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Inbreeding doomed woolly mammoths

Environmental causes, not hunting by humans, are blamed in a study.

June 09, 2007|Thomas H. Maugh II | Times Staff Writer

Paleontologists have long assumed that intensive hunting by humans led to the extinction of the woolly mammoths about 12,000 years ago, but new genetic analysis indicates that inbreeding and loss of genetic variability was the cause.

Paleontologists Ian Barnes of the University of London and Adrian Lister of University College London and their colleagues studied mitochondrial DNA samples from 96 bone, tooth and ivory specimens collected primarily in Alaska and Siberia. Their findings, published Thursday in the journal Current Biology, chart the animals' evolutionary history.

The animals apparently originated in Asia about 150,000 years ago and migrated over the land bridge connecting Siberia and Alaska in what is now the Bering Strait to form populations on two continents. A gradual warming of the Earth caused sea levels to rise, inundating the bridge and isolating the two groups, which became genetically distinct.

About 100,000 years ago, the land bridge appeared again, allowing the two groups to intermingle. By about 43,000 years ago, the Siberian lineage had died out.

The ice age 20,000 years ago stressed the remaining Alaskan population further, the researchers said, reducing their viability.

"A picture is emerging of extinction not as a sudden event at the end of the last ice age, but as a piecemeal process over tens of thousands of years involving progressive loss of genetic diversity," Barnes said. "For the mammoth, this seems much more likely to have been driven by environmental rather than human causes."

The 9-foot-tall creatures with their 20-inch-long strands of hair and gigantic curved tusks have long been an icon for the frozen tundra of the last ice age and the battles of humans to feed their families.

Their thick, shaggy hair, small ears -- to minimize heat loss -- and an extra layer of body fat helped them survive the cold, but did little to prevent their ultimately dying off.

thomas.maugh@latimes.com

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