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We humans like to take our time to grow. Among the great apes, only chimpanzees manage to lengthen the years between key stages of development.
But even chimps are ready to bite into a full set of chompers by the time they’re sexually mature. Homo sapiens don’t grow their last teeth until late adolescence.
This mystery of molars is difficult to solve, despite their emergence playing such a critical role in monitoring changes in our evolution. But researchers at the University of Arizona in the United States now think they may have deciphered it.
“One of the mysteries of human biological development is how the precise synchronization between molar emergence and the history of life occurred and how it is regulated,” says anthropologist and lead author Halszka Glowacka .
With the help of Gary Schwartz, a paleoanthropologist at the Institute of Human Origins at the University of Arizona, Glowacka put together examples of different skulls to compare their development.
By turning the bones and teeth of 21 primate species into 3D models, researchers were able to determine that the timing of our adult molars has a lot to do with the delicate balance of biomechanics in our growing skulls.
The adult forms of the teeth that we use to grind our food into a paste usually emerge from our gums in three stages – around 6, 12, and 18 (give or take).
Other primates get their adult molars earlier. Despite all our similarities in growth stages, the chimpanzee (Pan troglodytes) obtains its molars at 3, 6 and 12. The yellow baboon (Cynocephalic papio) has its last adult molars before the age of seven, and the rhesus macaque (Macaca mulatta) is all done by the time they are six years old.
An important factor limiting when teeth can appear is space. If the jawbone is not large enough for an adult size dental assembly, there is no point in pushing them in.
Humans don’t really have a lot of oral space as they are, with impacted wisdom teeth being a major problem for our species. But that doesn’t explain why they appear so late in our lives, or why the ones behind them seem to be causing more and more problems.
Having an empty space for a tooth to grow doesn’t make it a good idea to put one there, however. Teeth don’t creak on their own – there are plenty of muscles and bones supporting them, ensuring that enough pressure can safely tear and grind our food.
And it is “safety” that seems to be at the root of our late dental growth.
“It turns out that our jaws grow very slowly, possibly due to our overall slow life histories and, in combination with our short faces, delays when a mechanically safe space – or a ‘sweet spot’, if you will. – is available, resulting in our very late ages to the emergence of molars ”, explains Schwartz.
The posterior molars of primates are located just in front of two temporomandibular joints, which together form a hinge between your jawbone and the skull. Unlike other joints in our body, the two pivots must work in perfect synchronization with each other. They also need to transfer good force to one or more points to make you bite and chew.
In biomechanics, this three-point process is governed by principles within what is called the constrained level model. Put a tooth in the wrong place, and the forces produced by this pattern could be bad news for a jaw that just isn’t big enough to handle it.
For species with longer jaws, the time required for the skull to develop a suitable structure for the teeth closest to the muscles near the hinge is relatively short.
Humans, with our noticeably flatter faces, are not so fortunate and have to wait until our skulls have developed to such an extent that the forces exerted on each set of adult molars do not damage our growing jawbone.
Not only does this give us a new way to assess dental conditions, such as impacted molars, it could help paleontologists better understand the evolution of our unique jaws among our hominid ancestors.
“This study provides a powerful new lens through which the long-known links between tooth development, skull growth and maturation patterns can be visualized,” said Glowacka.
This research was published in Scientists progress.
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