What Makes us Human? Horizon

I'm Alice Roberts.

I'm expecting my second child in a few months

and I'm having a day out to visit some relatives.

Hello, hello.

Do you want this grape?

This is Le Puri.

She's a baby bonobo, or pygmy chimpanzee.

And of all animals alive today, she's one of my closest relatives.

Well, this is bringing out all the maternal instincts in me.

Oh, hello, Le Puri!

Le Puri may be cute, but having reached the grand old

age of one, she's much more developed than a one-year-old human.

She and I share 99% of our DNA and yet from the moment of birth,

our lives are so very different.

When my baby is born it will take him a year to even walk,

and yet with time, as a human, his life will develop a richness

far beyond that of our hairy ape cousins.

So what is it about our bodies, our genes

and ultimately our brains that sets us apart?

What is it that truly makes us human?

I've come somewhere I've long been keen to visit,

the great ape enclosure at the Max Planck Institute at Leipzig Zoo,

one of the biggest collections of great apes on the planet.

As well as humans, the family of great apes is made

up of gorillas, orangutans, chimpanzees and bonobos.

It's really lovely watching the little ones with the adults

because they're doing what you'd expect a toddler to do.

They're being annoying.

They're kind of going up and tickling the adults

and they've got this mischievous look in their eyes.

These apes are our closest living relatives.

And I'm here to find out what makes this particular ape,

the human one, different to all the others.

Very difficult to sketch them, really.

They stay still for a minute and then they're off again.

For me, as an anatomist,

the first thing to do is to look at the differences between our bodies.

Gorillas are the largest of the great apes

and looking at the massive silverback right

at the back of the enclosure there, he really is enormous.

He's magnificent.

They're sitting quite nicely still so you can get a real appreciation

of the similarities and differences between their anatomy and ours.

There's an awful lot about them which is very similar, in fact.

So if you look at the construction of the arms

and the legs, you've got all the same bones there.

But they are different shapes and different proportions,

so you can see that the arms are very long compared with the legs.

They've got very short legs compared with the rest of their bodies.

We've got very long legs, ridiculously long legs for an ape.

These differences relate to how we move around.

Whereas gorillas and other apes knuckle-walk on all fours,

we humans, uniquely, habitually walk around upright, on two legs.

And there's another obvious difference that becomes

apparent when you start drawing heads.

It's quite difficult sketching them.

I find the faces particularly difficult because you've got

such an idea in your head of what a human face looks like, and you

have to forget that entirely when you're sketching these apes, because

the proportions of their faces are entirely different from ours.

If you look at a human head,

the eyes are quite low on the head, perhaps about halfway down

if we measure from the top of the head to the chin.

If you look at a gorilla head, the eyes are right up on the top

because their brain case is so much smaller than ours.

They don't have this massive forehead

and a massive brain inside it.

We've long known that these two features, big brains

and upright walking, really are hallmarks of humans.

And somehow these big brains must explain the vast gulf that we see

between ourselves and our closest ape relatives, the chimpanzees.

Of all the great apes,

these are the ones to which we're most closely related.

And the amazing thing is that we now know from studies

of DNA, our DNA, theirs, and that of other apes, that, in fact, we are

more closely related to chimpanzees than either of us is to gorillas.

It's extraordinary.

There's this close-knit family of us, common chimpanzees and bonobos.

I think that says something really important about our place

in this primate family tree and it makes it even more

extraordinary that our lives are so different to theirs.

So what exactly has changed in the six million years

since we shared a common ancestor?

We certainly have bigger brains

and we think we're more intelligent, but chimps are full of surprises.

The Max Planck Institute is at the forefront of some ground-breaking

work, comparing intelligence in humans and in chimps.

For the past nine years,

Michael Tomasello has been closely studying this troop.

So she did something which I think in human society

would be considered rather odd.

She came along and presented her bottom to you.

So is that a kind of friendly sign?

-It's a kind of a friendly greeting.

-Right, OK.

A little friendlier than we might normally do in human society.

You see, there are these similarities,

but there are some quite important differences as well.

Exactly so.

'And his work on ape intelligence is casting a fascinating new

'light on what it means to be human.'

And when you first started doing this work,

were you surprised?

Did you, did you find that they were more or less intelligent

than you expected them to be?

