Evolution of the Hominin Shoulder: Early Homo

The morphology of the human shoulder seems well-suited to serving its role of providing a wide range of mobility for the upper limb. The dorsal position of a highly mobile scapula and the lateral orientation of the small, shallow glenoid fossa articulating with a medially directed, large round humeral head open up nearly a complete sphere of motion allowing the hand to be positioned almost anywhere around the body. For a creature that depends a great deal on its hands to manipulate objects in order to survive, such an ideal pairing of form and function would seem to be a wonderful example of the elegance of design in nature. Of course the human shoulder did not arrive at its present confi guration purely through natural selection for its current use. Humans share many features of upper limb morphology with extant apes suggesting that at least some aspects of human shoulder anatomy refl ect our phylogenetic heritage. In order to understand which features of the human shoulder have been inherited from a common hominin ancestor, including primitive retentions that continue to serve their original purpose and those that have been co-opted to serve a new function, and which are uniquely derived in humans, it is necessary to determine the course of evolutionary change in the hominin pectoral girdle and shoulder. This paper attempts to trace those changes, beginning with early hominins, but focusing on the characteristics of the shoulder region in early Homo erectus (see Larson, 2007, for a more detailed overview of hominin shoulder evolution).