Study finds that complex human childbirth and cognitive abilities are the result of upright walking.
Childbirth in humans is much more complex and painful than in great apes. It was long believed to be the result of the larger human brain and the narrow dimensions of the mother’s pelvis. Researchers from 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 in women during and after pregnancy and childbirth. The majority of these problems occur during pregnancy and are preventable or curable. However, childbirth remains dangerous. The World Health Organization estimates that 830 people die every day from causes related to childbirth and pregnancy. Moreover, for every woman who dies in childbirth, 20 to 30 others suffer 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 complications during childbirth. Other common problems include unsafe abortions and chronic conditions such as heart disease and diabetes.
All this shows how much more difficult and painful human childbirth is than that of great apes. This was long believed to be due to the larger brains of humans and the limited dimensions of the mother’s pelvis. Researchers from the University of Zurich have now shown, using 3D simulations, that childbirth was also a very complicated procedure in early hominin species that gave birth to relatively small-brained newborns – with important consequences for their cognitive development.
The fetus normally navigates a narrow and complicated birth canal by bending over and turning its head in different phases of human childbirth. This complicated procedure poses a significant risk of birth complications, which can range from prolonged labor to stillbirth or maternal death. These issues have long been thought to be the result of a conflict between humans adapting to straight walking and our larger brains.
The dilemma between walking upright and bigger brains
Bipedalism developed around seven million years ago and radically transformed the hominid’s pelvis into a veritable birth canal. However, larger brains only began to develop two million years ago, when the first species of the genus Homo emerged. The evolutionary solution to the dilemma caused 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 led by Pierre Frémondière from the University of Aix-Marseille have now discovered that Australopithecines, which lived there 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 exhibited morphological adaptations to bipedalism, they are ideal for studying the effects of these two conflicting evolutionary forces,” says Häusler.
The typical ratio of fetal and adult head size
The researchers used three-dimensional computer simulations to develop their findings. Since no newborn Australopithecus fossils are known, they simulated the birth process using different fetal head sizes to account for the possible range of estimates. Each species has a typical ratio between the brain size of its hatchlings and that of adults. Based on the ratio of non-human primates and the average brain size of an adult Australopithecus, the researchers calculated an average neonatal brain size of 180g. This would correspond to a size of 110 g in humans.
For their 3D simulations, the researchers also took into account the increased mobility of the pelvic joints 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, while the 180 g and 145 g sizes did not. “This means that australopithecine newborns were neurologically immature and dependent on help, like human babies today,” says Häusler.
Extended learning is the key to cognitive and cultural abilities
The results indicate that Australopithecines probably practiced a form of cooperative breeding, even before the appearance of the genus Homo. Compared to great apes, the brain developed longer outside the womb, allowing 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,” says Häusler. This conclusion is also supported by the first documented stone tools, which date back 3.3 million years, long before the appearance of the genus Homo.
Reference: “Dynamic finite element simulations reveal the early origin of a complex human birth pattern” by Pierre Frémondière, Lionel Thollon, François Marchal, Cinzia Fornai, Nicole M. Webb and Martin Haeusler, April 19, 2022, Communications Biology.