Not Merely a Constraint: How the Constrained Lever Model Shapes Mandibular Evolution via Integration

The Constrained Lever Model of vertebrate jaw biomechanics posits that the configuration of the triangle of support—demarcated by the bite point and the temporomandibular joints (TMJs)—limits muscle activity and thus feeding function. In particular, the resultant vector of the masticatory muscle forces must pass through the triangle or else the working-side mandibular condyle may be distracted out of the TMJ. We predict that the triangle acts as a functional module that is integrated with other aspects of mandibular morphology to facilitate coordinated evolution of the mandible as a whole. We tested whether skeletal traits in a triangle of support module facilitate mandibular evolution to a greater extent than random modules along a hypothetical selective trajectory. We applied viability selection modeling to simulate mandibular evolution from an ancestral population (common chimpanzees or Australopithecus afarensis) toward an adaptive peak representing modern humans. In these simulations, selection acts only on measurements assigned to the triangle of support or a random module, but other dimensions evolve via observed integration. The results demonstrated that selection on the triangle of support was more effective than expected by chance in producing a human-like mandible in a shorter amount of evolutionary time, when compared with randomly grouped sets of measurements.