June 3, 2022

Scientists Have Figured Out Why Childbirth Became So Complex and Dangerous

Pregnant Giving Birth

The World Health Organization estimates that nearly 300,000 people die every year due to pregnancy-related causes.

A study finds that complex human childbirth and cognitive abilities are a result of walking upright.

Childbirth in humans is much more complex and painful than in great apes. It was long believed that this was a result of humans’ larger brains and the narrow dimensions of the mother’s pelvis. Researchers at the University of Zurich have now used 3D simulations to show that childbirth was also a highly complex process in early hominin species that gave birth to relatively small-brained newborns – with important implications for their cognitive development.

Complications are common for women during and following pregnancy and childbirth. The majority of these issues arise during pregnancy and are either avoidable or curable. However, childbirth is still dangerous. The World Health Organization estimates that 830 people die every day due to causes related to childbirth and pregnancy. Furthermore, for every woman that dies due to childbirth, another 20-30 encounter injury, infection, or disabilities. 

Four major complications are responsible for 75% of maternal deaths: severe bleeding (typically after birth), infections, high blood pressure during pregnancy, and complications from delivery. Other common issues include unsafe abortions and chronic conditions such as cardiac diseases and diabetes. 

All of this shows how human birthing is much more difficult and painful than that of large apes. This was long believed to be due to humans’ bigger brains and the limited dimensions of the mother’s pelvis. Researchers at the University of Zurich have now shown, using 3D simulations, that birthing was likewise a highly complicated procedure in early hominin species that gave birth to relatively small-brained newborns – with significant consequences for their cognitive development.

The fetus normally navigates a narrow, convoluted birth canal by bending and turning its head at different phases during human delivery. This complicated procedure has a significant risk of birth complications, which may range from extended labor to stillbirth or maternal death. These issues were long thought to be the outcome of a conflict between humans adjusting to upright walking and our larger brains.

The dilemma between walking upright and larger brains

Bipedalism developed around seven million years ago and dramatically reshaped the hominin pelvis into a real birth canal. Larger brains, however, didn’t start to develop until two million years ago, when the earliest species of the genus Homo emerged. The evolutionary solution to the dilemma brought about 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 altriciality.

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

Birth Simulation Lucy

Birth simulation of Lucy (Australopithecus afarensis) with three different fetal head sizes. Only a brain size of maximum 30 percent of the adult size (right) fits through the birth canal. Credit: Martin Häusler, UZH

The typical ratio of fetal and adult head size

The researchers used three-dimensional computer simulations to develop their findings. Since no fossils of newborn australopithecines are known to exist, they simulated the birth process using different fetal head sizes to take into account the possible range of estimates. Every species has a typical ratio between the brain sizes of its newborns and adults. Based on the ratio of non-human primates and the average brain size of an adult Australopithecus, the researchers calculated a mean neonatal brain size of 180 g. This would correspond to a size of 110 g in humans.

For their 3D simulations, the researchers also took into account the increased pelvic joint mobility during pregnancy and determined a realistic soft tissue thickness. They found that only the 110 g fetal head sizes passed through the pelvic inlet and midplane without difficulty, unlike the 180 g and 145 g sizes. “This means that Australopithecus newborns were neurologically immature and dependent on help, similar to human babies today,” Häusler explains.

Prolonged learning is key to cognitive and cultural abilities

The findings indicate that australopithecines are likely to have practiced a form of cooperative breeding, even before the genus Homo appeared. Compared to great apes, the brains developed for longer outside the uterus, enabling infants to learn from other members of the group. “This prolonged period of learning is generally considered crucial for the cognitive and cultural development of humans,” Häusler says. This conclusion is also supported by the earliest documented stone tools, which date back to 3.3 million years ago – long before the genus Homo appeared.

Reference: “Dynamic finite-element simulations reveal early origin of complex human birth pattern” by Pierre Frémondière, Lionel Thollon, François Marchal, Cinzia Fornai, Nicole M. Webb, and Martin Haeusler, 19 April 2022, Communications Biology.
DOI: 10.1038/s42003-022-03321-z

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