Why Do We Have Eyebrows and Other Types of Facial Hair?

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.

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.

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.

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.

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.

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?