Wikipedia and the scientific community
Fat and buoyancy
Human beings are the fattest apes and most of our fat (adipose tissue) forms a continuous layer under the skin (subcutaneous). All mammals can store fat, but most of them store it internally, usually around the organs or in the abdominal cavities. However, there are a significant number of other species which, like us, store fat under the skin, for example: whales, dolphins, seals, penguins, pigs and bears, to name a few. They all either are or were aquatic, semi-aquatic or they are hibernators.
William Montagna, arguably the world's leading expert in mammalian skin, writing in the Journal of Human Evolution in 1985 writes "Together with the loss of a furry cover, human skin acquired a hypodermal fatty layer (panniculus adiposus) which is considerably thicker than that found in other primates, or mammals for that matter."  Current scientific theories are unable to offer any consolidated opinion as to why we so much fat although there are a number of theories. On the whole - just as they do regarding explanations for bipedalism and furlessness - scientists tend to ignore the question. Fat is important for storing energy and providing warmth, but it is also an excellent buoyancy aid as it 90% as dense as water (fat tissue 95%). The fatter a person is, the less energy they need to expend to stay afloat, with some particularly fat/obese people not needing to expend any energy at all to do so. In fact, although all large or medium-sized aquatic mammals have more fat than terrestrial ones, it would seem that the primary purpose for surface feeders is buoyancy, and not for keeping warm outside the water as Caroline Pond noted: "Bottom feeding species such as walruses and bearded seals have a thick, collagenous skin, relatively little subcutaneous fat, and a massive skeleton, while seals that feed nearer the surface have ... relatively thick blubber." 
Fatness at Birth (see also birth and babies)
Humans are among the fattest mammals at birth reflecting a rapid fat deposition during the third trimester of pregnancy. For most mammals, roughly 2-3% of birth weight is fat. A newborn human baby is made up of 16% fat, whereas a new-born chimpanzee averages 3% of birth weight as body fat. The only other animal that exceeds this percentage are seals, which are born with a comparable level of fat. There are two main types of fat: brown and white adipose tissue. Brown fat burns up quickly when body temperature falls, whereas white fat is only depleted when the body's intake of calories is insufficient to replace expended energy. Subcutaneous white fat is an insulator in some mammals; it was traditionally assumed that humans evolved extra baby fat to compensate for the loss of fur because they are unable to raise their temperatures by shivering, but most human baby fat is subcutaneous white adipose tissue, rather than internal heat-producing brown fat. The evolutionary importance of fat as a source of insulation outside the water in humans is debated, and there is minimal evidence that human baby fat evolved to serve this role. 
Adipocytes are the cells which store fat and which can expand to thrice their normal size when saturated. The more adipocytes an animal has, the more fat it can store. Human beings have around 25 billion of them, ten times more than any other land mammals our size and ten times more than we presently need. When 191 different mammal species were compared, Caroline Pond noted:
Hedgehogs, of course, are hibernators, and like bears, they need to store a lot of excess fat to survive the winter. When they wake up in the spring, they are lean once more . Fin whales, on the other hand, require a thick layer of fat (blubber) to insulate them in the cold Atlantic ocean. It was this, more than any other feature, which first alerted a young marine biologist called Alister Hardy to the idea that the ancestors of Homo sapiens may have acquired this fat layer in the same way that whales, dolphins, seals, penguins and hippopottomi acquired theirs.
Just because we don't need a specific acquired evolutionary feature anymore it doesn't mean that we will lose it. That's why we still have an appendix for instance and clearly, we still have an issue with fat. In the western world, we are seeing more and more overweight people, and an ever increasing number of obese and morbidly obese people. At the same time there is an increase in the number of people suffering from health problems: cardiovascular problems, diabetes, etc. It is natural to assume that being fat is in itself the cause of all these problems and this is what health professionals and the media have led us to believe. However, more evidence is appearing all the time to suggest that there may not be such a clear link to these issues (diabetes, heart disease) as previously supposed. Instead, it increasingly appears that it is poor dietary choices that are responsible for poor health, and often the same choices lead to an excess of fat, but one is not necessarily linked to the other. There is even a case for arguing that being overweight may be good for our health. If we lived in water all the time, we certainly wouldn't worry about most of the discomfort extra weight causes on land, just as a blue whale doesn't worry.
