It has always been difficult to explain why so many of the world's largest animals - the "megafauna" - had disappeared so quickly from different regions of the world over the past 100,000 years.
Climate change was considered the reason for the extinction of giant sloths in South America, huge flightless birds in Madagascar, the sabre- toothed cats of North America and carnivorous kangaroos in Australia.
In a few thousand years Australia lost all 19 species of marsupials over 100kg in size, and 22 out of 38 species between 10 and 100kg. South America fared even worse. It lost 46 of its 58 groups of large animals, including Glyptodonts (armadillos the size of vans), Macrauchenia, a long-necked camel-like animal, and Arctodus, a huge bear, half as big again as the grizzly.
Professor Gifford Miller, a scientist who dates fossils at the University of Colorado at Boulder, believes he has the first firm evidence linking the death of the megafauna with the emergence of Stone Age people at the end of the Pleistocene era - between 100,000 and 10,000 years ago. His team of researchers focused on the dating of pieces of fossilised eggshell left behind by Genyornis, a giant flightless bird.
The scientists found that none of the eggshell fragments they analysed were younger than 50,000 years old. Thiscoincides nicely with the date when the first humans arrived on the continent. "I think we have compelling evidence that the Genyornis extinction date is applicable to the majority of Australian megafauna," Professor Miller said.
The scientists believe that with the arrival of the first human colonisers, Australia's landscape underwent a radical change which ultimately killed off the larger animals, from giant horned tortoises as big as VW beetles and wombats the size of rhinos, to a 25-foot-long snake and a one-ton lizard.
The new arrivals are thought to have pioneered a system of land management used until recently by Aboriginal people, who would start bushfires in order to stimulate grass growth and encourage fresh game. "We suspect the systematic burning by the earliest colonisers differed enough from the natural fire cycle that key ecosystems were pushed past a threshold from which they could not recover," Professor Miller said.
Colin Tudge, a research fellow at the Centre for Philosophy at the London School of Economics, said there is now overwhelming evidence to suggest that humans were responsible for the "overkill" at the end of the Pleistocene era.
"Soon after you get humans arriving on a big continent or island, you get animals disappearing," Mr Tudge said. The only exception is Africa, where humans and big animals co-evolved for 2 million years.
"Unlike the animals of Africa, the creatures of the Americas and Australia were totally unused to human ways," Mr Tudge said.
"However it happened, it shows that the idea of a `noble savage' in tune with the animals around him is a lie. The only animals we are in harmony with are the ones that we failed to knock on the head," said Mr Tudge.Reuse content