Meeting Abstract

25.4  Monday, Jan. 5  Asymmetry in the human cranio-cervico-omo-clavicular complex suggests connection with bipedalism OSBORN, M.L.*; HOMBERGER, D.G.; Louisana State Univ., Baton Rouge; Louisana State Univ., Baton Rouge mosbor1@lsu.edu

The mastoid process and clavicle are larger in humans than in other mammals and have been interpreted as having evolved in connection with upright posture and bipedalism. We hypothesize further that these features evolved in connection with the suspension of the shoulder girdle from the skull and the freeing of hands to carry loads. The mastoid process and clavicle are part of a cranio-cervico-omo-clavicular (CCOC) complex, whose elements are connected primarily by the paired upper trapezius and sternocleidomastoid (SCM) muscles. When loads are carried, the upper trapezius muscles counteract the additional weight, thereby stabilizing the shoulders. Simultaneously, the SCM muscles prevent the head from being retroflexed by the contracting upper trapezius muscles. We predicted that the preferential use of one limb would be expressed as asymmetries in the paired elements of the CCOC complex, thereby providing a natural experiment to test the corollary of our main hypothesis that the unique development of the mastoid process and clavicle in humans may have evolved in connection with increased muscular forces on them. In right-handed individuals, the dorsal border of the attachment of the upper trapezius muscle was often higher on the right side of the skull. This asymmetry in the attachment area results in a thicker muscle and a greater mechanical advantage. The mastoid process width was often greater on the right side, which provides a larger attachment area for a thicker right SCM muscle to counterbalance the larger right trapezius muscle. Hence, greater muscular forces and increased use of the CCOC complex result in a stronger development of some of its biomechanically relevant features. These observations support our hypothesis.