Bones Origins of Us

The shape of your face...

Walking on two legs...

The way you see the world...

What makes you the person you are?

The story of each and every one of us

can be traced back millions of years to the plains of ancient Africa.

The answers to the question, "What makes us human?"

lie buried in the ground in the fossils and other traces of our ancestors,

but also lie deep within our own bodies, in our bones, flesh and genes.

As an anatomist, I'm fascinated by the way our bodies

have been sculpted

by our ancestors' struggle for survival.

But why did we leave behind the other apes in the forest...

..and stride out into the African savannah?

How did that change the way we looked...

..give us big muscles in the unlikeliest of places...

..and help us to acquire amazing new skills?

The story of how we became human describes how forest-dwelling apes evolved into us

and the story starts millions of years ago,

with an ape who stood upright and walked.

Our story began around six million years ago,

with apes who lived in an ancient African forest.

In many ways, they would have been similar to the apes

that still live in the forests here today.

I'm here in the ancient forest of Kibale in Uganda,

which covers about 700 square kilometres,

and I'm hoping to do something really special,

and that's to track down some of our closest living relatives - chimpanzees.

I want to get close enough to see how their bodies work,

but getting near to the wild chimps who live in this dense, wet forest isn't easy.

'Francis Mugurusi is my guide.'

Hello, where are the chimpanzees?

'He's been studying the chimps here for nearly 20 years.'

I think we're getting close now.

Francis, my guide, tells me that he can hear the chimpanzees.

He thinks there's two groups, one further away over there,

but also a group which is much nearer, perhaps only five or ten minutes away.

So this is really exciting.

CHIMPS SQUEAL IN THE DISTANCE

CHIMP CALLS

SEVERAL CHIMPS CALLING

Oh, there's lots of them, they're all around us.

This is just extraordinary.

This is my first sight of chimpanzees in the wild.

It's impossible to look at chimpanzees and think that we're not related to them.

Of course, they are our closest living relatives.

I mean, look at the way he's sitting there.

We are so closely related to chimpanzees,

we share nearly 99% of our DNA with them.

BREAKING WIND

I just want to say, that's not me.

But although we're genetically close,

we are not descended from them.

Looking at chimpanzees helps us understand where we've come from

and that's not because we've evolved from them, of course we haven't,

but if we trace back each of our family trees far enough

we reach a point where they come together.

We have a common ancestor with chimpanzees,

going back about six or seven million years ago.

So I'm here visiting my relatives.

Now, their ancestors stayed in the forests, whereas ours moved out.

And if we can find out how and why we did that,

well, that's the story of how we became human.

Our evolutionary journey is written into our bodies

and into the way we use them.

And a chimpanzee's body is built for a particular way of getting around.

(He's fast asleep.)

Literally, just a few metres away.

He's just having a quick look around, but basically he's dozing,

lying on his back with his limbs splayed out.

He's got these wonderfully long arms

and very short legs - he's a climber.

And his feet are wonderful.

He's still got this grasping ability in his feet that we've lost.

He's able to grip onto things and climb.

His great toe, his big toe, is out to the side like that,

so it makes his feet look like hands.

'Millions of years ago,

'our ancestors would have had feet which grasped like this.'

And that's something that we've lost.

'In six million years, our body plan has become very different,

'with our long legs and feet for walking on.'

SHE PANTS

It look as though they've moving quite slowly, but I can assure you they're not.

This is a fairly fast pace to be moving through the jungle.

'So what was it that set our ancestors off on a different path,

'a path that would lead us to colonise the globe,

'whilst other apes stayed in the African forest?

'And when did we start to change?'

It's always been a puzzle.

Until this extraordinary fossil was discovered just a few years ago.

This is Toumai, also known as sahelanthropus tchadensis,

and it's not putting it too strongly to say that his discovery caused something of a stir.

He certainly looks like an ape, and just to convince you of that,

I've got a modern chimpanzee skull

and you can see how similar the two are.

They've even got a similar sized brain.

But there's something very special about Toumai.

And just to explain that, first of all I want to show you

the foramen magnum underneath the chimpanzee skull.

This is where the spinal cord exits the skull.

If I hold the chimpanzee skull in that orientation,

as the skull would be in life,

with the eye sockets in a vertical plane,

we can see that the foramen magnum exits the skull at this angle.

