THE brave seamen whose great voyages of exploration opened up the world are iconic figures in European history. Columbus found the New World in 1492; Dias discovered the Cape of Good Hope in 1488; and Magellan set off to circumnavigate the world in 1519. However, there is one difficulty with this confident assertion of European mastery: it may not be true.

It seems more likely that the world and all its continents were discovered by a Chinese admiral named Zheng He, whose fleets roamed the oceans between 1405 and 1435. His exploits, which are well documented in Chinese historical records, were written about in a book which appeared in China around 1418 called “The Marvellous Visions of the Star Raft”.

Next week, in Beijing and London, fresh and dramatic evidence is to be revealed to bolster Zheng He's case. It is a copy, made in 1763, of a map, dated 1418, which contains notes that substantially match the descriptions in the book. “It will revolutionise our thinking about 15th-century world history,” says Gunnar Thompson, a student of ancient maps and early explorers.

The map (shown above) will be unveiled in Beijing on January 16th and at the National Maritime Museum in Greenwich a day later. Six Chinese characters in the upper right-hand corner of the map say this is a “general chart of the integrated world”. In the lower left-hand corner is a note that says the chart was drawn by Mo Yi Tong, imitating a world chart made in 1418 which showed the barbarians paying tribute to the Ming emperor, Zhu Di. The copyist distinguishes what he took from the original from what he added himself.

The map was bought for about $500 from a small Shanghai dealer in 2001 by Liu Gang, one of the most eminent commercial lawyers in China, who collects maps and paintings. Mr Liu says he knew it was significant, but thought it might be a modern fake. He showed his acquisition to five experienced collectors, who agreed that the traces of vermin on the bamboo paper it is written on, and the de-pigmentation of ink and colours, indicated that the map was more than 100 years old.

Mr Liu was unsure of its meaning, and asked specialists in ancient Chinese history for their advice, but none, he says, was forthcoming. Then, last autumn, he read “1421: The Year China Discovered the World”, a book written in 2003 by Gavin Menzies, in which the author makes the controversial claim that Zheng He circumnavigated the world, discovering America on the way. Mr Menzies, who is a former submariner in the Royal Navy and a merchant banker, is an amateur historian and his theory met with little approval from professionals. But it struck a chord: his book became a bestseller and his 1421 website is very popular. In any event, his arguments convinced Mr Liu that his map was a relic of Zheng He's earlier voyages.

The detail on the copy of the map is remarkable. The outlines of Africa, Europe and the Americas are instantly recognisable. It shows the Nile with two sources. The north-west passage appears to be free of ice. But the inaccuracies, also, are glaring. California is shown as an island; the British Isles do not appear at all. The distance from the Red Sea to the Mediterranean is ten times greater than it ought to be. Australia is in the wrong place (though cartographers no longer doubt that Australia and New Zealand were discovered by Chinese seamen centuries before Captain Cook arrived on the scene).

The commentary on the map, which seems to have been drawn from the original, is written in clear Chinese characters which can still be easily read. Of the west coast of America, the map says: “The skin of the race in this area is black-red, and feathers are wrapped around their heads and waists.” Of the Australians, it reports: “The skin of the aborigine is also black. All of them are naked and wearing bone articles around their waists.”

But this remarkable precision, rather than the errors, is what critics of the Menzies theory are likely to use to question the authenticity of the 1418 map. Mr Menzies and his followers are naturally extremely keen to establish that the 1763 copy is not a forgery and that it faithfully represents the 1418 original. This would lend weighty support to their thesis: that China had indeed discovered America by (if not actually in) 1421. Mass spectrography analysis to date the copied map is under way at Waikato University in New Zealand, and the results will be announced in February. But even if affirmative, this analysis is of limited importance since it can do no more than date the copyist's paper and inks.

Five academic experts on ancient charts note that the 1418 map puts together information that was available piecemeal in China from earlier nautical maps, going back to the 13th century and Kublai Khan, who was no mean explorer himself. They believe it is authentic.

The map makes good estimates of the latitude and longitude of much of the world, and recognises that the earth is round. “The Chinese were almost certainly aware of longitude before Zheng He set sail,” says Robert Cribbs of California State University. They certainly assumed the world was round. “The format of the map is totally consistent with the level of knowledge that we should expect of royal Chinese geographers following the voyages of Zheng He,” says Mr Thompson.

Moreover, some of the errors in the 1418 map soon turned up in European maps, the most striking being California drawn as an island. The Portuguese are aware of a world map drawn before 1420 by a cartographer named Albertin di Virga, which showed Africa and the Americas. Since no Portuguese seamen had yet discovered those places, the most obvious source for the information seems to be European copies of Chinese maps.

But this is certainly not a unanimous view among the experts, with many of the fiercest critics in China itself. Wang Tai-Peng, a scholarly journalist in Vancouver who does not doubt that the Chinese explored the world early in the 15th century (he has written about a visit by Chinese ambassadors to Florence in 1433), doubts whether Zheng He's ships landed in North America. Mr Wang also claims that Zheng He's navigation maps were drawn in a totally different Chinese map-making tradition. “Until the 1418 map is scientifically authenticated, we still have to take it with a grain of salt,” he says.

Most forgeries are driven by a commercial imperative, especially when the market for ancient maps is booming, as it is now. The Library of Congress recently paid $10m for a copy of a 1507 world map by Martin Waldseemuller, a German cartographer. But Mr Liu says he is not a seller: “The map is part of my life,” he claims.

The consequences of the discovery of this map could be considerable. If it does indeed prove to be the first map of the world, “the history of New World discovery will have to be rewritten,” claims Mr Menzies. How much does this matter? Showing that the world was first explored by Chinese rather than European seamen would be a major piece of historical revisionism. But there is more to history than that. It is no less interesting that the Chinese, having discovered the extent of the world, did not exploit it, politically or commercially. After all, Columbus's discovery of America led to exploitation and then development by Europeans which, 500 years later, made the United States more powerful than China had ever been.