Sydney - Kangaroos the size of horses and three-ton rhino- like diprotodons roamed Australia until humans arrived on the continent around 50,000 years ago.

Some scientists blame meat-eating mankind for hunting them to death while others say the megafauna were climate-change casualties and became extinct on their own.

The poster boy of the humans-as-killers school is Macquarie University ecologist Tim Flannery, who had a good go at the meteorologists in his 1994 book The Future Eaters.

But they won't lie down.

Sydney University archaeologist Judith Field leads those who are convinced that the culprit was the climate.

"I don't think that humans were the main agent because there's evidence of co-existence between humans and megafauna over long periods of time - which knocks Flannery's blitzkrieg theory on the head," she said.

Field and her friends are back on the defensive.

A joint Australian-British study published in August in the journal Proceedings of the National Academy of Sciences of the United States provides more ammunition for Flannery. Fossils carbon-dated to 41,000 years ago that the team found in a cave in Tasmania show the megafauna were alive when humans crossed a landbridge that then existed from the Australian mainland.

"The Tasmanian results echo those on mainland Australia, putting humans squarely back in the frame as the driving force behind the megafaunal extinction," Wollongong researcher Richard Roberts said.

His British counterpart, Chris Turney of the University of Exeter, said the "argument for climate change being the cause of this mass extinction has been seriously undermined", adding, "it's sad to know that our ancestors played such a major role in the extinction of these species - and sadder still when we consider that this trend continues today."

Field is undeterred. She says the new study proves nothing decisively because it showed only an overlap and not that people slaughtered the big animals. It's not the smoking spear.

"They have this article of faith now that humans arrived, swept across the continent and wiped out the megafauna," Field said.

"They've got one species from a cave where there's no evidence of people and suddenly its human causation." (dpa)