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Where the Wild Things Were

“What’s wrong with this picture?” asks Dr. Ross MacPhee, pointing to a projected image. It’s an Ice Age scene, depicting several hunters chucking spears at monstrous woolly mammoths in the middle of a blizzard—like something out of an old issue of Boy’s Life. A rumble of laughter rolls through the University Museum’s Harrison Auditorium. MacPhee, curator of vertebrate zoology at the American Museum of Natural History, has made his point: As with The Flintstones, the entertaining but historically flawed cartoon, much of the hunting illustration is simply preposterous.

Illustration by Regan Dunnick

MacPhee’s remarks came during a symposium—“Pleistocene Big Game Extinctions in the Americas”—that was sponsored by Penn’s Center for Ancient Studies, the Institute for Environmental Studies, and the University Museum. Held this past November, the symposium was one of a series of lectures devoted to natural disasters and funded by Bruce (C’47) and Peggy (Ed’47 Hon’85) Mainwaring.

While humans did hunt mammoths in the late Pleistocene (Ice Age) period, archaeologists have found no evidence that mammoths and other giant mammals were attacked en masse, much less in blizzards. Still, the prevailing theory about these prehistoric animals’ extinction blames overzealous human hunting.

The “overkill” hypothesis supposes that, after crossing the Bering Strait, humans marched through North and South America, leaving heaps of “megafaunal” carcasses in their wake. Some 71 species—including giant rabbits, woolly rhinos, mastodons, saber-toothed tigers, giant beavers, 400-pound sloths, and Volkswagon-sized turtles—suddenly died off around 12,000 years ago.

Recently, scientists have proposed a new theory: that human-borne diseases, rather than human slaughter, may have extinguished most of the New World’s giant mammals. These diseases could have arrived with the immigrants from Asia, leapt to new host species, traveled faster than their human originators, and thus wiped out many mammals before they had any direct human contact. MacPhee is a leading proponent of this theory, which he first published in 1997.

Dr. David Meltzer, an archaeologist from Southern Methodist University, has analyzed megafaunal sites across the U.S. in order to determine exactly how the animals died, and whether humans hunted them. Noting that the overkill theory was first proposed in 1967 by archaeologist Paul Martin, Meltzer said that the concept fit snugly with the environmental zeitgeist of the 1960s, when human over-development was seen as a threat to other species. Martin’s theory gained popular acceptance, even though it failed to answer such nettlesome questions as how some extinctions predated human settlement.

The overkill theory is basically “a guy thing,” and the belief in “Ice Age Rambos” goes against everything we know about hunter-gatherers, said MacPhee. “It’s just unimaginable to me that the people concerned would be interested merely in killing—especially large, dangerous animals like mammoths.”

Meltzer was also skeptical of Martin’s theory, even though his research confirmed that the humans did indeed hunt large mammals. The problem, said Meltzer, is that these people weren’t systematically killing big game, certainly not enough to cause extinction. In the Southeastern U.S., Meltzer studied remains and artifacts from the Pleistocene period, in particular Clovis spear points (first found in Clovis, New Mexico), in the hopes of finding a “smoking gun.” The Clovis points weren’t always found at mammoth sites, however, indicating that while the “aircraft carriers of the animal kingdom” may have been prize kills, they were not everyday meals. More important, Meltzer said, no Clovis points were found at other mega-mammal sites, thus showing that humans weren’t responsible for the disappearance of giant armadillos, giant sloths, and prehistoric horses. While overkill “is a great tale of human destruction and our destructiveness to other species,” said Meltzer, “this may be one case of environmental catastrophe that humans are not responsible for.”

MacPhee found inspiration for his new theory, “hyperdisease,” in a 1992 New Yorker article about the Ebola virus in Africa. The article, which evolved into a best-selling book, The Hot Zone, documented how a once-obscure disease jumped between species, somehow becoming lethally virulent in the process.

“Ebola had come out of nowhere and hit humans like a sledgehammer,” said MacPhee. Perhaps, he wondered, the same sort of disease could have crossed the Ice Age land bridge. The disease would have been non-lethal to its human carriers, just as Ebola wasn’t fatal to African rodents and bats, but when it was transmitted to New World mammals, which possessed no inherent immunity, it would have become extraordinarily deadly. The hyperdisease theory could also explain the sudden, catastrophic drops in Pleistocene mammal populations, a quandary that neither overkill nor climate-change theories had convincingly solved.

The bones and teeth of mammoths and other extinct mammals may hold evidence of a pathogen that led to their demise. MacPhee and his colleagues have traveled to remote places like Wrangel Island (off the coast of northeastern Siberia in the Chukchi Sea), where some of the best mammoth specimens can be found. They hope that marrow extracted from mammoth ulnas and femurs will contain an ancient killer microbe.

Another topic that informs the hyperdisease theory, albeit indirectly, is the debate over how populous the Americas were before European settlers arrived. Historians have long known that Native Americans suffered dramatic population declines after “biological first contact” with Europeans. Along with dreams of gold and conquest, explorers like Hernando de Soto also brought over diseases such as measles and smallpox. Oddly, the worst thing de Soto did, say experts, is introduce pigs, which retransmitted the Old World contagions to American wildlife. Like MacPhee’s theoretical hyperdisease, these diseases were effective biological weapons, which some experts believe killed off 96 percent of the Indian inhabitants. Recently, scholars have upped the estimates of these pre-Columbian populations; some suggest that the Americas may actually have been more populous than Europe. The total body count is still hotly debated, but if tens of millions of Indians died from diseases spread by a small number of pigs, perhaps the megafauna could have suffered similar losses due to disease brought by the Indians’ ancestors.

Though the ecological irony of this parallel is tempting, MacPhee stresses that hyperdisease is still only a theory. Until pathological evidence of disease is found in giant mammal DNA and RNA samples, his theory will have to compete with the other extinction hypotheses.

—Ted Mann C’00

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2003 The Pennsylvania Gazette Last modified 02/28/03