Why-brows?

 
Eyebrows are almost uniquely human!  Image credit: samhaseyebrows

Eyebrows are almost uniquely human!  Image credit: samhaseyebrows

 
 

Crazy Pinterest hairstyles are taking over the internet, but what about the hair that doesn’t grow on your head?  In our weekly Wax on/Wax off column we cover shaving, waxing, plucking, tweezing and just letting it grow, as well as what to do about troubles like razor bumps, ingrown hairs and eyebrows gone awry. 

Most mammals have hair all over the place.  Falling just behind whales, dolphins, and the naked mole rat, humans almost lead the competition for least hairy mammals on planet earth.  Yet, of all the mammalian species out there - most of them far more hirsute than us - only humans have weird little clumps of hair just above our eyes.  Over the centuries we’ve shaved, plucked, styled and shaped them, but why do we have them in the first place?  What on earth are our eyebrows actually for?

Eyebrows play a huge role in allowing us to recognise facial expressions.  Image credit: stocknesss

Unsurprisingly, there’s no simple answer.  Physiologists, evolutionary biologists and other scientists have been wondering for years just why humans evolved eyebrows.  One popular theory suggests that the hair helps wick moisture and sweat away from the eyes, which would be a real evolutionary advantage during rainfall or when running from predators.  The way eyebrows typically slant downwards, with the hair growing outward from the bridge of the nose, makes them ideally shaped for diverting sweat from the forehead down the sides of the face, and away from our sensitive eyes.  Our eyebrows might also help out the brow ridge in shading the eyes from bright light, making it easier to see when the sun is out.

Owl butterflies have eye-like markings on their wings to deter predators.  Our eyebrows may serve a similar function.  Image credit: Böhringer Friedrich

Another theory points to eyebrows as a surprising evolutionary tool to ward off predators.  Back when modern humans first evolved, what propelled them straight to the top of the food chain was their creativity and ability to use tools: advantages that disappeared entirely when an early human fell asleep!  Predators recognised this, and the most dangerous time to be an early human was at night, when predators recognised closed eyes as a sign of an easy and helpless target.

Certain scientists believe that the eyebrows actually evolved to help define the eye area, so in dim light or darkness a human might appear to have his eyes open even when he was asleep.  The dark brows would have created an eye-shaped area of contrast, effectively signposting “eyes are here” - and making potential predators think their prey was awake and watching them.  Today, in some rural areas of Africa and India, locals will wear masks on the back of their heads when they venture outside their villages, because just the suggestion that their prey might be watching them is enough to persuade big cats that it isn’t worth attacking.  Just like the false eyes on the wings of an owl butterfly, eyebrows might be evolution’s attempt to persuade predators we’re paying more attention than we really are!

Now that we’ve mostly moved out of the jungle and into cities, eyebrows serve a different, but no less important purpose.  An MIT study a few years ago showed that eyebrows are hugely important to facial recognition - more important even than the eyes themselves!  Study participants were shown pictures of celebrities with their eyes blanked out, and then with their eyebrows removed; the eyeless celebrities were recognised an astonishing 30% more consistently than the celebrities without eyebrows.  Eyebrows help people - particularly infants - recognise sexual dimorphism, and they are the single clearest indicator of a person’s mood.  Poetry might tell us the eyes are the windows to the soul, but science says the eyebrows have it!

Without her distinctive dark brows, even a beauty like Jennifer Connelly is barely recognisable!  Image credit: celebswithnoeyebrows