(♪SciShow Intro♪) Hank: Heads, shoulders, knees, and toes. The purpose of many body parts is fairly obvious. You need your knees and toes for walking, and how would you watch a YouTube video without your head? But, from baby teeth to butt hair, our bodies also do hold a few mysteries.

Not all of the things our bodies have or produce make it obvious why they are anything but a nuisance, so today, we're gonna show you why you owe a debt of thanks to all of these supposedly useless bits of the human body. First up, earwax.

Michael: I know you've all done it. You've either stuck a finger in your ear or a Q-tip and taken a good hard look at your earwax, and while I don't condone those activities, I completely understand why, because earwax might just be the strangest and most unique substance our bodies make. Earwax, or what doctors and scientists called cerumen, is a complex mix of things like oils, dead skin cells, and any dirt, dust, or microbes the wax has managed to trap. The stuff helps keep your ear canal moisturized, waterproofed, and safe from any invaders, which is pretty awesome for something that most people think is gross and sometimes smells funny.

But to really understand why we should marvel at our earwax, consider the anatomy of the ear. Your ear canal is the only spot on your body where there's skin with a tough outer layer that isn't fully exposed. When old skin cells flake off your arm, they can just fly away, but in your ear, they have nowhere to go.

That makes it really hard to clean your ear canal. These outer layer skin cells, which are called corneocytes, are actually already dead, and as your skin replenishes itself from the bottom up, they need to go somewhere. Earwax is evolution's rather ingenious solution to this problem.

Here's how it works. There are two kinds of glands embedded in your ear canal. There are sebaceous glands, which you have all over your body, except for your palms and soles of your feet.

They make an oily substance called sebum that's useful in lubricating the canal. Then there are also specialized sweat glands that you have only in your ears called ceruminous glands, which release even more lipids, as well as antimicrobial proteins.  The dead skin cells mix with the secretions from these two glands, forming a wax, and, as you talk or chew food, the wax naturally works its way out of your ear canal. So really, earwax is just the leftovers from your ears' self-cleaning process.

At the same time, the wax keeps the skin in the canal from getting too dry. Plus, it can capture dust or microbes and prevent anything dangerous from getting into the part of the ear that hears. So earwax is the ear's natural cleaner and defender.

You don't need to go in with a Q-tip to clean your ears out. The only time wax is bad is when too much builds up, and it starts to make it hard to hear, which is actually one of the most common reasons why people have hearing loss. Good news is, in most cases, all your have to do is remove the plug, but please, for the love of all things aural, do not use a Q-tip.

There are earwax removal kits that dissolve the earwax, and you can flush it out or you can let your doctor do it, but cotton swabs just push the wax in farther, and you risk rupturing your eardrum or introducing an infection. So, try to resist the urge, and know that, unless you have a blockage, the wax is actually doing you a favor.
Hank: So maybe there's no need to spend all that time messing around with cotton swabs. Earwax is weird and greasy, but it's doing its job. Another example of things we produce and spend hours trying to remove are baby teeth. Eventually, they'll probably get pushed out all the way naturally, but unlike earwax, I don't think it hurts to give them a good helping hand.

Just try to keep your next set of teeth in place, because we generally only get the two. Here's how your teeth stack up against those of other animals.

Missing teeth. Not so cute in the lead singer of the Pogues, but pretty dang cute on a smiling toddler. But why do humans have baby teeth, and why do we lose them? It is weird.

It is a weird, weird thing. Humans, and most mammals, are diphyodonts, meaning we grow two sets of teeth in our lifetimes, a permanent set of adult teeth and a deciduous set of baby teeth. Deciduous teeth are smaller and fewer in number, because a toddler's jaws are tiny and could never fit in a full set of 32 adult teeth, because that would be super horrible and terrifying and no one would get anywhere near them.

So instead, we begin life with 20 smaller teeth, which start erupting out of our gums when we're about to turn six months old and are fully in by the time we're two and a half. Just like our permanent teeth, deciduous teeth grow in pairs, meaning that when two incisors erupt from the lower jaw, you can bet that two incisors are gonna erupt from the upper jaw soon. This allows our mouths to bite down and chew evenly and helps ensure that our jaws grow and wear down evenly, too.

Now as we get bigger, we need new teeth, or there'd just be a bunch of space between all of them, but instead of wedging those teeth in between the old ones, we lose the old set and grow a whole new one. That's why baby teeth are called deciduous, just like the leaves on deciduous trees--they will shed at a specific stage of development. Four new molars erupt at the back of our mouths when we're around five or six years old.

