[MUSIC PLAYING] Hey, smart people.

Joe here.

Bodies, we've all got them, couldn't live without them, but why are they so dumb and unreliable?

I don't know if it's because I just got over a sinus infection, or because my wife just had knee surgery, or maybe it's just me getting older and hurting more.

I'm a millennial, but I have been noticing lately that the human body, it's got a lot of problems, and this video is a big, long rant about a bunch of them.

Don't get me wrong, humans are really awesome.

Look at all that we can do.

[MUSIC PLAYING] It's just that there's so much about our bodies that is flawed, like so many of our parts wear down or are easy to break, and others look like IKEA furniture would look if you accidentally threw away the instructions before putting it together.

It basically functions, but you're pretty sure something's backwards, and somehow you have, like, three of those little twisty things left over.

The great American poet John Mayer once said, "your body is a wonderland," but I think you meant your body is a blunderland.

From eyes that don't work right, and backs that ache, to needy diets and extra bones, what I'm saying is, sure, our bodies look cool, especially if they're wearing an awesome shirt, but who the heck designed these things?

Well, no one did.

We'll get back to that, but first, instead of talking about how great we are, let's talk about some of our critical weaknesses for a change.

[BUZZING] The first example is staring right at you.

I don't wear these things to look cool and smart.

They do make me look cool smart, but I wear them because I can't see.

Like nearly half of Americans and Europeans, or nearly 7 in 10 people in Asian countries, my peepers don't peep right.

I've worn glasses since elementary school.

Space shuttle, nice, very on-brand, younger me.

Anyway, before the invention of corrective lenses a few centuries ago, people who couldn't see just couldn't see.

Back in our prehistoric hunter-gatherer days, that could have meant starvation and death.

Bad eyes, empty stomachs, you lose.

Thing is, even if you don't wear glasses, you have eye problems.

While looking at this image, cover your left eye, and look at the dot, while keeping your face centered in front of the screen.

Slowly move closer or farther from your screen, and the cross will disappear.

Did it work?

Around 30 centimeters or 12 inches away works for me.

Pretty weird, huh?

You can try with the other eye too.

Cover your right one, stare at the cross, and move until the dot disappears.

That's your blind spot, and every animal with a backbone has a blind spot in each eye because of how the eye is built.

The light sensitive layer inside your eye is filled with tiny cells called photoreceptors.

They're like little microphones.

One end turns photons of light into electrical signals, and the other end is a wire that carries a signal away.

Except our retinas are built so the cables are pointed towards the light, like talking to the back of a microphone.

The cables from all those little microphones have to pass through a hole in the retina to get to the brain.

Where that hole is we have a blind spot.

We just don't usually notice it because our brain lies to us and fills in the image.

Why do we have it?

Well, because at some point way back in evolution, when our ancestors started to evolve the first light-sensitive tissues, that's just the direction the cells were facing.

And later, when those patches morphed into actual eyes, it was too late.

The backwards pattern was already set.

Evolution can't suddenly flip a whole eye around.

It can only make tweaks to what's already there.

But cephalopods, like octopuses, squid, and cuttlefish, they don't have a blind spot.

This branch of animals evolved eyes completely on their own, and in early octopus ancestors, the cables on all their microphone-shaped, light-sensing cells pointed towards the back, so their retina is unbroken.

Am I saying that cephalopods have better eyes than us?

Yes.

Point cephalopods, and another point for having eight legs.

OK, enough about eyes.

[BUZZING] Why is there so much empty space in our skulls?

You know I could take you off the set at anytime.

Right?

Watch it, Globey.

When we breathe, air enters our nose and passes through four chambers called sinuses, and there, the air gets warmed up, humidified, and filtered by mucous membranes.

The mucus then drains out and back down your throat, to your stomach.

Gross.

That works pretty well for the sinuses up on top.

They have gravity to help them, but the big ones behind your cheeks, they drain up, up.

That difficult drainage is why humans get so many head colds and sinus infections.

You know who doesn't get sinus infections?

Dogs.

Dogs and other animals that rely mainly on smell tend to have elongated nasal cavities, which drain down and back with gravity, the correct way.

But as our ancestors became more dependent on vision and less dependent on smell, our snouts got all smooshed up into our flat faces.

And now we have these tiny noses and get sick all the time.

[BUZZING] If you accidentally eat some air, well, no biggie, just burp it out.

But if you breathe in your food, you're going to choke and maybe die.

What's up with that?

It comes down to the fact that, like most other vertebrates, we breathe and eat through the same throat hole, another one of evolution's amazing bright ideas.

But I once saw a bird swallow a fish as big as its head.

It did not die.

It just had a delicious meal.

If I did that, I would die, but snakes and birds can swallow huge meals whole, because they're nostrils connect directly to their breathing parts without going through the throat.

It's like an alternate breathing system, but in every mammal, we've just got the one tube.

And all that separates the digesting part from the breathing part is a little flap called the epiglottis.