In Toumai it's completely different.

The foramen magnum is right underneath the skull,

which means the skull is balancing on an erect spine.

This isn't any old ape.

This is an ape who stood upright on two legs.

And not only that, this is a bipedal ape,

who dates to six to seven million years ago.

This is a hugely significant moment in our story.

It means that Toumai was moving around on two legs,

very close to the time our ancestors split from chimpanzees.

There's no question he's more chimpanzee-like than human

but Toumai puts standing up right at the start of our journey.

In the six million years since Toumai stood upright,

our skeleton has undergone many changes.

Our bones and muscles have been transformed

by this new way of getting around, upright, on two legs.

I'm a human anatomist - I've studied the structure of the human body

and I've mainly done that through dissection.

And in fact, that's exactly what anatomy means,

it means to take apart.

But today I'm trying out something a bit different.

I'm putting the human body, or at least the skeleton, back together again.

This skeleton is, as you might expect, white,

but in fact that's because these are dead bones.

Living bones are pink because they're full of blood.

Anybody that's broken a bone will know that.

A fractured bone bleeds like crazy.

Living bone in our bodies constantly changes

in response to the stresses and the strains we place it under.

So, over a period of years, all of the bone in your skeleton

is taken away and replaced with new bone.

But standing up on two legs

is dependent on a central yet vulnerable part of our anatomy.

Right in the centre of the skeleton is this wonderful structure,

the spine, built up of a series of repeating vertebrae,

and it forms this beautiful double-S shape.

But all of this anatomical beauty comes at a cost.

With this isolated spine, you can see the curves beautifully,

but you can see something else,

and that's the increase in size of the vertebrae as we go down,

until we get to here, the lumbar spine,

where the vertebrae are absolutely massive.

And that's because they're bearing the weight of everything above them.

So it's not surprising

that this is where we tend to get problems with our spines,

and, in fact, it's the most common reason for visits to GPs.

As we get older, the intervertebral discs start to dry out,

and the inside of them can pop out and press on the spinal nerves,

and that can be painful.

And also the weight that is borne by the spine moves backwards

and now is loaded onto these joints at the back,

so they can be painful, too.

So if standing upright causes us so many problems, why did we do it?

The answer is locked away in the dark recesses of time.

Six million years ago,

the world's climate was becoming colder and drier,

and the forests of Africa were thinning out.

And where dense jungles gave way to woodlands,

the apes who lived in them started to change.

BARKING AND CHATTER OF CHIMPANZEES

You can see what might have happened by looking at apes living today.

'Up in the trees,

'some of the best food is in the most inaccessible places.

'And being able to reach the highest branches is an obvious advantage.'

They are feeding on a fruit.

It's one of their favourite fruits that they feed on.

So they're eating fruits up there?

Yes, they're eating ripe fruits, and there are some that have fallen

with the leaves and branch here.

-Oh, right, yeah. Can I taste it?

-Yes, we can taste.

-So the little yellow ones are ripe?

-Yes, they are ripe, and they like it.

Mmm.

-It's somehow bitter.

-It is bitter.

But for them, they like it.

-It's not one of my favourite fruits.

-It can't be your favourite.

CHIMPANZEES HOOT AND SCREECH

In an increasingly patchy woodland,

being able to stand to reach fruit on the thinnest branches

must have been a great advantage for our ancestors.

And it's possible that this is what drove the changes in Toumai's body.

He could have been standing upright in the trees.

The latest discoveries

show that Toumai was the first of many bipedal apes.

Over the next two million years,

fossils like Orrorin tugenensis and Ardipithecus ramidus

show that other apes were also adapting

to their changing environment by standing upright.

They were still essentially climbers but as the forests thinned,

it's thought these apes were spending more time on the ground.

It's hard to know exactly when

our ancestors gave up a life in the trees for living on the ground.

But there is a clue, hidden away in our bones.

I've been watching the chimpanzees climbing,

and the way their ankles work,

so I want to compare that with my ankle.

Sorry about this, but the boot and the sock are coming off.

Now most of the time I'm walking around on the ground,

and my foot is at 90 degrees to my leg.

But I can move the ankle like this,

that's called dorsiflexion,

to about 20 degrees.

Now compare that with chimpanzees.

To climb efficiently on something vertical,

you need to be able to bend your foot up much more than we can.

