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? ? Science magic show. ? ?
? ? Science magic show. ? ?
? ? Science magic show. ? ?
? ? Science magic show. ? ?
? ? Science magic show. ? ?
? ? Science magic show. ? ?
? ? Hooray! ? ?
-Anna, the phone's ringing! -Thanks, Gladys!
Science help desk.
Anna speaking. -Hey, Anna.
John Cena here.
Just a quick question, one that's been on my mind
for quite some time.
Why do we have butts?
-Oh, my goodness.
I can't believe John Cena just called!
He asked such an amazing question.
We don't tend to think of our butts
as being particularly important parts of our bodies,
but they're actually pretty fundamental to who we are.
Every animal needs a way to take in energy and release waste.
We take in energy by eating through out mouths,
and we release waste, at least in part,
by pooping through our butts.
-Um, duh! -Right, Gladys.
We take our butts for granted.
But believe it or not, the advent of the anus
allowed for the evolution
of all sorts of diverse life-forms.
Here, let me show you.
The first animals didn't have butts at all.
In fact, they didn't even have digestive tracts.
No mouths, no stomachs, no intestines, no anuses.
These animals just let water wash through them,
carrying food in and waste products out.
-Ahh, the good life. -But then,
a few hundred million years ago,
there was a major innovation -- a gut.
The gut allowed animals to be way more efficient
at drawing nutrients out of their food.
But Not all guts are created equal.
Some early guts were just sacs with a single opening.
Actually, lots of animals today have this body plan.
Jellyfish, for example, poop and eat from the same hole.
-Ahh, the good life.
-No, sac-like guts work just fine,
but they're a bit limiting.
Imagine if, after every meal, you had to redigest your food
and go to the bathroom before you could eat again.
You simply couldn't eat as much each day,
which would mean you'd have to use less energy.
Sac-like guts also limit what you can look like.
You can't be very long if your food has to go in
and come out of the same place.
-Ew. -So, the next big thing
to hit digestive tracts
was what we call the through-gut.
For the first time ever, food went in through one opening,
a mouth,
and waste come out through something new,
something revolutionary --
an anus.
Having an anus is so useful that different lineages of animals
have independently evolved to have them.
And there's a huge variety.
Some creatures have anuses next to their mouths.
Some have more than one anus.
And some may even have transient anuses
that actually appear and disappear.
With an anus, metabolisms could get faster,
and bodies could get longer.
Guts could also get more specialized.
With food only traveling in one direction,
animals could have different segments of their digestive
tract perform different functions,
allowing them to eat things they never could before.
-Moo.
-So the great diversity of life on Earth
is due, in part, to the birth of the butt.
-Ahh, the good life. -Wow!
Amazing!
-I know!
Butts are way more fascinating than you might have thought.
-They are pretty interesting,
but that doesn't explain the human butt.
Why do you humans have tushes that are
so weirdly round and fleshy?
And why do so many of you find them so...
attractive? -Hmm.
That's a great question, Gladys,
and I don't know the answer to it.
But I do know someone who's an expert on the subject.
I'm gonna go ask her.
-Don't forget your coat!
You're gonna catch a cold!
Oy. Kids.
-Whoa.
Hey, Ashley. -Hey, Anna.
-Why do we have such big, round, fleshy butts?
-That's a great question.
-This is Dr Ashley Hammond.
She studies the evolution of apes and humans
and, in particular, she's an expert on their butts.
-You can call me a butthead.
-How long did you go to school to be a butthead?
-Eee.
Probably about 22 years, in total.
-What makes that part of the body so interesting to study?
-Well, butts are very specific to humans.
We have a very large, round, fleshy butt,
and you don't really see that in other primates.
-Our closest living relatives, the chimps,
have pretty unimpressive behinds.
Ashley studies the bones and muscles of the pelvis
to understand what makes the human tush different.
-Chimp butts, they have all the same muscles that we do,
but their muscles are different in size
and different in orientation,
because the bones are shaped differently.
-You see, chimps and other animals that walk around
on four legs have long pelvises.
But humans have a short, squat pelvis,
which brings our center of balance
above our feed and our knees.
That helps to stabilize us as we walk upright.
-Our pelvis has been completely reshaped
because we are walking on two legs.
-As a result, our butt muscles are shorter
and wider than those of apes,
which makes our rumps particularly round.
And the size of our muscles is different, too.
So, basically, the gluteus maximus
is like the biggest muscle in the human butt.
-It's the biggest muscle in the human body.
-Oh, man!
Our butts are so huge!
-Our butts are big. -The gluteus maximus
is not the biggest muscle in a chimp's body, though.
