Dynamic finite-element simulations reveal early origin of complex human birth pattern

Human infants are born neurologically immature, but whether this originates from conflicting selection pressures between bipedal locomotion and encephalization as suggested by the obstetrical dilemma remains controversial. Australopithecines are ideal for investigating this trade-off as they have a bipedally adapted pelvis, yet relatively small brains. Our finite-element birth simulations based on different pelvic reconstructions and a range of fetal head sizes indicate that australopithecines already possessed a human-like rotational birth pattern. Since only newborn head sizes smaller than those predicted for non-human primates leave adequate space for soft tissue between the bony pelvis and fetal skull, our data imply that australopithecines had secondarily altricial newborns and likely evolved cooperative breeding to care for their helpless infants. These prerequisites for advanced cognitive development therefore seem to have been corollary to skeletal adaptations to bipedal locomotion that preceded the appearance of the genus Homo and the increase in encephalization.