Independent Evolution of Knuckle-Walking in African Apes Shows That Humans Did Not Evolve from a Knuckle-Walking Ancestor

Department of Evolutionary Anthropology, Duke University, Durham, NC 27708-0383, USA.
Proceedings of the National Academy of Sciences (Impact Factor: 9.67). 09/2009; 106(34):14241-6. DOI: 10.1073/pnas.0901280106
Source: PubMed


Despite decades of debate, it remains unclear whether human bipedalism evolved from a terrestrial knuckle-walking ancestor or from a more generalized, arboreal ape ancestor. Proponents of the knuckle-walking hypothesis focused on the wrist and hand to find morphological evidence of this behavior in the human fossil record. These studies, however, have not examined variation or development of purported knuckle-walking features in apes or other primates, data that are critical to resolution of this long-standing debate. Here we present novel data on the frequency and development of putative knuckle-walking features of the wrist in apes and monkeys. We use these data to test the hypothesis that all knuckle-walking apes share similar anatomical features and that these features can be used to reliably infer locomotor behavior in our extinct ancestors. Contrary to previous expectations, features long-assumed to indicate knuckle-walking behavior are not found in all African apes, show different developmental patterns across species, and are found in nonknuckle-walking primates as well. However, variation among African ape wrist morphology can be clearly explained if we accept the likely independent evolution of 2 fundamentally different biomechanical modes of knuckle-walking: an extended wrist posture in an arboreal environment (Pan) versus a neutral, columnar hand posture in a terrestrial environment (Gorilla). The presence of purported knuckle-walking features in the hominin wrist can thus be viewed as evidence of arboreality, not terrestriality, and provide evidence that human bipedalism evolved from a more arboreal ancestor occupying the ecological niche common to all living apes.

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Article: Independent Evolution of Knuckle-Walking in African Apes Shows That Humans Did Not Evolve from a Knuckle-Walking Ancestor

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    • "Our data refute this hypothesis, demonstrating that there are indeed significant changes that occur ontogenetically in both ape and Homo medial cuneiform morphology. As described in the methods section, we quantified the osseous portion of medial cuneiforms of humans as young as 1-year-old and juvenile apes in dental stage 3 (only deciduous dentition, following Kivell and Schmitt, 2009). However, we recognize that the bones of the juvenile individuals in our study were still growing via endochondral ossification and that the shape of the preserved osteochondral interface may not mirror the shape of the joint surface, limiting any broad functional inferences. "
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    ABSTRACT: The medial cuneiform, namely the curvature and angulation of its distal facet with metatarsal 1, is crucial as a stabilizer in bipedal locomotion and an axis upon which the great toe medially deviates during arboreal locomotion in extant apes. Previous work has shown that facet curvature and angulation in adult dry-bone specimens can distinguish African apes from Homo, and can even distinguish among species of Gorilla. This study provides the first ontogenetic assessment of medial cuneiform curvature and angulation in juvenile (n = 68) and adult specimens (n = 102) using computed tomography in humans and extant ape specimens, including Pongo. Our data find that modern human juveniles initially have a convex and slightly medially oriented osseous surface of the developing medial cuneiform distal facet that flattens and becomes more distally oriented with age. The same pattern (though of a different magnitude) occurs developmentally in the chimpanzee medial cuneiform, but not in Gorilla or Pongo, whose medial cuneiform facet angulation remains unchanged ontogenetically. These data suggest that the medial cuneiform ossifies in a distinguishable pattern between Pongo, Gorilla, Pan, and Homo, which may in part be due to subtle differences in the loading environment at the hallucal tarsometatarsal joint-a finding that has important implications for interpreting fossil medial cuneiforms. Copyright © 2015 Elsevier Ltd. All rights reserved.
    Journal of Human Evolution 08/2015; DOI:10.1016/j.jhevol.2015.07.003 · 3.73 Impact Factor
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    • "Anatomical traits functionally related to knuckle-walking are located in the distal ends of the forearm bones, the wrist, the hand and the fingers. Many of these features have been functionally linked to the need to stabilise the radio-carpal and midcarpal joints (i.e. the wrist joint, broadly speaking) while weight is being supported (Kivell and Schmitt, 2009; Begun, 1992, 2004; Richmond and Strait, 2001; Richmond et al., 2001; Tuttle, 1967; Jenkins and Fleagle, 1975; Corruccini, 1978; Richmond, 2006) (Figure 1). The investigation of the interrelationships among the knuckle-walking traits, their distribution and variability in living primates and their presence or absence in hominin fossils are of critical importance for disentangling the context of emergence of upright walking. "
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    ABSTRACT: A central topic in human evolution pertains to the emergence of habitual bipedalism [i.e., upright walking (UW)], a behavioural characteristic that defines the human lineage and is unique among primates. To pinpoint the factors that led to the appearance of UW, one has to reconstruct the loco-motor behaviour of the last common ancestor of humans and their closest living relatives, the chimpanzees. Chimpanzees and gorillas adopt a peculiar mode of locomotion when travelling on the ground, knuckle-walking (KW). This article evaluates the evidence for and against the hypothesis that humans have descended from a KW ancestor. The conflicting nature of this evidence does not allow firm conclusions, but future avenues of research are proposed.
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    • "Primates have been a major focus of animal locomotion research for a number of reasons. Firstly they demonstrate an amazing array of different locomotor habits including unique forms such as arm swinging brachiation [1] and knucklewalking [2]. Secondly, their close evolutionary relationship to modern humans means that studies of primate locomotion give us great insight into the origin of upright bipedalism, our own unique locomotor form [3]. Thirdly, they demonstrate extreme specialisations in terms of arboreal locomotion being the largest entirely arboreal animals [4], with the largest leap distances [5], coupled with very low energy costs [6]. "

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