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Nornagest comments on Why Are Individual IQ Differences OK? - Less Wrong
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Not optimized for intelligence, optimized for neural efficiency. Wikipedia tells me that most mammals devote single-digit percentages of BMR to brainpower, but for anatomically modern humans it's closer to 25%. Maybe more in childhood; Wikipedia doesn't say but the brain-to-body-mass ratio there is higher. When you've got an overambitious monkey burning a quarter of its calories keeping its freaky big monkey brain happy, there are very good reasons for evolution to explore all the corners that it can easily cut, as long as they don't exist in a state of ridiculous abundance. And since we know of at least one population bottleneck in the Paleolithic, I'm pretty sure food scarcity was a thing at that stage.
Neural efficiency at doing what? Our contemporary idea of intelligence involves doing things that evolution did not optimize for. And again -- look at very smart people, look at very dumb people. Is the difference due to neural efficiency?
Sure, but evolution works slowly. Big brains are very new in evolutionary time, it's not like evolution had hundreds of millions of years to polish them.
On the neuron level, all the usual biochemistry that makes neurons go. On the architecture level, any structure that helped paleolithic humans do their thing, and none that didn't. Our thinking seems pretty general and flexible, for example, which means that a lot of the reflexive, special-purpose stuff we see in other mammals would have gotten pruned away at some stage of development.
No. That is, in part, my point. If I'm right about this, we should expect the efficient phenotypes to have reached fixation long ago.
(Strictly speaking, I'd probably expect some of the difference, especially on the low end, to be due to de novo mutations, some of which might have deleterious effects in this domain. But that's a corner case and I think we can ignore it for the purposes of this discussion.)
The usual biochemistry that makes neurons go, as far as I know, is not unique to humans and has been stable for quite a long time.
But I think we're getting a bit confused and start to chase our own tails. Let's circle back and see what the original disagreement was.
You said that "intelligence is metabolically expensive, and it seems likely that it shows some fairly steep diminishing returns in a subsistence farming environment". I countered by doubting that there's any metabolic difference (cost) between an IQ130 and an IQ70 human -- implying that while the returns are diminishing, the cost doesn't change in the range we're talking about. And after that it looks like the core of the discussion is whether somewhat higher intelligence (say, moving the average from 100 to 130) carries enough of a metabolic cost to make the evolution prevent that move.
My position is that the metabolic cost for IQ moves of a couple of standard deviations is sufficiently close to zero.
I would keep in mind that IQ differentials involving increased cranial capacity are more likely to have metabolic costs than ones that don't. It also seems to me that most of the pressure against higher IQs is going to be cognitive rather than metabolic.