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Dogs were living as companions to the early settlers of North America over 10,000 years ago.

of Durham University, UK and her American colleagues have re-examined the remains of three ancient dog skeletons, which had previously been excavated. Two of them, from the Koster site in a tributary of the Illinois River, were thought to be America鈥檚 oldest domesticated dogs, at around 9500 years old. The team performed fresh radiocarbon dating and found that these two dogs were even older: 10,110 and 10,130 years old.

A third dog from another Illinois site called Stilwell II was older still, at 10,190 years old. That makes it the oldest known domesticated dog in the Americas. It was discovered and excavated in 1962 and, like the Koster dogs, kept in the Illinois State Museum.

The researchers declined to comment on the study, as it will soon appear in a peer-reviewed journal.

The team concluded all three dogs were domesticated because their skeletons were found intact and unskinned, rather than butchered. They had also been carefully buried, further evidence they were valued by their human owners. The Stilwell II dog, which likely resembled a small English setter, was found beneath what seemed to be the floor of a living area.

Late arrivals

It鈥檚 unclear why it took so long for tame dogs to arrive in the ancient Americas, given that they had been domesticated at least 4000 years earlier in Eurasia.

By this time, the first settlers had begun crossing to America from Siberia, across land now covered by the Bering Strait. From Alaska, they progressed south into what is now Canada and the US, although it鈥檚 not clear by what route. Some were in Chile by 18,500 years ago.

Yet there is no evidence these first settlers from Siberia brought domestic dogs with them.

鈥淚t is generally accepted that dogs arrived in the US several thousand years after they were domesticated in Europe and Asia,鈥� says of Ghent University in Belgium. Earlier this year he reported evidence of a dog being deliberately buried 14,200 years ago in Germany.

鈥淲e don鈥檛 know if they were part of the first waves of immigration to the Americas,鈥� says Janssens. 鈥淚t could be so, but no archaeological bones have yet been found.鈥�

This may reflect a different attitude towards the dogs among the first settlers, says of Pennsylvania State University. 鈥淚f people brought dogs to the New World, which I think is overwhelmingly probable, [they] may not have had the time or the spiritual compulsion to bury them.鈥�

Research previewed at a conference in China last month may shed some light. of the University of Oxford and his colleagues sequenced a canine genital tumour that has spread around the world over thousands of years. Because the tumour is a rare form of transmissible cancer, the team could use genetics to trace its origins back to some of the earliest domesticated dogs in the Americas. The findings will be published in July.

Additional material by Catherine Brahic.