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Izeinwinter comments on [link] Back to the trees - Less Wrong Discussion
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Pfffthfhhf. The most likely cause of the shrinkage, in terms of selective pressure, is that maternal mortality and starvation were big killers, and being smart didn't help much when it came to not dying from them. That's no longer the case. More importantly, natural selection for human beings is over. Seriously, the opinion of the blind god is no longer relevant because he is very slow and tech is very fast. Something else will remake our species profoundly long before genetic pressures have time to do anything whatsoever. AI, Genetech, uploading, cyborging... If human beings continue to exist at all, as opposed to just getting a game-over ? Evolution just isn't going to get a say. At all.
If this were the case, how would they have gotten large in the first place? What would have changed to make starvation and maternal mortality larger killers than they were earlier?
"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.
Here is something weird - if EDSC is to be believed, there was no selection pressure outside humans for that. It was basically all human groups fighting. http://en.wikipedia.org/wiki/Evolution_of_human_intelligence#Ecological_dominance-social_competition_model
Wait, it gets better. Ecological dominance also suggests there is probably enough food to go around. No need to compete for that. What else would a mammal compete for? Um, I guess, females. There is a significant chance that the evolution of human intelligence is based on kidnapping brides.
This is utterly horrifying and fascinating. I always assumed intelligence evolved as a way to deal with nature. Turns out, we could deal with nature all right while being dumb. Intelligence evolved because we could not simply decide who fucks whom - so we went to war over it. Gives me the shudders.
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.