Researchers may have come up with one simple rule that explains how our teeth and those of our extinct ancestors grow: Baby teeth influence the size of neighboring teeth that develop later. The findings, published in Nature this week, would make it possible to predict the sizes of a whole row of adult teeth using just one isolated fossil tooth.
The size of the back teeth of hominins (that’s us and our extinct ancestors) have decreased over evolutionary time. Early hominins called australopiths, a group that includes Lucy’s species (pictured above), had larger teeth overall and their largest molar was nearer to the back of the mouth. Extinct species of our own genus, Homo, had smaller teeth and their biggest molar was closer to the middle of their jaw. This trend continues even today: Not all of us have our third and rearmost molars, or wisdom teeth. Researchers have attributed this pattern to changes in diet and the advent of cooking, but the mechanism underlying variation in tooth size isn’t well understood. Previous work with mice revealed that the size of one of their molars regulates the development of adjacent teeth. Under this mechanism – called the inhibitory cascade model – one tooth constrains the sizes of subsequently developing teeth.
To see if the developmental mechanism that controls relative molar size in mice could be applied to hominins and great apes, a team led by Alistair Evans of Monash University analyzed tooth size in modern humans and fossilized hominins. The team found a strong relationship between the absolute and relative sizes of the primary post-canine teeth – which include “baby” premolars and permanent molars.
From the size of an isolated tooth (white), the sizes of the remaining primary teeth (green) can be predicted using the inhibitory cascade. Illustrated here are the relative sizes predicted for the robust australopith Paranthropus boisei. Alistair Evans, Matt Skinner, Kierstin Catlett and E. Susanne Daly
"Most mammals, including all living humans and all extinct human ancestors, have two sets of teeth: a milk set, or 'baby' teeth, and an adult set," study co-author Kierstin Catlett of Arizona State University explains in a statement. Both milk molars and adult molars are needed to break food down into fuel. "Amazingly, the size of milk molars, which start developing prior to birth, have a powerful cascading effect on the size of later-forming adult molars," Catlett adds.
Furthermore, the team found that this tooth size pattern is constant with absolute tooth size in the australopiths, but the scaling relationship is different in Homo species based on the size of the first molar.