Mandibular premolar and second molar root morphological variation in modern humans: What root number can tell us about tooth morphogenesis.

This investigation of modern human mandibular premolar root variation tests the hypothesis that population-specific mandibular single-rooted premolar root size can predict a predisposition to root morphological complexity. Mandibular postcanines were examined and quantified from dental radiographs in a globally spread sample of 1,615 modern humans. Multirooted premolars and a fused molar root phenotype were investigated as probes into greater than, and less than, the normative root number. Twelve questions were addressed concerning root structure of mandibular premolars and second molars. A direct correlation was found between single-rooted mandibular premolar size and the predisposition to multirootedness. This correlation infers the following: 1) that postcanine primordia size during root formation predisposes to the development of more, or less, than the normative postcanine root number; and 2) that the epigenetic effect of tooth primordium size per se influences the induction of interradicular processes, which divides the root during its development. This simple developmental model helps explain the following observations: 1) population-specific variation in postcanine root number; 2) sexual dimorphism for multirooted mandibular premolar prevalence; 3) why microdont teeth are single-rooted; 4) the hierarchy of developmental canalization of interradicular processes; 5) megadont-hominin to late-hominin mandibular premolar root number transition; and 6) the fluctuation of mandibular premolar root number in primate evolutionary history.