Until about 11,000 years ago, mammoths, giant beavers, and other massive mammals roamed North America. Many researchers have blamed their demise on incoming Paleoindians, the first Americans, who allegedly hunted them to extinction. But a new study fingers climate and environmental changes instead. The findings could have implications for conservation strategies, including controversial proposals for “rewilding” lions and elephants into North America.

The idea that humans wiped out North America’s giant mammals, or megafauna, is known as the “overkill hypothesis.” First proposed by geoscientist Paul Martin more than 40 years ago, it was inspired in part by advances in radiocarbon dating, which seemed to indicate an overlap between the arrival of the first humans in North America and the demise of the great mammals. But over the years, a number of archaeologists have challenged the idea on several grounds. For example, some researchers have argued that out of 36 animals that went extinct, only two—the mammoth and the mastodon—show clear signs of having been hunted, such as cuts on their bones made by stone tools. Others have pointed to correlations between the timing of the extinctions and dramatic fluctuations in temperatures as the last ice age came to a halting close.

To get a higher resolution picture of what may have happened, archaeologists Matthew Boulanger and R. Lee Lyman of the University of Missouri, Columbia, decided to look at a region that had not been well studied in the past: the northeast of North America, including the states of New York, New Jersey, Pennsylvania, Massachusetts, Maine, and the Canadian province of Ontario. “This is a region that has been virtually absent from discussions” about megafaunal extinctions, Boulanger says, which have mostly focused on the Great Plains and the American Southwest. “Yet it is also a region with an incredibly rich record” of prehistoric animal remains. For example, the bones of at least 140 mastodons and 18 mammoths have been found in New York state alone.

Boulanger and Lyman compiled databases of radiocarbon dates from both megafaunal finds and Paleoindian sites for the northeast, throwing out any dates whose reliability had been or could be questioned. This gave a final sample of 57 megafauna dates from 47 different sites and 25 Paleoindian dates from 22 sites. When the two databases were compared, it became clear that most of the megafauna had already disappeared before humans came on the scene—suggesting that the humans had little to do with their demise.

The radiocarbon dates also suggest that northeastern megafauna underwent two major declines before finally going extinct. The first was 14,100 years ago, before any humans were in the region, but the number of animals then recovered after about 500 years; the second and final population crash began 12,700 years ago, when Paleoindians had just arrived in the region, according to the archaeological record. Moreover, the team reports in the 1 February issue of Quaternary Science Reviews, even though humans and megafauna continued to coexist for about 1000 years before the animals finally went extinct, the animals were already on their way out: Between 75% and 90% of the northeastern megafauna were gone before humans ever came on the scene. Yet even during the millennium of human and animal overlap, the team argues, there is no evidence for hunting: Neither megafaunal nor Paleoindian sites in the northeast contained animal bones that were butchered or otherwise modified.

The authors stress that their results can be directly applied only to northeastern North America, and not to other regions such as the Great Plains and Southwest. Nevertheless, given the large amount of megafauna in the northeast, and the lack of evidence for human involvement in their demise, they argue that overkill cannot have been the only or even the major factor for continent-wide extinctions: Climate and environmental stresses must have also played a key role. The timing of the second megafaunal crash, 12,700 years ago, corresponds with the beginning of a major, 1300-year-long cold snap called the Younger Dryas, which was followed by the warming trend (called the Holocene) we still live in today.

The new work bolsters the views of many researchers that “the arguments and evidence are stronger for environmental and climatic explanations,” says Lisa Nagaoka, a zooarchaeologist at the University of North Texas, Denton. By the time humans arrived, she says, the “tipping point” toward megafaunal extinctions may already have been reached.

And although these events occurred thousands of years ago, Lyman says that they have important implications today. Recently, a number of conservationists have begun advocating the “rewilding” of North America by reintroducing species such as elephants—which are closely related to extinct animals like mastodons and mammoths—and African lions, which are related to the extinct American lion.

This idea has received increasing attention in both the scientific literature and the popular media. For example, rewilding proponents advocate introducing elephants and Bactrian camels—which are now close to extinction in the Gobi Desert—onto the continent, with the idea that they would eat woody plants and weeds that threaten grasslands in the western United States, and that a new habitat would help protect them from extinction. But some researchers have argued that these proposals are based on faulty ecological logic and could end up hurting ecosystems rather than helping them, as well as threatening existing species.

And Lyman says that the strategy is based in large part on the ethical argument that because humans killed off relatives of these animals, they bear responsibility for now saving them and restoring their habitats. “The overkill hypothesis is a very weak foundation for rewilding.”

Meanwhile, advocates of the overkill hypothesis remain unconvinced by the new study. “The authors have engaged in an exercise in data analysis that neither proves nor disproves overkill,” says Gary Haynes, an archaeologist at the University of Nevada, Reno. Humans may have come into the northeast earlier than the radiocarbon database indicates, but their remains may not yet have been found, he says. Todd Surovell, an archaeologist at the University of Wyoming in Laramie, insists that the new study is entirely consistent with the view that humans dealt the final blow to the great beasts of North America: “The fundamental question is whether these animals would have suffered extinction if humans had not arrived.”