By Steve Connor, Science Editor
04 September 2003

The accepted theory of how prehistoric humans first migrated to America has been challenged by a study of an ancient set of bones unearthed in Mexico.

An analysis of 33 skulls found on the Mexican peninsula of Baja California suggests that the first Americans were not north Asians who crossed to the American continent about 12,000 years ago.

The skulls more closely resemble the present-day natives of Australia and the South Pacific, suggesting that there might have been an earlier movement to America across the Bering Strait separating modern Russia from Alaska.

The research, published in today's issue of the journal Nature, is the latest twist in the controversy over who were the first Americans and how did they arrive in the New World?

Native Americans today bear a close physical resemblance to north-east Asians and anthropologists have long believed that this is because they are both descended from the same ancestors, some of whom migrated to America across the Ice Age bridge that spanned the Bering Strait.

However, a team of scientists led by Rolando Gonzales-José of the University of Barcelona in Spain believes that a different scenario could have occurred prior to the accepted migration of northern Asians.

Dr Gonzales-José and his colleagues analysed the shape and dimensions of 33 skulls of a tribe of people who lived near the western Mexican coast of Baja California between 2,500 and 300 years ago.

These relatively long and narrow skulls share a closer affinity to the skulls of the present-day inhabitants of south Asia and the southern Pacific Rim.

This suggests that these particular people could not have shared the same ancestor as present-day native Americans whose skulls more closely resemble broad and short shape of northern Asians.

Tom Dillehay, an anthropologist from the University of Kentucky in Lexington, said the accepted idea of how America became populated with humans was probably far too simple. "More recent archaeological discoveries suggest that there were several different founding populations, arriving from different places," Dr Dillehay said.

"To complicate matters further, it is no longer certain that the first colonisers arrived about 12,000 years ago - some archaeological sites in South America date from 12,500 years ago, which suggests that the first humans arrived at least 15,000 years ago," he said.

Dr Dillehay said the ancient people who lived on the long peninsula of Baja California probably became isolated from the rest of the north American population. This meant they retained the much older ancestral trait of a long and narrow skull.