How is it that humans possess such remarkable stamina and endurance? While even juvenile chimps are stronger than humans, they cannot sustain physical effort for long periods like us. Part of the difference lies in our skins. Viewed under a magnifying glass, human skin has the same number of hair follicles per square inch as chimp skin, although in humans most of the hairs remain undeveloped. Humans have, however, approximately ten times as many sweat glands as chimps. The combination of high levels of perspiration and hairlessness make our skin an efficient cooling device that dissipates the chemical heat produced by exertion. This enables us to keep going and going. . . . 

This design feature is critical in more than just permitting sustained physical exertion: our big brains require a delicately controlled temperature range to function. Without our skin acting as an efficient radiator, our brains would quickly overheat during physical exertion – with potentially fatal consequences.