Proximal femoral musculoskeletal morphology of chimpanzees and its evolutionary significance: a critique of Morimoto et al. (2011).

Based on the analysis of computed tomography (CT) scan imagery, Morimoto et al. (Anatomical Record 2011; 294:1433-1445) concluded that the proximal femoral shaft attachment of the chimpanzee gluteus maximus (GM) lies in a position similar to that of modern humans (medial to a longitudinal bony structure that runs superoinferiorly along the lateral proximal shaft), contradicting the previous reports of similarity with the other extant apes. Based on a broader comparative osteological perspective and examination of some of the same CT imageries, we here demonstrate that: 1) although the chimpanzee insertion of the GM appears to lie more posteromedially than it does in gorillas and orangutans, the validity of the extent of this reassessment remains in doubt, pending crossvalidation of CT analyses by parallel dissections of the imaged specimens, and 2) the chimpanzee and human conditions are, nevertheless, distinct. We agree with Morimoto et al. (Anatomical Record 2011; 294:1433-1445) that these observations support the interpretation that superficially similar osteological topographies of the proximal femur were acquired independently by chimpanzees and gorillas, but we disagree about the significance of their suggested human-chimpanzee similarities. Although Morimoto et al. (Anatomical Record 2011; 294:1433-1445) considered these to be shared-derived features of the chimpanzee-human clade, we instead argue that the shared absence of strong anterolateral displacement of the GM attachment among chimpanzees, basal hominids (such as Orrorin and Ardipithecus), and humans likely reflects the primitive condition characteristic of a wide range of Miocene apes.