The real culprit of extinctions is climate change (Image: Sergey Gorshkov/Minden)

New forensic DNA evidence is painting a detailed picture of the death of the world’s megafauna – and it suggests that humans were not to blame.

Ever since a giant sloth was uncovered more than 200 years ago, hinting at the former presence of a menagerie of prehistoric giant mammals – the “megafauna” – humans have been on trial for their extinction. And the prosecution’s case has been strong.

“The overwhelming evidence is that the megafauna extinctions occur around the world whenever humans turn up,” says Alan Cooper, an ancient DNA researcher from the University of Adelaide in Australia.


And that chronology is hard to argue with, he says. “Whether it be 50,000 years ago in Australia or 13,000 years ago in South America or 1000 years ago in New Zealand: it’s a perfect match.”

But the real culprit, he says, is climate change.

Cooper and colleagues have simultaneously produced an unprecedentedly accurate map and timeline of changes in megafauna populations around Eurasia and North America, and precisely matched that timeline up with ancient climate records.

It punches a hole in a key argument of the prosecution. This states that climate cannot have caused megafauna extinctions because it has changed so much over the past 60,000 years. There were lots of warm and cool periods – interglacial and glacial epochs, respectively. If climate change is the real megafauna killer, why did the animals survive those events only to die when humans turned up in their region?

The new data show that they did not survive. Megafauna extinctions were actually relatively common during the past 60,000 years whether humans were around or not.

Invisible extinctions

To establish this, Cooper and his colleagues first compiled 10 years of ancient DNA work that has revealed a series of “invisible” extinctions. These are events involving two or more lineages with essentially identical skeletons but distinct genes – for example, two species of bison. If both lived in the same area in prehistory, one could have disappeared and we would not be aware of this just from examining the bones.

Secondly, Cooper’s team created a new ancient climate record that, uniquely, can be accurately linked with the carbon dates from bones to show when particular extinctions happened. Usually the climate change and carbon dating timelines are independent and difficult to link together. But Cooper and his colleagues found a marine sediment deposit near Argentina that contains a record of past climates – and because it contains marine microfossils, these past climates can be tied precisely to carbon dates.

Pulling everything together, the team was able to draw a picture that shows exactly what the climate was doing when various megafauna species vanished over the past 60,000 years. And a pattern emerges.

“Climate is the thing that is constantly sending these species out through time without humans even being involved,” says Cooper. “That’s the bit that is really completely new.”

The team were surprised by another finding that emerged: it wasn’t the long cold periods that wiped out megafauna, as some have suggested – it was warming.

“What we found, which we were staggered by: no matter how we analysed the data, abrupt warmings drove the extinctions or the replacements,” says team member Chris Turney of the University of New South Wales in Sydney, Australia.

It is most likely that warming was such a killer because it arrives rapidly – temperatures change far more abruptly than they do at the onset of a cooling period, says Cooper.

But Cooper is not letting humans off entirely. There was an uptick in extinctions at the end of the last ice age when, famously, the woolly mammoth met its maker. He says there is little doubt that humans played a part in that extinction because it correlates with a sudden explosion in human numbers around the world.

Overkill hypothesis

But perhaps humans did not simply hunt species to extinction – sometimes called the “overkill” hypothesis. Cooper says that the DNA research indicates that megafauna species were sometimes able to survive potential extinction events by migrating as climate changed. But those species-saving migrations became far more difficult to undertake 12,000 years ago because humans began disrupting the landscape in a big way through farming.

Brian Huntley from Durham University, UK, thinks the results prove more than Cooper claims. He says the assumption that humans played any significant part in the extinctions is unjustified. The magnitude of the last warming event was about twice that of the previous ones, and so would explain the increased rate of extinctions, he says.

But the prosecution is not giving up. Tim Flannery, a palaeontologist based in Sydney, Australia, says that the global correlation between human arrival and megafauna extinctions is just too big to be a coincidence, particularly in the southern hemisphere. “In New Zealand nothing happens through dramatic cycles in climate until 1000 years ago when people get there,” he says. And although Cooper’s work pins extinctions on periods of warming, in Australia the megafauna disappearance began 50,000 years ago during a period of slow cooling – just as humans arrived, he says.

Southern migration

Moving the work further south is the next frontier for Cooper and the human defence council. Right now, he is in Idaho, digging up more megafauna from Natural Trap Cave, trying to link what happened in warmer areas, south of the permafrost, to see whether the same sorts of things happened there. “We’re looking at South America too, and I think we can see the same pattern,” he says.

When it comes to Australia, Cooper is less sure of the sequence of events. But his ongoing work is pointing towards a long period of overlap between humans and megafauna. If that is right, it could be more evidence against the idea that humans are a sharp, fast killer of megafauna. Whatever happens, more data will be revealed in coming years.

Cooper’s findings also provide a stark warning for humans today. They show that periods of rapid warming cause major waves of extinctions and near-extinctions around the world. And what saved some animals during some of these periods was being able to migrate – which is less of an option today. “Human alteration or destruction of ecosystems is so pervasive that it is clear that many species, including groups such as birds, are unable to shift their ranges sufficiently rapidly to match current anthropogenic climatic changes,” says Huntley.

Journal reference: Science, DOI: 10.1126/science.aac4315