Fossilised skulls from a long-extinct tribe found in Mexico have reignited a debate about how early humans colonised the Americas after emerging in Africa and trekking across Asia.

The conventional view is that Native Americans are the descendants of Mongoloid people, who crossed over from Asia to Alaska in a migration sometime between 40,000 and 12,000 years ago. These intrepid settlers used the shallow island chain of the Bering Strait - as it then was - as stepping stones.

After they arrived in Alaska, they progressively headed southwards, eventually ending up in the tip of South America.

But that theory has come under pressure over the past decade, thanks to a mosaic of archaeological evidence suggesting that the very first Americans may have come from elsewhere. Powerful new backing for the dissenters has now come from a find of 33 skulls excavated from the tip of the Baja California peninsula in Mexico, detailed in today's issue of the journal, Nature.

The researchers, led by Rolando González-José of Spain's University of Barcelona, contend that the skulls do not bear the characteristics of northeast Asians. They say the remains have more similarities with the early peoples of South Asia and the Pacific Rim, who had relatively short, narrow faces and long and narrow braincases.

The authors do not deny the impact of Mongoloids in the wider colonisation of the Americas, nor do they suggest that the Mexican-based migrants did not use the Bering Strait - rather than rafts or boats - to get there.

But, they say, the discovery of these 'Paleoamericans' showed that the colonisation process was more complex, possibly entailing a broader genetic mix and a different timescale than is generally thought.

One of the researchers, Marina Sardi said that, instead of the colonisation being a gradual southward movement, it could have come in hops and jumps, with some tribes leapfrogging down the Pacific coast.

But in the specific case of the "Paleoamericans" in Baja California, there is no evidence that the tribe mixed with Mongoloids, she said.

They may have eventually died out, an isolated community, because of climate change and Baja California's geographic isolation, according to the study.

After the last Ice Age, which ended about 10,000 years ago, the peninsula would have become warmer and drier - an arid, disconnected place that would have been forbidding for travel.

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