Why do we grow two sets of teeth?
Most of our body parts grow with us, so what makes teeth different?
Picture the following:
It’s your sixth birthday party. The sun is shining and you’ve just enjoyed a glorious day outdoors playing with your best friends. But as you are called inside to enjoy the first bite of sweet, sweet birthday cake, your fortunes change. Your left middle finger starts to wobble and eventually falls off. While slightly annoyed and distracted — you can’t keep your hands off the empty cavity left behind — you return to your slice of cake with the knowledge that a new finger will shortly be growing in its place.
Absurd as the above story may sound, it does illuminate a peculiar question: why, unlike most other body parts, do we grow two sets of teeth?
Humans, like most other mammals, are diphyodonts — the technical term describing animals who grow two sets of teeth, including a deciduous set (aka milk teeth, baby teeth, drop teeth) and a permanent set. Our deciduous set of 20 teeth (ten top and bottom) already start growing in our embryonic phase. For most humans, these start to emerge between the first six months and two years of our lives, allowing sufficient time to comfortably breastfeed without causing mum too much pain. The last teeth (molars, found at the back of the mouth), generally erupt towards the end of the weaning period, which is also when the growth of our skull starts to significantly slow down. In some rare cases, the deciduous teeth may already emerge before birth, in which case they are referred to as “natal teeth”.
The arguments for deciduous teeth are refreshingly simple. Unlike most other body parts, teeth do not alter their size once fully developed (ignoring, of course, any effects of decay caused by that bar of chocolate you ate yesterday). Since a full set of 32 adult-size permanent teeth are simply too big to be held in a baby’s jaw, these are preceded by the initial deciduous set. Deciduous teeth play an important role in development by supporting the shape of the jaw during growth, aiding speech development and allowing hungry omnivorous babies to chew on protein-rich food. In addition, they help to maintain the spacing and channels through which the permanent teeth will eventually grow. Regarding the question of whether we could simply keep our original set of teeth as we grow, these would end up being too small and spaced out to efficiently bite through meat or chew on leafy greens.
Another evolutionary argument in favour of diphyodonts is that they increase their chances of maintaining a good set of teeth should any damage occur to the initial set. Polyphyodonts like sharks, geckos, crocodiles manatees, elephants and kangaroos (a unique group of mammals in this class), are even more protected as they can replace their teeth multiple times throughout their lives. A shark’s teeth are continually replaced along a form of “conveyor belt” up to 50,000 times in a 30-year lifespan. Elephants, which spend much of their lives chewing abrasive grass, replace their four main teeth around 6 times in a lifetime. Loss of teeth is one of the leading natural cause of death among older elephants, when the last set wears, the elephant may lose its ability to properly chew grass.
Luckily enough for the generously-cavitied author of this article, there is extensive research looking into applying polyphyodont magic to human tooth regeneration. Crocodilia showcase a particularly promising redundancy mechanism, with each tooth backed up by a smaller baby tooth and a stem cell, ready to replace each other in case the full tooth is dislodged (or slowly dissolved by a sugary pack of gummy bears).
