The unprecedented ecosystem disruption is now thought to have led to the extinction of Australia's large terrestrial mammals, which disappeared shortly after humans colonized the continent about 50,000 years ago, said Professor Gifford Miller of CU-Boulder's Institute of Arctic and Alpine Research.

Scientists have shown there were no significant swings in the continent's climate during that period, leading most to believe that humans had a hand in the extinctions through over-hunting, spreading disease or by altering the vegetation of the vast interior through systematic burning.

Using isotopic studies of fossil eggshells from both indigenous emus and the extinct, ostrich-sized Genyornis, a new study by Miller and colleagues publishing in the July 8 issue of Science magazine shows that the ecosystem's flora changed swiftly and dramatically after humans arrived.

The analyses, which scientists used to pinpoint particular plant groups ingested by the birds, indicated that emus living before 50,000 years ago preferred nutritious grasses characteristic of milder temperatures and warm summer rains, Miller said.

After 45,000 years ago, the eggshell evidence showed emus successfully switched to a diet of mostly shrubs and trees characteristic of drier conditions, he said.

But according to the research team, Genyornis - which also preferred the nutritious grass prior to 50,000 years ago - failed to make the dietary switch and became extinct shortly after humans arrived, he said.

"The opportunistic feeders adapted and the picky eaters went extinct," said Miller. "The most parsimonious explanation is these birds were responding to an unprecedented change in the vegetation over the continent during that time period."

Other study authors included Marilyn Fogel of the Carnegie Institution in Washington, D.C., John Magee and Michael Gagan of Australian National University in Canberra, Simon Clarke of Australia's Wollongong University and Beverly Johnson of Bates College in Lewiston, Maine.

The researchers analyzed nearly 1,500 fragments of fossilized emu and Genyornis eggshells dating back 140,000 years from three different regions in Australia's interior, including Lake Eyre, Port Augusta and the Darling-Murray Lakes. Each region has a distinct local climate and geography, he said.

They also looked at carbon isotopes in fossil wombat teeth collected from the Port Augusta and Darling-Murray sites, where such teeth often are found in association with fossil-bird eggshells.

While the analyses showed the diet of the vegetarian wombat consisted of a much larger proportion of the grasses favored by emus and Genyornis prior to 50,000 years ago, wombats, like emus, successfully switched to other vegetarian food sources after 45,000 years ago.

"Neither over-hunting nor human-induced diseases, the two most widely cited alternative agents for a human-caused extinction event in Australia, would result in the dramatic changes at the base of the food web documented by our datasets," wrote the authors in Science.

"The reduction of plant diversity, however it came about, would have led to the extinction of specialized herbivores and indirectly to the extinction of their non-human predators."

In January 2005, Miller and colleagues published a paper in Geology suggesting burning by ancient hunters and gatherers triggered the failure of the annual Australian monsoon over the interior thousands of years ago by altering the flora enough to decrease the exchange of water vapor between the biosphere and atmosphere.

Lake Eyre, a deep-water lake in Australia's interior that was filled by regular monsoon rains about 60,000 years ago, is now a huge salt flat that only occasionally is covered by a thin layer of salty water.

The earliest human colonizers in Australia are believed to have arrived by sea from Indonesia about 50,000 years ago, using fire as a tool to hunt, clear paths, signal each other and promote the growth of certain plants, Miller said.

More than 85 percent of Australia's large mammals, birds and reptiles weighing more than 100 pounds went extinct shortly after humans arrived, including 19 species of marsupials, a 25-foot-long lizard and a Volkswagen-sized tortoise, he said.

"This study shows that the environmental footprints of humans can have very large and unexpected consequences, which I think is relevant to what is happening with human activity on Earth today," said Miller.

"A cumulative series of small changes can have unintended large-scale, consequences, in this case a complete restructuring of the ecosystems."

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