May 30, 2005

Did climate shift kill giant Australian animals?

Bones suggest that early humans did not ruthlessly wipe out huge beasts.

by Michael Hopkin
Nature News

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An Australian research group may ease humanity's collective conscience over a spate of prehistoric extinctions on the southern continent. The die-off, they say, was not the rapacious work of newly arrived humans, but was due rather to changing climate.

Australia's large prehistoric animals, called megafauna, were as bizarre as anything that lives there today. King of them all was the marsupial lion, a 130-kilogram meat-eater who lived alongside giant kangaroos, huge lizards called goannas, and Diprotodon, which resembled a three-tonne wombat.

After the arrival of humans on the continent, at least 45,000 years ago, these weird and wonderful creatures began to die out. Experts blamed the colonizers, arguing that they launched a hunting 'blitzkrieg' that wiped out the megafauna within a few generations.

But the animals may have survived for a lot longer than people thought, argues Judith Field of the University of Sydney, who has analysed fossil remains. Her excavations seem to show that man and beast lived side by side for as long as 15,000 years.

"There's this image of spear-wielding hordes hacking through startled megafauna," she says. "But there's a range of stone tools, for butchery, woodworking, grinding. There's no evidence of a toolkit with specialized hunting gear."

Dig deep

Field and her colleagues collected animal bones from a ten-metre-deep section of earth at Cuddie Springs, New South Wales. They focused on bones from four layers: two with evidence of human settlement, such as stone tools, and two deeper ones with no evidence of tools.

They dated the bones by measuring the amounts of radioactive elements, such as uranium and thorium, that remained in the bones. They found that the various animal carcasses in each level would indeed have lived cheek by jowl with humans as recently as 30,000 years ago. The team report their research in Proceedings of the National Academy of Sciences.

Proponents of the blitzkreig model had previously argued that the dating of the Cuddie Springs material was not certain, but Field says their research clears up the matter. Well-preserved bones at other sites have been very hard to find, probably because they are too dry, whereas Cuddie Springs is the site of an old lake bed.

Drying up

A second group, reporting in the journal Memoirs of the Queensland Museum, has also unveiled evidence that climate change may have killed off many of Australia's animals. By looking at smaller animal bones from the Darling Downs in Queensland, they show that their disappearance seems to have coincided with increasing dryness.

But the matter is not settled yet, particularly as the timing of humans' first foray into Australia has still not been agreed. Fossil evidence from Lake Mungo in New South Wales suggests that they may have arrived 60,000 years ago. And it is possible that they hastened the megafauna's demise by burning habitats to make way for primitive agriculture.

Field remains convinced, however, that it was climate that drove the animals to their death. "The arid zone grew to encompass 70% of the continent by 30,000 years ago," she says. "There would have been very few opportunities once it got dryer."

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