Difficult birth and cognitive abilities are consequences of walking upright

Zurich – Human birth is harder and more painful than human apes. For a long time it was assumed that it was due to the large brain and the narrow conditions of the pelvis. With 3D birth simulations, researchers from the University of Zurich show that birth was already much more difficult in pre-humans about three million years ago, despite their still small brains – with consequences for their cognitive development.

To pass through the narrow birth canal, the human fetus must perform complex twisting and bending movements. This is associated with a high risk of birth complications up to and including childbirth and the death of mother and child. The common explanation for these birth defects is that they are the result of a conflict between our adaptations to walk efficiently and our large brain.

Dilemma between walking upright and bigger brain
The upright gait originated about seven million years ago and led to a profound remodeling of the pelvis with a shorter distance between the hip joint and the sacrum. However, the enormous increase in brain size did not occur until two million years ago, when the earliest representatives of the genus Homo appeared. The dilemma created by the two opposing selection pressures was solved by evolution by giving birth to neurologically underdeveloped, helpless newborns with relatively small brain sizes. We humans are therefore also referred to as secondary “nesting feces”.

The research team led by Martin Häusler from the Department of Evolutionary Medicine at the University of Zurich (UZH) and the team led by Pierre Frémondière from the University of Aix-Marseille have now shown that compared to apes, birth was already difficult in the australopithecines two to four million years ago. “Precursors like Lucy are ideal for studying the effects of the various evolutionary forces: they still had a relatively small, ape-like brain, but their pelvis already showed significant adaptations to upright gait,” says Häusler.
The ratio of fetal to adult head size typical of each species

The researchers succeeded in demonstrating this using three-dimensional computer simulations. Since no fossils of newborn australopithecines have survived, they simulated the birth with different fetal head sizes to cover the full possible range of variation. For each species, the brain size of the newborns is in a typical relationship to the brain size of the adults: based on the ratio of non-human primates to the brain size of an average adult Australopithecus, the researchers calculated the average brain size of the newborns to be 180 g. to a size of 110 g.

For the 3D birth simulations, the researchers took into account the increased mobility of the pelvic joints due to pregnancy and determined a realistic thickness of the soft tissue.

The result: trouble-free passage is only possible with a fetal head of 110 g, but not with 180 g or a medium size of 145 g. “This means that Australopithecus babies were as neurologically underdeveloped at birth and dependent on help as human babies are. today, “says Häusler.

Prolonged learning nurtured cognitive and cultural development
For this reason, Australopithecines have probably already practiced some form of raising their offspring together. Compared to humans, infant brains were able to grow longer outside the womb, allowing newborns to learn from other group members for longer periods of time. “This extended learning phase is generally considered a crucial factor in human cognitive and cultural development,” says Häusler. Archaeological finds also support the theory: The oldest stone tools, dated to 3.3 million years, date from a time when there were only australopithecines and no representatives of the genus Homo. (University of Zurich / mc / ps)

Literature:
Pierre Frémondière, Lionel Thollon, François Marchal, Cinzia Fornai, Nicole M. Webb, Martin Haeusler. Dynamic finite-element simulations reveal the early origins of complex human birth patterns. Communication Biology. April 19, 2022. DOI 10.1038 / s42003-022-03321-z

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