Fossils from a recently discovered cave in south western Australia suggest climate change was not solely responsible for the demise of early giant marsupials. A team led by Dr Gavin Prideaux from Flinders University in South Australia found that many species of megafauna had survived an earlier period of extreme climate 140,000 years ago and flourished beyond it. At the heart of the new research is a debate: How did 90% of Australia's megafauna die out by around 40,000 years ago?
     Prideaux who believes that humans were the cause. He argues there is evidence to suggest that the megafauna extinction occurred within 10,000 years of people arriving in Australia for the first time. The researcher and his team chose Tight Entrance Cave, near the Margaret River in south west Western Australia for their study, where the fossil record runs from 100,000 years before humans arrived, until well after they had settled in Australia.
     To track how the megafauna faired over time, the researchers recorded which species were present, and then analysed snail shells from the same sediments. Shells hold a record of previous climates, with moisture and temperature affecting which isotopes are present in the shell.
     "We show the megafauna were there immediately before and also really soon after the Penultimate Glacial Maximum (PGM - a period of extreme cool dryness around 140,000 years ago)," said Prideaux. "I think that completely knocks on the head the idea that the PGM had any kind of lasting impact on the megafauna," he added. "Our inference from this is that it was elevated human hunting pressure over an extended period of time that caused the extinction."
     But how could a handful of new arrivals have wiped out herds of giant animals that had been present on the continent for millions of years? Prideaux says although megafauna were large, their numbers were probably low, and the arrival of humans would have had an impact on these numbers. But, Prideaux adds that although the arrival of humans was decisive in the demise of megafauna in southwestern Australia, changes in climate and fire activity may also have played a role.

Edited from ABC Science (30 November 2010)