Humans, not climate, killed off Australia's big beasts

Humans exterminated an array of weird and wonderful Australian creatures within only 4,000 years of arriving on the continent, according to a study published Friday that shifted blame away from climate change.

Before the arrival of homo sapiens, Australia boasted 450-kilogramme (1,000-pound) kangaroos, wombats weighing as much as a rhino, eight-metre (25-foot) lizards, larger-than-human birds, and car-sized tortoises.

More than 85 percent of Australia's big mammals, birds and reptiles went extinct "shortly" after our species appeared, a team of scientists reported in the journal Nature Communications.

The cause of the megafauna die-off Down Under some 45,000 years ago has been the subject of much scientific debate.

A study in Proceedings of the National Academy of Sciences in 2013 said Australia's giant animals were already mostly extinct by the time humans arrived -- and pointed the finger at climate change.

That study, based on fossil finds, said there was no evidence that a human ever killed a single mega-animal -- many of which were herbivores.

The new research, on the contrary, concluded the mass extinction was much more likely caused by "overkill", specifically the hunting of juvenile animals.

The team based its findings on the remains of ancient megafauna excrement found in a sediment core drilled in the Indian Ocean, off the southwest Australian coast.

- Look back in time -

The core contains chronological layers of material that was blown or swept from the land into the sea, including dust, ash, and spores from a fungus that thrived in the dung of plant-eating creatures, the University of Colorado at Boulder, which took part in the study, said in a statement.

This allowed the team to "look back in time" and reconstruct climate and ecosystem conditions up to 150,000 years ago.

The abundance of fungus spores in the core "is good evidence for a lot of large mammals on the southwestern Australian landscape up until about 45,000 years ago," CU Boulder scientist Gifford Miller explained.

"Then, in a window of time lasting just a few thousand years, the megafauna population collapsed" -- and that within 4,000 years of human occupation of the continent.

The environment in southwest Australia changed from a dense eucalyptus woodland to arid, open shrubland about 70,000 years ago -- some 23,000 years before evidence for the presence of humans on the continent.

The team found no association between environmental change and megafauna extinction, or evidence that the animals suffering a slow demise as the area became drier.

"These findings rule out climate change, and implicate humans as the primary extinction cause," the researchers concluded.