Aisling Irwin reports from the annual meeting of the American Association for the Advancement of Science in Atlanta. Geneticists believe that the first humans reached America 30,000 years ago by crossing the land that joined it to Siberia. Their findings contradict the claims of archaeologists who say that humans did not appear in America until about 12,000 years ago.

Douglas Wallace, of Emory University school of medicine, has been trying to work out where the first Americans came from by analysing mitochondrial DNA in present-day native Americans.

Mitochondrial DNA is separate from the DNA helix in the cell nucleus. It is passed down the maternal line. Because it does not mix with the male line it is simpler to study, remaining more constant through the generations. Its only changes are due to mutations.

Dr Wallace and colleagues have discovered four types of mDNA in native Americans: "Each seemed to be a very distinct population," he said. He then studied the people whom he thought were the most likely originators of the Americans - the most north-eastern populations of Siberia. He found that the Siberian eskimos had three of the four types of mDNA (types A, C and D). There is no trace of the fourth among them - but it has been found on the Asian coast.

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"The next question we had was could we get some idea of the age of these migrations," he said. Using studies of a stable population with one type of mDNA and analysing how much the mDNA varies between individuals, it is possible to estimate the population's age by working out the rate of accumulation of diversity. A low level of variation between individuals' mDNA sequence, means that they emerged relatively recently.

Dr Wallace has studied the amount of variation within the groups that carry the A,B,C and D mDNA. Putting these findings together with studies of the mitochondrial DNA of Europeans and Africans, he has drawn a global map which shows that there were several waves of peoples populating the Americas.

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"The first palaeo-indian migrations occurred about 34,000 years ago from Siberia. A much later migration along the coast came about 15,000 years ago."

This was followed by a another migration from Siberia about 9,000 years ago.

Jan Klein, of the Max-Planck Institut for Biology in Berlin said: "Archaeologists say that the first signs of man in America are 12,000-14,000 years ago. They can't say who the people were but they can say when they appeared. The genetics support an older divergence. But mDNA is a single gene. Unless we have estimates from 50 genes it's very unreliable." But, he said, even when they take into account the ambiguities, geneticists' estimates are still placed at much more than 12,000 years ago.