What killed the great beasts of North America?

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.