Evaluating the Evolvability of Paranthropus Cranial Morphology in Relation to Feeding Biomechanics.

OBJECTIVE: Although disagreement persists as to the precise nature of the diet of Paranthropus, there is a consensus that the food resources consumed by Paranthropus were in some way mechanically challenging to process (i.e., by being "hard" and/or "tough"). While the highly derived feeding apparatus of Paranthropus likely conferred biomechanical performance advantages while consuming certain types of foods, it may also have limited the ability of these early hominins to respond to selection and evolve rapidly toward new adaptive peaks (i.e., reduced their evolvability). MATERIALS AND METHODS: We employed viability selection modeling to test this hypothesis. Viability selection simulations were performed using Paranthropus boisei (OH 5), Australopithecus afarensis (A.L. 444-2), and Homo habilis (KNM-ER 1813) specimens. We simulated the generation-to-generation evolution of biomechanically informative linear dimensions in a population where an individual's probability of survival (i.e., viability) was determined by its distance to a predetermined adaptive peak. The number of generations required for an evolving population to reach a new adaptive peak was used as a measure of evolvability. RESULTS: The results showed that the mean number of generations from P. boisei to H. habilis was larger than in the reverse direction when modeled using either chimpanzee or human estimates of population variance/covariance. It took longer for P. boisei to evolve toward Au. afarensis than in the reverse direction, but only with the chimpanzee estimates of population variance/covariance. DISCUSSION: The results suggest that P. boisei faced limitations in cranial evolvability, particularly if selection favored a cranial morphology similar to H. habilis.