Going under down under: early people at fault in Australian extinctions.

A lengthy, newly compiled fossil record of Australian mammals bolsters the notion that humanity's arrival on the island continent led to the extinction of' many large creatures there.

Archaeological evidence suggests that people arrived in northern and western Australia about 50,000 years ago (SN: 3/15/03 p. 173). By 5,000 years later, about 90 percent of the continent; mammals larger than a house cat had gone extinct, says Gavin J. Prideaux, a paleontologist at the Western Australian Museum in Perth. Casualties of that era include several species of kangaroos and wombats as well as marsupials that filled the ecological niches elsewhere populated by lions, hyenas, hippos, and tapirs.

By unearthing and cataloging specimens from a group of fossil-rich caves about 300 kilometers southeast of Adelaide, Prideaux and his colleagues assembled a nearly complete record of the past 500,000 years. Most of the 62 species of nonflying mammals on the list fell into the caverns via sinkholes, but some remains were brought in by owls that roosted there.

Scientists had compiled a long-term climate record for southeastern Australia by analyzing the caves' stalactites. Those structures formed and grew when rainfall was plentiful but not during dry spells.

During most of the past 500,000 years, the number and diversity of mammal fossils found in the Australian caves decreased only during intervals when the local climate was dry. When moisture returned, so did the animals. The only" exception is the die-off of mammals between 50,000 and 45,000 years ago, the team reports in the January Geology.

Those extinctions occurred at least 25,000 years before the most recent ice age began. "The climate was stable then, and mammals really shouldn't have been going extinct," says coauthor Richard G. Roberts, a geochemist at the University of Wollongong in Australia. "The only thing that's new during that period is people," he adds.

Scientists are debating how people might have caused the extinctions. Some researchers argue that the new inhabitants drastically altered Australian ecosystems by burning the landscape (SN: 7/23/0:7, p. 61). However, large species may have died off gradually when people preyed on the mammals' offspring faster than the animals reproduced, says Roberts.

The fossil record compiled by Prideaux and his colleagues shows that "the mammal fauna was resilient through time, despite climate fluctuations," says David W. Steadman, a paleontologist at the University of Florida in Gainesville. Changes in mammal populations during times of climate change "were nothing like those that occurred after people showed up," he notes.

"To think climate caused these extinctions is [now] untenable," comments Gifford H. Miller, a geologist at the University of Colorado at Boulder.