It is always a controversial topic, but let forget our animosity towards fat for a moment and consider a) why it exists and b) why does its location vary between sexes.
Why does it exist?
Fat has to have a purpose, otherwise why have it? There are four main reasons:
1) Storage of energy and nutrients to help us during times of hardship. A fairly well known concept.
2) Protection by creating a soft coating around our organs it absorbs shock associated with our everyday impacts (e.g. missing a step, bumping into a wall, etc) as well as the extraordinary events such as falls or blows.
3) Buoyancy to help keep us a float in water. Human have always lived near water and therefore swimming / fishing has always been a part of our life. This is why most professional swimmers do not look as muscular as other athletes, because they have and need more fat to help them float.
4) Insulation due to its location immediately under our skin (called subcutaneous fat) and it’s poor conduction of heat. In the modern world however, it’s insulating role is becoming more redundant with the development of clothes, central heating, air condition, etc.
Why does it exist where it does?
There is no definitive answer, however all current theories relate back to our ancestry. This is because our modern existence makes up a very small percentage of our life on this planet. Thus our bodies are still programmed for the “old way” of living.
The male pot belly
Humans used to live a hunter gathers life and it was the job of the males to hunt, climb and fight. Hence, the chance for an impact was much higher (be it a fall, a blow from a tree, or a fight with an animal or another human). The abdomen, chest and pelvis are all areas that contain our organs, thus they need protection. The chest and pelvis have that via our skeleton, i.e. the ribs and pelvic bones respectively. The abdomen however is relatively exposed. Therefore it makes protective sense for our males to grow fat in this area as it acts as a pseudo-shield around our abdominal organs.
Another advantage of abdominal fat is that it’s very close to our body’s centre of gravity. As hunters, fighters and climbers it was important that males were mobile and nimble. By placing fat near our centre of gravity dramatically reduces its impact on our mobility and positivity influences our stability.
Female buttocks and thighs
So why not use the same approach with females? Well it all comes down to perception and looks. If females were to have fat predominantly stored in their abdomen, it may give the impression they were pregnant. This for obvious reasons would put off a potential mate.
Females still needed to store fat and be nimble. Therefore, the location of the fat still needed to be close to their centre of gravity. The answer was the next closest place, i.e. the buttocks and thighs. Again, this had the added benefit of acting as a stabiliser.
So there you have it, who is to say that fat is all bad. Evolution certainly does not.