Then, our deciduous incisors, which are right here in the front, fall out and are replaced by permanent incisors. By the time we reach puberty, we have an almost full set of 28 permanent teeth. The last four emerge later in life.

These so-called "wisdom teeth" are molars in the back of the mouth. They were super helpful some 100 million years ago when our jaws were bigger, but evolution has made our mouths smaller, and now these molars crowd out other teeth and can cause pain, which is why a lot of people get them yanked out. Now, being a diphyodont might not be the best way to go.

While it sounds nice to have an extra set of something, we only get two sets of teeth in our lives. Polyphyodonts, on the other hand, can grow and regenerate teeth multiple times. These include alligators, fish, even some mammals like elephants, who can regenerate their teeth up to six times to help them enjoy long lives of grinding up plants.

But our second set of teeth will just keep wearing and breaking over time, so take care of them while you can, because they're the only ones you're gonna get. Ah, to be a polyphyodont. Imagine sitting at your desk at work, wiggling out your adult teeth to make room for your elderly teeth.

Maybe that would make us a little less preoccupied with braces. If you lose your adult teeth eventually, who'd care about straightening them? But why do some of our teeth grow crooked at all?

You might need to go to the dentist for a bunch of reasons, from filling cavities to wisdom tooth surgery. A lot of us have overcrowded or crooked teeth and need or want braces to straighten them out, but you don't see other species and think, "Aw, man, that beaver's teeth sure could be straighter." So why are we so prone to crooked smiles? Well, it turns out, there's just not enough space in our modern human jaw for all of our teeth to fit in easily, but it wasn't always this way. In fact, the problem might be linked to the rise of agriculture around 10,000 years ago.  Growing vegetables and raising animals went hand-in-hand with a big increase in how much humans processed their food, like by cutting it into smaller pieces and cooking it.

This made food much easier to eat, so we didn't need as much chewing and biting power. According to some anthropological evidence, this also meant that our jaws shrank, but the number of teeth we had stayed the same, leading to the orthodontic woes of today. One study looked at skulls from before, during, and after the transitions to agriculture in the Middle East and found changes in the jaws.

Jawbones from pre-agricultural times showed a lot of wear and tear on the teeth from intense chewing, but very little crowding. After farming, though, jaws became wider and shorter front-to-back, and they had more crowded, crooked teeth. Another study compared the skull and jaw shapes of 11 modern human populations from around the world, including some farmers and some hunter-gatherers, and the hunter-gatherers had longer, narrower jaws, especially lower jaws, than the farmers, which are better for intense chewing.

These changes in our jaw shape might not be because of changes in our genes, though. It's not necessarily natural selection at work, where the farmers with shorter jaws were more likely to survive. Instead, this might be an example of what evolutionary biologists call developmental plasticity, an organism's potential to grow in different ways depending on its environment.

A study on small rodents called hyraxes, for example, found that animals raised on softer, cooked food instead of raw or dried food had less growth in the bones of their jaws and lower faces. It's hard to test this directly in humans, though, because that would mean limiting what people could eat for a huge chunk of their lives, but it's possible that each of us could develop a bigger, stronger jaw if we only ate tough foods while growing up. So our love of warm, cooked meals, from tender meats to mac and cheese, might have doomed some of us to years of braces.

But in the long run, if we're talking, like, a few overcrowded teeth or, like, chewing your food forever until your teeth break, I'll take the first one, especially if it's got mac and cheese. For all the great things that agriculture has done for us, I guess we can let the crooked teeth slide. Our diet is actually super useful in cultivating gut bacteria that help keep us from getting sick.

The human immune system is complex, and the body has several hubs where immune cells and gut bacteria are centralized. And if there's one underappreciated body part involved in that process, it is the appendix. Here is Stefan to spotlight this undervalued organ.

Stefan: The appendix gets a bad rap. You probably never even think about it, unless it's the reason you're doubled over in abdominal pain, and, for the most part, it gets written off as a useless organ leftover from our evolutionary past. But even though you can usually ignore it, you've gotta give your appendix some credit, because it might not be as useless as you thought. The appendix has evolved in mammals at least 29 times, which is a pretty good sign that it does something.

