Compared to other animals, humans have a particularly difficult birth: Not only is the baby’s head exceptionally large in relation to the mother’s pelvis – the birth canal is also poorly shaped, so that the baby has to turn like a screw to fit through. This increases the risk of birth complications. But how could such an anatomy, which is sometimes life-threatening for mother and child, assert itself evolutionarily? A study has now come to the conclusion that this form represents an evolutionary compromise.
In the course of human evolution, the brain volume and thus the head circumference of the offspring increased considerably. The larger the baby’s head became relative to the mother’s pelvis, the greater the risk of birth complications. Why didn’t these evolutionary pressures lead to wider pools? And why did a twisted shape of the birth canal prevail instead, which made births even more difficult? One hypothesis is that too wide a pelvis would have made walking on two legs impossible. Another hypothesis suggests that the functionality of the pelvic floor muscles could also have played a role.
Pelvis not very willing to give birth
A team led by Ekaterina Stansfield from the University of Vienna has now examined this pelvic floor hypothesis and confirmed it using computer models. “For most women, the pelvic entrance is shaped like a longitudinal oval, while the exit is transversely oval,” the researchers explain. “Human birth usually involves a complex twisting motion of the fetal head, followed by the shoulders and the rest of the body as the baby passes through the birth canal.” In other animals, including great apes, the birth canal is evenly shaped, which creates complications in labor makes it less likely. “A uniformly designed birth canal would also be an advantage for our species during childbirth,” says Stansfield.
In order to find out what evolutionary advantage this pelvic shape, which is unfavorable for births, has, Stansfield and her colleagues modeled on the computer how the pelvic floor is stressed in different pelvic shapes. The pelvic floor muscles bridge the area between the bones. It ensures that we can hold urine and stool, supports the internal organs and, during pregnancy, the fetus. In view of our upright posture, this is important so that the organs and the fetus do not sag.
The wider, the more unstable
“In line with our hypothesis, we found that the ability to withstand pressure is actually influenced by the shape of the female pelvic floor,” the researchers report. The wider the pool, the more the pelvic floor go through – especially in round-cross oval bowl shape. “Our results show that the longitudinally oval lower birth canal is advantageous in terms of stability,” says Stansfield. This is consistent with clinical observations that women whose lower birth canal is more obliquely shaped are more likely to suffer from pelvic floor dysfunctions such as incontinence and sagging organs.
“This result then prompted us to ask why the pelvic entrance in humans isn’t also elongated,” says her colleague Barbara Fischer. “After all, a uniformly shaped birth canal would probably make childbirth easier because it would make the complex nature of human rotational birth superfluous.” The researchers assume that an oval pelvic entrance would place too much strain on the spine if the body was upright. If the diameter of the upper pelvis were larger from front to back, the spine would have to curve more, which would lead to back problems and impair the stability of the upright posture.
Evolutionary compromise
However, this has less to do with locomotion on two legs than previously assumed: “Researchers have long asked why the human pelvis has not widened in order to facilitate easier births and have come to the conclusion that a pelvis that is too wide is would have made walking on two legs inefficient, ”explain the authors. On the other hand, you yourself assume that the decisive factor was not so much the width of the pool as the depth. “On the other hand, we think that the transversely oval pelvic entrance is a consequence of the limitation of the pelvic diameter from the front to the back in the pelvis,” says Stansfield’s colleague Philipp Mitteroecker. “It is probably limited by our upright posture and not by the efficiency of the two-legged locomotion.”
The twisted shape of the human birth canal was created as an evolutionary compromise between different, sometimes contradicting requirements: “The longitudinally oval shape of the lower birth canal is an evolutionary adaptation to support the pelvic floor. In contrast to this, the transversely oval shape of the pelvic entrance probably developed due to the limitation of the longitudinal diameter by the upright posture, ”the researchers write. “The shape of the birth canal is therefore subject to the selection for birth, pelvic floor stability and upright posture.”
Source: Ekaterina Stansfield (University of Vienna, Austria) et al., BMC Biology, doi: 10.1186 / s12915-021-01150-w