Well, that's the great part, is that they were, in some ways,

more intelligent and in other ways maybe a little less so.

They will do some things that will just absolutely surprise you

and you just can't believe they're so clever, and then they'll

just turn around and do something that's just kind of thick.

We like to think we're the most intelligent species on the planet,

but we have to be careful about what exactly we mean by intelligence.

The first thing we have to get rid of in thinking about animal intelligence

is the idea that there's this ladder of intelligence

that goes from low to high, and animals can just be placed on it.

It's actually much more complicated than that.

Different animals have different intelligences, as it were.

So the best memorisers in the world are squirrels

and birds that hide their nuts in different locations and can remember

dozens and dozens and dozens of locations, more than we can.

Oh, I was going to say, so when you say best memorisers in the world, that includes us?

That includes us. Absolutely.

In the case of apes, what we think is that they're especially

good at cognising things about the physical world

and understanding space, and causal relations like when using tools,

what causes something to move and whatever.

They're very good at that and basically they're not

different from human children in that kind of understanding.

So here's this task.

I put a little peanut, this is for you, this is your reward

and I just put it in here.

'And to show me just how intelligent chimps can be,

'Michael's colleague, Daniel Hannus, has invited me to try my hand

'at solving a problem that they regularly give to chimpanzees.'

You just do whatever you want to retrieve the peanut.

'My task is to get the peanut out of the tube

'using anything that comes to hand.'

I wonder if I could use the chain somehow, and the teaspoon.

That's going to be really difficult, I think.

Slightly worried I'm going to lose the teaspoon as well.

You'll never get it out again.

I don't think that's the right thing to do.

It may take me a while to figure it out,

but the key to this puzzle is something that you might

think chimps don't have, the ability to use a bit of lateral thinking.

Am I allowed to use my water?

Whatever you want.

-Any idea you have, you could just try it out.

-OK.

Yes! Here it comes.

Yeah, wow!

Excellent.

It took me more than four minutes to get my peanut,

so now let's see how a chimpanzee manages.

Oh, here they come, so Daniel's just trying to get them

interested in the peanut and they're going to have to do exactly

the same test that I just did.

Oh, look, he's doing it.

It's just really clever, it really is,

watching this chimp doing that.

And he doesn't have a bottle of water like I had.

He's got to think about how to get the water in there.

He takes some from his drinking bottle into his mouth

and then he spits it out in the tube.

He hasn't quite done enough.

Can he reach it yet?

There's another mouthful of water gone in and, oh, it's just,

it's almost there.

It must be so frustrating.

Oh!

-Yeah!

-He's done it, he's done it.

You clever chimpanzee.

I think that was quicker than me.

Twice as quick, in fact.

But although this chimp has done it before, even when presented with

the task for the first time, many of the apes here figured out that water

could be used not only to drink, but also as a tool to make peanuts move.

At certain tasks, chimps are cleverer than you might think.

And what excites me is that Mike

and his team are now homing in on the specific aspects of human

intelligence and behaviour that set us apart from our hairy cousins.

What makes us really different is our ability to put our heads

together and to do things that neither one of us could do alone,

to create new resources that neither one of us could create alone.

It's really all about communicating and collaborating

and working together.

But you think there's some kind of add-on effect then of teaching

and of being in a society,

in a culture which kind of builds on those innate abilities?

It makes all the difference.

If you raised a child on a desert island with no social contact

so no teaching, no contact with humans, their intelligence

as an adult would be very similar to that of other apes.

It would be a little bit different,

but they're evolved to learn from others

and to communicate with others and to collaborate with others,

and if there was no-one there and no culture and no tools and no

language, then that natural human intelligence just wouldn't develop.

Fish are born expecting water, OK?

They've got fins, they've got gills, they're born expecting water

and humans are born expecting culture.

At the heart of being a human then is our culture,

and something that goes hand in hand with human culture

is our ability to co-operate, and Michael has devised an experiment

that he believes reveals a specific piece of behaviour that separates

us from chimps, that defines us a species, and truly makes us human.

So what's this test designed to look at then?

OK, this is a test of being able to collaborate or co-operate

by pulling in on a rope such that they each get their reward.

The rope is strong through these hooks, so that

if anyone individual pulls, it'll just pull the rope out loosely.

Ah, right.

And so you have to pull together in order to get the food.

