Evolvability of the Primate Pelvic Girdle

The ilium and ischiopubic bones of the pelvis arise from different regulatory pathways, and as a result, they may be modular in their organization such that features on one bone may be morphologically integrated with each other, but not with features on the other pelvic bone. Modularity at this gross level of organization can act to increase the ability of these structures to respond to selection pressures (i.e., their evolvability). Furthermore, recent work has suggested that the evolution of the human pelvis was facilitated by low levels of integration and high levels of evolvability relative to other African apes. However, the extent of morphological integration and modularity of the bones of the pelvic girdle is not well understood, especially across the entire order of primates. Therefore, the hypothesis that the ilium and ischiopubis constitute separate modules was tested using three-dimensional landmark data that were collected from 752 pelves from 35 primate species. In addition, the hypothesis that the human pelvis demonstrates greatest evolvability was tested by comparing it to all other primates. The results demonstrate that regardless of phylogeny and locomotor function, the primate pelvis as a whole is characterized by low levels of overall integration and high levels of evolvability. In addition, the results support the developmental hypothesis of separate ilium and ischiopubis modular units. Finally, all primates, including humans, apparently share a common pattern of integration, modularity, and evolvability in the pelvis.