According to a recent study, chimpanzees, similar to humans, face challenges with a narrow bony birth canal during childbirth. In humans, this issue is made worse by our upright posture, which leads to a twisting of the birth canal while our babies’ heads continue to grow. As a result, the so-called ‘obstetrical dilemma’ developed gradually through primate evolution, rather than appearing suddenly in humans as was previously suggested.
The birthing experience in chimpanzees and other great apes is generally viewed as easier compared to humans. This is mainly due to their larger pelvis and the smaller size of their newborn heads. On the contrary, human childbirth is comparatively more complicated and risky than that of other mammals. The original “obstetrical dilemma” theory posits that our childbirth difficulties arose from a conflict during human evolution between pelvic adaptations for walking upright and the increasing size of our infants’ brains. While the pelvis adapted to enhance balance for bipedal movement, the larger head of the baby still needed to pass through the birth canal. To tackle this dilemma, the shape of the pelvic bones varies between males and females, with females having wider dimensions, even if they are smaller in size. Additionally, human infants are born with less developed brains than other primates, allowing for brain growth to occur after birth.
A team of international researchers led by Nicole M. Webb from the Senckenberg Research Institute and Martin Haeusler from the Institute of Evolutionary Medicine, University of Zurich, simulated the birthing processes in both chimpanzees and humans, measuring the gap between the bony birth canal and the fetal head. Their findings reveal that the narrowness of the birth canal compared to infant head size is not exclusive to humans. Therefore, the “obstetrical dilemma” hypothesis, which was earlier attributed only to bipedalism and human brain size, evolved gradually through primate evolution and intensified in humans, leading to the higher rates of birth complications we see today.
Chimpanzee pelvis as narrow as that of humans
To validate the “obstetrical dilemma” hypothesis, researchers first assessed the space in the birth canal of both chimpanzees and humans, measuring the average distance between the fetal head and pelvic bones while considering the contribution of soft tissues. “Through a 3D virtual simulation of the birthing process, we demonstrated that the space in the chimpanzee pelvis is indeed as constricted as in humans,” says paleoanthropologist Nicole M. Webb. Significantly, a detailed analysis showed that female chimpanzees tend to have a more spacious pelvis compared to males, particularly in smaller females, indicating adaptations to manage these spatial constraints. Furthermore, they observed that great apes seem to approach human levels of neurological immaturity, or how secondarily altricial their offspring are when compared to monkeys, although to a lesser extent.
“Given these fascinating similarities, we propose a new theory that the obstetrical dilemma emerged gradually, becoming increasingly problematic throughout evolution. This challenges the earlier idea that our prolonged and challenging births appeared suddenly with the brain enlargement seen in Homo erectus,” explains Martin Haeusler. The increase in body size among early great apes made their pelvis stiffer, which limited how much their ligaments could stretch during childbirth. In early hominins, walking upright led to a twisted bony birth canal, necessitating complex movements of the fetal head. The researchers argue that this mechanism is likely the primary reason for the challenging childbirth in humans, rather than just the narrowness of the birth canal.
Complex childbirth is an evolutionary trade-off
The study indicates that the intricate nature of human childbirth is a result of gradual evolutionary compromises within the hominoid lineage. “The challenges of childbirth and the neurological immaturity of our newborns, along with the prolonged learning period that follows, are critical for the advancement of our intelligence. Additionally, humans represent just one extreme — we are not exceptional among primates,” remarks Haeusler. “There have even been isolated cases of birth assistance observed in captive orangutans. However, births among great apes in the wild are rarely documented — there is a pressing need for more behavioral observations,” Webb emphasized.
