October 24, 2001
The fate of ancient mammoths, mastodons and other large mammals, now extinct, has been the subject of debate for more than three decades
Now, a University of Washington archaeologist is stepping forward to dispute the so-called “overkill” hypothesis that blames “megafauna” extinction on human activity.
Writing in the Journal of World Prehistory, University of Washington professor Donald Grayson said there are dangerous environmental implications of using the overkill hypothesis as the basis for introducing exotic mammals, such as modern elephants, camels and other large herbivores, into arid 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,” said Grayson.
Instead of human activity killing off megafauna, 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. During the Pleistocene epoch, the massive ice sheets that covered much of the Northern Hemisphere began retreating.
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 today. 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. In addition, 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, about the same time when Clovis people were thought to be the first to enter North America. Clovis people were hunters who preyed on a diverse set of now-extinct large mammals.
Since records from islands show that human colonists cause extinctions, it was presumed Clovis people caused the extinctions of the large mammals they preyed upon.
“Martin’s theory is glitzy, easy to understand and fits with our image of ourselves as all-powerful,” said Grayson “It also fits well with the modern Green movement and the Judeo-Christian view of our place in the world. But there is no reason to believe that the early peoples of North America did what Martin’s argument says they did.”
More precisely, 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 big North American mammals became extinct at that time.
“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 ignored the possibility that the Clovis people were not the first humans in the New World. Evidence from a site in Monte Verde, Chile, demonstrates 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. 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,” said Grayson.
“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,” said Grayson.