[MUSIC]
In the animal kingdom, just about wherever we find brains, they're wrapped in heads.
But being so important, you'd think that evolution would have ... I dunno, looked after the brain
a bit better.
It just doesn't make sense that we wrap our most vital hunk of tissue in this thin candy
shell, dangling out in the open where any low-hanging doorway or fastball can crack
it like an egg.
BUT, when ya think about it there's a lot about the human body that doesn't immediately
make sense.
We breathe through the same hole we eat with, we grow more teeth than we can fit in our
mouths, we built our amusement park next to the garbage dump...
To craft this brain story, we're going to need some help.
And we need to head back to the beginning.
With BrainCraft!
The very first animals didn't have brains, or heads, or really...
much of anything going on.
But our ancestors DID know left from right.
Around 550 million years ago the first animals with bilateral symmetry branched off from
those with radial symmetry A radial animal like a jellyfish only has
two body axes to play with, from head to tentacles and out from the middle.
They're a bit ... two dimensional
Bilateral animals suddenly had a new dimension to play with.
Not only could they have a top and a bottom and grow out from the center, they could have
a front and a rear.
So many new possibilities!!
Every bilateral animal uses the same set of genes to control how its body is organized
head to tail.
These genetic choreographers, called Hox genes, control the developmental dance performed
by hundreds of other genes, instructing an embryo to put the head here or tail there.
Hox genes themselves are clustered on the chromosome, strung together in the same order
as the parts of the body they organize.
This is the basic body plan that nature has used to build everything from nematodes to
regular toads, from waterbears to polar bears, and humans to honeybees.
The Hox genes of complex animals like humans and mice aren't so much different from say,
a fly, we just have more of them.
Along our evolutionary journey, these genetic architects were duplicated, allowing the redundant
copies to specialize and sculpt new forms.
Early animals turned random jumbles of a few nerves into telegraph lines from tip to tail,
but they still had no central nervous system.
No brains, no elaborate spinal cords, just a simplest of wires connecting front to back.
Turns out that in order to spell "brain", you need an eye.
If you're in the business of eating, it helps to have them on the end of your body where
the food goes in....the other end is just... not pleasant.
So animals focused their eyespots and other sense organs on their eating ends, and so
more and more nerves began to cluster nearby...
this marks the beginnings of a brain.
Eyes evolved more than once in different branches of life.
And on those branches, bodies and nervous systems continued to grow more and more complex.
As these early brains became more powerful, and valuable, one branch of animals started
to protect their main nerve cord in a protective wrapper
Later, as sensory organs like eyes and scent receptors became increasingly powerful, so
did the brains that processed their information.
Suddenly, they were worth protecting.
A group of animals called the craniates began to wrap theirs in protective skulls, made
first of cartilage, and later bone.
Hardened skulls meant deadlier jaws, so some animals developed fins, all the better to
escape with.
This arms race led to more and more bony body parts.
Our ancestors must have been Daft Punk fans:
[SINGING]: "Harder, better, faster, stronger"
The first vertebrates that walked out of the water would diverge into animals as diverse
as dinosaurs and dogs, but their basic body plan was set.
Brain inside a skull, up at the eating end, close to the sensory organs.
This is why we don't see more variety in vertebrate shapes.
Evolution is kind of like Taco Bell, a seemingly infinite number of creations made from essentially
the same ingredients.
That's how evolution planted the seed that put your brain in your head.
Now why don't you put it to good use,
and head over to BrainCraft and find out how it grows.
Stay curious.
Hey, that's my line.
This video's part of a three part series all about "guts".
If you wanna know more about why your body is organized the way that it is, then check
out the rest of these videos.
If I've uploaded them yet