Toward solving the puzzle of thorax shape variation among early hominins (original) (raw)

Differences in thorax shape between humans and the great apes are well documented, with the narrow upper thorax and wide lower thorax of apes contrasting with the opposite pattern in fossil Homo and extant humans. Interestingly, current reconstructions of thoraces in small-bodied hominins, such as the Australopithecus afarensis A.L. 288-1, may feature a smaller, more apelike pulmonary thorax (upper thorax), while its larger-bodied conspecific KSD-VP-1/1 is suggested to be more human with a relatively expanded upper thorax. Toward understanding this dichotomy, we examine the relationship between body size and thorax shape, and model trajectories of the upper limb relative to differently shaped thoraces. We show that variation in thorax shapes facilitate or constrain certain locomotor patterns, as a small upper with an expanded lower thorax facilitates knuckle-walking, orienting the upper limb close to the sagittal midline and center of gravity during locomotion, and confers efficient scapular movement around the ribcage. Conversely, an expanded upper and small lower thorax better facilitates bipedality, facilitating more efficient arm swing and energy return from upper limb momentum, while lower thoracic expansion constrains arm swing in bipedal locomotion. Since lung volume and body mass scale isometrically, increases in lung capacity (and concomitant expansion of the thorax) would be best apportioned to the lower thorax in knuckle-walkers. By contrast, increases in lung capacity and thorax expansion in bipeds are best apportioned superiorly. Thus, locomotor constraints, in concert with the isometric relationship between body size and lung size, may explain thoracic shape variation in early hominins.