If you’re fossil hunting in Patagonia you might find some weird creatures: giant jaguars, 3-meter-tall sloths, and bears 10 times the size of grizzlies. The southern part of South America was once crawling with these great beasts, collectively known as megafauna. But around 12,000 years ago, they suddenly disappeared from Patagonia and many other parts of the Americas. What caused the mass extinction? A new study suggests it was a one-two punch: rapid climate warming and humans.

For many years, scientists believed that people were responsible. When humans arrived in the Americas shortly before 13,000 years ago, the story went, they almost immediately hunted the New World’s megafauna to extinction. Archaeologists turned up many butchered bones, so a “blitzkrieg” of hunting seemed to make sense. But now, most researchers think that people were already in the Americas by at least 15,000 years ago. That means that people and megafauna actually coexisted for a few thousand years before the mass extinctions began. What changed to drive these animals out of existence?

To find the missing ingredient, Alan Cooper, an evolutionary biologist at the University of Adelaide in Australia, radiocarbon dated nearly 100 fossils from Patagonia and sequenced their mitochondrial DNA, genes found in the power plants of cells and passed down only from the mother. When he lined up their ages with global climate records, he noticed a pattern: Many species of megafauna seemed to disappear during a period of extreme warming around 12,300 years ago, Cooper and his team write today in Science Advances. Ice cores from Greenland and West Antarctica suggest that average global temperatures quickly shot up during that time. Patagonia warmed by about 2°C over 1000 years, and the effects were devastating: All but one of the species Cooper studied went extinct. Even the lone survivor, the llamalike guanaco, experienced an extreme population crash at that time, the genetic effects of which are still detectable in its DNA.

Last year, Cooper spotted a similar pattern in North America, with megafauna going extinct during ancient warming events (which occurred at slightly different times in the Northern Hemisphere). The existence of complementary data from the two continents “is as close as you’re going to get to a replicated experiment,” he says.

Still, dramatic climate swings were not terribly unusual during this period in Earth’s history, and the Patagonian megafauna had made it through several warm periods before the one that finally did them in. The difference was that this time, humans were on the scene, says Cooper, hunting the animals, occupying territory, and changing the landscape in ways researchers are still trying to understand. “Warming allows the human impact to be so much worse,” Cooper says.

“It’s a really nice example of a key study where humans and climate seem to be intersecting in some way,” says Paul Koch, a paleontologist and geoscientist at the University of California, Santa Cruz, who wasn’t involved in the research. “It’s clear that without human impacts, there wouldn’t have been an extinction.” But what those impacts actually were is still unclear, he says. “Is it human populations growing rapidly once it warms, and hunting? Or is it humans disrupting the way the animals migrate and the way they use habitats?”

Other researchers want to know how rapid global warming changed Patagonia’s environment. Ice core data from the poles clearly show dramatic swings in average global temperatures, but researchers still don’t know how local ecosystems reacted to the change. “Whatever is going on at high latitudes is one thing. What that looks like on the ground further south … is quite a different thing,” says Ross Macphee, a paleontologist at the American Museum of Natural History in New York City who has studied South American megafauna and wasn’t involved in the current research. Did the plant life change in Patagonia, disrupting the food chain from the bottom up? Were habitats fragmented, or did they shrink to the point that they could no longer support large animals? “There’s a lot that’s left to be filled in there,” says Daniel Fisher, a paleontologist at the University of Michigan, Ann Arbor, who wasn’t involved in the current work. Fisher, who studies the extinction of mastodons and mammoths, suggests some answers could lie in nitrogen isotopes in the Patagonian bones, which can record changes in an animal’s diet and, thus, its environment. Cooper says his team has been working on such an analysis and hopes to publish the results soon.