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In today's episode Hank talks about hair: What's it good for, what's it made of, and why do we have less than other mammals?

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Here's a crazy statistic, the average man spends more than a month of his life shaving his face presumably. But the steampunk 'stache is just one of the many forms of human pelage or hair and fur which mammals use to regulate body temperature, stay camouflaged, enhance our sense of touch and look fabulous. But if hair is so great, why do we have so little of it compared to our closest evolutionary ancestors, why do we even have it at all, and what's going on up here, as opposed to everything below the up here. Well you might say SciShow has got you covered.

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All animal hair from the stuff on your head to a porcupines quills, to the twitchy whiskers of a mongoose is made of the same stuff, the protein keratin, but humans like other animals have several different kinds of hair, and they change in surprising ways over time. Fetuses in the womb for instance are covered in tiny hairs called lanugo, when a baby is born these hairs fall out and fine unpigmented vellus hairs or peach fuzz grow to take their place. Then of course when our awkward teen years roll around we start seeing certain vellus hairs morph into coarse new growths called terminal androgenic hairs, these develop in response to growing levels of sex hormones called androgens. Both men and women produce androgens, but men make a lot more which is why they usually have more body hair.

So thankfully all our different kinds of hair don't grow at the same rate, if they did we'd all look like Afghan Hounds, that's because your hair follicles go through different phases and rates of growth, turning on and off throughout your life time. For example the anagen, or active growing phase of the hair on your head lasts for years compared to your armpit or nether regions whose anagen phase lasts just for a few months. Your armpit hair can never grow longer than a couple of inches because it's growth pattern is limited, which is nice, you wouldn't want that going down to your knees. So your terminal head hair may be able to reach Rapunzel record book lengths if you never cut it, but sometimes hair needs a break, so it goes through a resting or telogen phase, which often results in breaking and shedding. It's these different phases along with things like shaft density and follicle size that make up the recipe for head hair versus leg hair versus eyebrow hair.

Still unless you suffer from the exceedingly rare condition of congenital hypertrichosis in which the lanugo hair of your childhood continues to grow your whole life unabated all over your face and body, humans appear to practically bare compared to other mammals. But here is a fun fact, we actually have the same number of follicles as our great ape cousins, about five million individual hairs. It's just that our hair is much shorter and finer than that of apes. Which raises the question, what happened to our fur? One theory suggests that it started with early hominid shift to bipedalism, human hair likely started to fade away around 1.7 million years ago, around the time early human ancestors began living the hot, shade less African savannah life and started walking upright exposing less of their bodies to direct sunlight.  

The fact that we still have hairy heads compared to the rest of our bodies may come from the fact that our domes needed built in shelter from the sun more than other parts, though that does beg the question why aren't our shoulders hairy. Another theory maintains that less hair meant fewer parasites and better health. A super hairy body provides lots of shelter for parasites whereas, less hair leaves fewer hiding spots for bugs. So bare skin may have been a good advertisements to potential mates for overall health and fitness, and considering that of course everything has an evolutionary purpose our head hair to help protect us from the sun and light body hair to deter bugs, we may likewise have kept our pubic and underarm hair to help us get it on. Both of these areas house apocrine glands which release the pheromones that help attract potential mates, we have extra thick hair in these hot moist areas to help trap and transmit these pheromones whether we like having it or not.  

So next time you pull out your razor take a second to think about all of the advantages your hair provides you and that it has been millions of years in the making, and then probably just shave it off because that's what society demands.

Thanks for watching this episode of SciShow. If you have any questions or comments or ideas for us we are on facebook and twitter and down in the comments below, and if you want to keep getting smarter with us here at SciShow you can go to and subscribe.         

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