C-sections and big heads

PZ Myers posted this story on his blog so I thought I’d share the comment I posted about it here.

“How do we know that big heads would be selected as opposed to small pelvises? The head to pelvis ratio doesn’t necessarily tell us which is the “culprit” unless we have measurements for each. Whereas the mother would have died in childbirth, she can now give birth to daughters who also have small pelvises and also require c-sections, but this would have nothing to do with an increase in cranium size of the baby. Furthermore, to increase in the population, these people would have to be more fit than the average and not just lucky to have been born at all.

Also, not all c-sections are for the same reason, so it makes no sense to assume that an increase in c-sections will lead to any one evolutionary trend — particularly given that in Western societies where childbirth has been heavily medicalized, c-sections are common whether medically necessary or not. It’s entirely probable that an increase in c-sections would have no effect other than to diversify characteristics in the population (as opposed to selecting for particular traits such as large brains).

Furthermore an increase in cranium size would not necessarily be linked to an increase in brain size.

Basically, I think this is bullshit.”

And, pressed for time, I didn’t even get into the limits on the size of the womb and the metabolic energy required for brain development in utero.


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