Reconstructing the form of locomotion that preceded the evolution of human bipedality is a key goal of paleoanthropology. Knuckle-walking (a semi-terrestrial form of locomotion in which weight is borne on the backs of the middle hand phalanges) is a plausible candidate because it is practiced by the African apes, which are humans' closest living relatives. However, arguments for a knuckle-walking ancestor for the human evolutionary lineage rely on accurate interpretations of certain anatomical features in chimpanzees, gorillas, and early hominins (fossil species more closely related to humans than chimpanzees) as adaptations to knuckle-walking. The proposed project, to be conducted by Caley Orr under the direction of Dr. Mary Marzke and Dr. Mark Spencer at Arizona State University, will test the hypothesis of adaptation to knuckle-walking for such features using the comparative method. The interdisciplinary project will proceed by integrating innovative techniques from the fields of anthropology, computer science, and bioengineering. Wrist joint shape in anthropoid primates will be quantified from laser scan-derived three-dimensional digital models. In addition, the three-dimensional movements of the wrist bones will be analyzed using serial computed tomography scanning of primate cadavers to test specific hypotheses about the functional consequences of differences in joint shape between species. The knuckle-walking African apes will be compared with the digitigrade baboons, which also use a "vertical manus" form of locomotion, during which the palm is held vertically when placed onto the substrate. These species will be contrasted with several primates capable of using extended-wrist palmigrade hand positions during which the palm is typically placed onto the substrate. Joint mechanisms shared exclusively by the African apes and baboons will be supported as adaptations to maintaining vertical manus postures, and these features can be used to infer such hand postures in fossil species.
The scientific merit of this study is that its novel integrative and quantitative approach facilitates the identification of features adapted to vertical manus postures. The appearance of these features in early hominins would support the hypothesis of a knuckle-walking ancestor. If a knuckle-walking phase in human evolution were to be demonstrated, it would limit adaptive explanations for the origins of bipedality to those consistent with a semi-terrestrial ancestor.
Broader impacts of the project are its value for education of students at all levels and its clinical as well as academic applications. First, the 3D laser-scan models and developed analytical software will be shared via an online database to facilitate further research. The data and software will be useful for researchers and educators in anthropology, paleontology, anatomy, and biomedical fields such as orthopedics. Second, the project will provide a research experience for an undergraduate student who will assist with data processing and analysis. Finally, the project will provide an interdisciplinary training opportunity for Caley Orr in techniques from anthropology, anatomy, bioengineering, and computer visualization that will facilitate his future research and enhance his teaching at the university level.
