the future of the world was bleak.

The population explosion was unstoppable.

Global famine was inevitable.

A cancer epidemic caused by chemicals in the environment

was going to shorten our lives.

The acid rain was falling on the forests.

The desert was advancing by a mile or two a year.

The oil was running out,

and a nuclear winter would finish us off.

None of those things happened,

(Laughter)

and astonishingly, if you look at what actually happened in my lifetime,

the average per-capita income

of the average person on the planet,

in real terms, adjusted for inflation,

has tripled.

Lifespan is up by 30 percent in my lifetime.

Child mortality is down by two-thirds.

Per-capita food production

is up by a third.

And all this at a time when the population has doubled.

How did we achieve that, whether you think it's a good thing or not?

How did we achieve that?

How did we become

the only species

that becomes more prosperous

as it becomes more populous?

The size of the blob in this graph represents the size of the population,

and the level of the graph

represents GDP per capita.

I think to answer that question

you need to understand

how human beings bring together their brains

and enable their ideas to combine and recombine,

to meet and, indeed, to mate.

In other words, you need to understand

how ideas have sex.

I want you to imagine

how we got from making objects like this

to making objects like this.

These are both real objects.

One is an Acheulean hand axe from half a million years ago

of the kind made by Homo erectus.

The other is obviously a computer mouse.

They're both exactly the same size and shape to an uncanny degree.

I've tried to work out which is bigger,

and it's almost impossible.

And that's because they're both designed to fit the human hand.

They're both technologies. In the end, their similarity is not that interesting.

It just tells you they were both designed to fit the human hand.

The differences are what interest me,

because the one on the left was made to a pretty unvarying design

for about a million years --

from one-and-a-half million years ago to half a million years ago.

Homo erectus made the same tool

for 30,000 generations.

Of course there were a few changes,

but tools changed slower than skeletons in those days.

There was no progress, no innovation.

It's an extraordinary phenomenon, but it's true.

Whereas the object on the right is obsolete after five years.

And there's another difference too,

which is the object on the left is made from one substance.

The object on the right is made from

a confection of different substances,

from silicon and metal and plastic and so on.

And more than that, it's a confection of different ideas,

the idea of plastic, the idea of a laser,

the idea of transistors.

They've all been combined together in this technology.

And it's this combination,

this cumulative technology, that intrigues me,

because I think it's the secret to understanding

what's happening in the world.

My body's an accumulation of ideas too:

the idea of skin cells, the idea of brain cells, the idea of liver cells.

They've come together.

How does evolution do cumulative, combinatorial things?

Well, it uses sexual reproduction.

In an asexual species, if you get two different mutations in different creatures,

a green one and a red one,

then one has to be better than the other.

One goes extinct for the other to survive.

But if you have a sexual species,

then it's possible for an individual

to inherit both mutations

from different lineages.

So what sex does is it enables the individual

to draw upon

the genetic innovations of the whole species.

It's not confined to its own lineage.

What's the process that's having the same effect

in cultural evolution

as sex is having in biological evolution?

And I think the answer is exchange,

the habit of exchanging one thing for another.

It's a unique human feature.

No other animal does it.

You can teach them in the laboratory to do a little bit of exchange --

and indeed there's reciprocity in other animals --

But the exchange of one object for another never happens.

As Adam Smith said, "No man ever saw a dog

make a fair exchange of a bone with another dog."

(Laughter)

You can have culture without exchange.

You can have, as it were, asexual culture.

Chimpanzees, killer whales, these kinds of creatures, they have culture.

They teach each other traditions

which are handed down from parent to offspring.

In this case, chimpanzees teaching each other

how to crack nuts with rocks.

But the difference is

that these cultures never expand, never grow,

never accumulate, never become combinatorial,

and the reason is because

there is no sex, as it were,

there is no exchange of ideas.

Chimpanzee troops have different cultures in different troops.

There's no exchange of ideas between them.

And why does exchange raise living standards?

Well, the answer came from David Ricardo in 1817.

And here is a Stone Age version of his story,

although he told it in terms of trade between countries.

Adam takes four hours to make a spear and three hours to make an axe.

Oz takes one hour to make a spear and two hours to make an axe.

So Oz is better at both spears and axes than Adam.

