Tooth emergence: timing is everything

08 October 2021
1 min read

The ages that most people get their three adult molars are six, 12 and 18 – much later than our closest living relative, the chimpanzee, who gets those same teeth at around three, six and 12.

Paleoanthropologists have questioned why humans evolve their molars at these specific ages, and equally, why those ages are later than apes.

Scientists at the University of Arizona and Arizona State University have recently unveiled a study in Science Advances this week that they think has finally solved the mystery.

Media platform Eureka Alert explains, “The one dental feature intimately associated with the pace of growth and life history is the ages at which our adult molars cut through the gumline. For many decades, evolutionary anthropologists have leveraged the very tight relationship — which exists across all primates — between the pace at which these adult molars emerge into the mouth with the overall pace of life. Modern humans, for instance, grow up incredibly slowly, have a very long and protracted life history, and emerge their adult molars very late in life, later than any other living or extinct primate.”

Lead author and assistant professor at the University of Arizona, College of Medicine-Phoenix, Halszka Glowacka, said of the study, “One of the mysteries of human biological development is how the precise synchrony between molar emergence and life history came about and how it is regulated.”

Halszaka’s study, conducted together with Gary Schwartz, a researcher with the Institute of Human Origins and professor in the School of Human Evolution and Social Change, concluded that it is the coordination between facial growth and the mechanics of the chewing muscles that determines not just where but when adult molars emerge. This delicate dance results in molars coming in only when enough of a “mechanically safe” space is created. Molars that emerge “ahead of schedule” would do so in a space that, when chewed on, would disrupt the fine-tuned function of the entire chewing apparatus by causing damage to the jaw joint.

Summarsing, Gary explains, “It turns out that our jaws grow very slowly, likely 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 at molar emergence,” said Schwartz.

Meanwhile, Halszka concludes, “This study provides a powerful new lens through which the long-known linkages among dental development, skull growth and maturational profiles can be viewed.”