Re-evaluating the correlated evolution of human hands and feet using viability selection modeling

Abstract

Objectives: Bipedalism and enhanced manipulative abilities are defining features of the hominin lineage, associated with derived foot and hand skeletal morphology, respectively. These unique morphologies are traditionally thought to have evolved independently, but previous work suggests that some aspects of hand and foot skeletal morphology coevolved, due to strong phenotypic correlations among serially homologous elements. Materials and Methods: Here, I further tested this hypothesis using viability selection modeling, which simulates phenotypic evolution based on the likelihood of individual survival, determined using individuals' distance to a predetermined adaptive peak, in this case modern Homo. Using chimpanzees (Pan) as a proxy for hominin ancestral hand and foot morphology, I quantified morphological changes in the hand when only homologous foot elements were targets of selection, as well as the reverse scenario in which only hand morphology influences viability. Results: Simulation results show that in both scenarios, one autopod evolves as correlated response to selection acting on the other. Importantly, however, simulation outcomes show that the adaptive evolution of the uniquely derived human foot can indirectly produce Homo-like manual proportions, but the reverse is not true: adaptive evolution of an African ape-like hand is not sufficient to produce a Homo-like human foot. Discussion: These simulations thus lend further empirical support to the hypothesis that human hands and feet coevolved, and indicate that selection acting on the foot may have produced derived hand morphology that facilitated the evolution of enhanced manipulative behaviors.