The 82nd Annual Meeting of the American Association of Physical Anthropologists (2013)

Ardipithecus ramidus proximal capitate morphology is most consistent with a locomotor ancestry of palmigrade arboreal clambering


1Department of Anatomy, Philadelphia College of Osteopathic Medicine, Georgia Campus, Suwanee, GA, 2The University Museum, University of Tokyo, Tokyo, Japan, 3Department of Anatomy, Case Western Reserve University School of Medicine, Cleveland, OH, 4Human Evolution Research Center and Department of Integrative Biology, University of California, Berkeley, CA, 5Department of Anthropology and School of Biomedical Sciences, Kent State University, Kent, OH

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We have previously argued that the total morphological pattern of Ardipithecus ramidus is consistent with an ancestry of palmigrade arboreal clambering (PAC). The chimpanzee-human ancestor was therefore likely primarily adapted to PAC as well, but only chimpanzees underwent further specialization to committed vertical climbing (CVC) (sensu stricto, cf., Fleagle et al 1981), ultimately leading to knuckle walking (KW).

Two proximal capitate features possibly suggestive of PAC ancestry are capitate head position and midcarpal articular surface size. The former was previously cited as evidence for PAC ancestry in Ar. ramidus. Our broad anthropoid sample demonstrates a clear phylogenetic pattern. Cercopithecoids have more dorsally positioned capitate heads, while those of ceboids are more palmar. Humans, Australopithecus afarensis, Ar. ramidus, and Asian apes have more palmarly positioned heads than those of African apes, likely reflecting the ontogeny of KW kinematics in the latter. The capitate head’s palmar position in Ar. ramidus suggests habitual palmigrady during arboreal climbing.

In great apes, humans, and Au. afarensis, the dorsal midcarpal articular surface extends approximately as far distally as the palmar, suggesting similar degrees of flexion and extension. This ratio in cercopithecoids and ceboids suggests a dominance of dorsiflexion. The Ar. ramidus (ARA-VP-6/500) range exceeds that of other anthropoids, but is most like ceboids, consistent with an unusually high range of midcarpal dorsiflexion. Taken together, these data support the earlier conclusion that Ar. ramidus evolved from a PAC ancestor and that hominids were never adapted to CVC.

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