Published online 23 January 2010 | Nature | doi:10.1038/news.2010.30

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Early humans wiped out Australia's giants

Climate not to blame for the extinction of Australia's big animals.

Palorchestes azaelThe half-tonne Palorchestes azael may have been exterminated by the activity of early humans.Peter Schouten

Humans, not climate change, caused the mass extinction of Australia's giant animals, such as huge kangaroos, tens of thousands of years ago.

Scientists have long argued over what killed off about 50 species of animals weighing more than 45 kilograms, including the gigantic kangaroo, Procoptodon, and the two-tonne wombat-like marsupial Diprotodon, late in the Pleistocene epoch, which stretched from 2.6 million until about 12,000 years ago.

Some have proposed that the ancestors of Australian Aborigines, who reached the continent between 60,000 and 45,000 years ago, rapidly hunted the animals to extinction. Others have argued for environmental disruption from human-lit fires — a 'slow burn' to extinction as people set the bush alight to clear pathways or flush out prey, altering the continent's vegetation, hydrology and climate. In the climate change camp are scholars who blame the most recent Ice Age, which peaked about 21,000 years ago.

Temporal overlap?

Evidence for a human cause has been mounting over the past decade. One study dated the extinction of the 2-metre-tall, 200-kilogram flightless bird Genyornis to about 50,000 years ago, soon after human colonization, and at a time when the climate was benign1. That work, on the bird's eggshell, was later backed up by a coast-to-coast project dating the extinction of giant marsupials, reptiles and birds across the continent to about 46,000 years ago2.

However, one site, Cuddie Springs in New South Wales, has been held up as evidence for a long overlap between humans and megafauna, seemingly clearing people of being the main agents of the extinction of the animals. It is the only site with megafauna remains and Aboriginal artefacts in the same sedimentary layers. Those layers had been dated by radiocarbon and luminescence methods to between about 40,000 and 30,000 years old.

But some researchers doubted the results, which dated the megafauna only indirectly, through charcoal and sand grains in the layers bearing the fossils and stone tools. They said the site had been disturbed, with megafauna fossils from older deposits working their way into younger deposits. Lacking the protein collagen, the bones could not be dated directly by the radiocarbon method.

Dating discrepancy

Now a team led by Rainer Grün, a geochronologist at the Australian National University in Canberra, has used electron spin resonance (ESR) and uranium-series techniques to date the megafauna teeth directly. Their laboratory is the only one in Australia — and one of only a few worldwide — using ESR in this way.

All of the specimens of extinct species are at least 50,000 years old, some much older, the team reports in a paper in press with Quaternary Science Reviews3. The results debunk claims of the late survival of the giant animals and a long period of coexistence between them and people. The findings weaken arguments for climate change as the main cause of the demise of the megafauna.

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But they do not discriminate between the two possible mechanisms of the catastrophe — rapid 'blitzkrieg' and slow burn — because the date of colonization and the date of extinction are not known with sufficient precision.

"Our results eliminate a strong argument against the blitzkrieg hypothesis but do not prove it," says Grün.

Richard Roberts, a geochronologist at the University of Wollongong, Australia, and biologist Barry Brook, of the University of Adelaide, Australia, say in a commentary4 in Science that "human impact was likely the decisive factor", possibly through hunting of young megafauna. Increased aridity during the last Ice Age might have reinforced this effect, but Australian megafauna were well adapted to dry conditions because they had survived repeated droughts in the past, they say.

Chris Johnson, an ecologist at James Cook University in Townsville, Australia, says the direct dates from Cuddie Springs mean the site now "falls in line with a mass of other evidence" for the rapid extinction of the Australian megafauna between 50,000 and 40,000 years ago.

Archaeologist James O'Connell of the University of Utah says the jury is still out. O'Connell, who has worked extensively in Australian archaeology — including on the Cuddie Springs site, says there might have been a long period of overlap between megafauna and people, regardless of which Cuddie Springs dates are correct. "Climate may not be the only factor, but it can't be eliminated as a significant consideration," he says. 

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