Sabre-toothed cats, one-tonne bears and sloths the size of elephants all happily coexisted with humans for up to 3,000 years, but were extinct within 300 years after the climate of South America rapidly warmed.

Key points: DNA in ancient bones shows South America's giant land animals died out 12,300 years ago

DNA in ancient bones shows South America's giant land animals died out 12,300 years ago This is up to 3,000 years after humans settled

This is up to 3,000 years after humans settled The disappearance coincides with rapid warming of climate

The disappearance coincides with rapid warming of climate This may have changed vegetation and boosted human hunting

These new findings counter the "Blitzkreig" theory that humans simply hunted into extinction the world's biggest beasts, or "megafauna".

Instead the scientists suggested climate change was the trigger for the ancient giants' demise.

The research was led by Professor Alan Cooper at the University of Adelaide and is published today in the journal Science Advances.

He and his colleagues studied ancient DNA from carbon-dated bones and teeth found in caves across southern South America, to trace the genetic history of the populations.

They found that the megafauna all disappeared within a narrow 300-year time frame around 12,300 years ago.

Yet the fossil record showed humans had been at Monte Verde, on the edge of Patagonia, from about 14,600 years ago.

The team, which also included researchers from Chile, the US and the University of New South Wales, discovered the extinction of the megafauna coincided with a rapid warming of South America's climate.

"The conventional way of thinking [about megafauna extinction] is that humans moved through North America in a Blitzkreig way, where rapid exposure to hunting and populations of humans running around all over the place killed off everything, in this front overkill," said Professor Cooper.

"But the data from South America is strong evidence that that didn't happen.

"We've got a very long period of overlap — at least 1,000 years and probably 3,000 years — where nothing happens.

"You've got humans, you've got megafauna and nothing is going on, which is kind of a revelation.

"The point at which megafauna do go extinct 12,300 years ago is right when the climate starts warming rapidly after a prolonged cold period."

The only large species to survive the extinction event were the ancestors of today's llama and alpaca — the guanaco — and even these species very nearly died out.

Luck of the draw – the larger ancestors of these guanacos in Patagonia were one of only two large ice age herbivores that survived the extinction. ( Bernard Gagnon/Wikimedia )

Professor Cooper said one small population of guanaco, by luck, arrived late into Patagonia and missed the extinction event.

"Because all the other animals are now extinct, no-one is eating the grass so you basically inherit the earth," he said.

Climate warming helped drive the extinctions, he explained, because it led to changes in vegetation with more rainfall and forests, which were unsuitable habitats for the big animals.

He said more evidence was needed to understand what role humans played at this time, but it was possible that as the climate warmed humans became more active, hunted more and the populations grew.

Professor Cooper believes it is important to better understand what happened to megafauna, given the planet is now warming and extinction rates are escalating.

"We might expect the same processes to be happening again. The extinctions we are seeing today from humans might have significantly to do with the fact it is warming as well," he said.

Implications for what happened in Australia

Professor Cooper and his team compared their new observations to what happened in North America and found that warming climate was again a key factor for extinction.

He said the findings have implications for the sometimes acrimonious debate over Australia's megafauna extinctions.

"What we see in Australia is a very prolonged period of overlap between humans and megafauna of about 11,000 years, so clearly it is not Blitzkreig."

"I think the climate signal might have something going for it [as a cause of extinction] but not in the way it is being pushed as the sole cause.

"It is the interaction of both of these factors — it is humans and warming that does the damage."