In contrast to their great ape relatives, humans have spread from their original home in a limited region of Africa to occupy and even dominate almost all terrestrial habitats. Their success has in part been attributed to the use of, and dependence on, technology of which stone tools represent the earliest evidence. The form of stone tools, together with data on their use and manufacture (knapping), also provide a long record of human cognitive development. Since the early period of tool production corresponds to major changes in the morphology of the hand, wrist and arm, researchers have suggested that increasing use of stone tools played a role in shaping modern human upper limb anatomy. The fracture mechanics involved in producing stone tools in different raw materials and the biomechanics of the upper limb during stone tool production, however, are not well understood. The interactive relationship of fracture mechanics and upper limb morphology on the resulting stone tool is also unclear. This project addresses these issues from an interdisciplinary perspective that integrates lithic analysis with experiments in fracture mechanics and biomechanics. Two main hypotheses will be tested: 1.) magnitude and direction of knapping forces required to produce specific flake morphologies can be predicted from raw material properties (i.e., toughness) and core shape, and, 2.) evolved upper limb morphology in Homo plays a key role in efficient stone tool production. The research design involves fracture mechanics experiments on relevant raw materials, followed by a two phase analysis of knapping motions using a digital motion analysis system to study upper limb motion patterns and forces acting across the hand and wrist. The motion analysis study will involve ten experienced knappers replicating stone flakes and tools in two different raw materials from four different successive tool traditions (Oldowan, Acheulian, Levallois and Middle Stone Age). The two phases will involve the same tasks but in the second phase the knappers' wrists will be restrained to ~30° of anterior-posterior motion to simulate the primitive condition found in African apes and early hominins. Flakes and tools from the two phases will be compared and analyzed for the accuracy and efficiency of motions during the natural vs. the restrained condition. The goal of this study is to determine how fundamental variables involved in stone tool production (material toughness, fracture behavior, core shape and upper limb kinematics) interact to determine flake morphology, knapping accuracy, and energetic efficiency. The study will provide new insights into the advent and development of stone tool production and the evolution of the human upper limb. This in turn has significant clinical implications for understanding upper limb joint motions, variations in morphology, and the impact of injuries and degenerative bone diseases.
