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

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Les différentes étapes de l’accouchement de Lucy avec un foetus à petit cr

Les différentes étapes de l’accouchement de Lucy avec un foetus à petit crâne (110g) modélisées par l’analyse en élément-fini : A engagement, B descente, C dégagement. © Pierre Frémondière

An interdisciplinary and international research team realized the first 4-dimensional simulation of hominin birth, constituting an important methodological advance for paleo-obstetrics. The researchers found that australopithecines gave birth to relatively small brained newborns, implying that they were helpless at birth similar to modern humans and their mothers thus needed the help of group members. Cooperative breeding therefore seems to have evolved secondary to bipedalism and thus prior to the appearance of larger brain sizes in the genus Homo. These results are published the 19th of April in Communications Biology.

Human delivery is traditionally viewed as a complex and difficult event that contrasts to the relatively straightforward and uncomplicated birth of most other mammals. Cephalo-pelvic disproportion is the leading cause of obstructed labour, resulting from a misfit of the relatively large fetal head with the dimensions of the mother’s pelvis, while the convoluted shape of the female birth canal contributes to the unique complexity of human childbirth. As a result, humans give birth to newborns with a relatively small brain size that are neurologically immature (i.e. secondarily altricial).

The emergence of a humanlike birth pattern is traditionally associated with two major events in human evolution: bipedalism, which dramatically reshaped the hominin pelvis to a real birth canal, and encephalization, which led to bigger brain sizes. Because australopithecines retained relatively small brain sizes but already displayed morphological adaptations to bipedalism, they are ideal to investigate these two conflicting evolutionary forces, which are the source of a trade-off known as the "obstetric dilemma".

The finite-element simulations of childbirth modelled ligamentous laxity at the sacroiliac joint, while three fetal head sizes were considered to reflect the possible range of cranial capacity estimates derived from the literature. Particularly when taking a realistic soft tissue thickness into account, these simulations suggested that, similarly to what happens in humans, australopithecines gave birth to neonates with small brain size. This implies a prolonged neurological maturation of early hominin infants, which in conjunction with cooperative breeding has been regarded as prerequisites for the development of advanced cognitive abilities. These evolutionary processes therefore seem to have been corollary to skeletal adaptations for bipedal locomotion that preceded the appearance of the genus Homo and the dramatical encephalisation that occurred in our species.

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