The first rib of hominoids.

Homo sapiens is unique among extant hominoids in displaying a univertebral articular pattern for the first rib; that is, the head of the first rib articulates only with the body of the first thoracic vertebra. All other hominoids, indeed virtually all other mammals, display a bivertebral pattern; that is, the head of the first rib articulates with the bodies of both the seventh cervical and the first thoracic vertebrae, as well as the intervening disk. Two fossil hominid partial first ribs, A.L. 288-lax and A.L. 333-118, show that the univertebral pattern was fully established in the hominid lineage by the appearance of Australopithecus afarensis. Four hypotheses, based in functional anatomy, can be postulated for the evolution of the univertebral pattern: (1), it increases the volume (via increased length) of the neck, which could, in turn, compensate for the functional loss of the laryngeal sac systems in hominid vocalization; (2), it is a consequence of the more barrel-shaped thorax in hominids; (3), it is a consequence of functional modifications in the hominid shoulder girdle; and/or (4), it is a consequence of modifications in hominid first rib motion while breathing in an upright stance. Fossil evidence supports all but the first hypothesis, and most strongly supports the third. However, evidence for the first hypothesis does suggest that the evolution of descent of the upper respiratory system in the hominid lineage may have been permitted by the presence of the univertebral pattern, while the reverse is probably not true. Furthermore, fossil evidence for the third hypothesis shows that, by the appearance of A. afarensis, the hominid upper limb had been freed from locomotor constraints, which concomitantly confirms full adaptation to upright posture. Thus, because of their potential relationship with upright posture, the two remaining hypotheses (i.e., "thoracic shape" and "first rib movement during breathing") also have support from the fossil evidence.