The capitular joint of the first rib in primates: a re-evaluation of the proposed link to locomotion

Evidence exists that in Australopithecus afarensis the head of the first rib articulated with the body of the first thoracic vertebra but not with the body of the seventh cervical vertebra (Ohman, 1983, 1986). Thus, the Hadar hominid would have differed from most primates, in which both these vertebrae are involved in formation of the first costal capitular joint. Indeed, Ohman (1986) has claimed that a univertebral pattern is unique to modern and fossil hominids among primates. He offered various theories on the adaptive significance of this trait, chief among which was a link to freeing the upper limb from any role in locomotion. Believing that Ohman's statement about the distribution of the univertebral pattern in living forms was based on inadequate samples, we have compiled data on the first costal capitular joint in a wider range of primate genera. Our observations demonstrate that the univertebral pattern, rather than being unique to hominids, is common among siamangs, occurs in an occasional gibbon, and is typical of the larger indriids. Consequently, one can no longer accept any contention that the univertebral first costal capitular joint of A. afarensis implies that it did not use its upper limbs for locomotion. Rather, the formation of this joint is correlated with orthogrady and body size. We discuss a possible explanation of this correlation in terms of movement of the first rib during breathing in an orthograde primate, be it one that stands on two legs, swings by two arms, or clings to trunks using all four appendages.