The percentage of fat in a woman's body is on average twice as much as in a man's and tends to be more evenly distributed with extra fat in the breasts, thighs and buttocks. We can suppose that fat women have been around for a while, as demonstrated by numerous idolised figurines such as the Venus (or Woman) of Willendorf and the Venus of Hohle Fels, which date back 25,000-35,000 years. We suppose that 'fatness' is a disadvantage of the modern age and that the original human model is 'thin' and society does little to dispel the notion that the ideal human form, especially for women, is as slim as possible. But what if our ancestors were in fact much fatter than we are today and fat women in particular seen as more fertile, or more attractive, the larger they were? Unfortunately, fossils do not give an indication of how much fat a person may have had on their bodies, so we can never really know for sure.
In human beings a woman needs a certain amount of body fat in order to conceive a child. Any woman whose fat levels fall below 17% of her body mass will cease to menstruate and will not ovulate again until her fat levels increase . As we have seen, the human foetus begins to rapidly lay down an enormous amount of fat during the third trimester of pregnancy, and continues to acquire this fat for several months after birth, and this requires extra resources from the mother, who has to increase her food supply through every season, including winter, when pickings may be slim. Having a sufficient store of fat in her body ensures that she can manage this. Therefore, in past times, the fattest women would have been the most successful breeders, and this in itself would have ensured that the trait was passed on.
As we have also seen, fat increases buoyancy, and in women, fat seems to increase with age, before finally levelling out around the time of menopause. Women, as well as most other mammals, on the whole cease to ovulate while they are lactating, which in natural societies can last for several years. During this time, an ancestral mother in the water would not only have to stay afloat herself, she would also have to have supported younger children and babies while she swims. The older she is, the more children she would likely have had to look after, and the fatter she was, the better able she would have been to do this. Once she reached menopause, like killer whales, she would still have been able to assist her grandchildren in the water and so help her daughters who still needed to dive for food.
Losing our fur while living on the savannah, makes little sense. Whether chasing prey or running from predators, the extra weight would have slowed our ancestors down and caused us to overheat. In contrast, at night, when temperatures drop considerably, it would have been a far less efficient insulator than a coat of fur, which is why, we can suppose, most savannah animals, including baboons, have retained their fur. Conversely, while a coat of hair provides the best insulation in air, a layer of fat gives the most efficient protection against heat loss in water. 
The only mammals which accumulate fat as a way to store energy are hibernators: bears, hedgehogs, marmots, dormice; or in some instances, animals that need to store energy because they live in deserts or in colder latitudes: fat-tailed sheep, the fat-tailed gecko, camels. The former have a seasonal fat layer, which burns off quickly when the animals are active, and the latter store the fat in specific locations where it will not impede locomotion. Neither of these arguments applies to Homo. And why would we need to accumulate a fat layer on the savannah to store energy when food is supposedly in abundant supply? This argument also doesn't explain why women and babies have more fat than man the mighty hunter.
The theory supposes that the sudden increase in carbohydrates from growing crops around 10,000 - 12,000 years ago, and the reduced calorie requirement from no longer chasing animals across the plains meant that we could afford to store excess calories as fat. But why would we need to store energy on our bodies when at the same time we were creating recepticles and barns to store our grains through the winter? Also, there are nomadic tribes alive today who have never practiced agriculture, and yet they still store fat in their bodies. Furthermore, the Venus figurines cited above predated agriculture by several tens of thousands of years, so it seems unlikely that our fatness stems from so recent a period.
Women became fat in order to be more attractive to men. As we have seen, fatness could be a sign of fertility in women and a promise of healthier babies more likely to survive, but if that is the case, why have men also acquired a layer of fat? Most epigamic markers (the peacock's tail, the lion's mane) are passed on only to the specific sex that benefits from it, and they only begin to show when the species reaches sexual maturity. Once again, this argument does not explain why human babies are born so fat.
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