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Genetic Study Supports 'Out-of-America' Theory for Woolly Mammoths

NEW YORK (GenomeWeb News) – North American woolly mammoths replaced and outlasted their Eurasian counterparts in Siberia, according to new research.
 
At one time it was thought that woolly mammoth populations were part of one large evolving group connected by the Bering Land Bridge. In contrast, research appearing online yesterday in Current Biology, underscores the diversity, dynamics and complexity of mammoth populations. The results also cast the North American woolly mammoths in a new light and deepen the mystery surrounding the disappearance of native Siberian woolly mammoths.
 
“Something really happened to those Siberian guys,” senior author Hendrik Poinar, a researcher at McMaster University’s Ancient DNA Centre, said in a statement. “There was a lot of turnover going on — something that’s difficult if not impossible to tell by looking at teeth, tusks, and bones.”
 
Poinar and his team extracted DNA from 135 woolly mammoth samples collected throughout Holarctica — an area that includes present day North America, Europe, and Asia. They subsequently amplified, cloned, and sequenced 743 base pairs of DNA from the cytochrome b gene and hyper-variable region I of the mitochondrial genome of 108 of these animals.
 
After incorporating data from 52 previously published mammoth sequences, the researchers used Bayesian and network methods to look at the phylogeographic patterns within the mammoth populations. They found five different mammoth haplogroups that fell into three clades: one found only in Asia, one found only in North America, and three that were found in both regions.
 
By mapping woolly mammoth populations over time, the team reconstructed the population dynamics leading up to the mammoths’ demise. The sequence data suggests that some North American woolly mammoths, which had been introduced to North America from Siberia roughly 200,000 years ago, returned to Siberia many years later.
 
This “Out-of-America” theory holds that a group of North American mammoths wandered back into Siberia and remained there until their extinction about ten thousand years ago. In the process, they replaced and/or succeeded the native Siberian mammoths, which disappeared roughly 40,000 years ago. It is unclear whether the North American mammoths contributed to the disappearance of their Eurasian counterparts or whether the events were independent.
 
The work may explain why genetically distinct groups of mammoths roamed Siberia — a finding that was reported by Pennsylvania State University researchers in the Proceedings of the National Academy of Sciences this past June.
 
It also pokes holes in the notion that the Bering Land Bridge was an easily and frequently traversed route. In contrast to the theory that woolly mammoth populations passed freely between North America and Eurasia, the work suggests that there were relatively few major mammoth migrations across the land mass.
 
“It’s interesting that our data can be explained by just four or five migration events [across the Bering Land Bridge],” Poinar said. “It was more of a filter than a freeway.”
 
The data also raises new questions about what led to the woolly mammoths’ ultimate downfall, since they seem to have maintained populations and genetic diversity through the Last Glacial Maximum about 20 thousand years ago, when ice sheets covered northern Europe and North America. “This puts the direct effects of climate change as a cause for extinction in question,” Poinar said.
 
“The classical understanding of Peistocene mammoth populations, typically told as a tale of two continents, should be revised to take these dynamic patterns into account,” Poinar and his colleagues concluded, “thus revealing a clearer picture of the tempo and mode of both evolution and extinction in Mammuthus.”

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