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JoshuaZ comments on [link] Back to the trees - Less Wrong Discussion
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"Agriculture" for the first. For the second, maternal mortality has always been the main limit on human cranial size, Giving birth is ridiculously dangerous to human beings compared with other mammals, and our infants are much less developed, both of which is directly linked to the fact that our heads are such outliers in size, and there is in addition a whole host of finicky adaptions to accommodate the largest head possible. - the infant soft spot in the skull, the extra wide pelvis.. There must have been really absurdly strong selection pressure for "bigger brains" for a long time to create all that, and it pushes the envelope on what is biologically practical for our bodyplan quite hard.
Agriculture seems plausibly relevant here, but I'm confused by your comment about maternal mortality. The fact that it has always been a selective pressure issue means that it isn't plausible that it would be a cause for a decline in brain size unless maternal mortality somehow became worse.