Many researchers have suggested that the greater robusticity and resultant strength of Neandertal femora, compared with those of modern humans, is due to extended use while foraging and hunting. This is known as the mobility hypothesis. This study examines that hypothesis by examining skeletal features and ranging behaviors in a large sample of living primates to determine if use equates with greater robusticity. Results will inform the fields of anthropology, primatology, orthopedics, engineering, and quantitative genetics. The study also impacts understanding of the femur's mechanical environment, which could aid the development of hip prosthetics. Furthermore, this study improves understanding of the correlation between genetics and femoral morphology, and promotes interdisciplinarity between engineering and social sciences.
The Neanderthal femur differs from that of recent modern humans in four ways that suggest it is more robust, or biomechanically stronger: it is more curved, has thicker bone, a round rather than elliptical shaft, and a smaller neck-shaft angle. These differences may be explained by the mobility hypothesis. Due to subsistence strategies relying heavily on the exploitation of animal resources, Neanderthals may have been traveling farther than modern humans in search of food, and would thus be loading their lower limbs to a greater extent. This study tests the mobility hypothesis using a three phase approach. The first phase establishes a comparative basis relating mobility to femoral skeletal features in 35 species of living primates with documented ranging behavior, using a method that will produce and disseminate hundreds of computerized 3D surface models. The second phase analyzes the degree to which key skeletal features are heritable or environmentally produced using 21 strains of inbred laboratory mice totaling 413 individuals. The final phase explores the strength of complete Neanderthal and modern human femora within each species' particular anatomical context, using an engineering technique (finite element analysis) designed to investigate how complex structures respond to loads, and include the impact of realistic muscle and hip joint reaction forces.
Thanks to funding from NSF, we were able to travel to and access primate skeletal collections from four museums across the country and abroad. This allowed us to create over 400 3-dimensional surface models of primate long bones from four genera of primates including macaques, guenons, tamarins, and baboons. Once our analyses are complete, the data gathered from the surface scans will inform us about the correlation between bone shape and ranging behavior, information relevant to the fields of primatology, osteology, anthropology, and others. These surface models will be entered into a database with access granted to other researchers dependent upon museum policies and restrictions. Computer models of a Neanderthal and modern human femur were created and several experiments modeling human walking were performed. Results from these experiments lead us to conclude that muscles play a significant role in the femur’s bending behavior. Muscles reduce the overall amount of femoral bending during walking, and also shift the bending direction through the shaft of the bone from side-to-side to front-to-back. These results are beneficial to anyone working with finite element analysis, especially those using this method for application to biological structures. Furthermore, although the Neanderthal femur is typically described as being stronger than the human femur, our results indicate the opposite. In each experiment, the human femur model experienced less stress than the Neanderthal femur, which indicates that humans are as well or better adapted as Neanderthals to walking long distances on level terrain, a finding that will impact our understanding of differences in lifestyle between Neanderthals and modern humans. In addition to allowing us to compare Neanderthal and modern humans specifically, this study improves our general understanding of the biomechanical environment of the femur during walking, a potentially important contribution to the field of orthopedics. The computer models created during this project will be uploaded to the BioMesh website (www.BioMesh.org) for use by other researchers interested in femoral biomechanics. The creation of these computer models is one of the most time consuming processes involved in modeling experiments, therefore sharing of resources and dissemination of previously constructed models with increasingly sophisticated morphology should be a great asset to others involved in this field.