-So now they each have their rope.

-I see. So that's a moveable plank?

It's a moveable plank.

-With their pieces of banana, which is their reward.

-Exactly.

-So they have to pull together?

-And they have to pull together.

If any one of them pulls alone, they just pull it out.

So now it's tightened up and they're ready to actually make it move,

but they have to be sensitive to what the other one is doing.

-Notice there was actually a look to the other?

-Yeah, this is amazing.

OK, and they both pull it in and get their rewards.

These are two of our best at doing this.

-That is stunning.

-They are very good.

Proper, proper co-operation. It's brilliant.

But co-operation in the chimp world is a fragile thing.

What happens if something goes wrong and one chimp gets her reward first?

-This one has tangled her rope.

-Oh!

She can't quite reach it.

And now she can't do it, as long as this one's let go.

You can now see the rope coming out.

-She's frustrated because they didn't pull exactly synchronously.

-Yeah.

So she's got her reward. She's happy now, she's gone.

And then her poor partner is left without a reward.

For our closest relatives, clever as they are, that's as far as it goes.

Chimps will co-operate, but only for selfish ends.

But the experiments get really interesting

when you start testing humans.

Michael has been comparing how young children perform in the same task.

This is a very similar test to the test that the chimpanzees

were doing with the bananas.

Yes, it's the same basic idea, same basic idea.

'Like the chimps, the kids have to collaborate by pulling

'on a string at the same time to release the marbles.

'And just like the chimps, they have no problem co-operating.'

'Instead of a piece of banana, their reward is

'the satisfaction of placing a marble in the plinck machine.'

THEY SPEAK GERMAN

'The experiment can be set up

'so that the children receive either an equal or an unequal reward.'

THEY SPEAK GERMAN

So, Mike, what's the idea of this test?

They have to work together to get the reward?

Yeah, so the idea of this test is that kids are not that

naturally generous with their own things, and so if they just

have some things and you tell them they can share, maybe they will,

maybe they won't, but when they work together and they generate together

these rewards, they have a tendency to want them to be equally split.

Here they come.

So she's setting it up...

..but this time making sure it's going to be an uneven distribution?

Yes. One of them's going to get more than the other and we'll see

if they need to even it out before they cash in their rewards.

Pulling.

Ah, so there's uneven rewards, no?

This little girl's got one and that one's got three.

Are they sharing?

Let's see.

They shared them out.

Yes, they shared them out.

So they ended up with two each, yep.

Isn't that interesting?

That was quite extraordinary,

because I wouldn't have naturally thought that kids of this age,

two-year-olds, three-year-olds, would be that into sharing.

It's only when they work together for it.

That's just fascinating.

'In these experiments, Mike and his team have uncovered a seemingly

'small but crucial difference between us and chimpanzees.'

OK, here they come.

'Human children do something that no other ape will do.'

-He rolls one over to him.

-He's rolled one over.

-Yeah.

'In that small act of sharing, they reveal something that really does

'lie at the heart of what it is to be human.'

'It's a tiny but profound difference between us and the other apes,

'and it's a way of thinking that underpins our ability

'to co-operate and create human culture.'

Somehow these huge brains that we've got encapsulate the main differences

between ourselves and our closest cousins,

because look at these chimpanzees.

They're naked and hairy, they're not wearing clothes,

they're not talking about me, they're not sketching me.

So there are some really massive differences between us and them

which must come down, in some ways, to what is going on

inside this huge organ in our heads.

'With just a few months left until I'm due to give birth,

'I'm off for a scan to see how my new baby is getting along.'

-So shall I just lie back on here, then, Chrissie?

-Yes.

-Right, this is some cold jelly.

-Yeah.

So I'll just have a little look around first of all.

'It is an emotional experience seeing my baby growing inside me.'

-That's the head.

-That's the head, yeah.

There's the heart beating.

Oh, that's wonderful.

That is my baby.

Look at that.

That is my baby inside my womb.

And looking at him, he's obviously small now.

He's only six months of gestation,

so he's got another few months to go,

but he looks like a perfect but small little baby at this point,

so everything is there.

He's got his fingers in place, his toes in place, and it is just

amazing that all of that has come from a single fertilised egg.

It never fails to amaze me.

