Humans are the only primates with beards – now we finally know why

Humans are the only primates with a beard, so biologists wonder why we acquired this unique trait. According to a new analysis of head anatomy in great apes, it probably didn’t evolve for a special reason of its own, but instead emerged as a side effect of other changes driven by natural selection.

“There has been a tendency to assume that every trait that varies significantly between species has been shaped by natural selection for a specific purpose, but this ‘purposive’ view of evolution is inaccurate,” he says. Noreen von Cramon-Taubadel at the University of Buffalo, New York. “Evolution is often more complicated and less controlled than people expect or assume.”

Simply put, the chin is the bony protrusion of the lower jaw that extends beyond the front teeth. Even among our closest relatives, no other human species has a chin, so it is used as a key identifying feature homo sapiens, but why this trait evolved is a mystery.

Some researchers suggest that it can reduce tension in the front of the jaw during chewing or that it supports our ability to form words. Others believe it evolved as part of sexual selection, with individuals preferring partners with this unique facial feature.

Others question whether the chin has any purpose at all, suspecting that the bony spur may have developed by accident as the skull and jaw underwent further evolutionary changes.

Von Cramon-Taubadel and her colleagues wondered if it might be none of these theories, but rather the result of genetic drift—essentially just a random evolutionary coincidence.

To find out, she and her colleagues examined 532 skulls belonging to humans and 14 other species and subspecies of modern apes—including chimpanzees, bonobos, gorillas, orangutans, and gibbons—housed in museums.

The researchers measured 46 distances between precise anatomical landmarks across the head and jaw—including nine along the region that forms the chin in humans—and mapped the results onto an evolutionary tree.

They then used this data to estimate the likely shape of the head and jaw of the last common ancestor of all great apes. Then they logged in standard quantitative genetic model to test whether the changes along each family branch were larger or smaller than expected from random drift alone.

They found that three of the traits associated with human chins were likely directly selected for — meaning that something about them was favorable enough to shape their evolution. But the other six characters appeared to be either unaffected by selection or simply by-products of evolution for other characters outside of the chin.

As our ancestors became more erect, the base of their skulls sagged and their faces were tucked under the braincase, rather than protruding forward as in chimpanzees, explains von Cramon-Taubadel. Meanwhile, larger brains and dietary changes reduced the need for large front teeth and strong masticatory muscles, shrinking the lower face and jaw. Over time, the bones of the upper jaw receded, so that the lower jaw protruded beyond the teeth – and thus the first chins arose.

This unique trait appears to have emerged as a consequence of the evolution of upright posture, larger heads and smaller teeth, highlighting how selection for one area of ​​the body can impact others, says von Cramon-Taubadel.

For Alessio Veneziano at the French National Museum of Natural History in Paris, the findings point to the beard as a “textbook example” of non-adaptation – a trait that appears without any direct action of natural selection. “It’s always fascinating to me to see confirmation of important evolutionary trends that emerge non-adaptively,” he says.

Evolutionary by-products like this are sometimes called spandrels – a term borrowed from architecture, where it refers to spaces that arise as a result of the shapes of other elements, such as arches. Human navel and small arms Tyrannosaurus rex they were also designed as spandrels.

The study highlights how tightly integrated the skull and jaw are as a single system — so when natural selection modifies one part, other traits can shift along with it, even if they weren’t the original target, he says James DiFrisco at the Francis Crick Institute in London. “Just because an observable feature like a chin looks like a distinct ‘thing’ doesn’t mean it’s actually evolving as an independent entity,” he explains.

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