He doesn't need Adam.

He can make his own spears and axes.

Well no, because if you think about it,

if Oz makes two spears and Adam make two axes,

and then they trade,

then they will each have saved an hour of work.

And the more they do this, the more true it's going to be,

because the more they do this, the better Adam is going to get at making axes

and the better Oz is going to get at making spears.

So the gains from trade are only going to grow.

And this is one of the beauties of exchange,

is it actually creates the momentum

for more specialization,

which creates the momentum for more exchange and so on.

Adam and Oz both saved an hour of time.

That is prosperity, the saving of time

in satisfying your needs.

Ask yourself how long you would have to work

to provide for yourself

an hour of reading light this evening to read a book by.

If you had to start from scratch, let's say you go out into the countryside.

You find a sheep. You kill it. You get the fat out of it.

You render it down. You make a candle, etc. etc.

How long is it going to take you? Quite a long time.

How long do you actually have to work

to earn an hour of reading light

if you're on the average wage in Britain today?

And the answer is about half a second.

Back in 1950,

you would have had to work for eight seconds on the average wage

to acquire that much light.

And that's seven and a half seconds of prosperity that you've gained

since 1950, as it were,

because that's seven and a half seconds in which you can do something else,

or you can acquire another good or service.

And back in 1880,

it would have been 15 minutes

to earn that amount of light on the average wage.

Back in 1800,

you'd have had to work six hours

to earn a candle that could burn for an hour.

In other words, the average person on the average wage

could not afford a candle in 1800.

Go back to this image of the axe and the mouse,

and ask yourself: "Who made them and for who?"

The stone axe was made by someone for himself.

It was self-sufficiency.

We call that poverty these days.

But the object on the right

was made for me by other people.

How many other people?

Tens? Hundreds? Thousands?

You know, I think it's probably millions.

Because you've got to include the man who grew the coffee,

which was brewed for the man who was on the oil rig,

who was drilling for oil, which was going to be made into the plastic, etc.

They were all working for me,

to make a mouse for me.

And that's the way society works.

That's what we've achieved as a species.

In the old days, if you were rich,

you literally had people working for you.

That's how you got to be rich; you employed them.

Louis XIV had a lot of people working for him.

They made his silly outfits, like this,

(Laughter)

and they did his silly hairstyles, or whatever.

He had 498 people

to prepare his dinner every night.

But a modern tourist going around the palace of Versailles

and looking at Louis XIV's pictures,

he has 498 people doing his dinner tonight too.

They're in bistros and cafes and restaurants

and shops all over Paris,

and they're all ready to serve you at an hour's notice with an excellent meal

that's probably got higher quality

than Louis XIV even had.

And that's what we've done, because we're all working for each other.

We're able to draw upon specialization and exchange

to raise each other's living standards.

Now, you do get other animals working for each other too.

Ants are a classic example; workers work for queens and queens work for workers.

But there's a big difference,

which is that it only happens within the colony.

There's no working for each other across the colonies.

And the reason for that is because there's a reproductive division of labor.

That is to say, they specialize with respect to reproduction.

The queen does it all.

In our species, we don't like doing that.

It's the one thing we insist on doing for ourselves, is reproduction.

(Laughter)

Even in England, we don't leave reproduction to the Queen.

(Applause)

So when did this habit start?

And how long has it been going on? And what does it mean?

Well, I think, probably, the oldest version of this

is probably the sexual division of labor.

But I've got no evidence for that.

It just looks like the first thing we did

was work male for female and female for male.

In all hunter-gatherer societies today,

there's a foraging division of labor

between, on the whole, hunting males and gathering females.

It isn't always quite that simple,

but there's a distinction between

specialized roles for males and females.

And the beauty of this system

is that it benefits both sides.

The woman knows

that, in the Hadzas' case here --

digging roots to share with men in exchange for meat --

she knows that all she has to do to get access to protein

is to dig some extra roots and trade them for meat.

And she doesn't have to go on an exhausting hunt

and try and kill a warthog.

And the man knows that he doesn't have to do any digging

to get roots.

All he has to do is make sure that when he kills a warthog

it's big enough to share some.

And so both sides raise each other's standards of living

through the sexual division of labor.

When did this happen? We don't know, but it's possible