Article

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: 9.81). 09/2009; 106(34):14241-6. DOI: 10.1073/pnas.0901280106
Source: PubMed

ABSTRACT 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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    • "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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    • "Much debate remains over the locomotor behaviour of the pre-bipedal common ancestor of Homo and Pan[8,17,18,20,29,81], in which the shared morphology of the wrist has played a central role [16-19]. One model envisions the pre-bipedal ancestor as a knuckle-walker that had already come down to the ground, similar to the locomotor behavior used by African apes [17,29,81,82]. "
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    ABSTRACT: The hominoid wrist has been a focus of numerous morphological analyses that aim to better understand long-standing questions about evolution of human and hominoid hand use. However, these same analyses also suggest various scenarios of complex and mosaic patterns of morphological evolution within the wrist and potentially multiple instances of homoplasy that require formal analysis within a phylogenetic context.We identify morphological features that principally characterize primate -- and, in particular, hominoid (apes, including humans) - wrist evolution and reveal the rate, process and evolutionary timing of patterns of morphological change on individual branches of the primate tree of life. Linear morphological variables of five wrist bones -- the scaphoid, lunate, triquetrum, capitate and hamate -- are analyzed in a diverse sample of extant hominoids (12 species, 332 specimens), Old World (8 species, 43 specimens) and New World (4 species, 26 specimens) monkeys, fossil Miocene apes (8 species, 20 specimens) and Plio-Pleistocene hominins (8 species, 18 specimens).ResultResults reveal a combination of parallel and synapomorphic morphology within haplorrhines, and especially within hominoids, across individual wrist bones. Similar morphology of some wrist bones reflects locomotor behaviour shared between clades (scaphoid, triquetrum and capitate) while others (lunate and hamate) indicate clade-specific synapomorphic morphology. Overall, hominoids show increased variation in wrist bone morphology compared with other primate clades, supporting previous analyses, and demonstrate several occurrences of parallel evolution, particularly between orangutans and hylobatids, and among hominines (extant African apes, humans and fossil hominins). Our analyses indicate that different evolutionary processes can underlie the evolution of a single anatomical unit (the wrist) to produce diversity in functional and morphological adaptations across individual wrist bones. These results exemplify a degree of evolutionary and functional independence across different wrist bones, the potential evolvability of skeletal morphology, and help to contextualize the postcranial mosaicism observed in the hominin fossil record.
    BMC Evolutionary Biology 10/2013; 13(1):229. DOI:10.1186/1471-2148-13-229 · 3.41 Impact Factor
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