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

Humans are the only primates with chins, which leaves biologists wondering why we got this unique trait. According to a new analysis of head anatomy in apes, it likely did not evolve for a specific reason, but instead emerged as a side effect of other changes driven by natural selection.

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

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

Some researchers suggest that it may reduce the load on 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 mates with this unique facial feature.

Still others question whether the chin has any purpose at all, and suspect that the bony protrusion may have developed randomly as the skull and jaw went through other evolutionary changes.

Von Cramon-Taubadel and her colleagues wondered if it might not be either of these theories, but rather the result of genetic drift—essentially, random evolutionary chance alone.

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 head and jaw shape of the last common ancestor of all apes. They then used a standard quantitative genetic model to test whether changes along each family branch were larger or smaller than expected under random drift alone.

They found that three of the human chin-related traits were likely directly selected—meaning that something about them was favorable enough to shape their evolution. But the other six traits appeared to be either unaffected by selection or merely byproducts of evolution for other non-chin traits.

As our ancestors became more erect, the scalp of their skulls bowed, and their faces tucked under the braincase instead of projecting forward as it does in chimpanzees, explains von Cramon-Taubadel. Meanwhile, larger brains and dietary shifts reduced the need for large incisors and powerful masticatory muscles, shrinking the lower face and jaw. Over time, the upper jaw bones receded, so that the lower jaw protruded above the teeth – which gave rise to the first chins.

As such, this unique trait appears to have emerged as a consequence of humans evolving an upright posture, larger heads and smaller teeth, highlighting how choices for one region of the body can have a knock-on effect on others, says von Cramon-Taubadel.

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

Evolutionary byproducts like this are sometimes called spandrels—a term borrowed from architecture, where it refers to spaces that arise as a consequence of the shapes of other features such as arches. The human navel and handgun to Tyrannosaurus rex have also been suggested to be spandlers.

The study highlights how closely integrated the skull and jaw are as a unified system – so that when natural selection adapts one part, other functions can shift along with it, even if they were not the original target, says James DiFrisco of the Francis Crick Institute in London. “Just because an observable feature like the chin looks like a distinct ‘thing’ doesn’t mean it actually develops as an independent entity,” he explains.

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