I mean, that's just extraordinary that the whole complexity

of the human body comes from that,

that single cell with genes from me and genes from my husband,

and that somehow, at the end of it, you end up with a human.

'But as well as being an expectant mother, I'm also an anatomist,

'so looking at the scan I can't help but be fascinated by the structures

'I can already see inside this brand new human of mine.'

So if we come back to looking at the head now...

You can almost see structures inside the brain, can't you? That's amazing.

You can. This is the cerebellum.

-Do you see this sort of dumbbell shape here?

-Yeah, yeah.

And this dark area here is the...

Yeah, so that's the back of the lateral ventricle, isn't it?

-That's right, that's the posterior ventricle there.

-That's amazing.

'After just six months, my baby's brain is already more than half

'the size of an adult chimpanzee's and it's still growing fast.'

How big is the head at the moment, Chrissie?

Well, shall we measure it and see?

So it says gestation just over 27 weeks,

and the head circumference is 25.6 centimetres.

What's the diameter?

The diameter, BPD 7.2 centimetres.

7.2, so it's going to get a little bit bigger.

Rather, yeah.

Oh, dear.

That's kind of big enough, I think.

'That growing head can't fail to remind me

'of something that's getting closer by the day.'

'Something that's particularly tricky for us humans.'

'Birth.'

'The enormous size of our brains,

'together with another uniquely human trait,

'our strange way of walking around on two legs,

'conspire to make human birth something of a squeeze,

'as any mother with tell you.'

WOMAN SCREAMS

And again.

Push, that's it, push.

WOMAN SCREAMS

It's quite strange being on a maternity ward and thinking that

I'm going to be back in a place like this in just two months' time,

ready for the appearance of my own little baby into the world.

I think it brings it home that human childbirth is really something

quite special, quite unique, even, amongst all other animals.

'By way of comparison, take a look at this,

'some rare film of a chimpanzee birth taken at Leipzig Zoo.'

'What's remarkable is just how quick and easy it is,

'certainly when compared with the rather more drawn-out

'and painful business of a human birth.'

WOMAN SCREAMS

Push harder, come on.

What I've drawn is essentially the anatomy of childbirth.

This is a human, female pelvis.

We're looking down on it from above, we're looking through

the birth canal and this is the baby's head

passing through that birth canal,

and you can see why childbirth is such a difficult process.

The birth canal is about ten centimetres in diameter,

the baby's head is about nine centimetres in diameter.

Now it's always been thought that there are constraints on the width

of the pelvis, which are all about walking on two legs,

that we can't actually push the hips any further apart

because that would make walking inefficient, and so that means,

for our big-brained babies,

they couldn't actually stay in the womb any longer,

because their heads would be too big to fit out through this birth canal.

And so our babies are born

at a relatively early stage of development.

Our new born babies are helpless.

'And that is one of the most puzzling paradoxes

'about being human.'

'For all our brilliance as a species, compared with other apes,

'our babies come into the world a bit useless.'

'For decades, we've assumed that our helpless babies are an unfortunate

'consequence of walking upright and having big brains.'

'It's called the obstetric dilemma.'

'It's in all the textbooks.'

'It's what I was taught at university

'and it's what I've gone on to teach others.'

'The female pelvis is struggling to do two different jobs,

'and we're stuck with these helpless babies.'

'If we could explain this dilemma, we'd start to open the door

'to a treasure trove of insights about being human...'

'..and there's some science emerging from the east coast of America

'that's shaking up the traditional view of women's hips.'

'Dr Holly Dunsworth decided that the female pelvis

'deserved a closer examination.'

It does seem peculiar, I mean, it really does mark us out amongst

all the other apes that childbirth for humans is...is difficult.

It's much more difficult than it is in chimpanzees and gorillas.

I know this from personal experience,

so what is it about our evolution that sets up this problem?

It's a dilemma. We have a tight fit.

We've got these two exceptional conditions.

We've got this funny way of getting around that we're doing right now,

and we've got these huge brains on top of our heads,

and natural selection acting on those two things

has come together and created this very difficult childbirth.

So is it a compromise, then?

The female pelvis is a compromise between something

that needs to be wide to let a large-brained baby out,

but needs to be narrow in order to make walking efficient?

Right, right.

And that's the obstetric dilemma, and it's unique to humans.

