Humans are the only species with jaws, a trait not present even in our closest relatives. In fact, it’s an anatomical feature so unique that it’s one of the main features anthropologists use to identify Homo sapiens in the fossil record.
However, despite these defining characteristics, surprisingly little is known about their evolutionary purpose. So why are we the only species with jaws?
This question is difficult to answer because experts do not agree on a single definition of chin. Some researchers claim that animals such as elephants and manatees have jaw-like projections, but they are not the same as the T-shaped structures that protrude beyond our bottom teeth. As a result, some scientists have stopped thinking of the jaw as a single feature and have instead begun to refer to it as the collective result of interactions between different parts of the head and jaw.
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“The jaw is very complex,” says Scott A. Williams, an evolutionary morphologist at New York University. “It cannot be quantified by a single metric, but rather consists of a collection of morphological features.”
A deeper understanding of jaw function could help scientists create a definition. Experts have suggested several possible purposes for the jaw.
Some argue that as we evolved and our teeth became smaller, our jaws strengthened our lower jaw and prevented our teeth from breaking when we chewed. The jaw also serves as an anchor point for the muscles of the tongue, leading some to believe that it may be connected to yet another unique human trait: the ability to speak. Additionally, some say the difference in the way our jaws stick out offers a hint that it may be related to sexual selection.
Rather, it appears that jaws are required structurally, but not because jaws evolved to have a specific function.
Noreen von Cramon-Taubadel, evolutionary morphologist at the University at Buffalo, New York
Nolen von Cramon-Taubadel, an evolutionary morphologist at the University at Buffalo in New York, set out to sift through that list to determine whether jaws evolved by chance or whether evolution directly influenced jaws.
To do so, von Cramon-Taubadel and her team studied dozens of features related to head and lower jaw size, including nine features related to the jaw. They then used the evolutionary tree of 15 hominids (a group that includes humans, their fossilized ancestors, gorillas, chimpanzees, orangutans, and gibbons) to examine whether their traits changed more or less over time, compared to random chance. Both results would suggest a role for natural selection in the evolution of the mandible.
Compared to other species, “human skulls are more different from those of our ancestors than you would expect, given the amount of time that has passed,” she says. However, only three of the nine jaw-specific traits appear to have been directly selected for.
Taken together, the team’s results, published in the journal PLOS One, suggest that the jaw may be a so-called spandrel. Spandrel is a term borrowed from architecture to describe a feature that is a side effect of something else. The spandrel concept, developed by evolutionary biologists Stephen Jay Gould and Richard Lewontin in 1979, was introduced to refute the view that all traits must serve a specific evolved purpose.
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“Rather, it seems that we need a jaw structurally, but not because the jaw evolved to have a specific function,” von Cramon-Taubadel told Live Science. “More and more studies are showing that what we thought was so important in terms of differences between humans and other apes can actually evolve just by random drift and gene flow.”
Professor von Cramon-Thorbadel said the group’s findings appeared to be strongly influenced by major known landmarks in human evolution, such as when humans first started walking upright and when our brains grew larger.
Despite these conclusions, von Cramon-Thobadell and Williams agree that the problem is far from resolved. For example, it is unclear when traits such as spoken language first appeared, making it difficult to link them to the evolution of the jaw. Williams acknowledges that the jaw may not have evolved for a specific purpose, but that doesn’t mean it’s arbitrary.
“It remains one of the defining characteristics of our lineage and is present in some form in every human being living on earth today,” he said.
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