What are advantages of eyebrows?

Eyebrows, we all have them, but what are they actually for? While eyebrows help to prevent debris, sweat, and water from falling into the eye socket, they serve another important function too – and it’s all to do with how they move and human connection.

We already know that our modern minds often reflect the ways our ancestors needed to work together to survive in the distant evolutionary past. But it seems our anatomy reflects the importance of getting on with other people as well. As our new research published in Nature Ecology and Evolution suggests, the ability to look either intimidating or friendly is reflected in our bones – at least where the shape of the skulls is concerned.

We all know that ancient species of humans, such as Neanderthals, looked a little bit different from us. But the most obvious difference is that archaic humans possessed a pronounced and very distinctive brow ridge which contrasts with our own flat and vertical foreheads. And for scientists, this difference between us and them has been the hardest to explain. It was even famously said that Neanderthals would go unnoticed on a New York subway if only they could wear something like a hat to cover this distinctive feature.

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But our latest research may have found an answer to explain why archaic humans had such a pronounced wedge of bone over their eyes (and why modern humans don’t). And it seems to be down to the fact that our highly movable eyebrows can be used to express a wide range of subtle emotions – which could have played a crucial role in human survival.

A sign of dominance

Research has already shown that humans today unconsciously raise their eyebrows briefly when they see someone at a distance to show we are not a threat. And we also lift our eyebrows to show sympathy with others – a tendency noticed by Darwin in the 19th century.

So with my colleagues Ricardo Godinho and Paul O'Higgins we looked at the iconic brow ridge of a fossilised skull (known as Kabwe 1) to find out more about its purpose. Ricardo used 3D engineering software, to shave back Kabwe’s huge brow ridge. And in doing so, found that Kabwe 1’s heavy brow offered no spatial advantage.

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The brow ridges in archaic humans also serve no obvious function in relation to chewing or other practical mechanics – a theory commonly put forward to explain protruding brow ridges. As when the ridge was taken away there was no effect on the rest of the face when biting. This means that brow ridges in archaic humans must have had a social function – most likely used to display social dominance as is seen in other primates.

For our species losing the brow ridge probably meant looking less intimidating, but by developing flatter and more vertical foreheads our species could do something very unusual – move our eyebrows in all kinds of subtle and important ways.

Although the loss of the brow ridge may have initially been driven by changes in our brain or facial reduction, it subsequently allowed our eyebrows to make many different subtle and friendly gestures to people around us.

Expressing emotions

Historically speaking, these marked changes in the face occurred at a time when the emergence of important social changes began to take place. Mainly the collaboration between distantly related groups of humans.

This was a time when modern human groups began to exchange gifts across large regions. Being able to create distant friendships probably helped early humans to colonise new environments – as they had friends they could rely on and retreat back to.

Modern humans also lived in larger and more diverse groups than previous species reducing interbreeding. So the impact of friendly and mutually supportive relationships with people outside one’s own group were far reaching. And the development of mobile eyebrows may have been a key part of all these changes.

But these changes weren’t just exclusive to humans – the developments seen when wolves became domesticated are in some ways similar. Dogs have more waggy tails and flatter faces than wolves. And dogs who are better able to look cuter by raising their brows are more likely to be selected from shelters.

It seems then that for humans (and dogs), being able to get along with others was key to survival. And for our ancestors, the evolution of the eyebrows performed an important function in expressing friendliness. All of which forms part of a process of “self-domestication” – where our human brains, bodies and even anatomy reflects a drive to get on better with those around us.

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We humans seem to have an on-again, off-again relationship with facial hair.

Prehistoric cave drawings reveal the myriad tools our ancient ancestors used to shave: shark’s teeth, sharpened flints and even clam shells. Nowadays, beards are back in style and people are taking a razor to their brows, instead.

But is there a reason we evolved to have these hairy baubles in the first place? And, if so, what evolutionary advantage might we be throwing away for the sake of staying on trend? 

Turns out, researchers have a lot to say on the matter. Get their answers to why we have eyebrows, what eyelashes are for and why we grow beards.

Why Do We Have Eyebrows?

Let’s start with those fuzzy caterpillars at the top of the face.

Eyebrows do a great job of preventing moisture like rain and sweat from running into our eyes. Everything from the angle at which these hairs grow to the shape of the brow’s arch are designed to direct moisture away, to the side of the face.

Of course, while protecting our peepers may have been their original purpose, eyebrows found themselves playing a secondary role somewhere along the line: conveying emotion.

What Are Eyebrows For?

A 2018 study published in Nature Ecology & Evolution parsed why early hominins’ brow ridges were so much larger and more rigid than modern humans’ ridges. They found that — contrary to some prevailing theories that the ridges structurally reinforced the skull or aided in biting mechanics — having more mobile eyebrows likely helped our ancestors to form relationships and ensure survival in groups.

Read More: What Did Ancient Humans Look Like?

Eyebrow hair, the researchers say, simply increased the visibility of this form of communication.

Funnily enough, evolutionary psychologists say dog eyebrows underwent a similar journey; centuries of domestication “transformed the facial muscle anatomy of dogs specifically for facial communication with humans,” write the authors of a 2019 study published in PNAS.

As if we needed yet another reason to love our canine companions.

What Are Eyelashes For?

The same goes for eyelashes, which have the added task of blocking solid particles like dust. And, according to a 2019 study that looked at lashes from a chemical engineering perspective, they may even help prevent dry eyes by limiting unwanted evaporation.

For those who are curious, the research team determined the “optimal eyelash length” to be around 15 to 30 percent of the eye’s width — resulting in an up to 30 percent reduction in evaporation. Time to throw away those eye drops and get out the measuring tape, perhaps?

When it comes to the lower half of the face, theories abound.

Why Do We Grow Beards?

In his Descent of Man, Charles Darwin suggested a reason for why we grow beards. He said that beards were an example of sexual selection and may have evolved “to charm or excite the opposite sex” — while also intimidating the competition. Researchers tend to think the same, for example, of lion’s manes, which may signal to other lions that the mane-bearer is in good health and a formidable opponent. 

Read More: It's Rare, But a Lioness Can Grow a Mane

But does this theory hold up in the modern age? 

A 2019 study published in Psychological Science suggests it’s possible; the researchers found that “the presence of a beard increased the speed and accuracy with which participants recognized displays of anger.” The same could not be said of displays of happiness or sadness, emotions which definitely do not serve to intimidate.

Are Beards For Protection?

But don’t assume that this means beards are all bark and no bite. Research recently published in Integrative Organismal Biology proposes that a fuzzy face protects vulnerable regions of the jaw from damaging strikes during combat. 

By crafting a fiber epoxy composite to serve as a faux bone and covering it with thick, wooly skin dissected from sheep, the researchers found that this model absorbed 37 percent more energy from a blunt impact than utterly hairless samples. Furthermore, the furred models “failed” — in other words, broke a bone — only around half the time, compared to nearly 100 percent of hair-free composites.

(They note that results may vary, however, depending on just how thick a beard can get.)

Now that you know why we have eyebrows, what eyelashes are for and why we grow beards, you may be wondering why we lose the hair on our heads.

Read More: Why Do Humans Go Bald?