So the idea is that ideally we'd kind of want to get our pelvis

a bit wider, but actually, female pelves

are already making us less efficient

than men at walking and running, and we can't push it any further?

Right.

But as this hypothesis goes, they can't get any wider

or else women would be even worse at walking and running

than we already are, and everything would fall to pieces.

Yeah, that you'd end up kind of waddling along,

and it would be really inefficient.

Right. You'd never escape a sabre-toothed cat, you know.

'What's amazing is that in all these decades, no one has ever thought

'to question the assumptions that underlie the obstetric dilemma,

'or the suggestion that women are rubbish at running.'

'Until now.'

'Together with her colleagues, Herman Pontzer and Anna Warriner,

'Holly set about exhaustively testing the assumptions

'about the female pelvis...'

'..and she's invited me to Herman's lab in New York to see the results.'

Awesome, thank you.

So, Holly, this was part of the original research that you did

with Herman and Anna, looking at the efficiency

of running and walking in women and men.

I saw them starting to do this sort of research,

and it fit really well with the doubts I was having about

all of this obstetric dilemma business.

Yeah, yeah.

So I'd been thinking about how kind of strange this idea was

that our pelvis was limiting our gestation length

and it was sort of, like, you know, an epiphany.

'The team set out to explore the assumption that women,

'with our wide hips especially adapted for birth,

'are less efficient at walking and running than men.'

'Using a motion capture system and a force plate, Anna devised an

'experiment to analyse the internal mechanics of hip and leg bones.'

'Until now it had always been assumed that women's hip muscles,

'being attached to a wider pelvis,

'had to work harder than those of men.'

Go ahead, Lesley.

And there she comes.

That's great, isn't it?

A pair of legs walking about.

'But what the experiments revealed was that throughout each step,

'the angle of the pelvis is constantly adjusted

'to minimise the necessary work.'

'Women's hips may be wider, but the wobbling makes a key difference.

As you sort of move through the course of the step,

she's adjusting her balance and her weight.

So from this wavering around,

you start to suspect that it's not going to be so simple

saying that women have wider pelves and therefore their muscles around

their hips need to work harder when you're walking and running?

Yeah, exactly. Exactly right.

'The result of all these measurements

'is to show that, for decades, we got it wrong.'

This is really important because it means that there isn't a constraint

on how wide the pelvis is in terms of being efficient at bipedalism.

These data indicate that there is no effect

of having a pelvis adapted for birth on your efficiency

during walking or running.

'The female pelvis is not, it seems, compromised at all,

'and women, with our wide hips,

'are just as efficient at walking and running as men.'

'So why didn't the female pelvis evolve to be even wider, to allow

'our babies to grow a bit bigger and to be a bit less helpless at birth?'

'The answer was revealed to me through an experiment

'that involved me drinking some specially-labelled water

'and then sending Herman frozen samples of my urine.'

So, Herman, I recognise these little plastic tubes.

That's right.

So we had you drink a small dose of what

we call doubly-labelled water,

and then we collected a bunch of urine samples as we have here,

and we can actually calculate how much carbon dioxide you're producing

every day, and therefore how many calories your body

is burning every day.

It's the gold standard for measuring energy

expenditure in people during normal life.

'The length of gestation, it turns out, has nothing to do

'with the width of the birth canal, but everything to do with energy.'

So here we have the energy that the foetus is using. This is

based on data from other studies, and we see it goes up exponentially.

As the kid gets bigger, it needs more and more and more energy,

and we take a look at the energy

that mums actually burn during pregnancy.

We see it goes up quite quickly at first, but then it levels off.

It hits a ceiling. You just can't do any more.

Your body is limited in how much energy it can burn.

Presumably then it doesn't matter if I were to eat more,

so if I were to eat a few more hundred calories, it doesn't matter.

I'm not going to be able to give that to the foetus

because I can't actually metabolise any quicker.

That's right. There's a limit to how much energy your body can put through.

There's a hard limit on that.

If gestation continued for another month,

you'd shoot through that ceiling.

It would be metabolically impossible to do.

So instead, what you do is, as you approach nine months,

you give birth.

Right.

So we take a look at your data, right?

We've got you plotted on here. You're right there.

Ah, right, so...

So you're about five months in.

That's exactly... So you could tell how many months pregnant I was

by looking at this without actually...without me telling you?

