May 31, 2005— Humans and ancient giant marsupials coexisted for at least 15,000 years, according to new findings that re-ignite the debate over how and when Australia's megafauna became extinct.

Archaeologist Judith Field, of the University of Sydney, says the team's findings put to rest one high-profile theory, that humans arrived in Australia and wiped out the megafauna during a relatively brief 1000-year "blitzkrieg."

"In some places people may well have had a role, but in other places they had no role at all," she says.

Field draws her conclusions, reported online in the Proceedings of the National Academy of Sciences, from an ancient lake bed at Cuddie Springs, in New South Wales, where she has been working for 14 years.

Field and team found evidence of human occupation including stone tools, charcoal, ochre and modified bone, dating from 36,000 years ago.

These were alongside remains of megafauna, including diprotodon (a giant wombat-like creature), protemnodon (a giant wallaby) and Genyornis newtoni (a giant flightless bird), surviving until 30,000 years ago.

Field said the 6,000-year overlap tells us people and megafauna coexisted there over an extended period. And given a conservative estimate of human arrival in Australia at 45,000 years ago, this means the two coexisted for at least 15,000 years.

She says this is incompatible with the blitzkrieg model, proposed by well known palaeontologist Tim Flannery, director of the South Australian Museum.

"Having a long overlap of humans and megafauna refutes the blitzkrieg argument of Tim Flannery," she says.

Field says her latest findings also challenge "simplistic" theories that climate change was to blame for the extinction, or that humans slowly killed off the megafauna over 10,000 years by hunting and burning their habitat.

She believes megafauna may have died out at different times across Australia, depending on a complex interaction of factors.

A Controversial Site

Cuddie Springs was excluded by scientists who in 2001 calculated the broadly accepted date of megafaunal extinction Australia-wide as being 46,500 years ago.

Those scientists, including Flannery and dating expert Bert Roberts of the University of Wollongong, argued that sediments at the site had been disturbed, making it difficult to use the dates of surrounding sediments to date the bones.

Field and team say they have now confirmed that the bones were not moved after death by measuring rare earth elements (REE) contained in them.

REEs are not found in bones during life, but are relatively common in soils. After an animal dies, they get taken up into the bone and leave a permanent fingerprint or memory of the original burial location, said Field.

But Roberts remains unconvinced.

He says the REE technique is generally used for much older deposits and may not be reliable for deposits as relatively young as Cuddie Springs.

Was Climate Change Responsible?

More evidence that points away from human intervention in the extinction of the megafauna comes from Queensland University of Technology research, published Monday, supporting the idea that climate change was responsible.

Research in the journal Memoirs of the Queensland Museum led by researcher Gilbert Price studied a 30-feet deep section of creek bed in the Darling Downs region in the state's southeast.

The researchers found 44 species ranging from land snails, frogs, lizards and small mammals to giant wombats and kangaroos.

The researchers say this suggests the extinction of Darling Downs megafauna was caused by a massive shift in climate rather than by the arrival of humans who over-hunted animals or destroyed habitats by burning the landscape.