Fossils from a recently discovered cave in south western Australia suggest climate change was not solely responsible for the demise of early giant marsupials.
The team led by Dr Gavin Prideaux from Flinders University in South Australia publish their findings this week in the Proceedings of the National Academy of Sciences.
Prideaux says the team's research shows 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 research is a debate that has occupied Australian archaeologists and palaeontologists for decades: How did 90% of Australia's megafauna die out by around 40,000 years ago?
On one side of the debate are those, like Prideaux, who believe that humans were the cause. They argue there is evidence to suggest that the megafauna extinction occurred within 10,000 years of people arriving in Australia for the first time.
Researchers on the other side argue there is no evidence for this assumption, and instead suggest the megafauna gradually died out as climate change altered the landscape.
Snail shell clues
Prideaux 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.
The team had to crawl down a vertical tunnel, 80 centimetres wide, and squeeze through a series of narrow chambers for 70 metres before emerging into a large dome shaped chamber where the fossils had been preserved. The animals most likely fell into the cave through an opening in the ground and remain trapped there, eventually dying and becoming buried in sediment.
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)," says Prideaux. The PGM was a period of extreme cool dryness around 140,000 years ago.
" I think that completely knocks on the head the idea that the PGM had any kind of lasting impact on the megafauna," he says.
"Our inference from this is that it was elevated human hunting pressure over an extended period of time that caused the extinction."
Small in number
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.
"When you are looking at the ecological makeup of these faunas - we only had a few predators such as thylacoleo (marsupial lion), megalania (giant goanna), and the thylacine. This suggests that if you have so few predators, you've got a limited number of herbivores."
He says the situation in Australia would have been very different to Africa today. "In Africa you might have 10,000 wildebeest, whereas in Australia you might only have had, say 100 Diprotodons (giant wombats) in a herd."
Prideaux says the arrival of humans would have had an impact on their numbers.
"Suddenly there's a new predator - and an efficient one. It doesn't really matter what the population of humans was - when people got to a particular area, they would have had some kind of effect," he says.
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.
No simple explanation
University of Sydney archaeologist Dr Judith Field says it's a good paper and a welcome addition to the work that's been done.
Field, who studies a site at Cuddy Springs in north west NSW, where both megafauna remains and human tools have been found side by side, says: "It's nice to see these guys moving away from the simple explanations they've run before, that is, human overkill."
"They still push that in this paper, but they are now saying there's no simple explanation, and it really is an ecological problem."
"They haven't provided any evidence to invoke human cause - they argue that they (humans and megafauna) overlap in time, but in Tight Entrance Cave, there's no archaeological record."