for National Geographic News
Climate and environmental change, not human hunters, forced the extinction of mammoths, saber-toothed cats, and many other large creatures that once roamed Siberia, Alaska, and Canada, a new study suggests.
Scientists say the changes also spurred the near obliteration of massive herds of bison that once thundered through the region of Beringia about 37,000 years ago. A land bridge formed during the last ice age, Beringia joined Asia to Alaska and northwestern Canada.
"This research shows it wasn't humans that instigated the loss of diversity," said Beth Shapiro, an evolutionary biologist at England's Oxford University. Shapiro led the study, which appeared last week in the journal Science.
Previous studies have suggested that early humans hunted mammoths, saber-toothed cats, and other large mammals to extinction and triggered the decline of bison herds. A basis of the theory is the belief that the ancient animal die-offs coincided with the migration of the first large human populations into North America.
Since the fossils of many of these animals are scarce, Shapiro and her team are attempting to infer the larger story of these animals from ancient bison fossils.
By analyzing the DNA from 442 bison fossils found in North America, Siberia, and China and preserved in museums around the world, Shapiro and her colleagues created a picture of how bison populations have changed over time.
According to the researchers' DNA analysis, bison populations surged and then inexplicably began to crash about 37,000 years ago.
The researchers say that whatever drove changes in bison populations also likely drove changes in the populations of other large mammals such as mammoths, short-faced bears, and saber-toothed cats.
Humans began their massive migration across the Beringia land bridge about 12,500 years ago, according to one theory (see related article)well after the bison decline and large mammal extinctions began.
Shapiro said the DNA analysis shows that the arrival of large human populations in North America was not the factor that initiated the animals' extinction.
"It could be that humans turned up just at the wrong time," Shapiro said. "The big mammals were so stressed from ecological changes that the influence of humans may have been what actually pushed them over the edge."
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