Without you telling me,

-I could know that you've approached that ceiling.

-Yeah.

You were just about at the maximum energy expenditure

-that we could expect your body to be able to do.

-Yeah.

It'll get to be unsustainable at just about nine months in,

and you'll give birth.

This is fascinating.

It means that the baby comes out at a moment in time

when it is just about to start demanding more energy

from the mother than the mother can possibly give it via the placenta.

That's right.

'This research is, I think, really revolutionary.'

'What it reveals is that however wide our hips became,

'our babies couldn't stay inside the womb a moment longer than they do.'

It makes me look at the female pelvis in a new light

and say, "Well, actually this isn't a design compromise."

"It works very well."

But also, it makes me look at those helpless babies

in a new light as well, because they work well, too.

-Right.

-You know, on the one hand, we say they're coming into the world

too early, but it works. It works within the context of human society

because otherwise we wouldn't be here in the numbers that we are.

We gestate as long as we should for primates of our body size,

and maybe a little longer.

We give birth to babies at the right size for primates of our body size,

or maybe a little larger.

It's just that once they're born, they have so much more growth

to experience, particularly in the brain,

and while we are achieving that growth,

we also have much more to learn about how to be a human

than a chimp has to learn about how to be a chimp.

'It turns out, then, that the very nature of human birth,

'the fact that I will deliver a seemingly underdeveloped baby,

'is in fact a key ingredient in my son's path to becoming human.'

'Our babies may be born helpless, but far from being a dumb idea,

'it turns out to be one of the smartest moves we ever made...

'..because in order to develop their full human potential,

'the brains of our human babies need the stimulation of other humans.'

'Somehow, the secrets of being human are locked away inside the brain,

'the most complicated, mysterious object in the universe.'

'But it's an organ that keeps its secrets wrapped up tight.'

So this is a brain which has been removed from a skull and you can

see that it's still got its coverings on it, its meninges,

so there are several layers of membrane

around the outside of the brain.

In order to see the brain, we're going to have to peel this back.

There we go, it's just going to come away actually.

I just need to get an edge and then when you've got an edge,

it peels off quite nicely.

It's rather like peeling the pith off an orange.

And once I've cleared this layer away,

we're starting to see really nicely the texture of the surface

of the brain, so this is the cerebral cortex that we're starting

to see here, and you can see how heavily folded it is.

This is one of the characteristics of a human brain,

that the cortex is incredibly heavily folded.

You get the impression that there's a lot of information

being packed into a small area.

So that's it, this is the brain, nicely cleaned up.

And I think that however many times I do this,

it is utterly extraordinary to be holding in my hands

the organ that, more than any other part of our body, is us.

It seems utterly extraordinary that this actually quite

unprepossessing physical object contains somebody's personality,

the seat of their emotions, and it was where they experienced the world

and where they held their memories.

It is just quite remarkable.

'Despite several hundred years of probing, exactly how the

'human brain achieves all that remains shrouded in mystery.'

'What little we do know only makes it seem all the more extraordinary.'

'We know that a human brain contains a staggering one hundred billion

'neurons, but it's not just the number of brain cells that matters.'

'What makes the human brain so incredible is the huge number

'of connections between those cells, the vastly complex internal wiring.'

So, I'm going to start slicing it

with this incredibly sharp brain knife.

'Human brains have about 40% more connections between cortical neurons

'than the brains of other primates.'

'That's around a hundred trillion connections in every brain.'

'We know the basic anatomy quite well

'but if we want to begin to understand this extraordinary level

'of complexity, we need to look at the brain with a whole new toolkit.'

'To discover exactly how our human brains came to be

'so highly connected, I've come to America to find out about the latest

'research into the human genome, the recipe for making a human.'

'There are three billion letters in the human genome,

'stored in the 23 chromosomes that hold this recipe

'in every cell of our bodies.'

'Each letter, A, G, C and T, represents one of the four bases,

'the chemical building blocks, which make up the long strands of DNA.'

'And for geneticists, like Franck Palleux, these letters hold

'the clues that could unlock the secrets of the human brain.'

So this is chromosome one written out?

Uh-huh.

-And how much of chromosome one is it?

-Just one fiftieth.

And there's a thousand pages here.

A thousand pages, 45,000 base pairs per page.

