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Scientists have discovered why birth has become so complex and dangerous

Pregnant Birth

The World Health Organization estimates that nearly 300,000 people die each year from pregnancy-related causes.

One study found that complex human birth and cognitive abilities are the result of walking upright.

Birth in humans is much more complex and painful than in great apes. It has long been believed that this was the result of the larger human brain and the narrow dimensions of the mother’s pelvis. Researchers at the University of Zurich have now used 3D simulations to show that birth was also an extremely complex process in early hominin species that gave birth to newborns with relatively small brains - with important developmental implications. their cognitive.

Complications are common for women during and after pregnancy and childbirth. Most of these problems occur during pregnancy and are either preventable or curable. However, childbirth is still dangerous. The World Health Organization estimates that 830 people die every day from birth and pregnancy-related causes. In addition, for every woman who dies due to childbirth, another 20-30 face injuries, infections or disabilities.

Four major complications are responsible for 75% of maternal deaths: severe bleeding (usually after birth), infections, high blood pressure during pregnancy, and birth complications. Other common problems include unsafe abortions and chronic conditions, such as heart disease and diabetes.

All this shows that the birth of man is much more difficult and painful than that of great apes. It has long been thought that this is due to the larger human brain and the limited size of the mother’s pelvis. Researchers at the University of Zurich have now shown, using 3D simulations, that birth was also an extremely complicated procedure in early hominin species that gave birth to newborns with relatively small brains - with significant consequences for development. their cognitive.

The fetus normally navigates a narrow, twisted birth canal by bending and turning its head at various stages during birth. This complicated procedure has a significant risk of complications at birth, which can range from prolonged labor to stillbirth or maternal death. These problems have long been thought to be the result of a conflict between people who adapt to walking upright and our larger brains.

The dilemma between walking straight and a bigger brain

Bipedalism developed about seven million years ago and dramatically reshaped the hominid pelvis into a true birth canal. Larger brains, however, did not begin to develop until two million years ago, when the oldest species of the genus Homo appeared. The evolutionary solution to the dilemma posed by these two conflicting evolutionary forces was to give birth to neurologically immature and helpless newborns with relatively small brains - a condition known as secondary altruism.

A research group led by Martin Häusler of the Institute of Evolutionary Medicine at the University of Zurich (UZH) and a team led by Pierre Frémondière of the University of Aix-Marseille have now found that australopithecines, which lived about four to at two million years old, it had a complex birth pattern compared to the great apes. “Because australopithecines such as Lucy were relatively small in brain size but already had morphological adaptations to bipedalism, they are ideal for investigating the effects of these two conflicting evolutionary forces,” says Häusler.

Lucy's birth simulation

Simulation of the birth of Lucy (Australopithecus afarensis) with three different dimensions of the fetal head. Only a brain size of up to 30% of the adult’s size (right) fits through the birth canal. Credit: Martin Häusler, UZH

The typical ratio between fetal and adult head size

The researchers used three-dimensional computer simulations to develop their findings. Because newborn australopithecines are not known to exist, they simulated the birth process using different sizes of the fetal head to account for a range of possible estimates. Each species has a typical relationship between the brain size of newborns and its adults. Based on the ratio of non-human primates to the average brain size of an adult Australopithecus, the researchers calculated an average neonatal brain size of 180 g. This would correspond to a size of 110 g in humans.

For their 3D simulations, the researchers also considered the increased mobility of the pelvic joints during pregnancy and determined a realistic thickness of the soft tissue. They found that only the size of the 110 g fetal head passed through the pelvic orifice and the middle plane without difficulty, as opposed to the 180 g and 145 g sizes. help, just like today’s human babies, ”explains Häusler.

Lifelong learning is the key to cognitive and cultural skills

The findings indicate that australopithecines probably practiced a form of cooperative reproduction, even before the emergence of the genus Homo. Compared to the great apes, the brain developed longer outside the womb, allowing children to learn from other members of the group. “This extended learning period is generally considered crucial for people’s cognitive and cultural development,” says Häusler. This conclusion is also supported by the oldest documented stone instruments, dating back 3.3 million years - long before the appearance of the genus Homo.

Reference: “Dynamic Finite Element Simulations Reveal the Early Origins of the Complex Model of Human Birth” by Pierre Frémondière, Lionel Thollon, François Marchal, Cinzia Fornai, Nicole M. Webb, and Martin Haeusler, April 19, 2022, Biology of communications.
DOI: 10.1038 / s42003-022-03321-z

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