In Australia during the Pleistocene epoch (which spans roughly two million years ago to 10 000 years ago) animals such as the rhinoceros-sized, wombatish Diprotodon, three metre-tall kangaroos, giant emus and goannas that were six metres long roamed the countryside. But, when humans showed up around 45 000 years ago, they disappeared.

Precisely what was to blame for the extinction of the so-called megafauna has been the cause of what could be politely described as 'robust debate' in the scientific world. Did other animals move in with the humans and compete with or eat the native ones? Did the humans just kill everything in sight? Was it disease brought by the newcomers? Did the humans set fire to the place so often that the existing ecosystem collapsed? Or was it simply down to climate change?

The extinction of megafauna was not confined to Australia; it happened in the Americas too. But what makes Australia interesting is that the climate was relatively stable here and no other large animals came with the humans. Could Australia's situation make for an independent test of the impact of humans on the megafauna?

Enter field ecologist, Professor David Bowman, and expert on population modeling, Dr Barry Brook, both from Charles Darwin University in the Northern Territory.

Now, the idea that humans cause rapid mass extinctions, known as the blitzkrieg model, has been around for a while -- it got a lot of attention when Tim Flannery published his best-selling book The Future Eaters. David and Barry preferred to call it 'uncertain blitzkrieg' because no one could say for sure what caused it. Until now.

Professor David Bowman and Dr Barry Brook: not from the Pleistocene Epoch.

"We've been able to show a biologically plausible reason whereby wherever humans go, with whatever level of technology they can get their hands on, there will be a corresponding inevitable extinction of animals because humans change the environment more quickly than the animals can adapt," David said.

Over two years of painstaking research, they compiled literature from around the world, looking at the animals that had survived and those that had become extinct. These animals were then ranked according to increasing body mass.

"What we saw was not a step, that is you got to a certain size and then you were dead -- we know that's not true because elephants exist. What we saw was a really nice sweeping curve so the bigger the animal, the more likely it was to go extinct," David said.

So they knew which animals had died, but not why. The pair created a computer model to see what event, or combinations of events, would produce the evidence they had before them. It's a bit like knowing that the answer is four, but not whether the original sum was two plus two, or two times two, or one plus three and so on.

"We discovered there are 101 different ways to extinction," David explained. "It can be by over-hunting, destroying habitats with fire, fixating on big animals as trophies, and there may have been some environmental disturbance -- climate change, drought and so on. But humans were always in the mix."

"It remains an inescapable fact that the extinction of the megafauna wouldn't have unfolded without humans being there," he said.

The pair agrees their theory supports the idea that human impact on the environment must be taken very seriously.

"The impact of humans can result in major ramifications that echo through evolutionary times," Barry said. "These megafauna have been wiped out forever, and there are many more animals and plants that will soon also be wiped out for good".