When chimps are climbing,

they dorsiflex their ankles up to 45 degrees.

The differences in ankle movement between us and them

could provide vital evidence in working out

exactly when our ancestors gave up climbing for walking.

To nail down when we became walking rather than climbing apes,

scientists at Boston University have been studying the bones

of our ancient ancestors with laser-like precision.

They've analysed the remains of every fossil

they could lay their hands on...

..including the bones of this truly remarkable fossil - Lucy.

She belongs to a species called Australopithecus afarensis.

This is a replica of Lucy, who is one of the most famous,

if not THE most famous skeletons in the whole of human evolution.

She's 3.2 million years old, and we have so much of her skeleton

that we can tell an enormous amount about her.

She would have stood just over a metre tall.

The length of her arms and her curved fingers

suggest that climbing was still really important

in the way she got around.

But recent research is challenging that idea.

There's one area of Lucy's skeleton

that's been the focus of Jeremy DeSilva's exciting new research.

Lucy has a spectacular ankle, uh... and we have some comparisons.

Great, so this is a chimp.

Right, and this is a human, and chimpanzees,

they do remarkable things with their feet and ankles.

They could take the top of their foot

and press it right up against their shin.

It's amazing flexion, which if you and I tried that,

we'd snap ligaments and our Achilles, we just aren't equipped for that.

A big ape like a chimpanzee,

putting all of its body on the foot and on the ankle

while it's climbing like that, leaves its mark on the bones.

On the left is the bottom of a chimpanzee's tibia,

or shin bone, where it forms the ankle joint,

and there's a very obvious trapezoid shape.

On the right is the same area of the human tibia,

and it's square.

The shape of your bones reflects

whether you use your ankles for climbing or for walking.

OK, so...let's have a look at Lucy and compare her.

Well, although she's tiny,

the shape of that joint just there

-is much more human-like.

-It is.

And that tells us that her feet were planted firmly on the ground

directly underneath her knees,

the adaptations we see in upright-walking creatures.

-Fantastic.

-Like us.

-Amazing to be able to tell so much just from the end of one bone.

-Yes.

And the magnificent thing about Lucy is that we have so many bones,

and each one of those bones tells a fascinating story.

Lucy still appears very ape-like,

and her brain was similar in size to that of a chimpanzee's.

But becoming a walking ape

had fundamentally changed the shape of her body.

By the time we see Australopithecines like Lucy,

we can be absolutely sure beyond a shadow of a doubt,

that our ancestors were standing and walking around on two legs.

And not only that, they were committed to walking.

It was their main way of getting around.

'Giving up climbing for walking suggests that our ancestors

'were moving beyond the confines of the forest,

'that they were exploring new habitats.

'But walking is a physical skill that takes time to learn.

'Just think about what these babies are trying to do.

'Balancing on their tiny little feet, defying gravity.

'Some of us get the hang of it quicker than others.

'And some of us aren't in a rush to do anything.

'But most of us will, at some stage in our early childhood,

'stagger to our feet and walk.'

These little ones are just learning

to do something that's incredibly difficult.

They've been quite happy for a few months crawling around on all fours,

but now they really want to get up onto two feet and start walking.

And at any point in time, when she cracks it,

she'll be balancing on just one foot.

'And with each step, this involves coordinating some 200 muscles.

'It's an amazing feat of learning, but there are physical changes too.

'As these toddlers learn to walk, their bodies are changing.'

They're using their muscles in different ways,

and the muscles will develop accordingly.

And deep inside their bodies, their bones are changing as well.

They'll start to develop the backwards curve in the lower spine

and the bottom of the spine will push down between the two hip bones.

The hip bones curve forwards,

and the thigh bone also starts to curve forwards and bend inwards.

But what's really interesting is that we don't know

how much these changes are programmed,

and how much they're appearing,

they're developing, in response to walking.

-Ooh!

-And jumping.

'It's obvious that the evolution of walking

'has had a profound impact on our bodies.

'And it all started with those ancestors who put one foot

'in front of the other.'

It took millions of years for our ancestors

to master the art of standing and then walking.

But walking would fundamentally alter the course

of our evolutionary history.

And the next critical step on the long road to becoming human

was driven by a new wave of drastic climate change.

From around three million years ago, East Africa started to dry

and the forests shrank back.

A brand-new habitat was born - the savannah.

