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Hank Green: Unless you've had it removed, you, like every other person in the world, have an appendix, which a kind of a stupid thing to have in your body. It doesn't do much; it kind of cuddles with your intestines (I imagine your intestines don't mind having someone to cuddle with -- it does provide that function), it also holds on to some gut bacteria, which is nice. But it pretty much does nothing for your entire life until the day when suddenly it has an uprising and decides to kill you.

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The appendix, along with some other completely useless things in your body like wisdom teeth and your tailbone... well, they did not escape the notice of one Mr Charles Darwin. He was a scientist. You may have heard of him.

So Darwin wrote this book -- it was kind of important -- called On the Origin of Species, and in that book he said that these things were vestiges of bygone eras, times when the ancestors of humans somewhere down the line had a use for a tail or some extra teeth for chewing meat or an organ to process a bunch of plant cellulose.

Now, Darwin here was not the first person to notice that humans (as well as some other animals) are tricked out with various equipment that they don't need. Darwin suggested within a certain set of circumstances and they develop body parts based on those circumstances, but when those circumstances go away, those body parts, they fall into disuse, but physical evidence will remain. And the amazing thing is that he was totally right.

They're called "vestigial structures". They're not always organs, so we say "structures", and they're basically one of the many things about our bodies that leads us to believe that we evolved from other species. Here are some other examples of cool vestigial structures that we have.

Goose bumps! Goose bumps are a reflex rather than an organ, but they are considered to be vestigial in humans. The pilomotor reflex is this thing that happens when a bird or mammal gets cold and wants to warm up, or when it sees a predator and it wants to warn that predator to step off. There's this tiny little muscle at the base of every single hair follicle that will contract and pull the hair erect. (Just because I said "erect", that's not funny.) And for a bird or a dog, that might help it get all puffy and look bigger to a predator and hold onto more air so it can stay warm.

The tailbone! Our tailbone, or the coccyx, is this little vestige of a tail because our ancestors used to have tails, and I'm sure Stan won't mind if we show off his butt here. And there you have a little tail! Can you get a zoom in on it? Zoom in on it. Yeah. See? A little butt tail there? He's got a little tail! And so do you. (That was excellent, excellent work. You're a really good actor.)

We see tailbones in all great apes, and we see them in the fossilized remains of our ancestors all the way back to the Miocene period when our ancestors were first trying to grab onto things with their newly opposable thumbs, but now all we have is this tiny, itty bitty stub of a tail.

But it is not totally useless. That little bone is now the anchor to all of the muscles that help us control our continents, if you know what I mean, are stuck to. So thanks, tailbone -- that is an important job and I appreciate it.

And our final vestigial structure, the wisdom teeth. We humans have become such delicate flowers that our mouths no longer have room for all the teeth we need in order to eat all of that woolly mammoth meat that we don't eat any more. So now that people are all agricultural and omnivorous and we generally tend to cook our food well, our jaws are now too small to accommodate all of the teeth that grow into them, and that results in modern humans frequently getting impacted wisdom teeth.

I actually have one of mine left, 'cause they took out three and they couldn't get one. They left the hole, but also the tooth was still there, so I'm worried about this happening to me, but 35% of people born today do not have wisdom teeth at all, so we're actually on an evolutionary trajectory to get rid of them completely, which I'm totally in favor of. Of course, people with wisdom teeth will have to die and not breed in order to not pass on that gene, and I've failed at that, sorry.

And actually one more, to end things on a slightly creepier note. You know this membrane that kinda just hangs out in the corner of your eye? Well, that is, uh, probably a vestige of something called a nictitating membrane, which birds still have to this day and is in fact a, uh, a third and fourth eyelid.

Thanks for watching SciShow. If you haven't, go to youtube.com/scishow and subscribe, because we're very interesting here. If you'd like to ask us questions or suggest video topics, we're on Facebook and Twitter and always in the YouTube comments below. Goodbye.

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