The passes of Karakoram and Torugut, linking China with western Asia - and ultimately with the whole of the western world - have only in recent years re-opened to a thin and tentative trickle of cross-border traffic. Yet a thousand years ago these were crucial and well-trodden trade routes.

The foundations for this famous road to the West , which was to become one of the most important arteries of trade and culture in world history, were laid over two millennia ago. In the second century BC nothing was known in China of the existence of people and lands beyond its borders, except by rumour. In 139 BC, the imperial court at Chang'an (Xi'an) decided to despatch an emissary, a man called Zhang Qiang, to investigate the world to the west and to seek possible allies in the constant struggle against nomadic marauders from the north. Zhang set out with a party of a hundred men; thirteen years later he returned, with only two other members of his original expedition - and no alliances. But the news he brought nevertheless set Emperor Wu Di and his court aflame, including tales of Central Asia, Persia and even the Mediterranean world. Further expeditions were soon despatched, initially to purchase horses for military purposes, and from these beginnings trade soon developed. By 100 BC a dozen immense caravans a year were heading west into the desert. Jade, porcelain, lacquerware and silk began flowing out of China.

The silkworm had already been domesticated in China for hundreds of years, but in the West the means by which this exotic material was manufactured remained a total mystery - people believed it was combed from the leaves of trees. The Chinese took great pains to protect their monopoly, punishing any attempt to export silkworms with death. It was only many centuries later that sericulture finally began to spread west, when silkworm larvae were smuggled out of China in hollow walking sticks by Nestorian monks. The first time the Romans saw silk, snaking in the wind from the banners of their Parthian enemies, it filled them with terror and resulted in a humiliating rout. They determined to acquire it for themselves, and soon Roman society became obsessed with the fabric which by the first century AD was coming west in such large quantities that the corresponding outflow of gold had begun to threaten the stability of the Roman economy.

Silk was not all that passed along the route. From China came oranges, peaches, pears, roses, chrysanthemums, cast iron, gunpowder, the crossbow, the wheelbarrow, paper and printing, and from the West came cucumbers, figs, chives, sesame, walnuts, grapes (and wine-making), wool, linen and ivory.

The entire route , from eastern China to the Mediterranean, was incredibly long and arduous. Starting from Chang'an, the Silk Road curved northwest through Gansu to the Yumen Pass, where it split. Leaving the protection of the Great Wall, travellers could follow one of two routes across the terrible deserts of Lop Nor and Taklamakan, braving the attacks from marauding bandits, to Kashgar. The southern route ran through Dunhuang, Lop Nor, Miran, Niya, Khotan and Yarkand; the northern route through Hami, Turpan, Kuqa and Aqsu. Oases along the route inevitably prospered as staging posts and watering holes, becoming important and wealthy cities in their own right, with their own garrisons to protect the caravans. When Chinese domination periodically declined, many of these cities turned themselves into self-sufficient city-states, or khanates . Today, many of these once powerful cities are now buried in the sands.

High in the Pamirs beyond Kashgar, the merchants traded their goods with the middlemen who carried them beyond the frontiers of China, either south to Kashmir, Bactria, Afghanistan and India, or north to Ferghana, Tashkent and Samarkand. Then, laden with western gold, the Chinese merchants would turn back down the mountains for the three-thousand-kilometre journey home.

As well as goods, the Silk Road carried new ideas in art and religion . Nestorian Christianity and Manichaeism trickled east across the mountains, but by far the most influential force was Buddhism . The first Buddhist missionaries appeared during the first century AD, crossing the High Pamirs from India, and their creed gained rapid acceptance among the nomads and oasis dwellers of what is now western China. All along the road, monasteries, chapels, stupas and grottoes proliferated, often sponsored by wealthy traders. By the fourth century, Buddhism had become the official religion of much of northern China and by the eighth it was accepted throughout the empire.

The remains of this early flowering of Buddhist art along the road are one of the great attractions of the Northwest for modern-day travellers. Naturally, history has taken its toll - zealous Muslims, Western archeologists, Red Guards and the forces of nature have all played a destructive part - but some sites have miraculously survived intact, above all the cave art at Mogao outside Dunhuang.

The Silk Road continued to flourish for centuries, reaching its zenith under the Tang (618-907 AD) and bringing immense wealth to the Chinese nobility and merchants. But it remained a slow, dangerous and expensive transport route. Predatory tribes to the north and south harried the caravans despite garrisons and military escorts. Occasionally entire regions broke free of Chinese control, requiring years to be "re-pacified". The route was physically arduous, too, taking at least five months from Chang'an to Kashgar, and whole caravans could be lost in the deserts or in the high mountain passes.

There was a brief final flowering of the trade in the thirteenth century, to which Marco Polo famously bore witness, when the whole Silk Road came temporarily under Mongol rule. But by now the writing was clearly on the wall for the overland routes. With the arrival of sericulture in Europe and the opening of sea routes between China and the West, the Silk Road had had its day. The road and its cities were slowly abandoned to the wind and the blowing sands.