Explained at Real Clear Science:
As a genus, humans, from Homo sapiens (that’s us) to our extinct ancestors Homo neanderthalensis and Homo erectus, are wanderers. Over the vast majority of our history, which spans hundreds of thousands of years, we have roved from place to place, inhabiting a wide range of habitats. We moved with the seasons, we moved to find food, we moved — perhaps — just to move. Our adaptability was our key adaptation, an evolutionary leg-up on the competition. The ability to store fat was vital to this lifestyle. Body fat cushions internal organs, but it also serves as a repository of energy that can be readily broken down and used to power muscles. Humans might fatten up at one environment, then move on to another. When food was scarce, we could count on our fat to sustain us, at least temporarily.
Chimpanzees, on the other hand, are localized to specific environments where food is often plentiful, primarily the forests of West and Central Africa. Fatty stores of energy aren’t required, but strength to climb food-bearing trees is. Natural selection favored brawn, causing chimps to shed fat as unnecessary weight.
Clever idea. But thoughts from readers?
(Who would want to be a chimp just to be thin?)
See also: Why human evolution did not go the way analysts would have predicted
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