Researchers simulated childbirth in chimpanzees and humans and measured the distance between the bony birth canal and the fetal head. The study shows that narrow birth canals relative to the size of the baby's head are not unique to humans
The birth process in chimpanzees and other great apes is generally considered to be relatively easy. This is often attributed to the relatively wide pelvis and small head of their births. In contrast, human birth is more complex and risky compared to other mammals. According to the original hypothesis of the "obstetrical dilemma", our difficulty in giving birth is due to a conflict that developed during human evolution between adaptations in the pelvis for upright walking and an increase in the size of the babies' brains. On the one hand, the pelvis was shortened to improve balance when walking on two legs, but the baby's large head still had to pass through the birth canal. As a solution to this dilemma, the structure of the pelvic bones is different between the sexes (where in women the pelvis has larger dimensions despite a smaller body size), and human babies are born less developed neurologically than other apes, so brain development is delayed until after birth.
An international team of researchers led by Nicole M. Webb from the Senckenberg Research Institute and Martin Hösler from the Institute for Evolutionary Medicine of the University of Zurich, simulated childbirth in chimpanzees and humans and measured the distance between the bony birth canal and the fetal head. The study shows that narrow birth canals relative to the size of the baby's head are not unique to humans. Therefore, the hypothesis of the "obstetrical dilemma", which was previously explained as a result of the development of upright walking and the enlargement of the human brain, did not appear suddenly with the development of modern man, but developed gradually throughout the evolution of primates - and intensified in humans, which explains the high rates of complications in births that we see currently.
The chimpanzee pelvis is just as narrow as the human pelvis
To test the "obstetric dilemma" hypothesis, the team of researchers compared the space available in the birth canal of chimpanzees and humans, using the average distance between the fetal head and the pelvic bones, and taking into account the contribution of the soft tissues. "Using a 3D virtual simulation of the birth process, we were able to show that the space in the pelvis of chimpanzees is actually as narrow as in humans," explains paleoanthropologist Nicole M. Web. It is interesting to note that after a detailed analysis of the shape of the pelvis it was found that the female chimpanzees have a wider pelvis than the males, especially in the smaller females, which provides evidence of adaptations to deal with these width limitations. The researchers also found that the great apes have a similar tendency to humans regarding the neurological prematurity of their children, or their secondary degree of prematurity compared to monkeys - a surprising similarity to humans, although to a lesser extent.
"Based on these parallels, we propose a new hypothesis according to which the obstetric dilemma gradually developed and worsened over evolutionary time. This is in contrast to the previous theory according to which our long and difficult births appeared suddenly with brain enlargement in Homo erectus," explains Martin Heusler. Increased body size in the ancestors of the great apes made the pelvis more rigid, which limited the stretchability of the ligaments during birth. In early hominins, upright walking also led to deformation of the bony birth canal, which required complex movements of the fetal head. The researchers claim that this mechanism, and not the narrowness of the birth canal, is most likely the main cause of difficulty in the birth process in humans.
A complex birth process is an evolutionary compromise
The study shows that the complex birth process in humans is the result of gradual compromises during the evolution of hominoids. "Difficult birth and neurological prematurity of our parents, with a long learning period after birth, are necessary conditions for the development of our intelligence. However, humans are only at one end of the spectrum - we are not unique among primates," notes Heusler. "Cases of assisted births have even been observed in orangutans in captivity. However, births of great apes in the wild are rarely observed - we urgently need more behavioral information," emphasizes Webb.
More of the topic in Hayadan:
