Anthropology

"This is a unique site, we've never seen anything like it before," Prof Higham, the director of Oxford's Radiocarbon Accelerator Unit, told BBC News. "The stone-tool evidence is very, very compelling."

The team excavated a 3m-deep (10ft) stratigraphic section - a sequence of soil layers arranged in the order they were deposited - and found some 1,900 stone artefacts made over thousands of years. Researchers were able to date bone, charcoal and sediment associated with the stone tools, using two scientific dating techniques. During the period known as the last glacial maximum, 26,000-19,000 years ago, sea levels were low enough for people to cross easily from Siberia to America via the Beringian land bridge. But what about during earlier times?

"Before 26,000 years ago, the latest data suggest that Beringia might have been a rather unattractive place for humans to be. It might well have been boggy and very difficult to traverse," said Prof Higham. "We still think the most likely scenario is for people to have come on a coastal route - hugging a coast - perhaps with some kind of maritime technology, which by that stage people in other parts of the world had certainly developed."

While people seem to have been in the Americas before the last glacial maximum, they were probably thin on the ground. It's only much later, between 14,000 and 15,000 years ago, that populations increase substantially.