And back in 2016, an international team of researchers set out to understand why it appears so many times. They started by looking at what kinds of things animals with appendixes have in common, but first they had to define what an appendix even is, since the thing we call an appendix comes in all shapes and sizes across mammals. As a starting point, the researchers defined it as a section of tissue extending from the cecum, the beginning of the large intestine.

They then used computer models to analyze data on hundreds of mammals. They gathered information about their habitats and social behavior and, of course, whether or not they had an appendix. They concluded two things.

The first is that the appendix has evolved more times than it's been lost, so it must have some kind of evolutionary advantage. And second, after finding high concentrations of lymph tissue, which protects the body against foreign invaders, they concluded that the appendix is involved in immunity across mammals. And that seems to be the case for humans, as well.

Within the inner layers of the appendix, we find all kinds of densely packed immune cells, including T and B cells and natural killer cells, which are all important parts of your body's immune response, but we also find a reservoir of good gut bacteria hanging out in there. It's the same type of bacteria that line the insides of the intestine, creating a protective barrier against invaders. Given its prime position just of the colon, researchers think the appendix might dish out emergency rations of gut bacteria in time of crisis, like during cases of extreme diarrhea.

Diarrhea can flush out your intestines, but the appendix may be able to provide a fresh population of the gut bacteria that keeps your digestion on track. They say you don't know what you have until it's gone, and it's true. One way to appreciate the immune function of the appendix is to see what happens when it comes out.

A study published in 2015 found that patients who contracted a particular bacterial infection called Clostridium difficile, were twice as likely to develop a severe infection if they didn't have an appendix. That study alone doesn't prove that the appendix prevents infection, but it does raise questions about whether or not surgeons should remove the appendix when they don't need to, since that might actually cause trouble rather than prevent it. That said, appendicitis can be deadly, so if your appendix needs to come out, it's coming out.

But otherwise, maybe we shouldn't be so quick to dismiss this organ that's usually just trying to be a pal.
Hank: Hopefully you feel a little more love for your appendix now, because, without surgery, you're stuck with it anyway, no ifs, ands, or buts. I did say that because I want to talk a little bit about butts. Why do we have them?

Michael: Why do we have butts? It might seem like a cheeky question, but, if you look around the animal kingdom, even at our closest relatives, big butts are pretty uniquely human. So why would evolution grant us such an asset? We have a few hypotheses.

So our butts are made primarily of two things--fat and muscle--and each likely has its own evolutionary role to play. Like, in general, fat is a resource that's useful for our brains as they develop. A 2016 study proposed that development for both our large brains and our fat stores coincided with the time our ancestors came down from the trees.

See, it takes less energy to transport all that fat when you're walking versus when you're climbing, so by walking on two legs, we could carry more fat, allowing us to develop and power a bigger brain, or so the hypothesis goes. There's still plenty we don't know about the evolution of fat, and bum fat in particular. It's tough to track, because it isn't preserved very well in fossils.

However, we're in slightly firmer territory with the muscle part of all of this, the gluteus maximus. It's the biggest muscle in our bodies, which is only the case for humans, not our ape relatives. We're also the only primates that habitually walk on two legs, so for nearly 200 years, scientists have thought that these two observations are probably related.

Now, your gluteus maximus isn't actually used very much when you walk on flat ground, but it sure is used when you run. As your heel hits the ground with each stride, your body angles forward, and it's that big gluteus maximus that's partly responsible for stabilizing your body and pulling it back. Like fat, muscles aren't usually preserved in fossils, but the bones they attach to are, and the functions of bones and muscles are closely related, so we can make inferences about the evolution of one from the other.

We know that, of our various ancestors, Homo erectus had a pretty human-looking pelvis around two million years ago, which could mean a human-looking butt. This came along with a bunch of other features that make us good runners, things like long Achilles tendons, short torsos, and long legs. Humans are the only primates that can run long distances, and scientists think that distance running helped us evolve new strategies for obtaining food by chasing prey to exhaustion or by quickly getting to carcasses to scavenge them, so butts would have been a key advantage for hunting.

In support of this idea, the evolution of the butt roughly corresponds to a time anthropologists think our diet increased in protein, possibly because we were better at running after our food, and in turn, all that protein was once again helpful for developing those big brains of ours. So the next time you're out for a run or even thinking through a tough problem, remember, thank your butt. It's a big part of being human.
Hank: So next time you have to run to catch a bus or make it to a meeting, thank you butt for making it a little easier. Now, butts let us do the most amazing things, like run or hike or sit on the couch and watch YouTube videos. They can be round or flat or sometimes in between. Some are smooth, and some are hairier, but when you sit on your butt to really think about it, why are butts hairy?

