2006年VOA标准英语-Study Suggests Climate Change Killed Off I

By David McAlary
Washington
10 May 2006

There is new evidence in the case of the missing woolly mammoth 1. In fact, several species of large mammals in addition to the mammoth went missing at the end of the last ice age more than 10,000 years ago, and scientific detectives have been seeking an explanation for their extinction 2. The new evidence points to a suspect other than human hunters, but that probably will not end the debate.

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"Oh give me a home

Where the buffalo 3 roam,

Where the deer and the antelope 4 play . . ."

Mammuthus columbi    
  
This traditional cowboy song pays homage 5 to some of the large game found in North America. Had it been written 13,000 years ago, it might have mentioned the woolly mammoth, giant beaver 6, saber-toothed tiger, wild horse, or a variety of other very large native animals that became extinct soon after. The extinction affected 7 75 percent of North America's big mammals. Scientists have long debated whether the cause was climate change or hunting by the human newcomers from Asia.

New scientific dating of 600 bones from that era from Alaska and Canada's Yukon Territory, including bones of humans, suggests that the warming climate was the culprit. University of Alaska scientist Dale Guthrie says it was a time when the landscape was changing from frigid 8 grassland 9 preferred by the large species to something less hospitable 10 to them.

"What happened during this changeover time was that more moisture and warmer temperatures allowed trees and tundra 11 to move in, a very unproductive landscape for large mammals but very dense 12 greenery," said Dale Guthrie.

The study of the bones' ages, published in the journal Nature, shows that the woolly mammoth was already declining in number when humans arrived in North America from Siberia over the Bering Strait, which was still frozen at the time. The research also points out that wild horses died off 1,000 years earlier than the mammoth.

"So if you think of it as a really potent 13 superkiller society coming in cleaning out all the animals, you would expect the mortality to be synchronous," he said.

Guthrie also notes that wild horse bones have never been found in early human hunting sites in North America. Abundant remains 14 of bison and elk 15 have been found, indicating they were the ones most hunted. Yet they did not become extinct.

"So that again says something about the human overkill theory," noted 16 Guthrie. "It makes it seem less plausible 17."

But others cling to the belief that humans forced the large mammal disappearance 18, not only in North America, but everywhere. University of Arizona geo-scientist Paul Martin has long described how the pattern of large animal extinctions coincides with human expansion and big-game hunting technologies.

"The arrival of the first people into a landmass [has] everything to do with the extinction of large animals at that time in that landmass," said Paul Martin.

Supporting this view is the dating of several burial sites of big Australian land mammals, reptiles 19, and birds in 2002. It revealed that the animals died off about 46,000 years ago, a few thousand years after humans arrived. That was far earlier than the North American extinctions. University of Melbourne researcher Richard Roberts says climate change cannot be the explanation for the loss of this so-called megafauna. If it were, extinctions would have occurred simultaneously 20 everywhere.

"All over the world, New Zealand, Madagascar, North America, South America - all of these islands and continents had megafauna that now no longer exist, and the only related theme is that people arrived at different points in time at each of those places," said Richard Roberts. "At all of these different points in time, soon after people arrived, the megafauna went extinct."

At the University of Alaska, Dale Guthrie agrees that animals are vulnerable to human colonization 21 and does not discount the notion that prehistoric 22 hunters might have decimated some populations of game. But he insists that people should not be singled out for blame.

"The story is more complex than any simplistic idea," he said. "So to have one simple solution to all of extinctions seems to be rather unwise."