![]() Keeping cool is a big problem for many mammals, not just the giant ones, especially when they live in hot places and generate abundant heat from prolonged walking or running. But our bare skin is related to staying cool, as our superior sweating abilities suggest. Neither is it the result of large body size. Human hairlessness is not an evolutionary adaptation to living underground or in the water-the popular embrace of the so-called aquatic ape hypothesis notwithstanding. But all of today’s megaherbivores live in sweltering conditions, where a fur coat would be deadly for beasts of such immense proportions. (On the flip side, mice and other small animals, which have a high surface-to-volume ratio, often struggle to retain sufficient heat.) During the Pleistocene epoch, which spans the time between two million and 10,000 years ago, the mammoths and other relatives of modern elephants and rhinoceroses were “woolly” because they lived in cold environments, and external insulation helped them conserve body heat and lower their food intake. The larger an animal is, the less surface area it has relative to overall body mass and the harder it is for the creature to rid its body of excess heat. The largest terrestrial mammals-namely, elephants, rhinoceroses and hippopotamuses-also evolved naked skin because they are at constant risk of overheating. This fur also protects their skin on land. In contrast, semiaquatic mammals-otters, for example-have dense, waterproof fur that traps air to provide positive buoyancy, thus decreasing the effort needed to float. To compensate for the lack of external insulation, these animals have blubber under the skin. In marine mammals that never venture ashore, such as whales, naked skin facilitates long-distance swimming and diving by reducing drag on the skin’s surface. In subterranean mammals, such as the naked mole rat, hairlessness evolved as a response to living in large underground colonies, where the benefits of hair are superfluous because the animals cannot see one another in the dark and because their social structure is such that they simply huddle together for warmth. Many of these creatures live underground or dwell exclusively in the water. Yet even though fur serves these many important purposes, a number of mammal lineages have evolved hair that is so sparse and fine as to serve no function. Furthermore, mammals can use their fur in social displays to indicate aggression or agitation: when a dog “raises its hackles” by involuntarily elevating the hairs on its neck and back, it is sending a clear signal to challengers to stay away. It also works as camouflage to confuse predators, and its distinctive patterns allow members of the same species to recognize one another. It provides insulation and protection against abrasion, moisture, damaging rays of sunlight, and potentially harmful parasites and microbes. Indeed, it is a defining characteristic of the class: all mammals possess at least some hair, and most of them have it in abundance. Hair is a type of body covering that is unique to mammals. ![]() To understand why our ancestors lost their body hair, we must first consider why other species have coats in the first place. ![]() In addition to explaining a very peculiar quirk of our appearance, the scenario suggests that naked skin itself played a crucial role in the evolution of other characteristic human traits, including our large brain and dependence on language. Thanks to these clues and insights gleaned over the past decade from genomics and physiology, I and others have pieced together a compelling account of why and when humans shed their fur. In recent years, though, researchers have realized that the fossil record does contain indirect hints about our transformation from hirsute to hairless. Finding answers has been difficult, however: most of the hallmark transitions in human evolution-such as the emergence of upright walking-are recorded directly in the fossils of our predecessors, but none of the known remains preserves impressions of human skin. ![]() How did we come to be so denuded? Scholars have pondered this question for centuries. Yes, we humans have hair on our heads and elsewhere, but compared with our relatives, even the hairiest person is basically bare. Every other member of our extended family has a dense covering of fur-from the short, black pelage of the howler monkey to the flowing copper coat of the orangutan-as do most other mammals. Among primates, humans are unique in having nearly naked skin.
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