Some 12,000 years ago, humans and climate change combined to kill off some of South America’s largest animals.
By Nathan Collins
A sabertooth tiger fossil. (Photo: Wikimedia Commons)
Around 12,000 years ago, at the end of the Pleistocene epoch, large animals throughout the Americas more or less dropped dead, and while theories abound, no one’s entirely sure why. Now, researchers report that a combination of climate change and human hunting conspired to make some of those extinctions happen—something neither one would have done on its own.
There are a number of theories for what killed off sabertooth tigers, giant beavers, and mastodons, among others. One is that humans simply hunted them to extinction, much as we almost did with American bison. Another is that warming temperatures, changing rainfall, and other climatic shifts at the end of the Pleistocene so altered American ecosystems that large animals could no longer survive. There are somewhat more exotic theories as well, including the possibility that, as humans expanded into new parts of the world, they brought with them diseases deadly to native animals.
As humans expanded into new parts of the world, they brought with them diseases deadly to native animals.
But which of those theories is right? To probe that question, University of Colorado-Boulder ecologist Jessica Metcalf and her colleagues analyzed DNA from bone and teeth samples collected in caves throughout the Patagonia region of South America, from which they identified a wide range of large cats, bears, and members of the camelid family. At the same time, they used radiocarbon dating to identify when each of those species roamed the land.
Two things stood out about those dates, Metcalf and her team found. First, many of the species they examined were present long after humans arrived in South America. Second, the period of overlap—from about 14,000 years ago until 12,300 years ago, when mass extinctions began—also overlapped with a period of significantly colder temperatures across the continent known as the Antarctic Cold Reversal. The extinctions only began, they point out, around the time that the extended cold snap ended and temperatures began to go back up.
“The initial presence of humans in the area during the ACR stadial conditions was apparently insufficient to drive megafaunal extinctions,” the authors write in Science Advances. “However, human presence, in combination with the rapid advance of forests and environmental changes associated with the ensuing warming phase, appears to have led to the collapse of the megafaunal ecosystem within a few hundred years.”