Mammals expend huge amounts of energy just keeping warm. A pelt is nature's insulation. Why would we forgo that benefit? The most imaginative explanation is that our ancestors went through an aquatic phase millions of years ago and jettisoned their fur, which is a poor insulator in water, just as cetaceans have done. Critics say that if you want to keep warm in water you need to be round and lardy, not long and limby. Worse, the "aquatic ape" theory lacks fossil evidence to back it up.
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