This was a whole new world, rich with opportunity,

and evolution went into overdrive.

There was an explosion of species taking advantage

of the expanding grasslands.

And alongside them were new species of walking apes,

who strode out on two legs into the changing landscape...

forming new branches of our family tree.

Giving up climbing for walking

meant that this group of apes

were in the right place at the right time.

As the forests receded, the walking ape came into its own.

In fact, we know from the fossils that around two million years ago

there were at least six different species

of these hominines, these apes which habitually walked on two legs.

It was a big, bushy family tree.

But while most of those lineages would eventually die out,

one would go on to be extraordinarily successful.

We don't really know why any of the others died out,

but the thought that any of our ancestors could have survived

in this arid, open environment is difficult to comprehend.

For a relatively puny forest ape,

life on the savannah would surely have been a dangerous proposition.

I am feeling quite nervous

and extremely vulnerable, out here on the plain.

I'm keeping my eyes peeled

and I can see some gazelles over there, and some zebra,

but I know that there are much more dangerous animals here as well.

I saw some lions earlier, and a cheetah.

And there would have been similarly formidable predators here

two million years ago.

So, how did our ancestors survive on the open savannah?

This extraordinary fossil skeleton of a young male,

unearthed here in Kenya,

gives us an insight into how our ancestors

managed not only to survive but to thrive on the savannah.

I've really enjoyed laying this skeleton out.

I've seen so many pictures of it

but there's nothing quite like being able to handle the real thing.

Well, actually, this is a replica,

but it is one of THE most famous early human fossils.

And it's just remarkable how much of the skeleton has been preserved,

how many bones we have here.

It dates back to one-and-a-half-million years ago.

He's called KNM-WT 15000,

or, perhaps more poetically, Nariokotome Boy.

And his bones tell us something really important

about a crucial change to our bodies in human evolution.

There are clues all over his skeleton,

but the most striking are in the lower half of his body.

Just look at the length of these legs. It is stunning.

If I put my leg down beside Nariokotome Boy's leg,

you can see that it's practically the same length.

His femur fits along my thigh,

his tibia...

fits quite nicely along my lower leg there.

And these long legs really are

an important step forward in human evolution.

This is the first time we've seen somebody who looks human -

he could be walking out there, in this landscape,

and you would not notice that he wasn't one of us.

Nariokotome Boy was a member

of a species of early humans known as Homo erectus.

He may be nearly two million years old,

but his body plan was obviously highly effective,

because from the neck down, he's so similar to us today.

But his brain was only two-thirds the size of ours.

He didn't get by on his wits alone.

So is there anything else about him that can tell us

how he survived out here?

There are plenty of adaptations here to efficient walking,

but there are also some surprising changes in this skeleton,

which don't seem to be related to walking at all.

He has very large knees

and big hips as well,

and in the trunk, he's got a waist -

he's got a long, narrow waist - the first time we've seen this.

His shoulders have also dropped down away from the head,

and on the back of his skull,

there's the sign of attachment of a very special ligament.

Now, all of those changes are to do with stabilising the trunk -

not something you really need while you're walking.

So what was this boy doing that destabilised him?

'The best place in the world

'to understand Nariokotome Boy's mysterious physique

'is not in Africa, but in Boston, at Harvard University.'

I've agreed to be the subject in an experiment,

so I'm wearing a gym kit and these rather odd items of footwear,

which are more like gloves than shoes,

but in them I'm effectively barefoot, like our ancestors.

'This is the lab of Professor Dan Lieberman.'

1.2m a second.

Here we go. Three, two, one.

'His ground-breaking research has revealed that the shape

'and structure of our bodies has been profoundly affected

'by a particular form of locomotion.'

Just pretend you're strolling along the African savannah.

All of a sudden, you've decided you have to run.

Maybe there's a kudu up ahead to chase - "OK, it's dinner."

We're going to get you up to a nice running speed,

maybe about a ten minute mile.

All right. Are you ready to speed up?

Yep, yep.

Here we go. Three, two, one.

All right. Well, you have a nice gait, nice forefoot strike.

As you're running, you're much less stable than when you're walking.

You're not falling over...

Yep.

..but you ought to be, because every time you hit the ground,

your body wants to fall forward on your chin.

'Staying balanced whilst running is hard.

'As we run, our legs throw our bodies out to the left and right.

