Metacarpal torsion in apes, humans, and earlyAustralopithecus:implications for manipulatory abilities

Abstract

Human hands, when compared to that of apes, have a series of adaptations to facilitate manipulation. Numerous studies have shown thatAustralopithecus afarensisandAu. africanusdisplay some of these adaptations, such as a longer thumb relative to the other fingers, asymmetric heads on the second and fifth metacarpals, and orientation of the second metacarpal joints with the trapezium and capitate away from the sagittal plane, while lacking others such as a very mobile fifth metacarpal, a styloid process on the third, and a flatter metacarpo-trapezium articulation, suggesting some adaptation to manipulation but more limited than in humans. This paper explores variation in metacarpal torsion, a trait said to enhance manipulation, in humans, apes, early australopithecines and specimens from Swartkrans. This study shows that humans are different from large apes in torsion of the third and fourth metacarpals. Humans are also characterized by wedge-shaped bases of the third and fourth metacarpals, making the metacarpal-base row very arched mediolaterally and placing the ulnar-most metacarpals in a position that facilitate opposition to the thumb in power or cradle grips. The third and fourth metacarpals ofAu. afarensisare very human-like, suggesting that the medial palm was already well adapted for these kinds of grips in that taxon.Au. africanuspresent a less clear human-like morphology, suggesting, perhaps, that the medial palm was less suited to human-like manipulation in that taxa than inAu. afarensis. Overall, this study supports previous studies onAu. afarensisandAu. africanusthat these taxa had derived hand morphology with some adaptation to human-like power and precision grips and support the hypothesis that dexterous hands largely predatedHomo.