Evolution and humans: evolving a bipedal birth canal (Evolution)

by David Turell , Friday, April 11, 2025, 21:30 (2 days ago) @ David Turell

Pelvic canal and head size co-evolved:

https://www.sciencemagazinedigital.org/sciencemagazine/library/item/11_april_2025/42649...

"... we identified 180 loci associated with seven highly heritable pelvic phenotypes. Birth canal phenotypes showed sex-specific genetic architecture, aligning with reproductive function. Larger birth canals were linked to slower walking pace and reduced back pain but increased hip osteoarthritis risk, whereas narrower birth canals were associated with reduced pelvic floor disorder risk but increased obstructed labor risk. Lastly, genetic correlation between birth canal and head widths provides evidence of coevolution between the human pelvis and brain, partially mitigating the dilemma.

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"...One area of contention centers on the relationship between pelvic shape and walking efficiency or walking speed. Some studies have found that there is an association between the two, whereas others have not. Another point of debate revolves around whether variation in birth canal proportions is associated with obstruction during delivery. Recently, appreciation has grown for the concept of a multifactorial pelvis, which proposes that the role of pelvic width reduction is not just to facilitate efficient bipedal locomotion but also to reduce the risk of pelvic floor disorders. Pelvic canal width reduction improves the pelvic floor’s ability to support the fetus and the internal organs and to prevent incontinence.

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"... recent data on a fairly large sample of great apes suggesting that human children are not born significantly earlier than those of the other apes. However, we observed a significant genetic correlation between adult head width and pelvic inlet width (rg = 0.22, P = 2.3 × 10–3) as well as a significant correlation between the width of the birth canal and neonatal birth weight, a proxy for neonatal head size [Pearson correlation, r ∼ 0.7 (58)] (regression slope = 0.01, P = 2.4 × 10–4). This suggests that natural selection might have led to genetic correlation between pelvic and head proportions, potentially reducing the risk of labor obstruction."

Comment: how female pelvic size adjusted to increasing fetal head size is amazing and suggests a dsigner at work.