What is head size for?
AFAIK, there is no proof that "intelligence" varies with distance from the equator. Equally, there is no evidence that "intelligence" varies with brain size (for "normal" brains). If there were, I'm sure we'd have heard about it by now :-)
Rather than assuming that distance from the equator allowed brains to grow to a more "productive" size, we should consider what other eveloutionary benefits might arise from moving to a cooler climate. I can think of one straight off: plumbing.
The brain is a very energy-intensive and blood-intensive organ. A larger head in a hot climate will presumably absorb more heat, which is inefficient for the body as a whole and the brain in particular (cheetahs can only run at top speed for a few seconds because their brains can overheat and they can pass out). By adapting the brain's plumbing to a cooler climate, evolution is making sure that growing a head is as simple as necessary (why waste effort - and risk failures - by fitting the plumbing into as small a space as possible?) and giving the body - and the brain in particular - another way to cool down.
Just because mankind's "intelligence" has allowed it to survive and thrive, we assume that it is more important than any other bodily function; nature doesn't care and is far smarter than us!
I'm not a biologist, BTW - please feel free to point out any errors in my assumptions :-)
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