If you are looking for an interesting case study on human evolution, look no further than the female pelvis. The shape of the pelvis is thought to be a compromise between two opposing evolutionary pressures. On one hand, a narrow pelvis is ideal for walking on two feet, a trait that gives us a competitive edge over other species. On the other hand, a wide pelvis is helpful in giving birth to our relatively large-headed babies. Traditional ideas about natural selection—the process whereby traits that help a species survive are passed on through generations—would suggest that humans have evolved one ‘best’ pelvis shape to allow for both safe childbirth and moving around on two feet. However, recent research now shows that this may not be the case.
Anthropologist Lia Betti and ecologist Andrea Manica from the UK teamed up to address this question by examining skeletal samples from 384 female individuals comprising 24 populations across multiple continents. Surprisingly, the researchers observed that pelvis shape was incredibly variable, and that the variation depended mainly on geography. Specifically, Sub-Saharan African populations had the narrowest pelvises, while Native American populations had the widest. Furthermore, differences in environment among the populations did not play a significant role in pelvis shape, despite prior evidence that climate has shaped human body evolution. It now appears that pelvis shape has become variable over time through random genetic changes.
Because pelvis shape plays a key role in childbirth, knowing how it varies in different populations could improve delivery practices in multi-ethnic societies like the USA. This work also holds implications for how scientists think about human evolution. The observation of multiple pelvis shapes reinforces the growing idea that natural selection does not necessarily produce a ‘perfect’ solution to each evolutionary problem, but will most often help produce a solution that is just ‘good enough.’
Managing Correspondent: Benjamin Andreone
News Article: Birth canals are different all over the world, countering a long-held evolutionary theory. Science
Original Article: Human variation in the shape of the birth canal is significant and geographically structured. Proceedings of the Royal Society B
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