By Elizabeth Mitchell
Science reporter, BBC News
Artist's impression: The iconic Ice Age woolly mammoth
Scientists have discovered that the last Siberian woolly mammoths may have originated in North America.
Their research in the journal Current Biology represents the largest study of ancient woolly mammoth DNA.
The scientists also question the direct role of climate change in the eventual demise of these large beasts.
They believe that woolly mammoths survived through the period when the ice sheets were at their maximum, while other Ice Age mammals "crashed out".
The iconic Ice Age woolly mammoth - Mammuthus primigenius - roamed through mainland Eurasia and North America until about 10,000 years ago.
Previous studies had hinted that the last mammoths left in Siberia were not natives - but immigrants from North America.
However, more evidence was required to strengthen the case for this "out of America" theory.
A team of researchers led by Professor Hendrik Poinar from McMaster University in Canada collected 160 mammoth samples from across Holarctica - a region encompassing present day North America, Europe and Asia.
Well-preserved DNA material - between 4,000 and 40,000 years old - was obtained from "almost every part of the animal - even from preserved hide, skin and hair", Professor Poinar told BBC News.
They analysed DNA from mitochondria - genetic material which is passed from mother to offspring via the egg - and can be used to track the ancestry of a species back many hundreds of generations.
The genetic information confirmed that a North American mammoth population overturned those endemic to Asia.
It is hard to speculate why the North American woolly mammoths returned to Siberia.
BERING LAND BRIDGE
a vast tundra plain that connected Asia and North America
about 1,000 miles from north to south at its greatest extent
was exposed and submerged as global sea levels changed during the Pleistocene
flooded and became the Bering Strait about 11,000 years ago
may have enabled migration of humans from Asia to the Americas
"Presumably, conditions were favourable on the Bering land bridge which was more of a large filter than an open highway," suggested Professor Poinar.
The expansion of North American forests may have "pushed the mammoths along", he added.
At the same time, the native Siberian mammoths, which may have been around for much of the Middle Pleistocene, completely disappeared.
It is unclear if the Siberian mammoths experienced a "natural decline" or if they were outcompeted by the North American immigrants.
The endemic Siberian population had different molar features and a "very unique DNA signature" - that was dated to be almost 900,000 years old.
It is possible that it may not have been a true woolly mammoth - but a more primitive species.
"Many people thought that this (primitive) species had become extinct way before 38,000 years ago," said Professor Poinar.
"Palaeontologists were not so happy because these are the intricacies of DNA that are very difficult to discern based on mammoth tusks and teeth," he added.
Scientists are now beginning to understand the dynamic evolutionary history of these Ice Age mammals.
"This study adds to a growing body of evidence about just how dramatic and tumultuous the Pleistocene climate actually was," Dr Beth Shapiro, a scientist from Penn State University in the US, told BBC News.
"With ancient DNA, we can actually go back in time and look directly at these old populations.
"Here we have evidence of local extinctions, replacements and long-distance dispersals," she explained.