Human skeletal variation: adaptive responses during growth of the bony pelvis

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Evolutionary explanation of the difficulty and risk to the mother and newborn associated with human childbirth have focused on the close fit between the size of the maternal pelvis and the size of the newborn as resulting from our distinctive method of walking on two legs and our large brains. This "obstetric dilemma" views female pelvic form as a compromise between the mechanical requirements of bipedalism for a narrow pelvis and the obstetric requirements for a roomy pelvic canal for our large-brained babies, resulting in strong selective pressures acting on the female pelvis among humans and over our evolutionary history. Physical variation within a population or species can be limited by such competing selective processes (as predicted by the obstetric dilemma hypothesis), yet morphology is also influenced by environmental stimuli acting during growth and development, through biological plasticity. For the pelvis these stimuli may include gait, activity levels and technological means of mitigating activity, diet, and health. Plastic responses generate variation among individuals and populations in anatomy and physiology. Hence, the cultural production of the human skeleton provides bioarchaeologists a window on the activities and adaptations of past populations. Medical models have tended to view both childbirth and female pelvic form in a typological framework, with variation from certain norms as increasing risk of obstructed labour. However, recent studies have shown significant variation among and within human populations in pelvic canal shape and capacity. Further, historical preconceptions of an "obstetric dilemma" may actually be the result of differential plasticity between the maternal pelvis and newborn size, in changing cultural environments. Under this model, human ecological and cultural changes may play a particular role in determining pelvic form. Yet, little research has to date examined the factors that influence pelvic growth and development. Human skeletal remains of juveniles from varied and well-documented archaeological contexts provide an opportunity to study factors influencing growth and development in specific ecological contexts (e.g., subsistence, technologies, and geographical location). In order to understand the potential role of biological plasticity in shaping the maternal pelvis, we ask: when and why does variation in pelvic forms arise, and what role does biological plasticity play in generating variation in relation to environmental stimuli? This study will use three-dimensional (3D) approaches to studying the form of juvenile and adult human pelvic bones from diverse skeletal samples to investigate the relationships among ecological and cultural attributes of populations and pelvic growth and development. This represents a novel approach to examining growth and development of the pelvis and the selective and plastic factors that produce morphological variation among human populations. Understanding the interplay between plasticity and constraint (evolutionary, functional, or developmental) in the generation of morphological variation aids in identifying environmental factors (climatic, ecological, cultural) that influence adaptation. These are of particular importance to evolutionary anthropologists, paleoanthropologists, and evolutionary anatomists. This patterning of human pelvic variation is also essential for understanding modern medical problems such as cephalo-pelvic disproportion and obstructed labour. An awareness of adaptive responses and constraints is of importance for bioarchaeologists where the outcomes of growth processes are increasingly being employed as indicators of population health through time in response to demographic and ecological changes.