Research has found that complex human births and cognitive abilities are the result of upright walking.
Childbirth in humans is much more complicated and painful than in monkeys. It has long been thought to be the result of people’s larger brains and the mother’s narrow pelvis. Researchers at the University of Zurich have already used 3D simulations to show that childbirth was also a very complex process in early types of hominins who gave birth to newborns with relatively small brains – with important implications for their cognitive development.
Complications are common in women during and after pregnancy and childbirth. Most of these problems occur during pregnancy and can be avoided or treated. However, childbirth is still dangerous. The World Health Organization estimates that 830 people die every day from birth and pregnancy. In addition, for every woman who dies in childbirth, another 20-30 face injury, infection or disability.
Four major complications are responsible for 75% of maternal deaths: heavy bleeding (usually after childbirth), infections, high blood pressure during pregnancy and postpartum complications. Other common problems include dangerous abortions and chronic conditions such as heart disease and diabetes.
All this shows how human birth is much more difficult and painful than that of great apes. It has long been thought that this is due to the larger brain of humans and the limited size of the mother’s pelvis. Researchers at the University of Zurich have already shown using 3D simulations that childbirth is also a very complex procedure for early hominins, which give birth to newborns with relatively small brains – with significant consequences for their cognitive development.
The fetus usually moves along a narrow, curved birth canal, bending and turning its head in different phases during human birth. This complex procedure has a significant risk of complications in childbirth, which can range from prolonged birth to stillbirth or death of the mother. These problems have long been thought to be the result of conflict between people who adjust to upright walking and our larger brains.
The dilemma between upright walking and bigger brains
The biped developed about seven million years ago and dramatically reshaped the pelvis of hominins into a true birth canal. However, larger brains did not begin to develop until two million years ago, when the earliest 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 altricality.
A research group led by Martin Heusler of the Institute of Evolutionary Medicine at the University of Zurich (UZH) and a team led by Pierre Fremondier of the University of Aix-Marseille have now found that Australopithecus, which lived about four to two million years ago, there was a complex pattern of birth compared to the great apes. “Because australopithecines like Lucy were relatively small in size, but have already shown morphological adaptations to the two-legged, they are ideal for studying the effects of these two conflicting evolutionary forces,” says Heusler.
The typical ratio of fetal head size to adult
Researchers have used three-dimensional computer simulations to develop their findings. Since fossils of newborn australopithecines are not known to exist, they simulate the birth process using different sizes of fetal heads to take into account the possible range of estimates. Each species has a typical relationship between the brain sizes of newborns and adults. Based on the ratio of non-human primates to the average brain size of an adult australopithecine, the researchers calculated the average brain size of a newborn 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 mobility of the pelvic joints during pregnancy and determined realistic soft tissue thickness. They found that only 110 g of the size of the fetal head passed through the pelvic entrance and the middle plane without difficulty, in contrast to the sizes of 180 g and 145 g. “This means that Australopithecus newborns were neurologically immature and dependent on help, like human babies today,” explains Heusler.
Lifelong learning is key to cognitive and cultural abilities
The results show that Australopithecus probably practiced a form of cooperative breeding, even before the Homo genus appeared. Compared to great apes, brains develop longer outside the uterus, allowing babies to learn from other members of the group. “This long period of learning is generally considered crucial to people’s cognitive and cultural development,” says Heusler. This conclusion is supported by the earliest documented stone tools, which date back 3.3 million years – long before the emergence of the genus Homo.
Reference: “Dynamic finite element simulations reveal the early origins of the complex model of human birth” by Pierre Fremondier, Lionel Tolon, Francois Marshall, Cynthia Fornay, Nicole M. Webb and Martin Heusler, April 19, 2022, Biology of communications.
DOI: 10.1038 / s42003-022-03321-z