The Functional Significance of Early Homo Pelvis Morphology

The transition from the Pliocene to the Pleistocene appears to be a time of major transition in the hominin lineage with likely adaptive shifts in behavior. Homo erectus is a temporally broad species with apparent behavioral plasticity and potentially related morphological variability. Some H. erectus postcranial material exhibits traits that indicate a general trend of mechanical reinforcement at loadbearing sites across the pelvis and femur. One such fossil, KNM-ER 3228, is a presumed H. erectus os coxae that is representative of this morphological pattern of robusticity, which includes pronounced acetabulosacral and acetabulocristal buttresses and rugose musculoligamentous attachment sites. This femoropelvic complex represents an evolutionarily stable morphological pattern across a significant portion of the Pleistocene, but its biomechanical significance is not well understood. Moderate increases in encephalization combined with an increase in body size seen in H. erectus may be implicated in early Homo pelvic evolution, as the mechanical consequences of a larger body and expansion of the birth canal for parturition may have led to increased loads during locomotion. The retention of ancestral traits in the H. erectus pelvis (such as laterally flared ilia), coupled with derived traits (such as the expanded birth canal), may have necessitated greater robusticity in the pelvis to reduce strain magnitudes in the bone. Alternatively, this robusticity may be serving to reduce strains within the context of behavioral adaptations, such as endurance running or long-distance walking, that may have increased the magnitude or frequency of loads relative to australopiths. This dissertation addresses two objectives relative to the biomechanical significance of the femoropelvic complex of H. erectus. Objective 1 is to investigate the degree to which the patterns we see in some H. erectus pelves are due to body size scaling of bony features resulting from an evolutionary increase in body size. Objective 2 is to determine what, if any, functional significance may be assigned to the femoropelvic complex, beyond that which body size scaling to keep strain magnitudes within physiological limits may explain. A gait analysis study is conducted in which kinematic and kinetic data from modern humans are collected and used as input for a series of finite element models. These models are created from a sample of modern humans, an Australopithecus africanus pelvis (Sts 14), and a novel reconstruction of KNM-ER 3228 (early Homo) created using 3D geometric morphometrics. The results of these finite element models lead to the rejection of the hypothesis that higher magnitude loads in the H. erectus pelvis resulting from increased body size and modest encephalization resulted in the increased robusticity of pelvic features. This mechanical robusticity was hypothesized to maintain strain levels within manageable physiological levels in the context of the retention of ancestral states. Instead, the lower strains incurred during normal bipedal walking in the H. erectus pelvis suggest a pelvis adapted to bipedal walking, beyond that which strain maintenance can explain. Further, the results of finite element analyses of KNM-ER 3228 and modern humans while running provide evidence to support a hypothesis that mechanical buttressing of the H. erectus pelvis is an adaptation to an increased number of loading cycles (relative to australopiths) in the context of long-distance walking and/or endurance running, and/or high magnitude loads in the context of endurance running. This dissertation contributes to the investigation of unanswered questions in Homo pelvic evolution and highlights the value of comparative modeling in paleoanthropology.