Evolution really dropped the ball when it came to our teeth, huh? Not only do the little biters start rotting out of our heads as soon as we stop adding special minerals to our water, but we don’t even get a full set to work with until we’re practically ancient. It takes a full 18 years for us to grow the last of our chompers, and those final molars can often be the worst of the lot, cramming their way into our mouths at such ridiculous angles that you’d be forgiven for thinking the name “wisdom teeth” was purely ironic.
Compared to other great apes, our molars come in very late – the only other primate that comes close is the chimpanzee, which grows their final pair of molars at age 12. But exactly why we have to put up with this state of affairs is something that scientists have puzzled over for a long time. Now, in a study published in the journal Science Advances, two researchers think they may have figured it out.
“Our results support the idea that the biomechanical constraint on masticatory form in adult primates operates throughout the duration of craniofacial growth,” reads the study. “This constraint regulates where molars can emerge safely into functional occlusion, which, when viewed within the mechanical context of overall orofacial growth, modulates the timing of when molars emerge.”
In other words: our wisdom teeth just can’t grow until our jaws are big enough to safely house them. Makes sense, right? But the trouble is, humans are weird. We have great big brains and tiny, receding snouts; we take decades to reach full adulthood but will reproduce multiple times before our first-born is old enough to reliably escape from predators. When you try to map the usual timings of primate molar emergence onto a species like that, things are bound to get hinky.
“It turns out that our jaws grow very slowly, likely due to our overall slow life histories,” explained study co-author Gary Schwartz in a statement. “[In] combination with our short faces, [that] delays when a mechanically safe space — or a ‘sweet spot,’ if you will — is available, resulting in our very late ages at molar emergence.”
That “mechanically safe” sweet spot depends on two things: the size of our jaws, but also the mechanics of our chewing muscles. Molars that erupt too early, the pair discovered, would end up in a space and masticatory system that wasn’t ready for it – and trying to actually use these premature wisdom teeth would result in a damaged jaw joint.
This delicate balance was found by the researchers in nearly two dozen species of primates – from small lemurs all the way up to gorillas. The team created 3D biomechanical models of these species’ skull and chewing muscle development, which they then combined with information about the rates of jaw growth in each animal. In this context, humans’ late-onset wisdom started to make a lot more sense.
“Our findings suggest that delayed molar emergence in H. sapiens is the result of extreme facial retraction coupled with a deceleration in, and extended duration of, orofacial growth,” explains the study. “[It] is the combination of orthognathic [with a moving jaw] faces with protracted jaw growth that results in a delayed appearance of alveolar space in which molars can safely emerge.”
The best part of the discovery – at least for anyone who knows the agony of an impacted wisdom tooth – is the implications for clinical dentistry. By exploring some of the finer details in the model, lead author Halszka Glowacka suggested they might be able to help understand the painful phenomenon.
“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,” Glowacka said. “This study provides a powerful new lens through which the long-known linkages among dental development, skull growth and maturational profiles can be viewed.”