A recent examination of pelvises from human skeletons indicate migration patterns and climate may be responsible for global variation in birth canal size and shape. This variation likely represents neutral evolution by random chance and not necessarily the selection for pelvic adaptations, challenging long-held theories for human evolution and modern obstetric protocols.
You might have heard that giving birth is akin to pushing a bowling ball through a straw. While I can’t verify the scientific accuracy of this claim, it turns out that twisting the bowling ball through the straw might actually provide a more accurate (and disturbing) picture. This is because the human pelvis is deeper at some parts and wider at others, so a fetus must rotate several times as it moves throughout the birth canal in order for its entire body, and specifically its head, to squeeze through.
You may wonder why a fetus (and its mother!) is subject to these discomfiting movements during childbirth, but it turns out we have the unique evolutionary history of the human pelvis to thank. The evolution of our pelvises has long been understood to be constrained by two competing forces: the space needed to successfully birth a large-brained infant and the compaction required for upright walking . As both of these factors are so important in human evolutionary history, it has been thought that there can be little room for variation in the shape and size of the human birth canal.
Reflecting this way of thinking, birth canals of women in the modern era have traditionally been classified into 4 predominant types . According to these criteria, pelvises could be classified as round, heart-shaped or oval, with the points of the oval directed either outward toward the hips, or front and back towards the pubic bone and spine respectively. And because each of these pelvic types are associated with a specific pattern of infant rotations during childbirth, deviation from these types is often thought to be anomalous and to potentially lead to high-risk childbirth.
However, a recent paper published in Proceedings of the Royal Society B  challenges this idea that pelvis variation is low due to a trade-off between upright walking and birthing large-brained infants. Authors Lia Betti and Andrea Manica examined the pelvises of 348 female skeletons, which ranged in origin from 2000 BCE to the modern era. Using these skeletons, they quantified global variation in pelvic morphology, and examined the evolutionary underpinnings to this variation.
The authors used 6 different measurements of pelvic diameter to calculate birth canal size for each skeleton, and examined variation in these measurements within and among geographic regions. They also included body mass predictions derived from femur measurements in their calculations in order to account for potential biases when measuring individuals of different sizes. Betti and Manica found that global variation in birth canal size and shape is substantial, with significantly more variation than seen in other parts of the body like the limb or skull bones. Sub-Saharan African populations have deeper-set birth canals from the front to the back of the body, whereas Native Americans have wider canals, and Asian, European, and North African populations have intermediate shapes. This indicates that the human pelvis varies considerably outside of the bounds of the 4 classic pelvic types. While this amount of variation suggests that the human pelvis is less restricted by strong evolutionary constraints than previously thought, the authors specifically sought to test whether selective pressures are responsible for this variation.
Betti and Manica tested the long-held pelvic evolutionary constraint idea by examining whether global variation is due to natural selection for a more advantageous pelvic shape or size, or whether variation is better explained by neutral evolution, the persistence of traits by random processes. They investigated this by quantifying the pelvic variation of the individual skeletons relative to the distance of their population from sub-Saharan Africa, where humans first originated. This distance is a proxy for the age of human populations—populations further away are presumed to be newer and smaller, relative to populations closer to sub-Saharan Africa.
This analysis is based on a prediction of neutral evolution: since newer populations are smaller than the original population, if a trait is evolving neutrally, it is expected to have lower genetic diversity in newer populations than the original due to random chance. Consistent with this prediction, the authors showed that pelvic variation of populations decreased the further away they were from sub-Saharan Africa. These results support the idea that the variation in pelvis structure is driven by neutral evolutionary processes.
Betti and Manica’s findings challenge the long-standing belief that modern-day human pelvic anatomy was solely shaped by the evolutionary constraints of a bipedal lifestyle and infant brain size. Instead, the authors claim that random genetic processes played a significant role in the evolution of the human pelvis.
Neutral evolutionary processes are not mutually exclusive with natural selection, but that does not necessarily mean that the commonly-considered constraint between infant head size and bipedal walking is best-supported. In fact, Betti and Manica suggest that ancient climatic patterns could have shaped pelvic structures. While overall birth canal shape was not correlated to climate, a relationship between climate and pelvic shape was revealed when the distinct pelvic measurements were analyzed separately: parts of the pelvis were wider in colder climates, supporting previous research that a wider core may help retain heat in the cold . This could represent a specific adaptation occurring in these colder regions.
These varied hypotheses serve as a reminder that the evolution of any given trait may be much more complex than we realize. The same trait could be evolving neutrally in some circumstances, under selection in others, or selection and neutral processes could be acting in tandem, even for traits that we think of as being incredibly important.
Examining the evolution of these traits and characterizing their variation is critical as it can have major implications for human health practices today. Of the 4 classic pelvic types, some have been correlated with higher risks of complications during childbirth , thus the extent the pelvic type of an individual conforms to these classic measurements can dictate delivery protocols. However, the measurements that generated these traditional pelvic types were taken from women primarily of European descent, and thus may obscure ethnic variation in birth canal size and shape. Not only does Betti and Manica’s study reveal the extent of the variation that these classic types could be missing, but the authors also urge practitioners to consider this variation when developing and implementing obstetrical protocol. This highlights how research rooted in elucidating our evolutionary history can also inform practices of modern medicine used today.
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