'Our shoulders and arms swing in the opposite direction,

'to try to keep us on the straight and narrow.

'But it's not enough -

'we need another crucial element to stay balanced...

'..our long, narrow waists.

'They allow us to twist whilst we run,

'which is vital to counteracting the destabilising forces of our legs.'

Another challenge when you're running is your head.

Every time you hit the ground,

your head wants to pitch forward really fast,

so your arm attaches to a ligament

that's unique to humans - the nuchal ligament - in the back of your head.

Just as your head wants to pitch forward,

the weight of your arm is connected in the mid-line to this ligament,

and it pulls your head back.

This ligament isn't huge, but it's vital for keeping us balanced when we run.

The attachment of that ligament is very obvious in the skull of Nariokotome Boy.

It fixes on this ridge.

Like us, it seems he had a nuchal ligament

to stop his head pitching forward whilst he ran.

So all these different parts of our anatomy - our long waists,

low shoulders and the nuchal ligament in the back of our neck,

seem to be adaptations to running.

They were there in Nariokotome Boy.

Our basic body plan goes back nearly two million years.

But there's one other really important bit of anatomy

when it comes to running, and that's in our bums.

You know what's nice about this? I'm not the person on the treadmill! Usually it's me.

'And it's not a bone, but a muscle.

'It's called the gluteus maximus.'

-We'll put electrodes on your gluteus maximus.

-Yep.

-The largest muscle in your body.

-There are different portions and we want to get the upper portion.

-Brilliant. On both sides?

-On both sides. Both cheeks.

-And I can use this stuff to get a good contact?

That's good, yeah.

'To see what effect the muscle has,

'I need to be wired up with some electrodes.'

And I expect that...they won't be filming you as you put these on.

-No, you WON'T be filming me as I put these on!

-All right.

So, with my bottom fully wired up,

and Professor Lieberman at the controls of the treadmill...

Go!

..it's time to fire up my gluteus maximus.

To begin with, all I need to do is walk.

And then Professor Lieberman turns up the power.

-I'm going to bring you up to a run.

-OK.

-A nice slow run.

Every time this muscle contracts, a signal is sent to the computer.

The stronger the contraction, the larger the signal.

All right, you can stop.

I'm going to stop you now.

The differences between how my gluteus maximus works

when I'm walking compared with when I'm running

are displayed on the computer screen.

So, this is you walking, right? And this is your left gluteus maximus in red,

and your right in green.

And you can see that when your right foot hits the ground in a walk,

-right at this moment, right here in time...

-Yep.

Your gluteus maximus turns on just a little bit.

And it's basically acting to push your leg back as you're walking.

OK.

OK, so now let's go to you running.

Bam. So here's walking, here's running, and you can see

the gluteus maximus, how much harder it's working. An enormous effect.

You don't really need your gluteus maximus to walk, but you can't run without it.

So really, in order to be a good runner, you have to have a good, strong butt.

You cannot run very easily as a biped without a big gluteus maximus.

So, the muscles in my bottom...

..your bottom...

and every human bottom on Earth...

..have been shaped by the fact

that our ancestors evolved a body built to run.

But this running body wasn't built for raw speed.

It evolved to run long distances.

Our ancestors were endurance runners.

In a developed country,

so few of us run on a regular basis that it really is remarkable

to reflect how much our bodies have been shaped by running.

And I think even the fittest amongst us

lead a relatively sedentary lifestyle

compared with our ancient ancestors, for whom running wasn't a choice,

it wasn't a recreational activity, it was essential to survival.

Being able to run long distances could have given

Nariokotome Boy an important advantage.

He could hunt, or compete with other scavengers for meat.

But running in this hot environment

may have changed our bodies in other unexpected ways.

In the searing heat of the African savannah,

running for any length of time can be deadly.

Keeping cool is critical to survival.

Other animals lose heat and control their core body temperature

by panting, and by avoiding the hottest part of the day.

Few animals hunt in the midday sun.

But it's thought our ancestors were able to exploit this niche,

because they developed something incredibly effective.

SHE PANTS

One of the really important ways that we keep cool

whilst running is this - sweat.

'But in order for sweating to work,

'we needed to lose our ape-like body hair.'

One of the most obvious differences between us and other apes

is our hairlessness, but in fact

we're not really naked apes at all,

because our bodies are covered in these very tiny, fine hairs.

