Traditional diets are not necessarily healthier than a good modern diet, although they will keep people alive unless something (like the local Communist/Islamic dictatorship) interrupts the food supply.
One of the big lacks in a lot of traditional diets is protein, either because it is actually unavailable (shortage of animal protein) or because it is artificially limited (for example, European peasants being forbidden to hunt game because the landlord wanted it all).
I'm trying to remember what I read about the switch from Paleolithic Caveman Diets of MEAT & FISH to agrarian plant based diets of early civilization -- I'm positive something was said about its impact on the brain????