When, at least 12,000 years ago, human beings first crossed into North America from Siberia, the continent teemed with large animals. Today, of course, our only encounters with giant short-faced bears, enormous sloths and dozens of other such extinct species come in museums. On this much, archaeologists and paleontologists agree. The causes of this mass extinction, however, remain clouded by conflicting findings and holes in the archaeological record.

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The mystery extends far beyond North America. Between about 50,000 and 10,000 years ago, near the end of the Pleistocene, much of the world's megafauna (usually defined as animals weighing at least 100 pounds) disappeared. At the same time, Homo sapiens was expanding from Africa into Eurasia, Australia and the Americas. The late Pleistocene also witnessed dramatic climate change, especially during the period of warming and deglaciation that followed the Last Glacial Maximum some 20,000 years ago.

This convergence of events makes for exciting—and sometimes contentious—science. High-impact human hunting, referred to by archaeologists as "overkill," and climate change are the two most cited possible causes of the extinctions, but the role of each remains contested.

The debate began to heat up in the late 1960s after Paul S. Martin, a professor of geosciences at the University of Arizona, first proposed a "blitzkrieg" model of human overkill for North America—basically, overkill on fast forward. In this scenario, humans moved rapidly through the continent, slaughtering mammoths, mastodons and other large prey as they went. Within about 1,000 years, most North American endemic megafauna were gone.

The blitzkrieg hypothesis has since been applied elsewhere, but it remains controversial. Criticism has focused on the lack of archaeological evidence, a charge Martin has responded to by arguing that, if the extinctions occurred quickly, there would be little trace of the massacre in the fossil record. Archaeologists Donald Grayson of the University of Washington and David Meltzer of Southern Methodist University have been particularly critical of Martin's response, calling it "faith-based" science.

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Two recent papers, both published in Proceedings of the National Academy of Sciences of the U.S.A., try to help settle the question. Todd Surovell and Nicole Waguespack of the University of Wyoming and P. Jeffrey Brantingham of the University of California, Los Angeles, studied the timing and location of Pleistocene encounters between humans and proboscideans (the order that includes mammoths, mastodons and elephants) and found evidence supporting the overkill hypothesis. Meanwhile, Clive N. G. Trueman of the University of Portsmouth and Judith H. Field of the University of Sydney were part of a multinational team that confirmed the age of megafauna fossils at a site in eastern Australia, concluding that their work weakens claims for overkill in the land Down Under.

Surovell, Waguespack and Brantingham outlined two possible extinction scenarios, one based on human overkill and the other on climate change. They then plugged into their models data from 41 archaeological sites in Africa, Europe, Asia and the Americas that contain remains of proboscideans hunted or scavenged by humans. If people hunted these animals to extinction, the authors argue, the kill sites should appear along the border between proboscidean and human ranges. So, as humans expanded south across North America, for example, the sites would also be located farther and farther south. If climate was the culprit, then people and proboscideans should have shared some of the same territory, at least until climate change shrunk proboscidean habitat. Thus, kill sites would be found both along and behind the frontier of human expansion.

The authors concluded that the location and age of the sites correlate closely with an overkill model. As humans moved north into Eurasia from Africa and, later, south from Alaska across the Americas, proboscidean range contracted correspondingly. Climate change, then, cannot account for proboscidean extinction "unless one were to invoke serial climatic change that perfectly tracks human global colonization." The odds, they're saying, aren't good.

Although the authors do not claim to have proved that humans drove other species to extinction, Surovell is skeptical of arguments for climate change. "I would like to see somebody explain how climate change could cause mass extinction on such a large geographical scale," he says. "Climate is constantly changing."

In Australia, much of the evidence for overkill relies on proving that many large animals became extinct within several millennia of the first appearance of humans, usually estimated at about 50,000 years ago. Unlike other parts of the world, nothing in Australia's fossil record proves that humans hunted megafauna. As Trueman and Field note in their paper, there aren't even any sites with evidence that early inhabitants had the tools to kill large animals.

Trueman and Field discuss the dates of a controversial archaeological site, Cuddie Springs, that might prove that at least some Australian megafauna survived much longer than previously thought, dealing a blow to arguments for overkill. The site includes remains of several extinct animals, including Diprotodon, a two-ton marsupial, and Genyornis, a large, flightless bird. Previous efforts, made using radiocarbon dating and other methods, have concluded that some megafauna remains found there are 36,000 to 30,000 years old, but the findings have been disputed. Trueman and Field used a newer technique in their recent work, an analysis of rare earth elements (REEs) in bone fragments, and confirmed these dates. As they're buried, bones adsorb REEs, leaving a "fingerprint" that links the bones to their original layer of deposition.

Proving that people coexisted with large animals for 10,000 years or more would not necessarily remove humans from the extinction equation, but it would make it more likely that other factors, such as climate, also played a key part. Field, for one, is convinced that the findings at Cuddie Springs disprove the possibility of blitzkrieg in Australia and cast doubt on the overkill hypothesis. It's about time, she says, "to start entertaining other ideas about the extinction process."

Not everyone is convinced. In a 2001 paper published in Science, Richard G. Roberts of the University of Wollongong and a team of investigators found evidence of widespread Australian megafauna extinctions by about 46,000 years ago, concluding that humans must have played an important role. Roberts says that he still has "some strong reservations" about the recent paper. He notes that REEs are usually used to date much older bones, for which an error of thousands of years one way or the other would be insignificant. Although he himself is not entirely persuaded by blitzkrieg, he does think that it remains a possibility.

If Pleistocene humans hunted some large animals to extinction but blitzkrieg is ruled out as a possibility in Australia, as the recent findings suggest, the search for an overarching theory may be futile. In a review of recent research on the extinctions, published in the October 1, 2004, issue of Science, coauthors Anthony D. Barnosky, Paul L. Koch, Robert S. Feranec, Scott L. Wing and Alan B. Shabel argued that it will be more productive to look for localized, species-by-species explanations than a single cause. Some combination of climate change and human activity, they think, probably determined the fate of much of the world's megafauna.

Implicating multiple factors might not be as satisfying as convicting a single perpetrator, but it may better explain the evidence at hand. And as the Science authors point out, the combination of climate change and human action can have a much greater effect on the world's animal species than either factor alone. There's no debate that both are today affecting the viability of the remaining megafauna.—Amos Esty