'The whole genome would fill 670,000 sheets of paper

'and at this rate, it would take me and Franck more than a week,

'working 24 hours a day to lay out the entire human code.'

It's amazing to think that our entire life, you know,

lies in this code.

'And if we want to find out what makes the human code unique,

'we need to compare our own recipe with others.'

'The breakthrough that lets us do this is that as well as the

'human genome, we now have sequenced the genomes of many other animals.'

'But finding the crucial sections of code

'that make us human is a monster puzzle.'

One of the important steps in this process to try to identify

what, in our genome, in the human genome, could underly what makes

us human, is to try to find differences at the base pair level,

in the coding sequences between us, basically our genome,

and the genome of our closest living relative at least, the chimpanzee.

'Franck has homed in on one particular change that is specific

'to humans which he believes could be fundamental.'

'It involves a gene called SRGAP2 that is found in all animals

'and mainly affects the developing brain,

'but in humans, and only in humans, this gene is duplicated four times.'

This gene starts at page 814 and the very beginning of the sequence

is this sequence, CACAGGAA, and so the gene starts here.

I can't believe you can recognise that.

The gene is about 125,000 base pairs long,

so it goes from page 814 to 840.

So that is a single gene, all of that?

That's a single gene. Exactly.

And the sequence basically ends right here, TGCTGCGT.

So this is a gene that we have in common with the other apes,

but we've got three more copies of it?

Correct. We've got three more copies of this gene.

That would be in volume 30.

Remember, this is only one volume for chromosome one.

The full sequence of chromosome one would take about 50 volumes, right?

So the copies would be in volume 30, 31, 32.

'What Franck discovered was that the human duplication of SRGAP2

'has a dramatic effect on the connectivity of neurons.'

'By splicing the human duplicate into mouse DNA, he showed that

'the mouse neurons increased their ability to form connections.'

So this is a normal mouse brain, and that's what happens

-if you put that duplicated bit of SRGAP2?

-Exactly.

You form many, many more spines, basically and we have other evidence

to show that those neurons are actually hyper-connected there.

You increase by about 40% the total number of connections

made onto these neurons.

Why is it so exciting?

Basically, humans stand apart completely.

Human neurons have about 40% to 50% increase in the total number

of connections made onto those neurons, which we know is a feature

that sort of distinguishes human neurons, basically.

For me, this is when genetics gets really exciting, because we've got

an actual observable difference in the brains of chimpanzees

versus the brains of humans, and now we've got something in the genome

which could explain that actual physical difference in our brains.

Exactly.

'Every nuance of human behaviour somehow springs from this massive,

'branching network of hyper-connected neurons

'in our huge brains.'

'It's what makes the human brain so brilliant,

'this complex wiring diagram of connections that holds our memories,

'our emotions, our ability to row a boat or to draw.'

'It's what makes us human.'

'But to even contemplate drawing this diagram,

'we need a whole new way of looking.'

'I've come to Harvard University to meet

'one of the world's foremost neuroscientists,

'who has set himself a task of overwhelming ambition.'

'Jeff Lichtman is planning to create the ultimate map,

'a wiring diagram of the human brain, one connection at a time.'

'If he can ever complete it, this monumental map could finally

'reveal the mystifying workings of the human brain.'

'But before he can begin, Jeff is first attempting to map

'the connections in the more modestly-sized mouse brain.'

This is a little plastic block where the brain is embedded.

That's not a whole mouse brain in there?

No, it's probably about a quarter of a mouse brain

so in order to see what's going on, we have to slice it extremely thin,

so we're slicing these brains with a diamond knife.

That diamond knife cuts off a section that's about 30 nanometres

thick, so that's 300 hydrogen atoms.

You know, it's just very small, it's about a thousandth the thickness

of a human hair, so that you end up with a very, very, very long tape

that has many, many, many thousands and thousands of sections on it.

I can just about see the sections on there actually,

those little rectangles.

Yes, and those sections are like frames of a movie.

'Every one of those thousands of wafer-thin sections

'must then be individually scanned.'

So this is actually real time, this is the images actually

coming in from the electron microscope here?

At 20 million pixels per second.

This will take 15 minutes, and then we do the next one,

and we have 10,000 to do in this data set.

That's three months.

The numbers are just astronomical, aren't they?

It is like looking at galaxies and counting stars.

