SPEAKER 1: I see ghosts of our ancestors in our bodies today.
And a fascinating piece of that past is found in our backs.
One of the reasons why it's falling on our backs
hurts so much is because of our inner monkey.
Each of us have a vestige of our tail inside of us.
We call that the coccyx.
And that sits at the base of our spine.
When we fall on that, it really can hurt.
The reason why it hurts is because it's not
covered by lush tissue or muscles or whatever.
So it's really kind of naked.
So ouch, thank you, inner monkey.
Although it might be nice to have a long tail
to help balance out here, our ancestors lost that appendage
some time ago.
I want to know when that change occurred.
So this is my local zoo, the Lincoln Park Zoo.
I come here with Michelle and the kids all the time,
but today we're here to do something special.
Look at that tail.
So you see tails everywhere here.
This is like tail central to you.
HOLLY DUNSWORTH: It is tail central.
SPEAKER 1: I'm here with anthropologist Holly Dunsworth.
She's taken me to the one place in the zoo where you definitely
won't find a tail, the ape house.
So this feels like home to you, here.
HOLLY DUNSWORTH: Yeah, it really does, it does.
They are, by far, the most like us
of any creature on the planet.
And it's not just because they're
so social and behaviorally complex,
it's also because of their bodies, of their anatomy.
SPEAKER 1: Apes, and that includes us, don't have tails.
It's one of the easiest ways to distinguish
an ape from a monkey.
Although scientists debate why our tails no longer
hang behind us, we roughly know when it happened.
HOLLY DUNSWORTH: We have pretty good evidence
for when we lost the tail from the fossil record for ape
evolution from Proconsul.
SPEAKER 1: Is that straight from Kenya to here?
HOLLY DUNSWORTH: Mm-hmm.
This is a cast of a very famous specimen of Proconsul.
It's an early fossil ape.
It lived about 18 to 20 million years ago.
And this is just one of the many partial skeletons
that we've got of this creature.
SPEAKER 1: By finding many similar Proconsul fossils,
scientists are able to complete a picture
of these ancient primates.
And a tiny bone from the base of the spine
reveals a lot about where its spinal cord ends.
The shape is a good indication that Proconsul
is one of the first ape species without a tail.
HOLLY DUNSWORTH: That is the last sacral vertebra
of a Proconsul.
SPEAKER 1: OK.
HOLLY DUNSWORTH: And I'm going to try to match it up--
SPEAKER 1: Yeah, if you could match it, that would be lucky.
HOLLY DUNSWORTH: --to the one on this chimpanzee.
This little bone here is this part of a Proconsul.
SPEAKER 1: Oh, I see.
So it's that hand.
HOLLY DUNSWORTH: It's a smaller-bodied animal
than a chimpanzee, but you've got that anatomy
there, that ridge, indicating to you that we're looking
at the last sacral vertebra.
It's tapered so much that it indicates that what follows
would have no tail.
SPEAKER 1: It wouldn't be so big.
HOLLY DUNSWORTH: If it was part of a tailed primate,
it wouldn't be tapered.
SPEAKER 1: So taper, no tail.
HOLLY DUNSWORTH: Right, exactly.
SPEAKER 1: Oh, that's awesome.
Proconsul answers an important question.
Sometime around 20 million years ago, our primate ancestors
lost their visible tails and left us with a little souvenir.
By adding this fact to our knowledge of that time,
we are one step closer to understanding
why that transition took place.
You didn't fall once.
Now we're going to go here, ready?
SPEAKER 2: Yeah.
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