Woolly mammoths, the only way we know 'em. Credit: Peter Bevan
This year may be remembered as the year the weight of climate change finally began to sink in. It only took climate scientists two decades of banging their heads against the wall to accomplish it.
While most observers call that cause for celebration, a few researchers are worried the climatologists have been too successful. They point to an increasing tendency to blame humans for ecological mysteries, a bias that's shaping funding priorities and hampering attempts to better understand natural history.
Both terrestrial and marine biologists, for example, are fighting over the fate of large animals, or "megafauna." The former group is grappling with the long-standing question of whether the first North American settlers catalyzed the extinction of mammoths, mastodons, and other sizeable terrestrial mammals. The latter debate involves the more recent decline of Steller sea lions.
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In both cases, the latest evidence suggests scientists were too quick to embrace the anthropogenic causes.
Paleontologists have long puzzled over the problem of what happened to North America's big land mammals 11,000 years ago, at the end of the Pleistocene Epoch. Since 1967, University of Arizona geoscientist Paul Martin's "overkill" theory—that humans wiped out these animals—has held sway.
It seems a plausible idea, given that most of the largest mammals seemed to disappear soon after humans stormed the Bering Land Bridge, and it remains popular. Evolutionary biologist Jared Diamond embraced it in his popular books, 1992's The Third Chimpanzee and again in Guns, Germs, and Steel: The Fates of Human Societies, which won the Pulitzer Prize in 1998. In a 2005 book, Twilight of the Mammoths: Ice Age Extinctions and the Rewilding of America, Martin extends his argument that "virtually all extinctions of wild animals in the past 50,000 years are anthropogenic."
But not everyone is convinced.
Russ Graham, director of Penn State's College of Earth and Mineral Sciences, cites a dearth of actual kill sites and other exceptions to the overkill chronology. "We've come a long way in the last 30 years as we've developed new techniques," he says, referring to isotopic analysis of fossils, computer models and new archeological discoveries. "All they have is timing," he says of overkill theorists.
Anthropologist Donald Grayson shares Graham's suspicion that unwarranted enthusiasm for anthropogenesis is "playing a major role" in diverting attention from alternative explanations. In a chapter of a forthcoming book on the arrival of the first North Americans, Grayson attributes the bias to a political agenda: "One may applaud the intent, but it is hard to avoid the fact that overkill's continued popularity in this context appears more closely related to the environmental movement than to any supporting evidence."
In a recent paper in the Journal of Archaeological Science titled "A requiem for North American overkill," Grayson and fellow archeologist David Meltzer point out that Martin's theory was published five years after the appearance of Rachel Carson's exposé of the dangers of chemical pollution, Silent Spring, and the same year as the founding of the Environmental Defense Fund.
"We suggest that the overkill argument captured the popular imagination during a time of intense concern over our species' destructive behavior toward life on Earth. It retains that grasp today," they wrote. "It is easy to show that overkill's continued popularity is closely related to the political uses to which it can be put."