BioEd Online

Australia's earliest settlers drove many animals to extinction through their use of fire, say palaeontologists who have studied the changing dietary habits of long-dead creatures.

Many large Australian animals are known to have died off after man first arrived on the continent around 50,000 years ago. But it has remained unclear exactly how, if at all, humanity caused this extinction.

Some experts have argued that early settlers unleashed a 'blitzkrieg' of hunting on the animals, wiping them out in a matter of generations. Others have argued that the aboriginals brought novel diseases with them from overseas.

Neither of those is the real story, argues Gifford Miller of the University of Colorado, Boulder. He says that humans' extensive use of fire altered the makeup of plant ecosystems, leading to a widespread die-off of creatures that fed on certain grasses.

Miller and his colleagues say they have found evidence that many animals changed their eating habits soon after humanity's arrival, and that those that were unable to adapt to new foods died out.

Emu eggs

The researchers studied preserved eggshell fragments from Lake Eyre, Port Augusta and the Darling-Murray Lakes in southern Australia. Some of the eggs came from the emu (Dromaius novaehollandiae), which survives as a species today; others belonged to the similar, but extinct bird Genyornis newtoni.

Miller's team reconstructed the birds' diets over the past 140,000 years by studying the levels of radioactive carbon isotopes in the eggshells. They found that Dromaius shifted from nutritious grasses, which the team identified by its distinctive levels of radioactive carbon, to less nutritious shrubs and trees around 45,000 years ago.

A similar trend was seen in wombat teeth, the researchers report in this week's Science¹. But Genyornis showed much less variation in its diet, which may explain why it failed to adapt and survive into the present.

The change in diet is down to man's extensive burning of grasslands, to clear passageways, open up hunting grounds or signal over long distances, Miller argues.

"Our evidence suggests that the enterprise of the first colonists altered ecosystems at their lowest level: the vegetation," Miller says. "And as vegetation changed, those animals with flexible dietary tolerances were able to adjust to the changed food sources, whereas those with more specialized dietary needs became extinct."

Cold shoulder

The researchers add that no climate shift is known to have occurred in Australia during the time of that extinction. So they argue that humanity, rather than climate, caused a change in vegetation, and widespread extinctions.

Others are not so sure. Fossils found at Cuddie Springs, New South Wales, seem to indicate that ancient fauna lived side-by-side with humans for several thousand years before finally succumbing to encroaching desertification as little as 30,000 years ago (see " Did climate shift kill off giant Australian animals?").

But Miller argues that more accurately dated fossils are needed to support this theory. "The Cuddie Springs dating remains very contentious," he says. "Most agree that the extinction event occurred between 50,000 and 45,000 years ago."