The earliest humans who peopled Australia some 55,000 years ago may have inadvertently disrupted the continent's food chain by burning vast areas of native vegetation, resulting in the extinction of most large animal species.

     Professor Gifford Miller, a geochronologist at the University of Colorado at Boulder, said recent dating evidence indicates an ostrich-sized bird known as Genyornis newtoni suddenly disappeared about 50,000 years ago. The research team speculated that many browsers like the flightless Genyornis -- and other animal species that fed predominately on shrubs and trees -- became extinct after centuries of burning by humans in the continent's interior changed the ecosystem's flora.

     "I think we have compelling circumstantial evidence that the Genyornis extinction date is applicable to the vast majority of Australian megafauna," said Miller. "There are certainly no secure dates to refute this supposition."

     Like any other group of people, the early Australians were just trying to keep their families fed, said Miller, currently on sabbatical in Australia. "We suspect the systematic burning by the earliest colonizers -- used to secure food, promote new vegetation growth, to signal other groups of people and for other purposes -- differed enough from the natural fire cycle that key ecosystems were pushed past a threshold from which they could not recover."

     A paper authored by Miller and Beverly Johnson of CU-Boulder's Institute of Arctic and Alpine Research, John Magee, Linda Ayliffe, Malcolm McCulloch and Nigel Spooner of Australian National University in Canberra, and Marilyn Fogel of the Carnegie Institution in Washington, DC, appeared in the January 8, 1999, issue of Science. Johnson is now a faculty member at the University of Washington.

     The debate on megafauna extinction in Australia has raged for more than a century, with some scientists proposing climate change and others touting human causes, such as over-hunting or fires.

     "By dating one element of the megafauna for the first time, we think we can evaluate which explanation is most probable," Miller said. Although fire was common before the first humans, the natural fire season occurred in November and December due to lightning strikes during the build-up to the wet season after sufficient fuel had accumulated, Miller said. The possibility of human burning at other times of the year and at a greater frequency may have inhibited the regeneration of the natural tree and shrub vegetation in the interior."

More than 85 percent of the Australian megafauna weighing more than 100 pounds became extinct at about the same time as the extinction of Genyornis, he said. The list includes 19 species of marsupials over 220 pounds, including a hippopotamus-sized relative of the wombat, a 25-foot-long, three-foot-in diameter snake, a 25-foot-long lizard and a Volkswagen-sized giant horned tortoise.

     Although the precise dates for many of the extinctions are still under debate, the evidence for extinction of Genyornis is more clear, said Miller. The team reported on more than 700 dates on Genyornis eggshells from three climate regions that documented their presence more than 100,000 years ago to their sudden disappearance about 50,000 years ago -- shortly after humans arrived in Australia.

     "The demise of Genyornis seems to have been triggered by a disruption of the food chain caused by the vegetation ecosystem being knocked out of balance over large areas of the continent," said Miller. "The simultaneous extinction of Genyornis at three different sites during an interval of modest climate change implies that human impact, not climate change, was responsible.

     The researchers used a dating technique known as racemization on the fossil Genyornis eggshells, in which changes in the amino acids present in the shells act as geological clocks. They also used radiocarbon dating and the decay of uranium to date the fossil eggshells directly. In addition, they used a luminescence dating technique to measure changes in quartz grains caused by trace levels of background radiation. The research team also was able to measure the dietary characteristics of Genyornis by using carbon isotopes in their eggshells. The results indicated Genyornis -- which had a strong, shearing beak -- was dependent primarily on shrubs and trees, while other bird and animal species with more versatile feeding habits survived.

     "We conclude than Genyornis was primarily a browser, and likely dependent on extensive shrub land, a dependency that may have made it susceptible to ecosystem disruption," the authors wrote. The extinctions appear to have been "taxonomically selective" in that animals with broader dietary tolerances, like the emu and cassowary-- half the weight of the 200-pound Genyornis, survived while Genyornis died out, said Miller, a professor in CU-Boulder's geological sciences department.

     "Systematic burning practices of the earliest human immigrants could have disrupted an especially sensitive ecosystem across the semi-arid zone, which in turn placed unprecedented stress on the dependent fauna. We postulate that this stress, possibly coupled with modest drying that occurred simultaneously and/or some direct human predation, led to megafaunal extinction," the authors wrote.

     Although some have speculated the demise of Genyornis was due to overhunting, evidence of direct predation on the birds by humans is limited to a single site, said Miller. In addition, kill sites for other megafauna are equally rare. The debate over the megafauna extinctions in Australia is somewhat similar to the debate over North American megafauna extinctions some 13,000 years ago. The North American extinctions occurred at a time when Paleo-Indians may have first arrived on the continent, which also was a time of rapid climate change. "Our evidence from Australia, where extinction clearly occurred when the climate was not severe, will likely rekindle the debate in North America," he said.

     In 1997, Miller proposed that systematic burning of vegetation by the earliest human colonizers beginning roughly 50,000 years ago may have altered the vegetation sufficiently to diminish the effectiveness of summer monsoons that periodically drenched northern Australia, triggering increased aridity over much of the interior.