On his current visit to Beijing, UK Prime Minister David Cameron has said China will soon reclaim its position as the world's biggest economy - a role it has held for 18 of the past 20 centuries. But how did the US, Britain and the rest of Europe interrupt this reign of supremacy? It comes down to location.

Why does the West dominate the world?

Europeans have been asking this question since the 18th Century, and Africans and Asians since the 19th. But there is still not much agreement on the answers.

People once claimed Westerners were simply biologically superior. Others have argued Western religion, culture, ethics, or institutions are uniquely excellent, or that the West has had better leaders. Others still reject all these ideas, insisting that Western domination is just an accident.

But in the last few years, a new kind of theory has gained ground.

What is the West? Distinctive ways of life began emerging in different parts of the world 11,000 years ago, when the first farmers created more complex societies. Great civilizations grew out of the original agricultural cores (in what we now call southwest Asia, China, Pakistan, Mexico, and Peru), all of which steadily expanded as population grew. The westernmost of the Old World's agricultural cores, in southwest Asia, was the foundation of what we now call Western Civilization. By 500 BC, the Western core had expanded across Europe, its centre of gravity shifting to the Mediterranean cultures of Greece and Rome. By 1500 AD it had expanded still further, and its centre was shifting into Western Europe. By 1900 AD it had expanded across the oceans, and its centre was shifting to North America.

People, it suggests, are much the same all over the world. The reason why some groups stuck with hunting and gathering while others built empires and had industrial revolutions has nothing to do with genetics, beliefs, attitudes, or great men: it was simply a matter of geography.

China and India are, of course poised to pick up the baton of global superpowers, but to explain why the West rules, we have to plunge back 15,000 years to the point when the world warmed up at the end of the last ice age.

Geography then dictated that there were only a few regions on the planet where farming was possible, because only they had the kinds of climate and landscape which allowed the evolution of wild plants and animals that could potentially be domesticated.

The densest concentrations of these plants and animals lay towards the western end of Eurasia, around the headwaters of the Euphrates, Tigris, and Jordan Rivers in what we now call south-west Asia. It was therefore here, around 9000 BC, that farming began, spreading outwards across Europe.

Farming also started independently in other areas, from China to Mexico; but because plants and animals that could be domesticated were somewhat less common in these zones than in the West, the process took thousands of years longer to get going. These other zones of complex agricultural societies also expanded, but the West long retained its early lead, producing the world's first cities, states, and empires.

But if this were all that there was to the story - that the West got an early lead and held onto it - there would be no controversy over why the West rules. In reality, when we look back across history, we see that things were more complicated. Geography determined how societies developed; but how societies developed simultaneously determined what geography meant.

The first city - 6,000 years ago in Iraq The ancient Greeks called it Mesopotamia, the land between two rivers - Tigris and Euphrates. But it is also the land between two seas - the Mediterranean Sea and Persia Gulf. It is also the land between mountain and desert, lagoon and salt marsh. All these geographical features have to be borne in mind when considering the birthplace of the first civilisations. Geography v history - it's impossible to know which takes precedence. There's no getting away from the brutal facts of nature - rivers that flood will dry up, rainfall that's intermittent, mountains that are impassable, deserts that are hostile. Applying this kind of analysis to Mesopotamia, where summers are hot, winters are cold and rainfall is low, I'd sum it up like this: difficult but not impossible. No garden of Eden, but no howling wilderness either. Richard Miles' Ancient Worlds on BBC Two Explore artefacts in the series

In the earliest days of agriculture, having the right temperatures, rainfall, and topography was all-important. But as villages grew into cities, these geographical facts became less important than living on a great river like the Nile, which made irrigation possible.

As states turned into empires, being on a river began mattering less than access to a navigable sea like the Mediterranean, which was what allowed Rome to move its food, armies, and taxes around.

As the ancient world's empires expanded further, though, they changed the meanings of geography again. The long bands of steppes from Mongolia to Hungary turned into a kind of highway along which nomads moved at will, undermining the empires themselves.

In the first five centuries AD, the Old World's great empires - from Rome in the West to Han China in the East - all came apart; but the political changes transformed geography once again. China recreated a unified empire in the 6th Century AD, while the West never did so.

For more than a millennium, until at least 1700, China was the richest, strongest, and most inventive place on earth, and the East pulled ahead of the West.

East Asian inventors came up with one breakthrough after another. By 1300 their ships could cross the oceans and their crude guns could shoot the people on the other side. But then, in the kind of paradox that fills human history, the East's breakthroughs changed the meaning of geography once again.

Western Europe - sticking out into the cold North Atlantic, far from the centres of action - had always been a backwater. But when Europeans learned of the East's ocean-going ships and guns, their location on the Atlantic abruptly became a huge geographical plus.

Before people could cross the oceans, it had not mattered that Europe was twice as close as China to the vast, rich lands of the Americas. But now that people could cross the oceans, this became the most important geographical fact in the world.

The Atlantic, 3,000 miles across, became a kind of Goldilocks Ocean, neither too big nor too small. It was just big enough that very different kinds of goods were produced around its shores in Europe, Africa, and America; and just small enough that the ships of Shakespeare's age could cross it quite easily.

The Pacific, by contrast, was much too big. Following the prevailing tides and winds, it was an 8,000-mile trip from China to California - just about possible 500 years ago, but too far to make trade profitable.

Geography determined that it was western Europeans, rather than the 15th Century's finest sailors - the Chinese - who discovered, plundered, and colonised the Americas. Chinese sailors were just as daring as Spaniards; Chinese settlers just as intrepid as Britons; but Europeans, not Chinese, seized the Americas because Europeans only had to go half as far.

Europeans went on in the 17th Century to create a new market economy around the shores of the Atlantic, exploiting comparative advantages between continents. This forced European thinkers to confront new questions about how the winds and tides worked. They learned to measure and count in better ways, and cracked the codes of physics, chemistry, and biology.

As a result, Europe, not China, had a scientific revolution. Europeans, not Chinese, turned science's insights onto society itself in the 18th Century in what we now call the Enlightenment.

Will China soon rival the US? Many observers think so, but not George W Bush. In an interview with the Times this week, he said that "internal problems" meant it was unlikely to rival the US any time soon. "Do I think America will remain sole superpower? I do." The Times [subscription required]

By 1800, science and the Atlantic market economy pushed western Europeans into mechanising production and tapping the power of fossil fuels. Britain had the world's first industrial revolution, and by 1850 bestrode the world like a colossus.

But the transforming power of geography did not stop there. By 1900 the British-dominated global economy had drawn in the resources of North America, changing the meaning of geography once again. The US, until recently a rather backward periphery, became the new global core.

And still the process did not stop. In the 20th Century, the American-dominated global economy in turn drew in the resources of Asia. As container ships and jet airliners turned even the vast Pacific Ocean into a puddle, the apparently backward peripheries of Japan, then the "Asian Tigers", and eventually China and India turned into even newer global cores.

The "rise of the East", so shocking to so many Westerners, was entirely predictable to those who understood that geography determines how societies develop, and that how societies develop simultaneously determines what geography means.

When power and wealth shifted across the Atlantic from Europe to America in the mid-20th Century, the process was horrifyingly violent. As we move into the mid-21st century, power and wealth will shift across the Pacific from America to China.

The great challenge for the next generation is not how to stop geography from working; it is how to manage its effects without a Third World War.

Why the West Rules - For Now: The Patterns of History, and What they Reveal About the Future is published by Profile.