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As a SciShow viewer, you can keep building   your STEM skills with a 30 day free trial  and 20% off an annual premium subscription at Brilliant.org/SciShow. [♪ INTRO]  It’s happened to all of us. You’re going about  your day, checking yourself out in the mirror.   And all of a sudden, you see a dark, thick hair  growing where it never used to.    To be clear, you always had a hair growing there.

It just  didn’t look like that before.   And scientists do know why some hairs transform from peach fuzz to something a little more luscious. But that   doesn’t make it any less random. While some parts  of your body, like your head and your armpits,   really call attention to their hairy adornment,  pretty much all of your body is covered in hair.   In fact, you have the same number of hairs packed  into each square centimeter of skin as a chimp.

It   just doesn’t look like it because you’ve generally  got two distinct kinds of hair. The light and thin   peach fuzz found in places like your arms and back  is what those in the biz call vellus hair. And the   long, dark, and handsomes more associated with  the top of your head and your unmentionables are   terminal hairs.

That’s not to say that everyone’s  back looks like a baby’s butt. We humans vary in   how dark the hair on different parts of our  bodies can get, so consider those examples   as places that are lighter and darker than the  hair on other parts of your body. And we love   and appreciate both types of hair for their  unique contributions to our survival.

First,   the top of your head needs protection from the  Sun. Specifically, the ultraviolet radiation that   it sends toward the Earth. And thicker, darker  terminal hairs can absorb more UV rays, blocking   them from getting to the skin below.

But instead  of getting that UV protection all over our bodies,   we have vellus hairs to keep us from overheating.  When humans overheat, we sweat. And humans are   particularly sweaty animals. We have significantly  more sweat glands than other primates, but if we   had just as many terminal hairs, they’d cover up  those glands and the sweat they make couldn’t wick   away from our bodies to cool us down.

So in some  parts of your body, it’s really helpful to have   barely there vellus hairs. But in other super  sweaty parts, like your armpits and buttcrack,   you’ve got a bunch of terminal hairs stuck where  the sun don’t shine. They don’t really seem   poised to protect you from a bunch of UV rays.

So  instead, some research suggests that they might   help you spread your musk and attract a partner.  That might explain why these terminal hairs tend   to appear when your body goes through puberty.  But regardless of why your armpit hairs become   voluminous at puberty, it’s clear that that stage  of life is when lots of dark hairs emerge. And I   know it seems like they appear out of nowhere. But  it turns out that they were there all along!

Those   suddenly dark hairs used to be vellus hairs.  And they were nudged into the transformation   of a lifetime by your hormones, specifically your  androgens. When androgens bind to a vellus hair   follicle, they can activate a gene with the catchy  name of hHa7. And once that gene gets turned on,   the vellus hair follicle starts working with more  of the protein keratin, and the hair grows longer,   thicker, and darker.

Now, not every vellus hair  cell in your body has this hHa7 gene turned on,   but they are in some unexpected places, like your  scalp. So they're not just in the places that are   famous for getting super hairy after puberty.  On top of that, some of these cells rocking   hHa7 also have more receptors, which makes it  easier for androgens to swoop in and order the   switch. So some cells, no matter where they  are on your body, are predisposed to a darker,   thicker fate.

As humans move through various life  stages, like puberty, pregnancy, and menopause,   the body makes different amounts of androgens  than it did before. And any one of these could   trigger the transformation of whole patches of  vellus hairs into terminal ones. But what about   those single terminal hairs that show up on just  a random day?

Well, if it isn’t a wayward androgen   molecule activating that hHa7 gene, it could also  be due to an enzyme called 5 alpha reductase. One   of this enzyme’s jobs is to turn the very famous  androgen testosterone into dihydrotestosterone,   or DHT. And when DHT interacts with a vellus hair  follicle, it begins a process that ends with the   hair expressing some genes that aren’t hHa7 that  turn it into a terminal.

In fact, research has   shown that some people who have more terminal  hairs don’t have increased androgen levels. They   just have more active 5 alpha reductase enzymes.  And while we don’t know exactly what causes that   increased activity, it could be genetic. The  science behind the vellus to terminal hair   transition can get a little, well, hairy.

But it’s  not as inexplicable as it may seem in the moment.  And thank you to Brilliant for supporting this  SciShow video! Brilliant is an interactive online   learning platform with thousands of lessons  in math, science, and computer science,   including a course all about Search Engines!  If you’ve googled this question about your body   a thousand times and never come up with these  answers, it could be because the SciShow video   didn’t exist yet. Because usually, search engine  technology can comb through billions of web pages   in less than a second to give you exactly what  you were looking for.

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