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

Area of Science:

• Paleoanthropology
• Human Evolution
• Reproductive Biology

Background:

• Human infants are born neurologically immature, a state potentially linked to the obstetrical dilemma—conflicting pressures of bipedal locomotion and brain expansion (encephalization).
• Australopithecines, with their bipedally adapted pelvises and smaller brains, offer a unique model to study this evolutionary trade-off.

Purpose of the Study:

• To investigate the birthing capabilities and infant altriciality in Australopithecines using finite-element birth simulations.
• To determine if bony morphology alone can predict rotational birth and to estimate australopithecine newborn head size relative to body size.

Main Methods:

• Finite-element birth simulations were employed using various pelvic reconstructions and estimated fetal head sizes.
• Soft tissue thickness was approximated to enhance the accuracy of the simulations.

Main Results:

• Bony morphology alone is insufficient to infer rotational birth in australopithecines.
• Simulations suggest australopithecines gave birth to immature, secondary altricial infants with head sizes smaller than predicted for non-primate relatives of similar body mass.
• These findings were particularly evident when accounting for soft tissue approximations.

Conclusions:

• Australopithecines likely required cooperative breeding to care for their secondary altricial infants.
• The evolution of cooperative breeding and advanced cognitive development prerequisites may have preceded the genus Homo and significant encephalization.
• Skeletal adaptations for bipedal locomotion appear to have preceded the conditions necessary for increased brain size and complex social structures.