The Cause of the Extinction of the Australian Megafauna is...

...Humans!?!

Yes, several findings and papers link the migration of humans to the Australian continent to the eventual extinction of many of Australia's existing megafauna, including giant marsupials such as the gigantic kangaroo, Procoptodon, the two ton wombat-like marsupial, Diprotodon, and the half ton Palorchestes azael, an extinct Australian giant marsupial which was similar to a ground sloth, giant reptiles, and flightless birds.

Humans arrived on the Australian continent between 60,000 and 45,000 years ago and fossil records place the megafauna extinctions to have completed by around 46,000 years ago. For some time, the large Australian animals and humans might have coexisted but eventually most of the megafauna were driven to extinction. Researchers believe this was because humans were hunting the megafauna for a food source. The conflicting argument that other scientists have is that climate caused the extinctions, similar to many of the other extinctions that have occurred in history.

There is supporting and conflicting evidence for both hypotheses. The fact that the large animals went extinct shortly after the supposed arrival of humans supports the theory that humans were a driving force for the extinction of these great animals. However, several sites, especially one particular controversial site known as Cuddie Springs (it is controversial because some researchers have said the layers of rock may have moved or been moved, with some much older fossil rock pushing up into more recent rock with human tools), have shown sedimentary rock layers with both prehistoric man-made tools and megafauna fossils in the same time period, showing that humans and these large Australian animals may have coexisted for several thousand years before some other event occurred or maybe until humans eventually hunted them to extinction anyway. That other event could have been a climate change, because evidence has shown that that period of the Last Ice Age had above average aridity. However, evidence against climate being a factor was that these animals were already evolved for the drier conditions since the climate had been fairly arid for several thousand years, and when these animals went extinct, the climate was fairly calm and tolerable.

The argument goes both ways on how these great megafauna went extinct: climate or humans, and many scientists see the situation as unresolved. So, maybe it was not the Australian ancestors who had the major role in the extinction of the Australian megafauna?

(The image above is an artist's reconstruction of the half-tonne Palorchestes azael, an extinct Australian giant marsupial which was similar to a ground sloth. (Credit: Courtesy of Peter Schouten))

Source Articles
http://www.sciencedaily.com/releases/2010/01/100121141109.htm
http://www.nature.com/news/2010/100123/full/news.2010.30.html