Functional Morphology of the Midcarpal Joint in Knuckle-Walkers and Terrestrial Quadrupeds

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5. Conclusions

The functional anatomy of the forelimb has played a critical role in reconstructing locomotor and postural adaptations of fossil anthropoids, including hominins. This study examined the functional anatomy of the midcarpal joint and wrist ranges of motion in extant hominoids and terrestrial cercopithecines to investigate ways that knuckle-walkers are similar to, and distinct from, other terrestrial catarrhines. New data on maximum ranges of motion in Pan, Hylobates, Papio, and Erythrocebus support the conclusion that Asian apes have unusually mobile wrist joints as adaptations for suspension and/or climbing activities, and that African ape wrists have limited ranges of extension as an adaptation to knuckle-walking.

Results provide some support to hypotheses about aspects of midcarpal morphology being functionally related to limiting wrist extension in the knuckle-walking African apes. The depth of the concavity on the scaphoid/centrale facet of the capitate, the distal extent of articular surfaces, and dorsal ridge development appear to distinguish knuckle-walkers. Although variability exists in any single characteristic, the total morphological pattern is fairly distinct in African apes. The likely role of the capitate-scaphoid joint in limiting extension during knuckle-walking supports the hypothesis that early ontogenetic fusion of the os centrale to the scaphoid is a knuckle-walking adaptation.

African apes and the terrestrial cercopithecoids have significantly broader midcarpal joints compared to Asian apes, supporting the hypotheses that the morphologies in the former taxa represent adaptations to pronograde weight support, and in the latter are adaptations for increased mobility. The broad midcarpal morphology of the early hominin (∼3.5 Ma Australopithecus) KNM-WT 22944 supports evidence that hominins evolved from an ancestor adapted to knuckle-walking and climbing, rather than a climbing ancestor with a locomotor repertoire that lacked a terrestrial component.