Regarding sugar and evolution, it’s interesting to note which animals like sweetness and which do not. It’s all about environment and diet. Environments offer niches of habitable conditions. As organisms, by chance, come into contact with those conditions, genetic variations within the species may allow some organisms to benefit from the conditions. Over countless generations the conditions of a niche will select out and shape a new species. If sweetness was not a condition of the niche, the new species may have lost its sweetness receptors.
Felines are a family of cat species that moved into environmental niches in which savory was supreme and sweetness was not a factor. Consequently they have lost their sweetness receptors. Offer a cat two portions of the same food, except that one portion has been sweetened, and the cat will show no preference for, or avoidance of, either portion.
Grazers, on the other hand, have a heightened sense of sweetness, which allows them to taste the sugar in grass. They have a preference for sweetness. Cattle will fight over the sweetest hay.
Omnivorous humans are somewhere in between. We like sweet things, but not to the exclusion of savory. We evolved from a line of hominins that began with a mostly vegetarian diet, but gradually took advantage of environmental niches that included meat. Nevertheless, our sweetness receptors are well developed—too well developed for modern conditions, which leads to obesity.
Infant humans do not learn to like sweetness from mother’s milk. They are born with it. In one study, newborn babies were offered sucrose before they had tasted milk and showed clear signs of satisfaction and pleasure with the taste, even exhibiting faint smiles. The sweetness receptors are already there and send the appropriate signals to the brain.