.)
Evolution of the Brow
The supraorbital ridge, also called the supraorbital arch or supraciliary ridge, lies above the eye sockets and is common to all primates, including humans, although the size of the ridge varies between species.7 Scientists refer to a more prominent ridge as the supraorbital torus, which presents as a continuous shelf of bone that projects above the orbits and nose and can be seen in our closest living relatives (the gorilla and chimpanzee), as well as in most fossil hominids (our ancestors). With the expansion of the cortex and development of the frontal lobe, the prominent brow gave way to the high, straight forehead of modern Homo sapiens, leaving behind only 2 hairy reminders of our evolutionary past along the lower margins of the ridge. What purpose do they serve?
Researchers believe the primary function of the eyebrow to be one of protection, the slope of the brow and arch drawing moisture or other debris around to the sides of the face. Because of their position and curvature, the eyebrows act as a shield against direct bright light, and stimulation of the hairsâ€”abundantly innervated and sensitive to tactile stimulationâ€”causes a reflex blinking of both eyelids.8 But why did