Our ancestors survived through a bias for action (e.g. eating every scrap of food I can get my hands on or clubbing a creature that looks at me sideways) not a bias for planning (thinking about retirement from hunting and gathering).
We do need some fat -- it transports fat-soluble vitamins (A, D, E, and K), carries flavors and helps you feel satisfied after eating -- but look for monounsaturated and omega-3 types and avoid saturated and trans fats.
Various health studies have shown that eating a handful of almonds every day -- a food high in magnesium, Vitamin E and B2 -- can lower cholesterol and help reduce the risk of heart disease.