True. And if that's the case, it becomes a bit like wondering why people wear more clothing to keep warm rather than just trusting their bodies' natural defense

True. And if that's the case, it becomes a bit like wondering why people wear more clothing to keep warm rather than just trusting their bodies' natural defense against the elements---given that their ancestors clearly got by for thousands of years with just their skin and a maybe few scraps of cloth or animal hide if the going got REALLY tough, who needs down-lined jackets?