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DNA sheds light on an American die-off

From the journal Science: Ancient bison DNA may help explain why many large mammals disappeared from North America about 10,000 years ago.
Bison and pronghorn antelope graze together in Wyoming, in this diorama created by James Perry Wilson for the American Museum of Natural History. The bison are descendants of Beringian bison that almost went extinct more than 10,000 years ago.
Bison and pronghorn antelope graze together in Wyoming, in this diorama created by James Perry Wilson for the American Museum of Natural History. The bison are descendants of Beringian bison that almost went extinct more than 10,000 years ago.
/ Source: Science

Ancient bison DNA may help explain a longstanding mystery of North American wildlife: why mammoths, North American lions, short-faced bears and many other large mammals disappeared about 10,000 years ago.

The authors of a new study show that bison populations in present-day Siberia, Alaska and the Yukon transitioned from boom to bust about 37,000 years ago. The bison population drop began well before large numbers of humans lived in the New World, and this timing suggests that climate and environmental change — not human hunting pressure — drove bison to near-extinction on the landmass known as Beringia. Placing the start of ancient bison declines before large numbers of humans lived in the area calls into question the oft-cited idea that hunters prompted the high-profile extinctions of mammoths and many other large mammals.

The research appears in Friday's issue of the journal Science, published by AAAS, the nonprofit science society.

‘Cockroaches of the Pleistocene’
“Bison are the cockroaches of the Pleistocene,” said Science author Duane Froese of the University of Alberta. The Pleistocene epoch spanned from 1.8 million to 10,000 years ago. Ancient bison are cockroaches in the sense that their fossilized remains are extremely common and widespread, said Beth Shapiro of Oxford University, lead author of the Science study.

Shapiro and colleagues sampled DNA from about 350 of these fossilized bison bones and reconstructed ancient bison history based on changes in genetic diversity.

Extracting ancient mitochondrial DNA from a bison bone.
Extracting ancient mitochondrial DNA from a bison bone.Henry Wellcome

This history provides the first information on when bison populations transitioned from exponential expansion to rapid decline. The timing of this turning point agrees with recent research pointing to local extinctions of brown bears and wild horses in Alaska around 35,000 years ago. Together, these findings support the possibility of a larger-scale environmental change affecting populations of large mammals and possibly triggering their downfall.

The start of the bison population crash coincided with the warmest years between the two most recent periods of rapid glacier expansion. The period was marked by reductions in tundra grasslands — with warmer temperatures, forests marched northward and invaded grasslands. This warm period was followed by cold, dry conditions that accompanied the glaciers that moved across much of North America. The authors suggest that some component of these ecological changes may have caused the widespread bison decline across Beringia.

Many of the other super-sized mammal species disappeared more than 25,000 years after the start of the bison decline. Differences in how individual species responded to environmental stress may help explain the staggered timing of the extinctions, the authors say. Shapiro added that human hunters could have exterminated species that were already weakened by climatic and environmental changes.

The smell of bison DNA
The mitochondrial DNA sequences used by the researchers are best-preserved in dense, heavy bones, such as the base of the skull and the longest leg bone, Shapiro said.

If the insides of a freshly drilled fossil bone smells “a bit like burned hair,” the bone probably contains the well-preserved DNA necessary for tracking changes in bison genetic diversity.

“This technique shows the genetic effects of environmental change and climate change,” said Science author Alan Cooper of Oxford University.

For the bison, the “genetic effect” was a dramatic decline in genetic diversity that accompanied a crash in bison numbers. While bison rebounded, the “genetic effects” for other charismatic species may have been more severe.

Rewriting bison history
The North American bison alive today are related to the ancient bison of Beringia, but there is disagreement in the details of the familial connection. In contrast to previous assertions, the Science authors report that today’s North American bison are the descendants of bison that were living south of the glaciers that formed across much of North America about 20,000 years ago, at the end of the last ice age.

In the past, scientists have argued that today’s bison are the descendants of populations that weathered the last ice sheet expansion north of the glaciers and moved south after the two major glaciers began to retreat.

The genetic history also provides new insights into the prolific, yet confusing and contradictory, classifications of Beringian bison. The new DNA analysis shows that ancient bison with big horns and big heads are not all grouped together in one small area of the family tree. The fact that bison with similar horns and skulls are scattered throughout the bison lineage suggests that food and environment — and not strictly genetics — determines these characteristics. It also highlights the possibility that ancient bison, and perhaps other species, need to be re-categorized after incorporating genetic information into their family trees.

In this way, the DNA history of ancient bison could also have significant implications for other parts the fossil record, Cooper said.