Other researchers, however, have suggested that phalangeal curvature is probably largely genetically determined, and its presence among fossil hominins may be functionally unimportant and simply a primitive retention from an earlier arboreal primate ancestor ( 15, 16). According to this view, hominin phalangeal curvature can be interpreted as direct evidence of routine engagement in arboreal activities during life ( 13, 14). Most researchers believe that phalangeal curvature is a phenotypically plastic trait that develops throughout life in response to mechanical loads experienced by hands and feet during arboreal locomotion ( 9– 12). This begs an obvious yet contentious question: Does the phalangeal curvature of fossil hominins indicate a significant degree of arboreality, and maybe even frequent suspensory locomotion? At the crux of debates are sharply differing assumptions about the mechanistic causes of phalangeal curvature. Across living primates, pronounced phalangeal curvature is found only among taxa that spend substantial amounts of time living and moving in trees, with the greatest curvature among chimpanzees and other species that frequently suspend their bodies below branches ( 7, 8). An important implication of this finding is that phalangeal curvature among fossil hominins is evidently best interpreted as a primitive trait inherited from an arboreal ancestral species rather than proof of engagement in arboreal activities during life.Īmong the most hotly debated anatomical features of ancient hominin fossils is the marked degree of longitudinal curvature seen in the proximal phalanges of the hands and feet of Australopithecus, Paranthropus, and certain early Homo species ( 1– 6). Thus, rather than being a direct effect of mechanical loads produced by lifetime arboreal activities, phalangeal curvature appears to be shaped largely by genetic factors. We show that the degree of hand and foot phalangeal curvature in this individual is indistinguishable from that of wild chimpanzees and distinct from humans. Here, we describe the phalangeal curvature of a chimpanzee who was raised during the 1930s in New York City to live much like a human, including by having very few opportunities to engage in arboreal activities. Intriguingly, ancient fossil hominins also exhibit substantial phalangeal curvature, which, too, has been interpreted as a direct result of habitual arboreality during life. Arboreal primates such as chimpanzees exhibit pronounced curvature in their hand and foot phalanges, which is assumed to develop throughout life in response to mechanical loads produced by grasping and hanging from branches.
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