Hokkaido: Home of the Ainu, who were among the US's first immigrants

They say the results show two distinct groups of Native Americans. The first arrived about 15,000 years ago, and formed most of the pre-colonial population of the Americas.

But the other main migration, they say, came less than 5,000 years ago. It accounted for the modern Inuits of Arctic America, the Aleuts of Alaska, and for peoples who penetrated the south-west of what is now the United States to become the Navajo Indians.

Japanese relations

Dr Brace believes that the first group's closest relatives in the Old World are likely to have been the ancient Jomon people of Japan, and the Ainu, who remain to this day in northern Japan.

Their anatomy in many ways resembles Europeans more than other East Asians.

The second wave of American immigrants, Dr Brace maintains, came from China, Mongolia and South-East Asia.

His theory might go some way towards explaining the great variety seen in the most ancient skeletons found in North America, some of which appear to look more like Europeans than modern Native Americans, or the Asians usually supposed to have been their ancestors.

But scientists increasingly look for genetic evidence to back up findings derived from measuring bones, and that has yet to be found in this case.