Scientists have been picking over the bones and evidence for more than three decades but cannot agree on what caused the extinction of many of the continent's large mammals.

Now, in two new papers, a University of Washington archaeologist disputes the so-called overkill hypothesis that pins the crime on the New World's first humans, calling it a "faith-based credo" that bows to Green politics.

"While the initial presentation of the overkill hypothesis was good and productive science, it has now become something more akin to a faith-based policy statement than to a scientific statement about the past," said Donald Grayson, a UW anthropology professor.

Writing in the current issue of the Journal of World Prehistory and in a paper to be published in a forthcoming issue of the Bulletin of the Florida Museum of Natural History, Grayson said there are dangerous environmental implications of using overkill hypothesis as the basis for introducing exotic mammals into arid western North America."

He looks askance at the idea of introducing modern elephants, camels and other large herbivores into the southwest United States.

"Overkill proponents have argued that these animals would still be around if people hadn't killed them and that ecological niches still exist for them. Those niches do not exist. Otherwise the herbivores would still be there."

If early humans didn't kill North America's megafauna, then what did?

Grayson points to climate shifts, during the late Pleistocene epoch, which ended about 10,000 years ago, and subsequent changes in weather and plants as the likely culprits in the demise of North America's megafauna.

The massive ice sheets that covered much of the Northern Hemisphere began retreating. In North America, this icy mantle prevented Arctic weather systems from extending into the mid-continent.

Seasonal weather swings were less dramatic and didn't reach as far south as they presently do. But with this change, the climate became more similar to today's, marked by cold winters and warm summers.

As a result, an unusual patchwork aggregation of plant communities ceased to exit and there was a massive reorganization of biotic communities. At the same time, new data developed by Russell Graham, a paleontologist with the Denver Museum, shows that small mammals such as shrews and voles were moving about the landscape and becoming locally extinct. And there were the extinctions of some 35 genera of large North American mammals, including horses, camels, bears, giant sloths, saber-toothed cats, mastodons and mammoths.

The overkill hypothesis was proposed by retired University of Arizona ecologist Paul Martin in 1967 and its basic arguments haven't changed since. It claims:

• large mammal extinctions occurred 11,000 years ago;
• Clovis people were the first to enter North America, about 11,000 years ago;
• Clovis people were hunters who preyed on a diverse set of now-extinct large mammals;
• records from islands show that human colonists cause extinction;
• therefore, Clovis people caused extinctions.

First of all there is no compelling evidence that the majority of the extinctions occurred during Clovis times, said Grayson. Only 15 genera can be shown to have survived beyond 12,000 years ago and into Clovis times.

For 30 years, overkill proponents have assumed that since some genera can be shown to have become extinct around 11,000 years ago, all the big North American mammals became extinct at that time, he said.

"That's an enormous assumption, even though there is no compelling evidence of it in North America," Grayson said.

He also said overkill proponents have consistently ignored the possibility that the Clovis people were not the first humans in the New World. They reject evidence from a site in Monte Verde, Chile, showing human occupation that dates some 12,500 to 12,800 years ago. Monte Verde also has yielded some material that may push human occupation back to 33,000 years before the present.

Well-accepted Clovis sites dating between 10,800 and 11,300 years ago have been found in North America, and distinctive, fluted projectile points mark this culture. Clovis artifacts have been found with mammoth remains in more than a dozen sites across the Great Plains and the southwestern United States.

Grayson said there is no reason to doubt that these people scavenged and hunted large mammals. But he cautioned that while mammoths, mastodons, horses and camels were the most common large mammals in the late Pleistocene -- 10,000 to 20,000 years ago -- only mammoths are found at kill sites associated with Clovis people.

As for the claim that human colonization of the world's islands resulted in widespread vertebrate extinction, Grayson said this did not occur simply because of human hunting.

"No one has ever securely documented the prehistoric extinction of any vertebrate as a result human predation, though it may certainly have happened. In virtually all cases, when people colonize an area many other changes follow -- fire, erosion and the introduction of a wide range of predators and competitors.

"We do know that human colonists caused extinctions in isolated, tightly bound island settings, but islands are fundamentally different from continents," he added.

"The overkill hypothesis attempts to compare the incomparable and there is no evidence of human-caused environmental change in North America. But there is evidence of climate change. Overkill is bad science because it is immune to the empirical record."

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