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shminux comments on Brain shrinkage in humans over past ~20 000 years - what did we lose? - Less Wrong
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The hypothesis that decrease in brain size corresponded to some partial loss of function is simplest. The only simpler prior is that brain size is entirely unrelated to functionality.
This is pretty much ruled out by fact that brain size (and ratio of brain size to body size) increased through the evolution up to very recently, implying that larger brains must have had an advantage.
With regards to notion that the decrease in size was compensated for by increase in efficiency of the brain, that's just some backwards reasoning starting from "we couldn't possibly have got dumber" and arriving at "we must've gotten smarter by increasing efficiency".
If you try to reason this forwards - why exactly would increase in brain efficiency lead to shrinkage of the brain? It could as well lead to growth, if the more efficient brain pays off better for the extra size. Why would the increase in brain efficiency lead to intelligence-preserving shrinkage, while the change of lifestyle and improvements to the survival due to accumulated knowledge - increase in efficiency of survival if you wish - would not lead to shrinkage as well?
Here is another, just as plausible hypothesis: given that intelligence is determined largely by the amount of grey matter of the neocortex, which is a relatively small part of a mammalian brain, the absolute increase in the grey matter volume would allow for much larger absolute reduction in the brain size without reducing intelligence.
There is nothing inherently "default" about either hypothesis, both require experimental testing just the same. If you privilege one of them, you are committing a cognitive fallacy.
If all you know about two mammals is that they have different brain sizes, then it seems plausible to guess that the one with the larger brain (especially if the brain is larger by mass and as a ratio to body size) has greater overall functionality. This doesn't seem like a particularly privileged hypothesis, just the baseline observation.
Look at the title: "...what did we lose?". It assumes that we lost something, seems like clearly privileging this hypothesis.
Keep in mind that white matter is the wiring that is connecting the gray matter. The glia are the cells that support the neurons. And so on. It doesn't really make most sense to just assume that you can enlarge gray matter, and shrink any of that, without making it work even worse than with more uniform shrinkage.
See, that's precisely why hypotheses are not equally plausible. The hypotheses are:
a: recent shrinkage was accompanied with some loss of function of the shrinking organ
and
b: recent shrinkage would have been accompanied with some loss of function of the shrinking organ, BUT there was a hypothetical low hanging fruit that was picked at same time [proceed with making extra hypothetical assumptions about what the low hanging fruit might be].
The latter hypothesis is strictly more complex than former. Probably complex enough that we wouldn't even have been talking about the latter hypothesis had we not arrived at it via backward reasoning, starting from the notion that we didn't get dumber.
I don't think you have looked at the brains, and at evolution, and have said - ohh, it makes the most sense that shrinkage of the [whatever you think is shrinking] and growth of the gray matter, is the way how the intelligence and efficiency would have been increased. I think you picked a notion that we didn't get dumber, and then thought of the ways how you think things could have went so that we didn't get dumber.
I just reason forwards. I have the shrinkage, I have the relation between size and function, I have the evolution trying to shrink the brain without loss of function for millions years before that timespan, yet the size increased. I reason forwards to loss of function in the recent shrinkage, likely loss of intelligence (which doesn't bother me as I don't believe evolution uniformly leads to things we call better).