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 in the journal Science Advances Friday, 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.

The results are a sobering reminder of what can happen to creatures simultaneously stressed by humans and climate — a condition faced by most animals alive today, Cooper noted. But there's also a lesson about resilience: DNA evidence shows that the ancestors of modern llamas were very nearly lost in the wave of die-offs that killed other Ice Age creatures. Their strategy for survival, he continued, could inform our modern conservation efforts.

But first, the extinctions. 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. This 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 only recently arrived at the end of the Pleistocene. Proponents of what scientists call the "blitzkrieg hypothesis" argued that this caused the death of the megafauna, which had evolved in the absence of human hunters and were ill-adapted to avoid them.

In order 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. "That might have been unpleasant for them, but it's very convenient for us."

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.

This 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. 

"It's really interesting," Cooper added, "because we've warmed the planet up through the last 100 years by not much short of the temperature that we’re talking about in this particular study. If you look back in time and you see warming associated with this destructive influence, you have to think, are we doing that right now? Have we been doing that for the past 100 years?"

The species that vanished from Patagonia during the period of the study included Arctotherium, an enormous snub-snouted bear; Mylodon darwinii, the giant ground sloth; and Panthera onca mesembrina, a jaguar the size of a modern tiger. A llama relative called the guanaco also disappeared from the fossil record — which was confusing to scientists, since wild guanacos still exist in the mountains of Peru, Bolivia, Chile and Argentina.

DNA analysis revealed that the entire modern population — and their domesticated llama cousins — descend from a small population that waited out the extinctions in a remote mountain refuge, then repopulated Patagonia 1,000 years later.

"It goes to show that the connectivity between areas allows species to survive these events," Cooper said.

Fast forward 12,000 years, that principle can be applied to wildlife corridors — the strands of preserved habitat that link parks and wildlife refuges otherwise separated by cities, highways and human activity.

"Data shows you can stick a fence around something but sooner or later a warming or flood or fire or disease is going to knock out that small refuge," Cooper said. "It's only going to survive if all the refuges are connected together."

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