The genetic architecture and evolutionary consequences of the human pelvic form

Human pelvic shape has undergone significant evolutionary change since the divergence from the chimpanzee lineage. This transformation, involving the reduction of the pelvic canal size to support bipedal locomotion, is thought to give rise to the obstetrical dilemma, a hypothesis highlighting the mismatch between the large brain size of infants and the narrowed birth canal in females. Empirical evidence for this classic hypothesis has been equivocal, largely due to a lack of sample size and appropriate types of data. To elucidate the genetic underpinnings of pelvic morphology, we applied a deep learning model to 31,115 dual-energy X-ray absorptiometry (DXA) from the UK Biobank, extracting a set of seven pelvic proportion (PP) phenotypes, including measures of the birth canal. All PPs were found to be highly heritable (∼25-40%) and a genome-wide association study of these traits identified 179 independent loci. Unlike other skeletal proportions including long bone lengths, the subpubic angle associated with the birth canal exhibits a genetic correlation between sexes significantly less than 1, in line with sex-specific reproductive function. PPs were also left-right asymmetric but not heritable and instead associated with handedness. We conducted phenotypic and genetic association analyses to link PPs to 3 facets of the dilemma: locomotion, pelvic floor function and childbirth. Larger birth canal phenotypes were associated with reduced walking pace, decreased risk of back pain, and increased risk of hip osteoarthritis - phenotypes linked to locomotor efficiency. We also observed that a narrower birth canal width was associated with a reduced risk of pelvic floor disorders. When examining childbirth-related outcomes, narrower birth canal phenotypes were associated with increased risk of emergency cesarean sections and obstructed labor due to insufficient dilation, but not obstructed labor due to positioning of the fetus. Finally, we examined whether the dilemma might have been alleviated through evolution. We found no association between any PPs and gestational duration, contrary to the initial prediction by Washburn in 1960. However, we found that the birth weight of the child, a proxy for skull and brain size, was genetically correlated with birth canal width but not with other PPs. Collectively, our study offers fresh insight on a 60-year-old debate in human evolutionary studies. Our results support the idea that the obstetrical dilemma has played a central role in the co-evolution of the human brain and pelvis, while also highlighting the potential role of associated factors such as pelvic floor health.