Evolutionary selection and morphological integration in the foot of modern humans

Objectives: To advance our understanding of the evolution of the hominin foot by quantifying integration and responses to selection in the foot of modern humans.Materials and Methods: The sample includes 247 female and male adult individuals from Euro-American, Afro-American, European, and Amerindian populations. We collected 190 linear measurements from the 26 skeletal elements that constitute the modern human foot. With these data, we calculated the magnitudes of integration and the ability of the foot to respond to selection demands.Results: The results revealed that distal phalanges are less integrated, more evolvable, and more flexible than proximal elements (i.e., proximal phalanges and metatarsals). Also, bones from the medial ray (e.g., hallux) show stronger integration and weaker evolvability than their counterparts from the lateral column (e.g., fifth ray), following this trend from medial to lateral positions. Among the tarsals, the talus and calcaneus are the most integrated, least evolvable, and flexible elements from that module.Discussion: These results suggest that selection for bipedalism would have reorganized the variance/covariance matrix of the foot. The hallux might have been under strong functional selection pressures for bipedal requirements, resulting in a strong integration and low evolvability. Also, differences in the developmental process of each bone seem to have played an essential role in the degree of evolvability, showing those elements that develop earlier have less ability to respond to selection demands.