Giant killer possums, rhino-sized wombats and five-metre-long pythons all sound like animals out of a bad sci-fi movie. Ludwig Leichhardt, one of Australia's great explorers, believed he might find some of these creatures still roaming in Australia's north. We now know that he was at least 20,000 years too late. But why did the megafauna die out? Was it changes in the physical environment or hunting by humans?

At the time that woolly mammoths and sabre tooths were roaming Europe, Australia boasted some of the most unique animals ever known. Then, in an episode of mass extinction, they disappeared, just like the dinosaurs. Why did these giant animals, or megafauna, die out? Scientists have been debating this question for over 140 years.

There are many theories as to why the megafauna became extinct, but two simplistic and hotly-contested ones are the 'blitzkrieg' model, and climatic change.

1. Blitzkrieg them!

Some scientists believe that the extinction of the megafauna in Australia was either caused by a 'blitzkrieg' of human-induced extinction, or through disruption of the ecosystem by humans. This infers that Australian Aborigines were the reason for the demise of the megafauna.

This hypothesis is based on the timing of the extinction of the megafauna coinciding with the time that humans advanced into Australia. Since these animals were the biggest and slowest in the environment, they were very vulnerable. They were easy prey, and humans hunted them to extinction.

However, there is also evidence that may discredit this theory. It has been found that there was a long overlap between the times that the megafauna existed and humans entered Australia.

2. The climate changed?

Dr Judith Field, of the University of Sydney says, "It can be argued that climate change might have driven vegetation change, periodic drought, and increased seasonality. (The climate) weaves backwards and forwards —it might just need one more extended drought and bang, the megafauna are knocked off."

The find of mass fossils at Alcoota, Northern Territory may support the theory that megafauna became extinct due to climatic change.

Dr Peter Murray, from the Museum of Central Australia, and a team of researchers have been excavating fossils in a remote part of the Northern Territory, called Alcoota, for the past 12 years. Recently they made a rare scientific find—a large number of 'thunder birds', or Ilbandornis, in the one spot.

Thunder birds belong to the family Dromornithidae, the largest of which was about three metres tall, weighed about 500 kilograms, and has been nicknamed the 'giant demon duck of doom'

There are two theories for the grouping of the bird fossils at Alcoota. It may be that the water in the environment sorted the bones into similarly-sized accumulations, or that the birds may have been flocked together around a water hole during a drought.

"Animals tend to gravitate towards the last few remaining water holes during a drought. When you get many animals congregating around one water hole, they eat all the food. Ultimately, if the drought doesn't break, they die of starvation," says Dr Murray.

3. Set of complex factors

Henk Godthelp of the University of New South Wales believes that the extinction of the megafauna was caused by a complex set of factors.

"As with anything, it's always more complicated than just the arrival of humans or just the change in climatic conditions. I think that a range of different factors caused this. Nature is never that simple" he says.

"Whether people were around and knocked off the last megafauna (some scientists believe) is incidental, because they would have died anyway," he says.

Why is it so hard?

One reason that discovering what caused the death of the megafauna is difficult is due to the lack of exact methods of dating

"How are we expected to draw a simple conclusion from a very patchy fossil record where you've got huge sampling biases, and where you've got time resolutions of no better than 5,000 years in any one place? We've looked at recent extinctions that occurred over 100 or 200 years, and we can't explain them. How do we expect to resolve these sorts of issues when we can't see more than a 5,000-year block of time?" says Field.

Who cares?

Is it worthwhile discovering the reason why the megafauna became extinct? "Absolutely," says Godthelp. "I think that there are lessons to be learnt from observing changes in ecological time and ecological events."

Field agrees, "We only learn by looking at the fossil records, by looking at the past and how things have happened, and by what events have taken place."

But we must act with caution. Field believes that blindly supporting simplistic theories about the demise of the megafauna is fraught with danger. "All of these ideas have implications for modern-day events, and when you start putting forward solutions (based) on the selective data, then there are going to be big problems."