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why do we have hair on our arms

At first glance, the underside of a human arm may look hairless. But a closer inspection will reveal that tiny, colorless hairs cover it like soft peach fuzz. That s because modern human beings ( Homo sapiens ) are covered with hair it s just difficult to see, said Yana Kamberov, an assistant professor of genetics at the University of Pennsylvania. We are actually very hairy, Kamberov told Live Science. For instance, our foreheads, ears and, yes, even the undersides of our arms, are covered with tiny hairs called vellus hairs, she said. The only places without hair on the outer human body are the palms, soles of the feet, lips and nipples, Kamberov said. [
Essentially, humans are just as hairy as chimpanzees, according to research comparing hair density between the two species, she said. But whereas chimps are covered with scruffy, black hair that s easy to see, most human hair is less visible because it s minuscule and colorless. About 2 million years ago, an adaptation led the genus Homo to miniaturize its body hair, Kamberov said. In addition, Homo underwent an adaptation that increased its number of eccrine sweat glands the glands that most mammals have only on their palms and the soles of their feet. The density of those glands exploded, so if you look at the relative density of these glands in a human and a chimpanzee and a macaque, our density is much higher than what you would expect for a primate of our body size, Kamberov said.


These adaptations helped the Homo genus become exceptional, Kamberov said. Most animals need to take breaks during long runs to cool off by panting, Kamberov said. A horse, for instance, can t pant when it s galloping,. In contrast, humans can run long distances, even marathons, without having to stop, because we can cool off by sweating with our vast number of. In addition, if humans had a lot of scruffy hair, as chimps do, the sweat would just coat the hair and not the skin. When most of our body hair is miniaturized, sweat can coat the skin, keeping it cool by making it wet, and then by evaporating off of it, allowing humans to continue walking, trekking or running without overheating, Kamberov said. That leads to another question, however: Why isn t all of our hair miniaturized peach fuzz? The answer has to do with puberty, Kamberov said. When humans go through puberty, trigger some of the tiny vellushairs to trans-differentiate, or change into that have color, grow longer and cycle, Kamberov said. It s unclear why some vellus hairs respond to hormones and others don t, she said. The same goes for arm hair it remains a mystery why the top part of people s arms have terminal hairs and the bottom side does not.


Perhaps those longer hairs are meant to keep the exposed part of the arm warm, Kamberov said. Another idea is that it might be an adaptation to minimize friction during arm swing, but that s a wild guess, said Daniel Lieberman, a biological anthropologist at Harvard University. And still another idea is that the terminal hairs on certain parts of the human body are. Original article on. Hair is great for running your fingers through and growing make-a-statement goatees, but having hair was once far more purposeful than simply serving as bodily ornamentation. For early humans, hair kept them warm, protected them from cuts and scrapes, provided camouflage, and even served as a nice handhold for the young. They were much hairier than modern humans, and the reason that we lost a lot of body hair over time isn t because we invented heaters and parkas. More likely, our ancestors started hunting in hot, tropical areas -- and bare skin adds to the efficiency of our cooling system. The reason why we kept the tuft at the top? Many experts agree that it had to do with a mating ritual that went a little something like this: The male with the most impressive hair -- or he who could make it look that way -- frightened away his rivals, got his girl, and fathered the next generation. Hence, head hair played a major role in obtaining a partner and successfully producing offspring.


Today, our hair still performs many useful functions, in addition to keeping barbers employed. The hair on our scalps protects us against the sun, and our eyelashes act as our first defense against bugs, dust, and other irritating objects. In the phase of human development when our ancestors had lost their full-body follicular coverage but clothes were still as scarce as skyscrapers, the hair in our nether regions camouflaged our reproductive parts from generation-threatening spears. And by lining our armpits -- we docs call this the axilla -- and groins, our dry hair actually acts as a lubricant, allowing our arms and legs to move without chafing. Also, both then and now, our body hair serves as a protector against malaria. The Anopheles mosquito -- a low-flying bug that likes the legs -- hates hair, in part because hair warns its victim to start swatting. While their bite is painless, our hair signals the presence of mosquitoes before they bite (it s why kids are at greater risk -- they have less hair on their legs). That s most likely the original purpose of hair: it served as an early warning system of bodily threats. We seem to ignore the armor function of our hair today, removing it every chance we get, except on our heads and eyes.

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