Expressive Eyebrow Wiggling Serves a Very Important Evolutionary Purpose

There's a good reason our arches are so mobile.


Human eyebrows can be mobile to a fault: A raised brow might signify interest, even flirtation; but too high an arch might suggest skepticism or incredulity. Their expressiveness might be their entire reason for being, argue scientists in a paper published Monday. In their research on why humans have movable brows while other hominins had thick forehead ridges, they argue that our dancing brows made communication possible when language wasn’t available.

The existence of our mobile eyebrows and smooth foreheads, the University of York team write in their study in Nature Ecology and Evolution, is unique among humans. All of our hominin and ape relatives, like Neanderthals and gorillas, have a protruding brow ridge in its place — hence the insulting misnomer ‘Cro-Magnon forehead’. For a long time, scientists have tried to figure out what role that brow ridge and its more delicate human counterpart play in the body, arguing that it has a physical role in protecting the head. But using computer modeling to manipulate ancient hominin skulls, the researchers throw out those theories in favor of communication-focused explanation.

Mandrills, like many other monkeys and apes, have a protruding brow ridge, whereas humans usually have a smoother forehead and mobile eyebrows.

Flickr / Jim Bauer

The two major theories explaining the protruding brow ridge contend that it either fills the gap between the brain case and eye sockets or that it stabilizes the head when the jaw clenches. Using computer modeling to manipulate the 3D skull of Kabwe, a fossilized Homo heidelbergensis skull dating back some 200,000 to 600,000 years, they show that Kabwe’s eye sockets and jaw would’ve been just fine if he’d had a more delicate brow, ruling out physical explanations for the protruding forehead.

An alternative explanation, they argue, is that that those big foreheads were used for communication, and our more delicate, movable ones simply represent improvement in that regard. The brows of apes and monkeys might be akin to antlers on a deer, signaling social status; the crude communication these brows afforded eventually gave way to the nuanced emotions our dancing arches convey. “[Jutting brows]’ conversion to a more vertical brow in modern humans allowed for the display of friendlier emotions which helped form social bonds between individuals,” said senior author Paul O’Higgins, Ph.D. in a statement on Monday.

Our jutting forehead ridges may be less prominent now, but if the eyebrow beauty industry is any indication, we’re doing whatever it takes to make what’s left of them as prominent as possible. Nodding to ‘Her Eyebrowness’ Cara Delevigne, the Guardian reported in 2016 that the eyebrow industry was worth some $28 million in the United Kingdom alone. Eyebrow trends, like eyebrows themselves, wax and wane, but the importance of our arches is timeless: As MIT researchers pointed out in 2003, they’re one of the most important features in face recognition.

Though our brows and their communication skills are much more sophisticated than those of our ancient humans, that doesn’t mean they should be wiggled wantonly. Nobody wants to come across like this guy.

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