Unfortunately we’re in a vicious cycle here. If what you say is true, we should all be eating higher fat diets; but fat is relatively difficult to digest due to the compromised digestive systems of modern civilized people, which I imagine is why the eating of fat then easily leads to heart and circulatory diseases. It’s inescapable that diet is always set in the context of lifestyle and environment.

However, the rest of the body, including much of the brain, can get energy from fatty acids or from ketones, which are a by-product of the breakdown of fatty acids for energy. In fact, the brain has been shown to overall run more efficiently on ketones than on glucose (except for those parts with not enough mitochondria to use the ketones). It makes sense from an evolutionary standpoint because there’d be no use storing fat in case of food shortage if you could not then turn around and actually use that fat for energy, and because if you’re short on food supply you will need to get smart enough to figure out where to find some more.

Judging by the diets of those indigenous peoples who have been documented in modern times, our ancestors who hunted (over 90 percent of them) deliberately sought out fat-heavy foods because eating a high-plant diet wasn’t practical for much of the year, especially during the Ice Age, and because a diet heavy in protein but low in fat and sugar leads to rabbit sickness. As it happens that fat-heavy diet also nourishes our hearts and our brains. On top of that, a fat-heavy but extremely low-sugar way of eating also allows our bodies to rely on fatty acids and ketones for energy, which keeps insulin very low and prevents hyper- and hypoglycemic episodes.

Pretty neat stuff.