and there were no grocery stores,

no restaurants, no supermarkets.

No problem.

You'd probably go out and forage for your food

like a traditional forager.

Could you do it?

Well, the truth is, in today's world,

with seven billion humans to feed,

foraging couldn't come near to providing enough food.

Farming, like what you see around me in this rice paddy

near Cheongju in South Korea, is absolutely essential

to the survival of the modern world.

But what is farming anyway?

To understand farming,

it may help to go back to the world of foragers.

Remember, foragers travel around their environments

selecting the plants, the animals

and the other raw materials they need

and using them without processing them too much.

Farmers are very different.

They select a small range of species,

they tend to them very, very carefully

as in this paddy field,

and they keep them in protected environments.

Now, we humans are not the only species that do this.

For example, honeypot ants herd aphids.

They protect them, they feed them,

they help them reproduce,

and in return, they get nutritious honeydew.

When species get dependent like this in the natural world,

over time they tend to change very, very rapidly indeed.

For example, the ants, after a time,

get so dependent on the aphids that if all the aphids died,

the ants would starve to death.

And the aphids get so dependent that they cannot reproduce

without the honeypot ants.

Biologists refer to such relations

between species, such relations of close dependency,

as symbiosis.

But when humans are involved in such relations,

we tend to call it farming or agriculture.

When humans started farming, the species they tended to

began to change quite dramatically.

This was because humans selected

the most nutritious wheat grains, or rice grains,

or corn grains,

or the most docile and fattest animals.

And the result was that within a few generations,

new domesticated species began to appear

and they were created not by natural selection,

but by artificial selection.

You can see the difference

very, very clearly if you compare

a modern corn cob, which is fat and nutritious and tasty,

with its ancestor, teosinte,

which is a rather pathetic-looking, weedy plant.

Or if you compare a modern sheep,

domesticated sheep, which tends to be fat and docile

and frankly not too smart,

with its ancestor, a mountain goat,

a very intelligent, athletic creature.

Humans change too,

but they tended not to change genetically

so much as technologically, socially and culturally.

They learned to clear trees, to build paddy fields like this,

to divert rivers, to protect their crops and animals

from pests such as rats or wolves.

And the result of all of this was that humans found

that they could produce more of what they wanted

from a given area, from a small area,

than they had before and that meant that humans

could now live in sedentary villages

rather than being nomadic.

But to get a real sense of what's going on with farming,

we need to think about it from a big history perspective.

Remember, all the energy that supports the biosphere,

or most of it, comes from sunlight

through photosynthesis.

So what farmers were really doing

was diverting more of that energy

into species that they could use and away from other species.

The result was a sort of huge energy grab

by one species, our own.

No wonder, with farming, human populations

began to grow so rapidly.

So where and when did farming and agriculture begin?

At the moment, it looks as if farming really began

about 11,000 years ago

in the highlands east of the Mediterranean

that we know as the Fertile Crescent.

And here they grew wheat.

It may also have grown... appeared at about the same time

in the Nile Valley, slightly further south.

Then from about 8,000 years ago,

we have evidence of rice growing in China

and at about the same time,

we have evidence of the growing of taro and yams

in the highlands of Papua New Guinea,

though it took several thousand years

for agriculture to really flourish here.

Then, from about 5,000 to 4,000 years ago,

agriculture pops up in several different parts of the world.

In West Africa,

farmers start growing sorghum and millet.

And farming also appears in the Americas

right across the Atlantic,

in Mesoamerica,

where they're growing maize and squash,

and also in the Andes,

where potatoes were a very important crop.

From these core regions,

agriculture then spreads to neighboring regions,

but there's a real puzzle here

because none of the core regions seem to have been connected.

So what was really going on?

So why, after almost 200,000 years

of living as foragers,

did humans in so many parts of the world

that had no connection between each other

begin to behave in such similar ways

in such a brief period of time?

Well, there seem to be two main reasons.

One factor is overpopulation, the other is climate change.

And these factors operated

in many different parts of the world.

Let's take overpopulation first.

During the Paleolithic era, if populations grew too much,

you could generally solve the problem

by migrating into a new region.

But after the settlement of the Americas

from about 15,000 years ago,

there were no large areas left to migrate into.

So from now on, if populations began to grow,

you had to try to get more resources from a given area.

In other words, you had to farm.

Now, that's the first factor.

The second, climate change, is subtler.

Most of the Paleolithic era was dominated by the Ice Ages.

And during the Ice Ages,

for the most part, climates were so cold

and so unpredictable

that farming was more or less impossible.

Then, from about 18,000 years ago,

global climates began to change.

They began to get warmer.

Glaciers began to retreat, sea levels began to rise,

and in area after area, you began to get warmer

and wetter climates.

There was a brief period between 13,000 and 12,000 years ago

when climates were cold again,

but then from about 12,000 years ago,

climates became warmer and wetter,

and we entered the interglacial period

that we're still living in today.

Now, as a result of global climate change,

humans and animals and plants everywhere in the world

had to start changing their behaviors.

In some areas such as the Fertile Crescent

east of the Mediterranean, as climates changed,

resources became more abundant.

There were more plants and more animals.

And in these regions,

these particularly favored regions,

some foragers began to settle down

because they found they could live in one place

for most of the year without traveling around.

And they began to form villages.

They became sedentary.

In the Fertile Crescent,

archeologists refer to the people

who settled in these villages as Natufians,

and they've excavated many of their villages.

But as they settled down, their behaviors changed

and, in particular, populations began to grow.

We're not sure why, but one reason may be

that if you're a villager,

you don't have to carry children around

so there's less pressure to reduce

the number of children.

In any case, populations grew and that posed a problem.

Within a few generations, they found they didn't have

enough resources to feed everyone

in what had once seemed an area of abundance.

So what are they to do?

Well, perhaps they could go back to foraging.

The problem was they'd probably forgotten

many of the old techniques of foraging,

and, besides, neighbors probably occupied those lands now.

So what can they do?

Well, they can start tending their crops

and animals more carefully.

They can start providing

the plants they like with extra water.

They can start clearing away weeds.

They can start penning

particular animals in enclosures.

In fact, they can start farming.

Now, something like what happened to the Natufians

seems to have happened in many other areas.

In the case of the Natufians, we have lots of evidence

about this expansion of villages and population growth.

But something like this happened in many areas

and everywhere, the same two factors

seemed to have been involved.

First, global climate change, which made agriculture possible,

and secondly, overpopulation, which made it necessary.

Agriculture is our seventh major threshold

of increasing complexity.

As we've seen, these are pivotal events

that allow the creation of new, more complex things

with new emergent properties.

Agriculture is not just a matter of tastier fruit or fatter cows.

Agriculture unlocked forces much more powerful than that

and they would transform human history.

How and why?