The scientists pulled DNA from the teeth and compact bones, such as jaws, of 135 mammoths and took a closer look at 108 of the best samples.
The scientists also considered previously published information on DNA of 52 more woolly mammoths.
The research included DNA from more than 60 North American woolly mammoths, only a handful of which had been carefully studied before.
The Siberian and North American permafrost has preserved much genetic material from woolly mammoths, allowing scientists a clearer glimpse into the ancient DNA than is possible in other climates.
After radiocarbon dating the woolly mammoth samples, the scientists concluded that the ancient creature's first migration from Siberia across the Bering Strait may have taken place as long as 700,000 years ago, lead author Debruyne said.
The migration was possible via a land bridge north of the Bering Sea that linked Siberia and Alaska during icier times, when sea levels were lower.
(See related story: New Land-Bridge Evidence Adds to Mystery of 1st Americans [October 18, 2006])
For several hundred thousand years, the animals evolved separately in North America, developing unique genetic markers distinct from the Siberian populations, the scientists said.
Then another migration took place back over the strait about 300,000 years ago, they said.
"Until recently everybody has assumed the migration of the woolly mammoths into North America was a late event," Debruyne said.
The original Siberian population disappeared after that—about 40,000 years ago—while the North American crew came to dominate the next 30,000 years, the scientists concluded.
"What we don't know now is why those Siberians went extinct first, and if it was somehow the Americans' fault," Poinar said of the woolly mammoths.
"Out on a Limb"
Critics worry the McMaster team jumps to conclusions.
Stephan Schuster, a Pennsylvania State University biochemist who studies woolly mammoth genes, said the authors "go out on a limb" in interpreting their information.
The theory that woolly mammoths crossed back and forth over the Bering Strait and that the North American descendents outlived the original Siberians is plausible enough, he said.
But the genetic evidence presented is insufficient, he said, noting that the conclusions are based on only a single marker from mitochondrial DNA—the genetic information passed down from mothers.
"This would have been a much more significant study if this scientific team could have provided the complete mitochondrial DNA sequences and not just a short stretch," he said.
Schuster said the final word on mammoth extinctions will require an analysis of the genetic markers found in conventional or "nuclear" DNA, which is more difficult to isolate from ancient bones.
The authors of today's paper, however, stand by their science.
"Of course more data is good, but I'm not sure he's asking for the right data," said Debruyne, adding that more detailed genetic analyses have not so far challenged their conclusions.
Debruyne said DNA studies of more individual woolly mammoths is critical to understanding about the whole population, while Schuster emphasized more detailed study of the existing samples.
Molecular paleobiologist Ian Barnes of the Royal Holloway University of London said the new study both confirms and builds on conclusions he published last year based on the remains of far fewer woolly mammoths.
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