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

Humans are the only primates with a chin, leading biologists to wonder why we acquired this unique feature. It likely did not evolve for a specific reason, but rather arose as a side effect of other changes driven by natural selection, according to a new analysis of ape head anatomy.

“There has been a tendency to assume that every characteristic that differs significantly between species was shaped by natural selection for a specific purpose, but this ‘deliberate’ view of evolution is inaccurate,” says Noreen von Cramon-Taubadel of the University at Buffalo in New York. “Evolution is often more complicated 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, which is why it has been used as a key identifying characteristic of Homo sapiens, but why this trait evolved is a mystery.

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

Still others question whether the chin serves any purpose, suspecting that the bony protrusion might have evolved accidentally while the skull and jaw underwent other evolutionary changes.

Von Cramon-Taubadel and his colleagues wondered whether this might not be due to any of these theories, but rather the result of genetic drift – essentially, evolutionary chance alone.

To find out, she and her colleagues studied 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.

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

They found that three of the traits linked to the human chin were likely directly selected for, meaning that something in them was favorable enough to shape their evolution. But the other six traits seemed either unaffected by selection or simply byproducts of the evolution of other traits other than the chin.

As our ancestors straightened up, the base of their skulls flexed and their faces tucked under the skull instead of projecting forward as is the case in chimpanzees, says von Cramon-Taubadel. Meanwhile, larger brains and dietary changes reduced the need for large front teeth and powerful chewing muscles, shrinking the lower face and jaw. Over time, the upper jaw bones receded, leaving the lower jaw protruding beyond the teeth, giving rise to the first chins.

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

For Alessio Veneziano of the National Museum of Natural History in Paris, the results point to the chin as “a classic 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 that occur in non-adaptive ways,” he says.

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

The study highlights how closely the skull and jaw are integrated as a unified system – so that when natural selection changes one part, other features can shift at the same time, even if they weren’t the original target, says James DiFrisco of the Francis Crick Institute in London. “Just because an observable feature like the chin looks like a separate ‘thing’ doesn’t mean it actually evolves as an independent unit,” he explains.

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