So maybe it's more accurate to say that we are furless.

And amongst those fine hairs on our skin are the pores

of up to four million sweat glands,

which can pump out as much as three litres of sweat an hour.

So combined with that furlessness,

this means that we can very effectively

and efficiently lose body heat

from the surface of our skin, through the evaporation of sweat.

Now, when you're running,

you're generating much more internal body heat

than you do whilst walking, and when you're running in a hot place

like this, the need to get rid of all that heat is even more pressing.

So this combination of furlessness

and sweatiness has been put forward

as just one of the physical adaptations that evolved

in our ancestors for endurance running.

And that means, in the heat and over a long distance, we can run down

any animal on the planet, because we can keep cool and they can't.

Our long distance runner's body became our secret weapon.

It took nearly five million years of evolution to get from Toumai

to Nariokotome Boy.

In that time,

our ancestors had abandoned the forest for the savannah,

and had gone from being four-limbed climbers to two-legged runners.

And standing up on two legs had an important knock-on effect.

It freed up our arms.

The anatomy of our legs was completely transformed

as our ancestors became consummate runners and walkers.

But what about our arms and our hands?

I've got a really mobile shoulder.

I have a forearm which I can rotate 180 degrees, and a grasping hand.

Now, these are all relics of our tree living ancestry,

but we took those old adaptations

and used them for something completely new,

something that, in turn, would shape our future.

And that was making tools.

As far as we know, the first stone tool maker was Homo habilis,

appearing around two and a half million years ago.

And every human species since has refined and developed

that tool-making ability.

But we aren't the only animals who use tools.

So what is it about being human that makes our tools so special?

To find out, I've come to the Uganda Wildlife Education Centre.

Hello, I've got stuff in my pockets here. Hello!

Hello, little one. Hello, hello, hello.

'This is a place of sanctuary for young chimpanzees

'rescued from poachers.'

He's biting me. Oh, hello.

'I'm here to see how they use tools, but they just want to play.'

No, no, don't look at that, don't look up there.

So this is Nipper, who's three-and-a-half

and has about as much energy as a human toddler, I would say.

You don't want to walk, do you? You want to be carried.

Come on, then, Nipper.

'As part of their rehabilitation, the chimps here are encouraged to

'do things they naturally do in the wild.'

What's this on here?

'One of them is termite fishing.'

Look at that.

'The centre has built this concrete copy of a termite mound,

'which is full of honey, rather than insects.'

This little three-and-a-half-year-old

certainly knows what he's up to.

Look at that. He's poking this twig into the hole,

and then pulling it back out again with honey on it.

There's no doubt these chimps can use tools,

but it falls a little short of human tool use,

and this might be linked to the way they hold them.

If I was holding this twig, I'm choosing to hold it like that,

and pushing my thumb down to anchor it on my hand.

Nipper is holding it like that.

It's less dexterous,

and it's actually more difficult to guide the twig in.

So could the secret to human tool use be in the way we use our hands?

Our hands move with incredible precision.

They contain a quarter of the bones in the body.

Surprisingly, our fingers themselves have few muscles in them,

they're mainly moved by tendons from the forearm.

Yet anatomically, our fingers and thumbs are very similar

to those of our chimpanzee cousins.

The extraordinary thing about chimpanzee hands

is when you look at them, they look quite similar to ours.

And in inside, they've got the same bones, the same muscles.

So why do we use them so differently?

There must be something going on which makes our hands unique,

and uniquely able to make and use tools.

To unlock the mystery of the human hand,

I've come to the capital of the United States, Washington DC.

Here, new research is shedding light on the evolution of our hands.

This is Professor Brian Richmond.

And for his test, I need to have one of my hands wrapped up

in some very technical electronic equipment.

OK. Does that feel awfully tight?

You probably won't be extending it all the way back like this.

This very strange glove-like contraption looks like

I'm about to play a bizarre virtual reality computer game.

But in fact, these blue strips are pressure transducers,

which are going to allow me

to capture information about how my hand works in real time.

And you can see it on the screen behind me.

The special strips in the glove

measure the pressure I'm generating through each of my fingers.

From left to right on the screen, you can see the force applied

to the little finger, ring, middle finger, index finger and thumb.

The bones and joints of our hands, the muscles

and the nerves that supply them, are set up in such a way

that we have incredibly fine control over the movement of our hands.

