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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.
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