Not all warm places are easy. It seems to me that it would take plenty of brains to survive in the Kalahari, or to chart the islands of Polynesia. And it’s probably a lot easier to store food in Norway, than in the Amazon.
It's interesting that you mention Polynesia. There was a book that came out at the tail end of the 90s called Guns, Germs and Steel. One of the hypothesis it also puts forth has to do with a culture's ability to thrive and evolve relative to its ease of access to high-quality protein. Apparently, absent the right kinds of protein, the brain's ability to use it's higher reasoning centers becomes compromised.
One of the geographic areas the author talks about is Papua New Guinea - yes, not exactly Polynesia, but right next door. The indigenous tribes in Papua New Guinea have compromised high-learning centers of the brain, and it's due in large part - according to the author - because of their inability to get consistent and quality sources of protein.
Perhaps Norway is a harsh climate physically, but it might also have abundant year-round sources of good protein that's high in Omega-3.
You are quite correct. There is a fallacious notion out there that farming in temperate northern climates somehow requires more foresight than farming in tropical climates, and therefore the northern farmer acquired greater intellect than the lazy tropical farmer who merely has to pick bananas off a tree.
This idea is completely false, and is spread by those who have no notions of the difficulties of tropical agriculture. Predators, plant and animal diseases, and crop destroying insects, as well as thin calcareous soils, and extended dry seasons followed by torrential wet seasons are just a few of the factors that make tropical agriculture far more difficult than temperate climate agriculture.
Also note that simply clearing a plot of jungle for planting is highly laborious