In fact, it's not even the biggest muscle in their butt.
They use their gluteus maximus
primarily for something called hip abduction.
If you're abducting your limbs, you're spreading them out wide.
-And this is important if you're a chimpanzee or an orangutan
or really any ape that's up in the trees,
because you have to move through the tree branches,
or you will fall to your death.
So you have to be able to reach real far
and grab the branches around you.
-The human gluteus maximus is what's called a hip extensor.
It helps us pull our legs backward.
It also stabilizes our trunks as we move around on two legs.
And in modern humans, it's especially important
for walking on an incline,
squatting, and running.
So the size of our gluteus maximus reflects
just how necessary it is.
From the bones to the muscles,
our backsides look the way they do
because they help us with the most human of traits --
moving about on two legs.
There's just one more question I need Ashley to answer.
As the epic poet Sir Mix-a-Lot once said,
"I like big butts, and I cannot lie."
So why do many humans have a particular attraction to butts?
-Interest in butts has a long history in primates.
If we look at other kinds of primates,
they are also very interested
in the butts of other animals of their species.
-Some male monkeys have rainbow-colored butts.
It's their way of getting the attention
of the females in their group.
And so their butts are, like, kind of the flashy clothing.
-They're neon signs -- "Look at my butt."
-Some female monkeys and apes have extravagant butts, too.
At certain times of year,
their backsides will blow up like balloons.
-And what that is, it's also a signal,
but it's signaling to the males in their group
that they are capable and ready to have babies.
-What are the differences in humans
between boy and girl butts? -Okay.
So, generally, women have wider butts than males,
and this is because women have to do
one really important task that males do not do,
and that is give birth.
-Humans are smart,
and our babies have particularly large brains,
so human mothers have to have wide pelvises
to let their big-headed babies pass through.
So, while no one knows for sure,
our butts might be a sign of fertility,
because they could indicate
that we're able to have healthy babies.
Ashley, thank you so much
for sharing all of your butt knowledge with us!
Buttheads unite!
See ya!
-All right, butts, where were we?
-Wow!
Who knew that your behinds were so important
to the survival of your species.
-I know, right?
-Well, seems like you've now
thoroughly answered John Cena's question.
Want to break for lunch?
There's a great new Korean place around the corner.
Their bibimbap is fantastic.
-Hold your horses, Gladys.
There's one more butt-centric question I'd like to answer.
We now know why we have butts, but when do we have butts?
-Huh?
-What I mean is, when does your butt appear in your development?
Turns out the timing depends on what sort of animal you are.
Here, let me show you.
Come on, Gladys.
-Oh, geez.
-Let's say you're a human, like me.
You started off as a single cell,
and that cell divided and divided and divided
until it became a hollow ball of cells, balled a blastula.
Then, something unusual happened.
The blastula started to pucker.
A depression began forming in the ball of cells.
Eventually, it poked all the way through to the other side,
creating almost a doughnut shape --
a sphere with an outside wall, an inside wall,
and a hole in the middle.
That inside wall became your gut, and that depression,
that first defining feature in the ball of cells,
became your anus.
The other side of the doughnut hole became your mouth,
and the timing of this development is so important
that our entire lineage is named after it.
All the animals with backbones, plus sea cucumbers, starfish,
and lots of others are called deuterostomes,
which means "mouth second."
In other words, our anuses came first.
However, if you're a snail or an earthworm,
you're known as a protostome, meaning "mouth first."
That initial depression in the ball of cells
became your mouth, and your anus appeared second.
-So, you're telling me
that your butt was your first defining feature?
-That's right, Gladys.
-Wha ha ha ha ha!
-Well, on that note, I think we really have
comprehensively answered
John Cena's question.
So there's only one thing left to do.
-Sing! -
? ? Bright and flashy butts in primates. ? ?
? ? Guess that's one way of getting a date. ? ?
? ? Mouth in the front, and the butt in the back. ? ?
? ? Two different places to poop and snack. ? ?
? ? Butts, butts. ? ?
? ? It's butts for us. ? ?
? ? Butts, butts. ? ?
? ? It's butts for us. ? ?
? ? Gotta go to bed ?
? It's getting late. ? ?
? ? Glad I got a butt. ? ?
? ? Everything's gonna be... ? ?
? ? pretty good. ? ?
? ? Pretty good. ? ?
Hooray!
-Hooray! -
? ? Science magic show. ? ?
? ? Science magic show. ? ?
? ? Science magic show. ? ?
? ? Science magic show. ? ?
? ? Science magic show. ? ?
? ? Science magic show. ? ?
? ? Hooray! ? ?
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