The Killing Fields What robbed the Americas of their most charismatic mammals?

 A little over 12,000 years ago, Kansas looked like Kenya's Serengeti Plain on steroids.

 Cheetahs stalked the Midwestern prairie, cheek by jowl with lions, camels, sloths, tapirs, yaks, llamas, and other mammals that survive today only on other continents. These familiar creatures shared the American landscape with a bizarre bestiary of woolly mammoths, saber-toothed cats, 500-pound armadillos, giant ground sloths, and many more unfamiliar species.

 Then, in the span of 4 short centuries, the curtain slammed down on most of these large mammals. Some 135 species winked out of existence on North and South America, despite having made a home in this hemisphere for millions of years. Scientists have spent decades debating what catastrophe slashed the rolls of American mammals so severely.

 Most paleontologists and ecologists have concluded that the species died out because of substantial shifts in climate accompanying the end of the ice age. But a new study of the extinction pattern across the hemisphere adds weight to an alternative idea, called the overkill hypothesis, that lays the blame squarely on human shoulders. In this scenario, the first waves of Asian hunters entering the Americas triggered the mammalian disaster by killing off the big game.

"You can't make a climate story out of this," says paleontologist John Alroy of the University of California, Santa Barbara. "This pattern is a positive confirmation of the overkill hypothesis." Alroy and Karen Sears of the University of Chicago presented their findings in Denver at a meeting of the Society of Vertebrate Paleontology in October.

 Alroy began this study by looking at how the biological crisis swept across North America at the end of the last ice age. Using a published list showing where fossils had been discovered, Alroy devised a statistical method for mapping the variations in extinction rates.

 As the ice retreated, the climate of North America changed most dramatically in the northern states. There, some habitat started out as tundra and within a thousand years, it sprouted forests. If climate had caused the extinctions, Alroy would have expected to find a much higher rate of die-offs in the north relative to the south.

 The pattern he observed, however, fit the opposite trend. Only 40 percent of the large-mammal species went extinct in the northern states, whereas 60 percent disappeared from Florida, Texas, and California. "That's a firm refutation of the climate idea," he says.

 When Alroy and Sears next looked at published fossil records further south, they expected to see a mirror of the North American pattern: lower rates of extinction in the temperate zones and higher ones in the tropics.

 The data threw them a second curve, however. Instead of peaking in the tropics, the extinction rates kept climbing the further south they looked. Down in temperate Argentina-the most southerly site in the study-more than 90 percent of the large mammals had disappeared.

 Trying to understand what made the southern species so vulnerable at this time, the researchers recognized an additional clue buried in the extinction lists. The American animals that survived the calamity-brown bears, moose, elk, wolves, deer, sheep, musk ox, wolverines-had coexisted with people in Asia for millions of years. These species passed back and forth into the New World across the Bering Strait land bridge, which emerged from the ocean during the ice age when sea levels dropped by 120 meters.

 By contrast, almost all the animals that died out were homegrown American species that had never had any contact with humans before the first hunters arrived just after the end of the ice age.

"If a species had populations in Eurasia and North America, it had a much better chance of making it through, even though Eurasia and North America were the two areas where the deglaciation was the most intense," says Alroy.

 This discovery, he contends, backs up the overkill hypothesis, first proposed in 1967 by Paul S. Martin of the University of Arizona in Tucson. Martin and other researchers discussed the hypothesis most recently in Extinctions in Near Time (1999, Ross D.E. MacPhee, Kluwer Academic), the outgrowth of a conference held 2 years ago at the American Museum of Natural History in New York. According to these scientists, the first human hunters who crossed the land bridge had great success catching animals that had never evolved any defenses against the most efficient predator on the planet.

 The new evidence, however, does not win over opponents who have long battled the overkill hypothesis. "I'm skeptical that the arrival of humans on the scene had much to do with it. I think the dominant factor was the changing climate," says paleontologist Ernest L. Lundelius Jr. of the University of Texas at Austin.

 The climate of 11,000 years ago was particularly harsh, with more extreme seasons than animals had experienced during the last ice age or the warm periods between previous glacial epochs, says Lundelius. At the time of the last interglacial, for instance, a large ground tortoise lived in central Illinois, in a region where winter temperatures today would prohibit the survival of such a reptile, he says.

 Anthony D. Barnosky of the University of California, Berkeley questions Alroy and Sears' results because the fossil data for South America remain much poorer than for North America. "Probably what they're seeing is a real sampling bias," he says.

 As for cause of the extinctions, he takes a Solomonic approach. "You had a correspondence in time of both purported causes. In fact, what probably happened was a one-two punch," he says, invoking both climate and human hunters. "If only one or the other had happened, it would not have been as severe," he says.

 In the past 2 years, a third possible explanation has entered the ring. People could have sparked the extinctions without hunting the species to death, but rather by introducing new epidemic diseases to the hemisphere, says Ross D.E. MacPhee of the American Museum of Natural History.

 The pathogens might have jumped from people or from the animals that march alongside them, such as rats, fleas, and other parasites. "There is this invisible biotic baggage that accompanies humans wherever they go," says MacPhee. He considers disease a much more plausible explanation for extinction of dangerous animals like mammoths than is a band of hunters sweeping over the continents.

 Alroy discounts the disease hypothesis, saying that animals had been crossing back and forth between Asia and North America for 65 million years without ever causing a crisis of the magnitude seen 11,000 years ago. MacPhee, however, sees the arrival of people as adding a new mode of transportation by which species-jumping diseases could enter the Americas. He and a colleague are currently trying to culture microorganisms from the bones of animals that died at this time.

 Defenders of the overkill hypothesis have long noted that large animal extinctions on different continents didn't all happen at the end of the last ice age. In Australia, for instance, an arkful of some 60 species died out around 50,000 years ago, about the same time that people first arrived.

 Evidence reported earlier this year supported the link between human invasions and animal extinctions, but it raised a new hypothesis to explain the connection. The early human residents of Australia may have killed off animals indirectly by setting widespread fires that altered the mix of vegetation, according to researchers from the University of Colorado at Boulder.

 Future studies of fossils from the Americas and elsewhere may ultimately reveal whether people had a hand in all of these extinctions and whether that hand was bloody, sooty, or simply loaded with infectious germs.