Over the last year, you may have noticed a comment on, we think, every single SciShow video, asking us one question. We have ignored this question long enough. It is time we took it on. Litojohnny wants to know, why does he have hair around his anus?

Well, Johnny, the reason we haven't answered you is because, you know, despite the fact that everyone gets their own personal, pocket-sized supercomputer and that we can send robots to Mars and convert the entire face of the planet to human use, we still do not really know why humans have butt hair. And it may not surprise you to learn that not a whole lot of research has been done on the "why" part of this question about butt hair, but a fair amount of study has gone into the medical problems that butt hair can cause. For example, pilonidal disease is a chronic skin infection caused by hairs that get embedded near the top of the butt crack, which, if you want to impress your doctor, you can describe by its technical name, the intergluteal cleft.

So as the owner of a butt yourself, you probably know that butt hair does seem to have more downside than upside, so given that, what, if any, purpose does it serve? Well, there are a few theories out there, and maybe some enterprising scientist out there watching right now can do some research on them, but here's what's been proposed. Theory number one: Butt hair exists because there's just no significant evolutionary pressure against butt hair.

Sure, it's sometimes inconvenient, and, depending on the moment in cultural history, it might be considered unsightly, but it appears that butt hair has never been a significant reason for one human not to make babies with another human. It's important to keep in mind that not every bit of our physiology needs an evolutionary purpose, so butt hair might just be another side effect of unintelligent design Theory number two: Scent communication. Body odor definitely has a negative connotation in today's world, but there's little doubt that communication through scent has played an important role in the evolution of humans.

After all, that's likely why we have body hair in the same areas where we produce body odors. The hair is there to hold onto sebaceous, or oily, secretions that have their own smell and are also consumed by bacteria that create even more smells. Since we all produce different smell compounds and all have our own microbiomes, each individual human actually smells different.

And if our early human ancestors were anything like other animals, and they probably were, their personal smell probably helped with everything from broadcasting territorial rights to attracting mates. Butt hair, then, may be just another way our oldest human ancestors enhanced their smell profiles. Theory number three: Friction.

In addition to giving off smells, humans have also always done a great deal of walking and running, and skin rubbing on skin, especially in areas where that skin may be moist and dirty, can cause irritation, rashes, and even serious, debilitating infection. It's even possible that those sebaceous, or waxy, secretions that help produce body odor are held in place by body hairs to provide an added benefit, acting like a kind of natural anti-chafing cream. Now this theory, of the one that we've talked about, is most appealing to me personally, but it's very difficult to test, because shaving, or otherwise removing butt hair and then having someone run 20 miles on a treadmill, is not a good experimental design, because there's no way to know whether any irritation is caused by the lack of hair or whatever technique was used to remove the hair, none of which sound fun to me.

But I have come up with an alternative experimental design that I like quite a lot--just interviewing a few hundred runners about how much they need to worry about butt chafing and then measure the density of their anal pelage to see if there's any correlation between whether they chafe and how hirsute their butts are, which is not an experiment that I want to do personally. But if there's an expert out there in anatomy and physiology who's up for tackling this prickly problem, please take it on, and if you get any useful data, definitely let us and Litojohnny know how it went. So we can only guess at the uses of butt hair for now.

Ultimately, a few of these body parts don't do all that much for us, but, at some point in evolutionary history, it was probably super important for something, which is a bigger category of body parts than you might think. So let's talk about why stuff we don't need sometimes sticks around.

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. And so Darwin wrote this book, and 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 that animals evolve 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 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? See the little butt tail there? Got a little tail! And so do you. That was excellent work; you're a really good actor.

We see tailbones in all great apes, and we even 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. So 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 which all of the muscles that help us control our continence, 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 anymore. 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, because 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 are actually on an evolutionary trajectory to get rid of them completely, which I'm totally in favor of. Of course, people with wisdom teeth would have to die and not breed in order to not pass on that, and I failed at that, sorry.

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

At this point, we hope you know a little bit more about yourself, and if you have other questions about random body parts, you can send them to us on Patreon at Patreon.com/SciShow. And if you want to smile more and show off those crooked teeth, check out this video on the science of happiness. 

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