Upright walking is the defining feature of modern humans and their extinct fossil relatives (hominins) and understanding the transition to this unique form of locomotion is a central problem in paleoanthropology. Considerable disagreement surrounds how the functional significance of hominin fossil anatomy is interpreted, especially with regard to characteristics retained from a primitive, apelike ancestor. Some researchers reject primitive traits when reconstructing behavior from fossil hominins, considering only derived features (such as those adapted for bipedalism), while others consider both primitive and derived traits in asserting that early hominins practiced both arboreal (climbing) and bipedal behaviors. Several characteristics of the shoulder have been cited as primitive and therefore potentially indicative of climbing in members of the genus Australopithecus. This study will focus on shoulder anatomy because previous work has highlighted morphological convergence of this region based on locomotor similarities and also because several new fossils preserving the shoulder fill key gaps in the hominin fossil record. To investigate the relationship between shoulder function and anatomy, this project will use 1) an experimental approach with wild-type mice exercised under two conditions (vertical climbing and horizontal running) and a consideration of transgenic, hypermuscular mice to examine the morphological effects of different locomotor styles and increased musculature, 2) a comparative analysis of primates to examine how the development of shoulder anatomy varies with changes in function during growth, and 3) a reanalysis of early hominin fossils to develop hypotheses about the functional significance of primitive features in light of the experimental and ontogenetic results. These approaches will evaluate the relationship between locomotor activity, muscular anatomy, and shoulder skeletal development to test long-standing hypotheses about interpreting primitive features in early hominin fossil anatomy.
In addition to the doctoral student's training, this project will provide research experience to an undergraduate student assistant. This research has broader implications beyond physical anthropology in that experimentally testing the influence of different styles of locomotion on muscle development and bone shape in mice will contribute to our understanding of skeletal health and biology. Moreover, exploring the influence of locomotor differences in a model organism and morphological development in primates has the potential to transform our ability to make functional interpretations of early hominin skeletal anatomy. By examining whether or not primitive features in hominin shoulders are directly related to locomotor style, this project will shed light on developmental anatomy of the shoulder and will evaluate different theories about the origin and evolution of human bipedalism.
