By Jonathan Amos
BBC News science reporter
It is unlikely humans exterminated in a short killing spree the immense marsupial Diprotodon and other huge beasts that once roamed Australia.
The Diprotodon was an immense, hippo-sized beast (Image by BBC Bristol Design)
Two new studies reject the theory that humans moving on to the continent more than 42,000 years ago took out its megafauna in a 1,000-year "blitzkrieg".
The studies suggest instead a more complex pattern to the extinctions.
Their authors say humans certainly had a role but it was not as important as the period's climate changes.
The studies are published this week in the Proceedings of the National Academy of Sciences (PNAS) and in Memoirs of the Queensland Museum.
However, other researchers who believe the continent's megafauna did suffer a catastrophe at the hands of humans have told BBC News they are not persuaded by the new data.
'Guilt by association'
Migrating humans have been blamed for the quick disappearance of large creatures both in the northern and southern hemisphere.
In North America, for example, the demise of mammoths and sabre-toothed tigers at the end of the Pleistocene Epoch is coincident with the arrival on the landmass of new stone-spear technologies carried by humans about 12,000 years ago.
And in Australia, the extinction of great beasts - such as the marsupial lion (Thylacoleo carnifex), the immense wombat-like Diprotodon optatum and the 400kg lizard Megalania prisca - also occurred at roughly the same time humans appeared on the scene.
Previous research had even indicated a very rapid removal of the megafauna, in perhaps as little as a thousand years.
But in their PNAS paper, Clive Trueman, from the University of Portsmouth, UK, and colleagues in Australia argue evidence for the involvement of human overkill in the southern extinction is largely circumstantial - "guilt by association".
They report detailed new dating data on fossils found at Cuddie Springs in New South Wales. These suggest humans lived side by side with the great beasts of Australia for at least 10-12,000 years.
It gives the lie, they claim, to the notion that humans rapidly removed the large animals either by hunting or by changing the landscape through burning.
Instead, the team argues for a more complex explanation of megafaunal extinction in which large climate shifts played a significant role.
These saw temperatures plummet and the lush landscape become arid by about 30-35,000 years ago.
Australia had an exotic collection of large mammals, such as marsupial lions
"These findings demonstrate that extinction was a gradual process, strongly implicating climate change as the driving mechanism," said co-author Dr Judith Field, from the University of Sydney.
"The role of humans in this process has yet to be established.
"More broadly, these findings suggest that the spread of modern humans to new regions did not necessarily result in unsustainable hunting and mass extinction of the native fauna."
Also, the researchers believe humans simply would not have had the hunting technologies to take out so many large creatures.
"There is not a single stone-spearpoint in Australia until, at the very earliest, about 15,000 years ago - long after anyone thinks the megafauna went extinct," co-author Dr Stephen Wroe, from the University of Sydney, told the BBC News website.
"You try taking out a two-to-three-tonne wombat with a pointy stick.
"I don't doubt the first Aboriginals did hunt megafauna but the argument that they did it with the efficiency required to effect near-instantaneous extinction is not, in my view, credible."
Small and big
This analysis fits with the second paper, published by Gilbert Price, of Queensland University of Technology.
His view is that the colder, drier climate changed the types of animals that could survive in the region of Australia he studied.
He found that the patterns of fossils in a creek bed in the Darling Downs area of south-east Queensland suggested that other, smaller species also disappeared with the larger ones.
Gilbert Price studied fossils from Queensland
The shift in the fossils found in the 10m-deep section of creek bed mirrored the changes to the environment as woodland and scrubland gave way to grassland, he told Memoirs of the Queensland Museum.
Mr Price said that many of the fossils found pre-dated human activity in the area, absolving humans from any involvement in their extinction.
"We've done a little bit of radiocarbon dating on the deposits themselves and we know that the age of the deposits pre-dates the first humans on the Darling Downs by about 30-35,000 years," he told the Australian Broadcasting Corporation.
"We know that there're no human or cultural artefacts in the deposits as well, and we know that all the cut marks on the bones are related to predation by some of the other species that lived on the Darling Downs, such as marsupial lions."
The possible role of humans in the demise of the megafauna is a hot topic in Australian palaeontology.
The controversy was given extra impetus in 2001 when a paper published in the journal Science reported new dating data for fossil-containing sediments from 28 sites across Australia and Papua New Guinea.
The results suggested the great beasts had all gone by about 46,000 years ago - long before the dramatic climate changes brought on by the last glacial maximum, but late enough for the extinctions to have occurred in the presence of humans.
Some took this as clear support for the blitzkrieg theory. Others still think events unfolded more slowly, and hold to the idea that humans caused catastrophic damage to animal habitats as they burned the landscape to drive game out into the open.
The Cuddie Springs site is crucial to the debate because it contains what appear to be some of the youngest fossils. However, it is one of a number of sites which could have been disturbed, with previously buried bones being introduced into younger sediments.
If correct, this would mean that dates from such sites are unreliable.
The PNAS team has attempted to resolve this issue by comparing the rare earth elements (REE) incorporated into bone fragments found at different levels within the Cuddie Springs sediments.
"When an animal dies and is buried, [these] elements in the local environment are adsorbed into its bones, forming a permanent 'fingerprint' of the original burial locality. Fossils deposited at different times will have different concentrations of these elements," said Dr Trueman.
The team believes this REE approach has proved that bones in each layer came from animals that lived and died together - that the sediments were not disturbed and the younger dates stand up.
But the lead author of the 2001 Science paper, Professor Bert Roberts, of the University of Wollongong, told BBC News he still had doubts.
"The problem with using REE on such young deposits is the assumption (made by Trueman et al) that REE are taken up soon after burial of the bones and that they then stay 'locked in'.
"One of the papers they cite in support of this 'rapid uptake' assumption in fact says that REE take 10,000 to 30,000 years to accumulate in bone.
"This length of time is trivial for bones that are millions of years old, but it matters if - as at Cuddie Springs - you're looking at bones that are only a few tens of thousands of years old."
In other words, REE information in older, eroded bones could get "overprinted" with younger dates if the specimens had been disturbed into more recent sediments.
He added: "I would have been convinced if they had dated the bones directly by a reliable means - no easy task, I can assure you.
"Radiocarbon dating has already been tried twice, and failed on both occasions."