A Brisbane researcher is on the verge of debunking claims that Aborigines were solely responsible for the extinction of Australia's megafauna by hunting 18,000 to 30,000 years ago (see: Early Human Activity May Have Led to Animal Extinctions).

Queensland Museum's Scott Hocknull, who has just completed his honours thesis, has discovered evidence of dramatic climate change in Australia at the time the giant animals are believed to have died out."

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."

For almost 4 million years, giant marsupials, birds and reptiles roamed Australia and the rest of the world. In Australia, the megafauna reached its maximum size about 100,000 years ago, with the largest beast a herbivorous wombat-like creature the size of a rhinoceros, called Diprotodon optatum. Other megafauna included a sheep-sized echidna, the tree-dwelling marsupial lion, 3m-tall kangaroos and a 600kg carnivorous goanna. Large Tasmanian tigers and Tasmanian devils inhabited the mainland. In 2000 scientists found one of the most fearsome Australian megafauna was a 3m, 300kg, meat-eating duck. The animal had a large brain and is thought to have hunted large mammals.

Most scientists agree that many megafauna species were still around when the Aborigines arrived in Australia more than 40,000 years ago. Australian Museum principal researcher Tim Flannery achieved some notoriety in 1995 with his book The Future Eaters, that argued the megafauna species were killed off quite quickly by the Aborigines when they arrived.

Mr Hocknull has added to evidence showing the great climatic changes in Australia when the megafauna became extinct. "What I'm finding with my research is that climate has had a profound effect on the environment," Mr Hocknull said. "Before we can come to grips with the effects of humans on the environment, we have to know what was happening before humans arrived. There is little evidence to suggest a massive extinction, there's little evidence to suggest a gradual extinction. What that means is you can't pinpoint any period of time when the megafauna went extinct. The fact is, people have jumped on the bandwagon saying humans definitely killed them off."

Mr Hocknell has been digging up evidence west of Rockhampton to look at how far rainforests extended and when they began to contract to their present levels.

He has round rainforests existed in the Rockhampton region 2 million to 3 million years ago, and have retracted more than 1000km since. Within a 10sq km area, he has found evidence of deserts, rainforests and everything in between, having existed over the past 3 million years. "That just illustrates how fast changes have taken place," Mr Hocknull said.

In the northern hemisphere, glaciers created in the ice ages were blamed for the extinction of megafauna such as mastodons and giant elks.

In Australia, even though there were few glaciers, similar environmental factors were responsible for megafauna extinction, Mr Hocknull argues. "People have always been faced with the conundrum that if there is no ice how do we know the climate change was so dramatic," he says. "I'm finding that we went from rainforest to open woodlands to savannah to desert in a relatively short period of time."

Mr Hocknell believes that hunting may have been the final threat that drove the megafauna to extinction, but only after the effects of climate change.