>> The thumb is divergent, that is it forms an angle
with the index finger.
So that shows you that it could bring its thumb into opposition
with the rest of the digits.
>> Notharctus also has long fingers
and nails instead of claws.
This is one of the first times in the fossil record
that we see a hand that looks like our own.
The hand of a primate.
It's a turning point in the history of the human body.
An anatomical change that would eventually let us shape the
world around us.
But why did primates evolve such a hand in the first place?
To find out, I'm going to look for clues
where my ancient ancestors lived, up in the trees.
Thankfully, I've got ropes.
Ready. A little knotty up here.
>> Okay.
>> Woo hoo.
These creatures are
so unbelievably agile all through here.
This is their home.
They are able to live on small branches,
big branches, throughout the tree.
They can jump branch to branch.
You know when you're in the tree, what you see is
that the canopy is not one place.
There are lots of environments.
Lots of niches up here.
And one special one is on the ends of the branches.
Because it's there where the flowers
and the fruits and the insects are.
So the rewards are great to be out there.
We call it the fine branch niche.
And for the earliest primates,
being able to navigate the small branches there was vital.
And that gives us a clue as to why their hands were
so different from what came before.
Creatures with really short fingers and claws,
they're really great
at corralling these big thick branches.
But not so much when you get to the ends of the branches.
That's because this sort of hand can't grasp,
lengthening the fingers
and better still adding a divergent thumb means you can
curl your fingers around even the thinnest branches,
grasp them tightly and remain stable.
We think that's why the hands and feet
of early mammals changed.
What you end up with are primate hands, well adapted to moving
around the fine branches of trees just
like these monkeys here.
Embedded in our bodies is our distant past.
The hand I use to write with, to type on a computer,
to throw a baseball, that hand has a long evolutionary history
and one important point in that history was here in the trees
on the fine branches that made a hand
with longer fingers and a thumb.