Archaeologists say they may have finally solved one of Australian's biggest puzzles - how its giant prehistoric animals, collectively known as megafauna, were driven to extinction. Researchers say that unlike previous theories, which blamed the continent's early human inhabitants for the animals' demise, it was climate change which eventually killed off the continent's giant flightless birds, three-metre-tall kangaroos and car-sized wombats. Judith Field and Richard Fullager, from Sydney University, have found evidence that humans co-existed with the megafauna for at least 6,000 years. The reason the giant animals died out about 36,000 years ago, they claim, was because Australia gradually became more arid and food sources more scarce. 'There is evidence to support the notion [that] climate change was the driving factor in the extinction of the megafauna,' Dr Field told a scientific conference this week. Australia was once roamed by a bizarre collection of giant animals, from enormous emu-like birds to marsupial lions, giant wallabies and lizards, as well as the rhinoceros-like diprotodon, which weighed nearly three tonnes. The only survivor from that era is the red kangaroo, the world's largest marsupial, which stands taller than a man and still ranges across Australia's deserts and open woodland. After excavating a site rich with megafauna remains and evidence of human occupation, at Cuddie Springs in outback New South Wales, Dr Fullager said ancient Aborigines did not have sufficiently deadly weapons to carry out a mass slaughter of megafauna species. The scientists found the bones of several megafauna species in the same sedimentary layers as stone artefacts and ancient fire sites, suggesting a lengthy co-existence between humans and the giant animals.