Evolutionary transformation of the hominin shoulder

Abstract

Despite the fact that the shoulder is one of the most extensively studied regions in comparative primate and human anatomy, two recent fossil hominin discoveries have revealed quite unexpected morphology. The first is a humerus of the diminutive fossil hominin from the island of Flores, Homo floresiensis (LB1/50), which displays a very low degree of humeral torsion1, 2 (Fig. 1; see Box 1). Modern humans have a high degree of torsion and, since this is commonly viewed as a derived feature shared with hominoids,3-6 one would expect all fossil hominins to display high humeral torsion. The second is the recently discovered Australopithecus afarensis juvenile scapula DIK-1-1 from Dikika, Ethiopia, which seems to most closely resemble those of gorillas.7 This specimen is the first nearly complete scapula known for an early hominin and, given the close phylogenetic relationship between humans and chimpanzees suggested by molecular studies,8-13 one would have expected more similarity to chimpanzees among extant hominoids.