Evolutionary dynamics of the hominin pelvis

As the pelvis is at the intersection of two distinctly human traits - efficient habitual bipedalism and a birth canal that allows for passage of a relatively large-brained neonate - it is arguably uniquely important for understanding our evolution. Most previous studies of pelvic evolution, and paleoanthropology in general, use interspecific morphological comparisons among extant and extinct primates to make inferences as to the behavior, phylogenetic relationships, and selective pressures that resulted in observed changes. While the findings of these studies have yielded many fundamental insights on human evolution, the differences seen in these comparisons are the pattern, rather than the process, of evolution. As such, the particulars of the evolutionary dynamics that underlie morphological change are generally unknown. This study attempts to fill that void by applying evolutionary quantitative genetic theory and methods to the study of hominin pelvic evolution. There are three sets of results. First, stepping back from paleoanthropological issues, this study explores the effects of sample size and transformations on the statistics that form the backbone of the analyses to follow. These statistics are interpreted in the light of what they mean biologically rather than merely statistically, with an emphasis on taking a measurement theory approach to understanding the links between data and reality. In addition, this analysis provides recommendations to future researchers with regards to experimental design. Second, this study explores the role of selection in changing not the just trait means but the genetic relationships between traits, revealed in patterns of integration. Integration between traits is fundamental to evolution as such relationships can bias and constrain evolutionary trajectories. In addition, as evidence suggests that patterns of integration can evolve in response to selection, comparisons of these patterns could provide a new source of information to complement fossil analyses. The results are the first to demonstrate that changes in the pattern of interrelationships among traits (integration) in the hip bone have occurred during hominin evolution, that the human hip is less evolutionarily constrained than that of other great apes, that the changes in the human pattern of integration could have facilitated the transition between a pre-hominin and hominin pelvic morphology, and that these changes in integration are likely to be related to the evolution of bipedalism. Third, this study addresses one of the classic issues in paleoanthropology: what is the ultimate cause of the dangers and difficulties experienced during modern human birth? Though hypotheses relating evolutionary constraints on hominin obstetric dimensions to bipedal function have been proposed, the role of genetic constraints in reducing the ability of the birth canal to respond to selection has not been previously explored. The results of this study show that genetic constraints among obstetric traits and between obstetric and other pelvic traits can negatively influence the evolutionary potential of the human birth canal, but there is a significant reduction in these constraints in humans, and likely other later hominins, when compared to other apes. These findings suggest that natural selection for increased obstetric dimensions in later hominins reduced ancestral hominoid genetic constraints on the birth canal, allowing for morphological evolution along a trajectory that might have previously been difficult or impossible to traverse. In total, the results presented here suggest that natural selection led to the evolution of hominin integration patterns in a way that permitted and facilitated the complex morphological changes that serve to distinguish our lineage from that of the other great apes