My, how big are your teeth? The sizes of teeth vary widely in mammals. It seems that the diet of a mammal has a strong influence on how big they are, but how is tooth size controlled during their development in the embryo and beyond? Our work has shown that the sizes of molar teeth in a row along a jaw is tightly constrained by a developmental rule we call the 'inhibitory cascade' (Kavanagh et al. 2007, Nature). The rule is very simple: in a tooth row, any tooth is the average size of the two teeth either side of it.
The inhibitory cascade rule applies to not just molar teeth, but also the milk (or baby) teeth in front of the molars. Humans have two baby premolar teeth at the back of each jaw, and behind them the larger molar teeth erupt, which are kept into adulthood. The third of these molars is the wisdom tooth, which causes so many problems in people, often because there is not enough room for them. Our recent work demonstrated that human teeth follow the inhibitory cascade rule very strongly, as do our fossil relatives like the Lucy fossil from Africa and the Neanderthals in Ice Age Europe (Evans et al. 2016, Nature). This pattern is so strong, in fact, that for these five teeth in a row, from just the size of one tooth, we can predict the size of the remaining four teeth without even finding the fossils!