But it's not really about moving our hands freely in space,

it's about the pressure that we can apply to objects.

That looks so easy, but tell that to a chimpanzee.

Chimpanzees usually hold a piece of fruit in two hands to eat it.

They don't seem to be able to apply enough pressure with their fingers

to bite into it whilst holding it with just one hand.

In chimpanzees, all of the fingers are very firmly attached

within the hand. But in our hand, the third is firmly attached

and the others are more mobile, particularly the fifth finger.

So we can move that little finger within the hand

much more than an ape can, and we can even rotate that little finger around

to meet the thumb. It's almost like having a thumb on the other side.

It's facing the thumb across the palm.

Precisely. It lets you grasp around an object.

So, with my electronic glove fully activated,

it's time to test just how powerful my flexible little finger is.

And look at that. You can see the pressure on your little finger,

and your thumb on the other side.

Our hands are so mobile that they can conform in any variety of ways

to handle any variety of objects, and that's what makes our hands special

compared to the hands of other monkeys and apes.

But there's something else we have and chimpanzees don't.

It's very obvious when you compare the bones.

The thumb in a human hand is just so much longer and thicker.

If you think of how powerful a chimpanzee's hand is,

ironically, the thumb is quite weak

-compared to the big powerful thumb that we have.

-Yeah.

'But that big thumb is a relatively new bit of anatomy.

'It's only been around for the last two and a half million years.

'And it first appears in Homo habilis,

'our ancestor who made those early stone tools.

'It seems more than a coincidence that big thumbs appear

'at the same time as stone tools,

'and it's always been thought that the two are linked.

'Fortunately, we have the technology to put that theory to the test.'

OK, I'm ready. I've got the hammer stone in my hand

that's strapped up to the monitors, so stand well back.

OK.

There you go, good.

So, if our big thumbs are important for making stone tools,

you'd expect to see a large pressure spike on the screen for my thumb.

What was actually going on with my fingers and thumbs?

So we can see right here that you have force on your thumb,

but you have just as much force on your other fingers as well.

We don't see particularly high force on the thumb.

Why on Earth, then, did our thumb become so big and strong?

If it's not making stone tools,

could it be linked to how we use them?

Let's see what happens when I cut some meat.

That's great. You can see it's sharp, it's really cutting.

It's incredible, yeah. Look at that.

OK, and let's see how your thumb's doing.

Oh, look at that. The thumb pressure is very high.

It's as high or higher than it is on the fingers.

That's interesting. It's a very different pattern from when I was making the tools.

Absolutely, and that tells us that your thumb is having

to really forcefully pinch that tool while it's being used.

And that's not what we saw when you made a tool.

So this tells us that maybe it's using a tool

that helps explain the evolution of a robust thumb,

instead of making a tool.

For the first time, it's becoming clear

that it's how our ancestors used the tools they made

that shaped our anatomy.

The bones in our hands developed as our ancestors' behaviour changed.

It's fascinating to look at the shape

and the function of our hands today,

and to realise how that has been brought about through evolution.

We think about our thumbs being so important,

but it turns out our little fingers are incredibly important as well.

And what's really amazing is that our hands have changed

because of something that we've done. It's not just

about adapting to our environment,

it's about adapting to things that we've made.

The tools that we have created have shaped our hands.

And that ability to use tools didn't just transform our anatomy,

it utterly changed our world.

Dexterous and powerful hands were fundamentally important

to the success of our ancestors.

Our species, Homo sapiens,

only appeared on the planet around 200,000 years ago,

but we are the most successful human species ever.

With our hands, we could make the tools

and technology which allowed us to colonise every corner of the globe.

But they also enabled us to do much more than that.

They gave us the means to transform the world around us.

But it all started back in Africa,

with an ape who got up on two legs and walked.

Our bones and muscles form the foundations of two fundamentally

human characteristics. We are bipedal apes and we are tool makers.

On our long legs, we strode out of our continent of origin

and went on to colonise the globe.

But the dexterity of our hands enabled us to make tools

and transform our environment.

And I think it's really humbling

to realise that our greatest achievements,

our most advanced technology, soaring architecture,

exquisite art and music,

they all depend on an unpredictable series of anatomical adjustments

that changed our ancestors

into walkers and runners and sculpted the hand of the tool maker.

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