Compared to other primates, the ability of humans to store body fat is extraordinary: not only do baby humans have more fat, but even the thinnest human has more than any other mammal.
For example, a baby monkey has about 3% body fat, while human babies are born with about 15%.
This is because during evolution our ancestors, who adapted to storing a lot of fat, managed to survive, thrive and reproduce much more efficiently even when there was a calorie shortage and so they genetically passed on the tendency to store fat to future generations.
Fat molecules are an excellent source of highly concentrated energy for our organism, and we can obtain them from the digestion of fatty foods, as well as from complex carbohydrates.
As Daniel E. Lieberman, a paleoanthropologist at Harvard University, explains in his book The Human Body: "A single gram of fat stores nine kilocalories, more than twice the energy per gram of carbohydrates or protein.
The reason we store so much fat in our bodies is not only because of the shortage of calories our ancestors were subjected to but because they depended exclusively on a diet based on the fruits they could harvest or the animals they could hunt on the run.
There is also another reason that contributes to this need: the brain.
Our brain requires a lot of energy to function. So much so that a baby's brain, which is only a quarter the size of an adult's, already consumes about 100 kilocalories a day, i.e. 60% of the energy budget at rest (the adult brain consumes between 280 and 420 a day, between 20 and 30% of the body's energy budget).
Skin fat also acts as a thermal insulator that helps keep us warm, and much of the energy we need for physical exercise comes from fat cells. In other words, for our ancestors, more fat meant less cold, and also more energy to hunt animals or to look for fruit during long walks.
It is unknown when this tendency to store fat began, but it probably started with Homo erectus, which would have helped to feed a growing brain.
The problem is that today there is no shortage of calories. We don't need to travel long distances to get food, and the food we eat is either processed or high in fat and sugar.
Our bodies, however, still behave like our ancestors', which has triggered an epidemic of overweight and obesity.
Quite simply, our bodies continue to be large stores of fat because this is how they were shaped to process it over millions of years in which little body fat was synonymous with little chance of survival. If there were ancestors who did not feel pleasure in consuming fat and who did not tend to store it, they were those who probably did not survive, did not reproduce... and from whom we have not inherited their genes.