A guanaco in Patagonia today. The animals’ earl ancestors only survived here because they arrived after the competition had died out. Frank Lukasseck

Bears the size of cars and savage sabre-toothed cats that ruled the plains of ice age Patagonia were no match for a fatal combination of humans and climate change that wiped them out about 12,300 years ago, according to a new study.

But contrary to previous research, humans alone did not cause their downfall. The fossil record shows humans had been at Monte Verde, on the edge of Patagonia, from about 14,600 years ago. That means the South American megafauna had coexisted with humans for up to 3,000 years. But when the climate heated up, almost all the megafauna were extinct within 300 years, research led by the Australian Centre for Ancient DNA (ACAD) at the University of Adelaide has found.

Ice cores from Greenland and Antarctica suggest that average global temperatures quickly shot up around 12,300 years ago and Patagonia warmed by about 2°C over 1,000 years, with disastrous consequences.

“Patagonia turns out to be the Rosetta Stone – it shows that human colonisation didn’t immediately result in extinctions, but only as long as it stayed cold,” says study leader Alan Cooper, ACAD Director.

Ancient DNA extracted from radiocarbon-dated bones and teeth found in caves across Patagonia, and Tierra del Fuego, reveal the genetic history of the populations.

Species such as the South American horse, giant jaguar and sabre-toothed cat, a sloth the size of an elephant and one-tonne short-faced bear seemed to disappear across Patagonia shortly after humans arrived.

The only large species in South America to survive were the guanaco and vicuna – ancestors of today’s llama and alpaca –¬ and even they almost went extinct.

Only the late arrival in Patagonia of a population of guanacos from the north saved the species, all other populations became extinct, the study found. Because they arrived so late, all the other species were dead, leaving an abundance of food.

“Because all the other animals are now extinct, no-one is eating the grass so you basically inherit the earth,” Cooper told reporters.

In order to determine the impact of humans, his team decided to look at how human migration compared with changing climate.

“The Americas are unique in that humans moved through two continents, from Alaska to Patagonia, in just 1,500 years,” says Chris Turney, from the University of New South Wales, who was also involved in the study.

“As they did so, they passed through distinctly different climate states – warm in the north, and cold in the south. As a result, we can contrast human impacts under the different climatic conditions.”

The research was published in Science Advances.