One-two punch killed off Ice Age giants

Human hunting, climate change proved lethal, study finds

Patagonia is littered with the bones of giants: saber-toothed cats with huge, arcing cheek teeth, ground sloths that weighed as much as cars, bears that stood 10 feet tall. Throughout the last Ice Age, these creatures roamed the grasslands and mountain plateaus at South America’s southernmost tip.

And then, suddenly, they died out. While examining ancient specimens for a study on evolutionary history a few years ago, molecular biologist Alan Cooper realized that the species all seemed to disappear at exactly the same time, around 12,300 years ago.

“There’s been this long-running debate about what it was that killed the megafauna: climate change or human overkill,” said Cooper, the director of the Australian Center for Ancient DNA at the University of Adelaide. “We found that it was really both.”

In a study published Friday in the journal Science Advances, Cooper and his colleagues report that human hunting and rapid warming delivered a deadly one-two punch to the giants of the last Ice Age: a thousand years of human presence weakened the populations, leaving them unable to cope with the swift environmental change that followed.

wBut there’s also a lesson about resilience: DNA evidence shows that the ancestors of modern llamas were nearly lost in the wave of die-offs that killed other Ice Age creatures. The modern population descends from a group that waited out the extinctions in a remote mountain refuge, then repopulated Patagonia 1,000 years later.

The end of the Ice Age, roughly 12,000 years ago, was marked by die-offs of large mammals around the globe. A warmer climate helps change landscapes from grassland — which sustains populations of large mammalian grazers and the carnivores that feed on them — into forests. That led many paleontologists to believe that hot weather was what killed the megafauna.

Yet the extinctions were particularly pronounced in the Americas, where humans had recently arrived at the end of the Pleistocene. Proponents of what scientists call the “blitzkrieg hypothesis” argued this caused the death of the megafauna, which had evolved in the absence of human hunters and were ill-adapted to avoid them.

To figure out who was correct, researchers had to disentangle the two factors, and Patagonia was the perfect place to do it.

“Humans arrived in South America just before it entered a prolonged cold snap, called the Antarctic Cold Reversal,” Cooper said.

The cold reversal delayed the warming that was affecting other areas for about a millennium. During that time, the megafauna continued to thrive. But DNA analysis and radiocarbon dating of fossils conducted by Cooper’s team suggest that at least six large species went extinct in a span of a few hundred years after warming resumed.

That is the opposite of the trend seen in North America, which underwent a cool spell called the Greenland Stadial just after the end of the Antarctic Cold Reversal. There, megafauna died out just before the stadial, and just after it, but had a brief respite while the continent was cold.

“There’s this really strong pattern that the extinctions are always happening in the warm phases, then they’re particularly intense if there are humans present,” said Jessica Metcalf, an evolutionary biologist at the University of Colorado in Boulder and the lead author of the study.