Shoulder Morphology in Early Hominin Evolution

Among the enduring debates about the earliest stages of human evolution is whether or not the early hominin upper limb continued to serve a locomotor function following the adoption of bipedal habits. While the morphology of the entire upper limb is relevant to this debate, the shoulder is a region of primate anatomy that past studies have shown strongly reflects the differing functional demands imposed by differences in locomotor modes and posture. This study, therefore, reviews what is currently known about the pectoral girdle and proximal humerus of early hominins. This morphology is compared to that of humans and extant apes to attempt to establish what the last common ancestor of the African ape/human clade might have looked like, and to determine to what degree early hominins depart from, or have retained that primitive condition. Available information regarding the functional interpretation of traits is also reviewed to help shed light on how the upper limb of early hominins was actually used. The results of this review indicate that the early hominin shoulder retained many features of the presumed ancestral condition. These include a dorsal scapula positioned high on the thorax, a well-developed supraspinatus muscle, a relatively short, oblique clavicle, and a low to modest degree of humeral torsion. While most of these features would have been compatible with a limb that either continued to serve a locomotor role or was completely freed from it, possession of modest humeral torsion seems counterproductive for a limb involved only in manipulation. Since this characteristic displays some developmental plasticity, maintenance of modest torsion implies continued functional importance. A persisting role in arboreal locomotion would seem the most likely selective force resisting the development of high humeral torsion as came to characterize later hominin evolution.