There you go again, guessing why your opinion may be correct but not providing anything substantive. In Good Calories, Bad Calories, Gary Taubes describes a early 20th Century British group who happily and healthily lived for a year among the Inuit and adopted their all meat diet. Upon returning they accepted a Royal Society challenge to their assertions and did the same diet under controlled circumstances back in London. They stayed thin and fit despite weekly blood tests to catch out cheating.

Evolution does not work so quickly that whole populations (Inuit, Masai) shift over to radically different metabolisms in a few hundred or thousand years. To a large extent, what they can do, we can do.