ABSTRACT
The presence of nutritional energy has a critical role in evolution, and the ability to store fat is a key factor in the survival of species in seasonal habitats. Animals hibernating during severe winters, or those that estivate during the dry season, need fat stores to survive. Early fossil apes lived in the tropical forest canopy in Africa and subsisted primarily on fruit, and this diet was retained through the first four million years of human evolution. A mutation of the uricase gene in the common ancestor of great apes and humans at ca. 15 Ma had the effect of raising uric acid levels, which has a role in causing metabolic syndrome where fructose is the main food source, leading to production of fat in the liver. The appearance of the genus Homo was associated with a transition to a hunter-gatherer diet, which was aided by the use of fire for cooking and was associated with a marked increase in brain size. Obesity was probably rare in early human ancestors, but agriculture marked a return to carbohydrate-based diets, with some obesity and diabetes, but the introduction and increase of sugar in post-Medieval diets led to the spread of these diseases.
