By STEVE CONNOR
The accepted theory of how prehistoric humans colonised America has been challenged by a study of bones unearthed in Mexico.
An analysis of 33 skulls found on the Mexican peninsula of Baja California suggests that the first Americans did not migrate across the Bering Strait separating modern Russia and Alaska 12,000 years ago.
The skulls more closely resemble the present-day native people of Australia and the South Pacific, suggesting that there might have been a much earlier colonisation of America via a different route.
The research, published in the journal Nature, is the latest twist in the controversy over who the first Americans were and how they arrived in the New World.
Native Americans 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 land bridge that spanned the Bering Strait about 12,000 years ago.
However, a team of scientists led by Rolando Gonzales-Jose of the University of Barcelona in Spain believes that a different scenario could have occurred many thousands of years prior to the accepted migration of the first Americans.
Dr Gonzales-Jose and his colleagues analysed the shape and dimensions of 33 skulls of a tribe of people who lived a Stone Age existence near the western Mexican coast of Baja California.
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 Stone Age people could not have shared the same ancestor as present-day native Americans whose skulls more closely resemble the broad and short ones of northern Asians.
Tom Dillehay, an anthropologist from the University of Kentucky in Lexington, said that the accepted idea of how America became populated was probably far too simple.
"More recent archaeological discoveries suggest that there were several different founding populations, arriving from different places, each with different lifestyles and technologies.
"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.
Last December, scientists in Mexico announced that they had dated the oldest skull in the New World. It belonged to a 26-year-old woman who lived near present-day Mexico City abut 13,000 years ago.