People fired up Aussie extinctions.

Early human colonists of Australia apparently lit massive fires that reshaped the continent's landscape nearly 50,000 years ago and drove many animal species to extinction, according to new chemical analyses of ancient emu eggs and wombat teeth.

Emus and wombats dramatically changed their diets in comparable ways between 50,000 and 45,000 years ago, say geologist Gifford H. Miller of the University of Colorado at Boulder and his colleagues. At three sites in central and southeastern Australia, these creatures began to eat only shrubs, trees, and herbs, after having spent the previous 100,000 years consuming a variety of grasses as well, Miller's team reports in the July 8 Science.

New eggshell evidence also indicates that a large, flightless bird species that died out in Australia around 50,000 years ago had eaten mostly grasses. Those now-extinct animals couldn't adapt quickly enough to survive in areas where shrubs rapidly replaced burned-out grasses, the scientists propose. Many other animals hit evolutionary dead-ends for the same reason, they add.

Miller and his coworkers examined specific forms of carbon in eggshells and teeth to determine whether the ancient animals ate primarily grasses or shrubs.

There's no evidence of a marked climate change in Australia around 50,000 years ago, the investigators note. People first reached Australia at about that time, though, and probably set fire to large swaths of land for reasons that include clearing passageways and hunting along the fire front, the team speculates.

The relative contributions of human settlers and a changing climate to ancient Australian animal extinctions remain controversial (SN: 6/18/05, p. 397).--B.B.