Early Australians to Blame for Mass Extinctions, Study Finds

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"We saw a quick shift in dominance from C4 drought-resistant trees, shrubs, and grasses to C3 desert plants," said Fogel.

The timing of the abrupt change in diet, and the extinction of up to 50 giant marsupial species, coincides with the arrival of humans.

Many of the extinct marsupial megafauna were large, herbivorous browsers, some weighing several tons. The emu, with its wide range of food sources, was able to adapt and survived the changes to the ecosystem. Animals like the Genyornis, which had a more specialized diet, did not.

In a commentary accompanying the report in Science, Christopher Johnson, a biologist at James Cook University in Australia, raises the possibility that over-hunting by humans may have caused the extinction of the large browsers, triggering a shift in the landscape from abundant grasses to desert scrub.

But the authors maintain that neither over-hunting nor human-introduced diseases could have caused such dramatic changes at the base of the food chain.

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