So in three months, you will have imaged a cube,

a three-dimensional cube,

which actually measures a quarter of a millimetre in each direction?

That's right, roughly.

In order to image a millimetre cubed, then,

that would be 16 times.

Yeah, so about four years.

Then how long to image a whole mouse brain?

You'd have to do that about a thousand times,

so that's about, what, 4,000 years.

OK, and how long to image a human brain?

That would be a thousand times longer, so about...

Four million years!

So not in my lifetime, at this speed.

Jeff, you've got to hope it gets quicker.

'A multi-million year timescale may sound daunting,

'but technology advances, and already Jeff and his team

'are giving us an incredible glimpse

'into the inner workings of the brain.'

I've asked one of my colleagues, Bobby Kestheri,

to hold one of these wafers that is being imaged really still

so that we can zoom up on here, and he's very courageous.

He's going to jump into the electron microscope in a second.

And this is one of those sections.

So that's just like one of the sections that we saw

-coming off the...

-That's right.

'The images are so detailed they allow Jeff to zoom in right down

'to the scale of individual neurons...'

Those big white circles are nerve cells.

'..revealing, at the smaller scale, the cross-section

'of the maze of wires at the heart of the brain.'

As we zoom up more,

we see, finally, an axon making a synapse onto a dendrite of a cell.

'Jeff can then reassemble the tiny cube of brain inside a computer,

'piling up the brain slices, tracking the complex path

'of each neuron with a different colour and creating a 3-D model

'of the individual wires that connect the brain.'

'The wires are packed incredibly densely.'

'This shows the wiring in just one five-millionth

'of a cubic millimetre of brain.'

So that's the circuitry, that's your three-dimensional wiring diagram?

It's quite beautiful to look at the brain this way and to realise

this is an infinitesimally small piece of a very large brain.

I think for humans trying to contemplate this,

the difficulty is that it's very hard for a human brain

to understand the extraordinary complexity of a human brain.

We think we're really smart and that we can understand everything,

but, in fact, the machine we're using to allow us

to understand things is way more complicated

than the rather simple thoughts that come out of our minds.

'Our brains are not only large, they have many more connections

'than the brains of any other animal.'

'Ultimately, by reaching down to these individual neurons,

'by mapping the trillions of connections,

'we may be able to pinpoint exactly how these hyper-connections

'translate into the psychology and behaviour of human beings.'

For most animals, their brains are largely encoded by their genes.

A fruit fly does not have to go to school to fly

and doesn't even have to learn how to fly.

It knows how to fly from the get-go.

In humans, it's very hard to know

what kinds of behaviours we have intrinsically.

Probably coughing, pooping, peeing and a few other things

we definitely can do, breathing,

but learning how to button your shirt or read or use the language

you think with, all of that requires learning.

You're an obligate learner.

It's not an extra, it's an essential ingredient of being a human being.

So humans have essentially got more behaviour which is learned

and less behaviour which is programmed right from the beginning?

Yes, we end up with brains that are capable of all these amazing things,

but we come into the world seemingly knowing much less about the world

than almost any other animal.

It takes us a year to walk, 18 years to leave the nest,

and during all that time, humans are building up information

about how to behave, and the neural circuits for behaviour

based on experience, rather than based on genetic information.

A human today, as an adult, is doing an entirely different set of things

than humans were doing thousands of years ago,

and any young person will tell you that their parents seem

old-fashioned and their grandparents seem positively ancient,

but imagine, you know, what people were doing

thousands or tens of thousands of years ago.

It's because humans constantly evolve in a cultural way,

even though our genetic heritage has not changed very rapidly.

That's the genius of being a human being.

I really love the beauty in Jeff's work,

those fantastic rainbow-coloured neurons all connected together

in incredibly complex and dense networks.

And, of course, all those connections are being made

at the moment inside the brain of my baby inside my womb...

..and that's an extraordinary thought in itself, but I think the

point at which he will really start to become human is the point where

we get that interplay between nature and nurture,

the process that really carves out a human mind,

and that starts at birth.

'And here he is, my beautiful baby boy.'

'He's very, very new, and he's certainly very helpless.'

'He's also got a big head.'

'He is full of potential, having emerged into the world,

'and he's ready to learn to become a human being.'

Subtitles by Red Bee Media