This had, of course, been almost possible for thousands of years. Messages and artefacts passed between neighbours had long been the stuff of cultural diffusion. But there were gaps. Even in AD 900, no human being had visited New Zealand, or crossed from Europe to the Americas. No one was yet commuting to Australia across the Torres Strait.
A hundred years later, however, these gaps were tenuously bridged, inviting a thought-experiment. Imagine a vital piece of information despatched by word of mouth from, say, the heart of the Islamic world, being passed across cultural borders from messenger to messenger. Ignore practical problems. Assume peaceful progress, perfect translation, a steady pace.
In AD 1000, Islam derives slaves and gold from sub- Saharan Africa. The message heads quickly south, across the Sahara and down the Nile, being carried on the length of the continent by Bantu farmers and herders, until it reaches the Khoisan in the Kalahari Desert. Islam also trades with Byzantium, which is in close touch with proto-Russia, whose people are just now adopting Byzantium's Orthodox Christianity.
The Rus, dominating the north-south trade routes along the major rivers, still have close dealings with their ancestral Viking culture. The Vikings are not only everywhere around Europe's coasts, but also commute regularly to Iceland. Icelanders have recently colonised Greenland. Greenland Vikings are at this moment doing their best to establish a colony in Newfoundland, and are trading, as well as fighting, with Inuit from northern Canada.
Here the message divides. One route heads south, through the eastern woodlands and the arid south-west to central America, and thence to the Andes and down the Amazon. In the north, the Inuit of the Thule culture, who are spread across the Canadian Arctic, pass the word along to relatives in Alaska, who paddle it across the Bering Strait to Siberia. Inuit there obtain iron from the borders between Siberia and China. From China, following Silk Road routes, the message flows to Central Asia, then down through India, where the expansive Chola empire transmits it to Sri Lanka.
Meanwhile, messengers are advancing southward through South-East Asia, via the Khmer empire of ancient Cambodia. Offshore, three local trade kingdoms, create alternative routes through the Indonesian archipelago. The word passes along Pacific island chains to the Polynesian frontier, even being carried with the very first settlers to the virgin land of New Zealand. Back to the north and west, on the coast of north Australia, traders are just beginning to exploit sausage-like marine creatures, beches- de-mer (sea cucumbers, or trepang) much prized as a delicacy in China, then as now. From sea-cucumber gatherers in Arnhem Land, Australian Aborigines carry the message across the continent. Back north of the Hindu Kush, where we left one of our Silk Road messengers, it is a small step through Muslim Afghanistan back to the heart of Islam.
Imagine all this to have occurred at walking pace, 24 hours a day. Our message has covered 35,000 miles in one year, right around the world plus 10,000 miles for the twists and turns. Minor cultures - Amazonian tribesmen, Easter Islanders - will remain untouched. But it does not seem too fanciful to look for the roots of today's "one world" in the world of 1,000 years ago.
John Man is the author of `The Atlas of the Year 1000' (Penguin, pounds 10.99)