Nicole Dieneke Sybille Grunstra

Among mammals, the human birth process is difficult with high rates of complications and mortality, because our pelvis is small compared to the size of the baby. Scientists have long speculated on this ‘obstetric conundrum’. The classic explanation is that selection for a bipedal gait in our ancestors led to a narrow pelvis; the subsequent increase in brain size has made childbirth increasingly difficult. Alternative, but less prominent, explanations include the selection for a narrow pelvis to support the pelvic floor in resisting the weight of the viscera and a large fetus. A greater incidence of pelvic floor disorders among women with wider pelvises supports this hypothesis. Due to the multitude of confounding factors, evolutionary hypotheses like these cannot be tested adequately in humans; our present ecological and sociocultural environments deviate from the environment that has driven our anatomical evolution. I propose to test hypotheses about the evolutionary origin of human childbirth in primates and non-primate mammals. A detailed study of pelvic shape in differently sized species with different combinations of locomotion (e.g. quadrupedalism, vertical clinging and leaping, or bipedal jumping), posture (upright vs. horizontal) will allow me to disentangle the selective forces exerted by locomotion and pelvic floor support.