We know that humans live relatively long lives, and we certainly know that we spend a larger proportion of those lives as children than other species. The question remains: how did we manage to extend this critical period of our growth? When and where did our ancestors start to stretch out the limits of physiology and build that long childhood? And where can we find evidence of this evolutionary process? The very surprising answer is: in the mouths of babes—specifically, their teeth. But to understand how the timing of teeth tells us the story of, well, us, we need to first put teeth in contex…