Island extinctions.

People arrived in Australia about 50,000 years ago. Soon after, many of the island's large mammals disappeared, new evidence suggests.

Among the animals that went extinct were several species of kangaroos and wombats and some other creatures found nowhere else. Known as marsupials, these animals had pouches but filled ecological niches populated elsewhere by lions, hyenas, hippos, tapirs, and other large animals.

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This illuminating new look into the past comes from a group of caves in southeastern Australia. Fossils fill the caves, which lie 300 kilometers (186 miles) southeast of Adelaide.

Researchers led by a paleontologist at the Western Australian Museum in Perth collected, identified, and dated fossils that covered some 500,000 years of history. The bones that they found belonged to 62 species of mammals that didn't fly. Most of these creatures fell into the caves through sinkholes in the ground. Owls brought in others.

Previously, scientists had used icicle-like rock formations, called stalactites, to piece together a history of climate change in the area. When the weather was wet, water dripped down the stalactites, making them grow. During dry times, stalactite growth stopped.

Over the past 500,000 years, the Perth scientists found, the number and types of mammals in the caves decreased only during long dry spells. The animals came back when the rains returned.

Between 50,000 and 45,000 years ago, however, many of Australia's creatures that were cat-sized or larger disappeared, even though there was no major climate shift. The next ice age wouldn't begin for another 25,000 years.

"The climate was stable then, and mammals really shouldn't have been going extinct," says Richard G. Roberts, a geochemist at the University of Wollongong in Australia.

"The only thing that's new during that period," he adds, "is people."

Scientists aren't yet sure how people might have caused the wave of extinction among large animals in Australia. People often burned much of the landscape, and some experts argue that animals died as fire destroyed their habitats. It's also possible that large species dwindled gradually as people hunted and ate them faster than they could reproduce.

Whatever the explanation, the data are clear. People had a more profound effect on the lives (and deaths) of Australian animals than climate change did.--E. Sohn