Evolutionary selection and morphological integration in the vertebral column of modern humans

Objectives The main objective is to quantify integration, modularity, and response to selection in the presacral vertebral column of modern humans. Materials and methods Seventeen linear variables on each presacral vertebra were collected in 108 modern humans producing a total of similar to 39,000 measurements. Then, we studied patterns and magnitudes of integration at regional, vertebral, and intra-vertebral levels. Additionally, we calculated the ability of vertebrae to respond to selection by quantifying differences in evolvability, flexibility, and constraint throughout the spine. Results The results indicate that caudal vertebrae are more evolvable than those located more cranially in the presacral vertebral column, following an increasing pattern of evolvability from the cervical to the lumbar region. Additionally, the atlas and fifth lumbar vertebra show the lowest values of integration, while central thoracic vertebrae display the highest magnitudes of integration. Discussion These results could be related to three main factors: body plan organization expressed by the Hox genes, the strong developmental constraints that determine the number of mammalian vertebrae, and, finally, the functional requirements of an adaptation to bipedal locomotion in the human lineage.