The morphology of the gluteus maxìmus during human evolution: Prerequisite or consequence of the upright bipedal posture?

Abstract

The human gluteus maximus differs from that of the other hominoids because of its size and bony attachments. These differences raise questions concerning their sequence of appearance in human evolution. Given that humans practice a unique locomotor style, one wonders if the human gluteus maximus morphology is a prerequisite or a consequence of upright bipedal locomotion. This question is addressed using a computer model that evaluates muscle leverage in a variety of locomotor postures. In this model, the human-like, or ape-like, muscular pattern is imposed upon a representative hindlimb of each of the five extant hominoids. Shapes of the skeletal elements (i.e., ilium and ischium lengths) are adjusted in the computer to simulate an evolutionary progression from an ape to a human skeletal morphology. Changes in the leverage of different parts of the gluteus maximus (measured as moment arms) are monitored during this transition. The results show how the mechanical leverages of the gluteus maximus would have changed in a variety of hypothetical evolutionary sequences that describe an ape to human transition. Although the hominoid models exhibit minor differences in these simulations, they all show that the postural and locomotor functions of the gluteus maximus would become more difficult if musculoskeletal morphology changed to the human-like pattern before erect bipedal posture was adopted. Conversely, small adjustments in the ape-like musculoskeletal condition could support an erect bipedal posture. These results suggest that a humanlike posture would have preceded the appearance of the humanlike musculoskeletal morphology. Human gluteal morphology, therefore, is a consequence and not a prerequisite of the upright bipedal posture.