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Human infants are born neurologically immature, potentially owing to conflicting selection
pressures between bipedal locomotion and encephalization as suggested by the obstetrical
dilemma hypothesis. Australopithecines are ideal for investigating this trade-off, having a
bipedally adapted pelvis, yet relatively small brains. Our finite-element birth simulations
indicate that rotational birth cannot be inferred from bony morphology alone. Based on a
range of pelvic reconstructions and fetal head sizes, our simulations further imply that
australopithecines, like humans, gave birth to immature, secondary altricial newborns with
head sizes smaller than those predicted for non-human primates of the same body size
especially when soft tissue thickness is adequately approximated. We conclude that aus-
tralopithecines required cooperative breeding to care for their secondary altricial infants.
These prerequisites for advanced cognitive development therefore seem to have been cor-
ollary to skeletal adaptations for bipedal locomotion that preceded the appearance of the
genus Homo and the increase in encephalization.