Humans are the only species with a chin – a feature lacking even in our closest relatives. In fact, it is such a unique anatomical feature that it is one of the main features anthropologists use for identification Homo sapiens remains in the fossil record.
Yet, for such a defining feature, we know surprisingly little about its evolutionary purpose. So why are we the only species with chins?
This question is difficult to answer because experts have not agreed on a single definition of a chin. While some researchers have argued that animals such as elephants and manatees have chin-like protrusions, they are not the same T-shaped structures that protrude beyond our own bottom teeth. As a result, some researchers have moved away from thinking of the chin as a single feature, instead referring to it as the collective result of interactions between many different parts of the head and jaw.
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“So much about the chin is complicated,” said Scott A. Williamsan evolutionary morphologist at New York University. “It cannot be quantified by a single metric, but rather is composed of a constellation of morphological features.”
A better understanding of the function of the chin can, in turn, help researchers create a definition. Experts have suggested several possible purposes for the chin.
Some have suggested that as we developed smaller teeth, it looked like the chin strengthen our lower jaw and prevent our teeth from breaking while we chew. Others believe that the chin may be linked to yet another uniquely human trait – our ability to speak – with the chin providing a anchor point for our tongue muscles. And still others say that the variation in how distinct our chins are gives a hint that it may be related sexual selection.
Instead, it seems that structurally we must have a chin, but not because the chin evolved to have a particular function.
Noreen von Cramon-Taubadel, evolutionary morphologist at the University of Buffalo in New York
Noreen von Cramon-Taubadelan evolutionary morphologist at the University at Buffalo in New York, set out to win this list by determining whether the chin could have evolved by chance or whether development have acted directly on it.
To do so, von Cramon-Taubadel and her team studied dozens of traits associated with head and mandible size, including nine traits associated with the chin. Then, using an evolutionary tree of 15 hominoids – a group that includes humans, their fossil ancestors, gorillas, chimpanzees, orangutans and gibbons – they looked at whether these traits have changed more or less over time compared to random chance. Both results suggest a role for natural selection in the evolution of the lower jaw.
Compared to other species, “the human skull is more different from our ancestors than we would expect given the amount of time that has passed,” she said. However, only three of the nine chin-specific traits appeared to be under direct selection.
Together, the team’s results, published in the journal PLOS Onesuggests that the chin may be what is known as a spandrel – a term borrowed from architecture to describe a feature that is a side effect of something else. Coined by evolutionary biologists Stephen Jay Gould and Richard Lewontin in 1979, the concept of a spandrel was introduced to argue against the view that each function must serve a specific, evolved purpose.
“Instead, it appears that structurally we must have a chin, but not because the chin evolved to have a particular function,” von Cramon-Taubadel told LiveScience. “More and more studies are showing that things that we used to think were terribly important in terms of differences between humans and other apes could actually evolve just by random drift and gene flow.”
Von Cramon-Taubadel said the group’s findings appear to be more strongly influenced by known major landmarks in human evolutionincluding when we started walking upright and grew bigger brains.
Despite these takeaways, von Cramon-Taubadel and Williams agree that the issue is far from settled. For example, it is unknown when traits such as speech first appeared, so it is difficult to link them to chin development. While Williams accepts that the chin may not have evolved for a specific purpose, it does not do so arbitrarily.
“It remains one of the defining characteristics of our lineage that is present in one form or another in all humans living on the planet today,” he said.
