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50,000 years ago, Australia was home to a whole range of huge and bizarre creatures such as 7-metre long lizards, flesh-eating kangaroos and tortoises the size of a sports-car. Then, quite suddenly, they all disappeared - and scientists have never really understood why. Until now...American journalist Daniel Grossman went Down Under to get the story.
Dr Gifford Miller is a geologist at the University of Colorado in the United States and he has a theory to explain why Australia's so-called "mega-fauna" became extinct around 50,000 years ago. Dr Miller thinks that man was responsible for the loss of these creatures - and specifically man's fires.
The very first people to colonize Australia arrived around the same time that the super-sized tortoises and hippo-sized wombat-like marsupials died out. "We suspect", explains Dr Miller, "that systematic burning by the earliest colonizers, differed enough from a natural bush-fire cycle that key ecosystems were pushed past a threshold from which they could not recover." In other words, massive burning caused plants to die, dry rain-forests to disappear and the local climate to change. "The big fires set by humans," continues Dr Miller, "disrupted the brush on which the animals depended for food. As a result, all marsupials over 100 kilograms - along with three large reptiles and the ostrich-sized bird called Genyornis newtoni went extinct."
Wolfe Creek Crater
The theory is engaging - because it explains a number of changes that happened at that time - but it's also difficult to prove. Which is why Dr Miller and his team travelled to Wolfe Creek Crater in Western Australia's Great Sandy Desert. Scientists believe that the crater was formed when a huge meteorite slammed into the earth 300,000 years ago; however the local Aborigines believe that the crater was dug by bandicoots and is the sacred home of the mystical rainbow serpent. Whatever the crater's origins, Dr Miller set out to examine the changes in weather patterns - specifically the wet and dry-cycles and associated changes in vegetation - as revealed by the different layers of sediment in Wolfe Creek.
Sacred Ground
However, when work at Wolfe Creek was well underway and the results were looking promising, Dr Miller and his team hit a snag. The aboriginal Tjurabalan people of Wolfe Creek asked the scientists to stop their drilling as the ground is sacred and should not be disturbed. So respecting the wishes of the tribal elders, Dr Miller's team had to put an end to their 
research in the crater but were allowed to drill a core in a sand dune just outside the crater. Preliminary results show that there was a change in vegetation around the crater, which could be evidence of the fire-induced drying Dr Miller thinks happened 50,000 years ago.
Giant Bird Eggs
In the meantime, Dr Miller's team has been following up another line of research - that of analysing fossil eggs from the big Genyornis bird. Examination of eggshells of this large, flightless bird in three different sites 
across Australia indicate that indeed, this giant species died out around 50,000 years ago. The researchers are also studying ancient eggs of emus that lived alongside Genyornis. It turns out that at the same time the Genyornis died out, the emus changed their diet. The scientists believe this means rainfall changed, altering the vegetation available. The emu adapted, the Genyornis didn't.
Rain Loss
Dr Gifford Miller has a theory for why rainfall might have declined. He theorises that the many fires set by the first humans in Australia cleared the dry rainforest, which burns easily, and this in turn reduced recycling of water from the plants back into the atmosphere. "And that caused an increase in aridity across the continent," says Dr Miller, "from which it never recovered."
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Tags: Australia, dinosaurs, marsupials, Wolfe Creek