Morphological integration and the evolution of knuckle-walking Scott A. Williams Department of Anthropology, University of Illinois,109 Davenport Hall, 607 S. Mathews Ave., Urbana, IL 61801, USA article info Article history: Received 7 September 2009 Accepted 10 March 2010 Keywords: Homology Homoplasy Modularity Functional complex Bipedalism African apes abstract The evolution of knuckle-walking has profound implications for our understanding of the emergence of bipedalism. The modern debate surrounding its evolution is concerned with whether or not it is homologous in chimpanzees and gorillas. Here, this problem is approached using the methods of morphological integration to test hypotheses of patterns and magnitudes of integration in the third manual ray and capitate. If knuckle-walking morphologies are highly integrated and evolve in a corre- lated bundle (i.e., comprising a functional complex), it seems reasonable that they could have been recruited independently relatively easily in gorillas and chimpanzees, thus increasing the likelihood of homoplasy. If, however, there is no evidence for a knuckle-walking complex, then it seems less likely that chimpanzees and gorillas would have evolved knuckle-walking independently. Results indicate that chimpanzees and gorillas are not characterized by high magnitudes of integration or unique patterns of integration that distinguish them from non-knuckle-walking taxa. This does not support the hypothesis of a knuckle-walking complex, nor does it support the contention that knuckle-walking could have been easily evolved independently in chimpanzees and gorillas. Implications for trait analysis and the evolution of bipedalism are discussed, as are recent analyses supporting the independent origins of knuckle-walking. Ó 2010 Elsevier Ltd. All rights reserved. Introduction The debate surrounding the evolution of knuckle-walking has a long history and its inception predates the discovery of an early fossil hominin (Dart, 1925). Although W. K. Gregory and D. J. Morton agreed in general about the evolution of orthogrady in apes, their views on the evolution of bipedalism were not congruent. While Gregory (1916:334) proposed that the terrestrial locomotion of gorillas and chimpanzees is a “peculiar method of taking strides with the forearms, in a semi-erect posture” and “forms a necessary prelude to fully erect bipedal progression,” Morton (1926:158) rejected the view “that the terrestrial semi-erect posture of the great apes represents an approach toward the upright posture of man,” instead arguing that “there is nothing homologous in their respective terrestrial postures.” This early disagreement marks the beginning of a long debate over the role of knuckle-walking in human evolution (see Richmond et al., 2001; Crompton et al., 2008 and sources therein). Currently, the debate is concerned with whether knuckle-walking is homologous in chimpanzees and gorillas, having evolved only once in the common ancestor of hominines (gorillas, chimpanzees, and humans) or whether it has evolved multiple times independently (Dainton and Macho, 1999; Inouye, 2002; Inouye and Shea, 2004; Orr, 2005; Williams, 2006; Filler, 2007; Kivell and Schmitt, 2009; Lovejoy et al., 2009). The possibility that knuckle-walking may have evolved inde- pendently in gorillas and chimpanzees has been offered as an alternative to the homologous origin of knuckle-walking for some time (e.g., Pilbeam, 1970; de Bonis, 1983; Boaz, 1988), but it was not explicitly supported by morphological data until Dainton and Macho (1999), in a study of the ontogeny and allometry of chim- panzee and gorilla wrist bones, concluded that the parallel evolu- tion of knuckle-walking was more parsimonious than a single origin. Recently, Kivell and Schmitt (2009) came to the same conclusion based on a similar study of wrist bones. Lovejoy et al. (2009) have most recently expressed support for the independent evolution of knuckle-walking based on their interpretation of the forearm of Ardipithecus ramidus. If knuckle-walking is homologous, then the last common ancestor of not only hominines, but also that of humans and chimpanzees, was a knuckle-walker. This evolutionary history would place the emergence of bipedalism in the context of a knuckle-walking background. Alternatively, if knuckle-walking evolved independently in the chimpanzee and gorilla clades, then it is likely that bipedalism was preceded by some other type of locomotor mode. This issue is fundamental to our understanding of the evolution of bipedal locomotion. In order to attempt to explain E-mail address: sawill@gmail.com Contents lists available at ScienceDirect Journal of Human Evolution journal homepage: www.elsevier.com/locate/jhevol 0047-2484/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jhevol.2010.03.005 Journal of Human Evolution 58 (2010) 432e440