Deciphering the correlated evolutionary responses of the hands and feet in modern humans

The coevolution of the hands and feet in modern humans has been a subject of significant interest due to their unique morphological features that differentiate humans from other primates and their implications in human evolution. This study aims to investigate the degree of correlated responses to selection between hands and feet and to determine whether one of the autopods has exerted a greater influence on this coevolution, focusing on their homologous elements and morphological traits. We analyzed the 38 long bones of the hands and feet from 96 modern human specimens, employing a comprehensive methodological framework that includes morphological analysis, assessments of modularity, integration, and covariation patterns under random selection. Additionally, Bayesian analyses were conducted to test whether foot morphology drives hand morphology or vice versa. Our findings indicate a high degree of morphological integration between the hands and feet, revealing a trend of increasing correlation from the first to the fifth ray. Consistent with previous studies, our Bayesian model provides robust evidence that the feet drive the morphological coevolution of human autopods, likely in response to functional selection pressures associated with bipedalism. However, our results also highlight that the intertwined evolutionary trajectories of the hands and feet are not a simple unidirectional model, underscoring the complexity of morphological integration and the diverse coevolutionary patterns among different rays, reflecting their specialized functions and evolutionary adaptations.