The epiglottis open, you're breathing.

Epiglottis closed, you can eat or drink.

If you mess up that order, well, here's how to do the Heimlich maneuver.

Now, lots of animals can choke; even whales can choke if fish get stuck in their blow holes.

Yes, that actually happens.

But humans are especially prone to choking because our voicebox, or larynx, has moved up so high in our throats.

I tell you, these throats were made for talking.

Some languages even make vocal sounds using the epiglottis, like in some African languages.

[NON-ENGLISH SPEECH] That higher voicebox has squished up the swallowing parts of our throat, so there's not a lot of room for error, but on the plus side, we can yodel.

So maybe call this bad evolutionary trait a tie.

[BUZZING] So walking upright-- pros, all the cool stuff we can do.

Cons, so many unique and painful ways to injure ourselves.

Some of your body's joints are beautiful.

I'm a huge fan of the jaw, oh, and the hip.

That ball, that socket, it's like Michelangelo sculpted it, but the human knee and ankle look like an elementary school art project held together by rubber bands.

Back when our ancestors walked on all fours, they had twice as many limbs and muscles to carry their weight, but when they transitioned to walking on two legs, it put a lot more stress on our knees and ankles.

When you quickly change direction while running, the anterior cruciate ligament is basically the only thing holding the two halves of your leg together.

It has basically no blood vessels, and if you tear it, the only way to fix it is surgery which we only invented like 100 years ago or something.

I have personally known at least a dozen people who have torn their ACL.

Now, if we were hunter-gatherers or ancient hominids, every one of them would probably be dead.

I don't even-- I don't even know I'm laughing.

That's horrible.

And right under that is the Achilles tendon.

Since we walk on the balls of our feet, that tendon takes basically all the force of the lower leg, like a big, fleshy rubber band.

If you tear that one, you also can't walk.

It's maybe the most important tendon in your body.

So of course, it just sits there on the back of the leg, completely exposed, waiting for the person behind us at the grocery store to just ram it with their cart or, I don't know, maybe your mythical arch nemesis to hit you with a poisoned arrow.

This is not how you'd engineer bipedal legs from the ground up.

It's way too many weak spots for any crucial structural system, but when the assignment was, "turn an animal that walks on all fours into a fancy dancing ape on two legs," evolution had to work with what it was given.

[BUZZING] Body parts are one thing, but evolution has messed up our insides too, like, we are really poorly cut out for eating.

Pretty much every animal needs the same nutrients in order to function-- stuff like amino acids, vitamins, and a few minerals.

But most animals make most of these things for themselves, but we have to get a literal grocery list of nutrients from our diets.

Take vitamins, that's what we call essential macronutrients that we have to get from our diet to survive.

Vitamin C, for example-- more than half a dozen proteins need vitamin C around to do their job.

Without it, your bones get brittle, your tissue breaks down, you just bleed, oh, and your teeth fall out.

Scurvy is no fun.

Pretty important stuff, this vitamin C. So of course, we can't make any, like at all.

We have to get every bit we need from our diets.

Now, almost every animal on earth makes their own vitamin C. My dog never has to drink orange juice, neither does a cow or a cat, but I do.

Strangely, humans have all the genes necessary to make vitamin C in our DNA.

Yet, somewhere in our evolutionary history, in some ancestor of all primates, one piece of that vitamin C machinery mutated and broke.

Now we have to eat it or die, along with all of these.

Of the 20 amino acids we need to build proteins, our bodies only make 11.

Now, many animals can make all 20, but we have to get almost half from our diet.

Needing to have ready sources of these essential nutrients has placed restrictions on where and how our species could live, at least before we could walk into any pharmacy and get them all in pill form.

Which one of these pills is the corned beef, and which one is the cabbage?

Pretty much everywhere you look, it seems like our body has room for evolutionary improvement.

Our teeth-- most people grow a third set of molars, wisdom teeth, that won't even fit in their mouth and have to be removed.

Do I need to mention the fact that a male's gamete-producing organs sit dangerously exposed outside the body?

And the pelvis, most women can't deliver a baby without medical assistance, because the human head is so large.

[MUSIC PLAYING] Who came up with all these bad ideas?

The answer, of course, is no one.

Thanks to science, we know that the human body isn't engineered or designed.

It's evolved.

Everything is the way it is because it got that way, making tiny tweaks to what was there before.

That means that our backs hurt because we're walking upright with a spine that used to be horizontal.

We get fooled, and we fool ourselves, because our brains evolved in a different world from the one that we invented in the past few decades.

Sure, our bodies are full of parts that barely get the job done, full of things that could be built way better, and that can be frustrating, sometimes even painful.

But nobody-- and I literally mean no body-- is perfect, because surviving isn't about being perfect.

It's about being good enough.

It's about being imperfect in the perfect way, and if you're watching this today, then you are good enough, because you're a survivor of a four-billion year story.

Our flaws make us who we are because evolution and natural selection made us who we are, flaws and all.

Stay curious.