Washington D.C. - A group of U.S. and Australian researchers have used the eggshells of an enormous flightless bird to show that humans were probably responsible for a major extinction event that saw the demise of more than 85% of Australia's large animals. In the 8 January 1999 issue of Science, Gifford Miller of the University of Colorado and his colleagues report that the extinction probably occurred about 50,000 years ago and speculate that its cause may have been ecological disruption from burning by the continent's early human inhabitants. This evidence should help clear up a decades-old debate as to whether human activity or climate change was behind the extinction events that rocked ecosystems around the world during the Quaternary period (1.8 million years ago to the present).
Over a century ago, explorer Alfred Russell Wallace wrote that "we live in a zoologically impoverished world, from which all the hugest, and fiercest, and strangest forms have recently disappeared...." The dearth of these animal species (collectively known as "megafauna," for their moderate to large size) is a result of the mass extinction events of the Quaternary period, which Timothy Flannery of Harvard University describes in a Perspective that accompanies the Report by Miller and his colleagues. These events were mild or absent in some regions of the world, such as Africa and Southeast Asia, moderate in others such as Europe, and extreme in the Americas, Australia, and many oceanic islands.
However, in recent decades, agreement as to why the megafaunal extinctions occurred has been as non-existent as the long-gone giant sloth. Parts of the Quaternary were periods of rapid and extensive climatic change that could have been capable of driving many species to extinction. On the other hand, modern humans, who first spread across the globe during this time, might also have brought about the megafauna's demise, either through hunting or less direct means. Most researchers have sided with one of these two possibilities, and the debate-which has focused on the Americas and northern Eurasia-has been nearly deadlocked for years.
The earlier and less-studied Australian megafaunal extinction provides a fresh start for investigating these events. It was a dramatic affair that cost the continent many large terrestrial vertebrate species like none known today, including carnivorous kangaroos, 20 foot-long monitor lizards, and a horned tortoise nearly the size of a small car. Until now, however, a fundamental obstacle has been nailing down the actual timing of the event, largely because its age pushes the limits of radiocarbon dating.
Miller's group cleared this hurdle when they documented the timing of the extinction of Genyornis newtoni, a ponderous flightless bird with thick, short legs that weighed around 200 pounds--twice as much as the modern day emu. They used a variety of alternative dating techniques (not based on radiocarbon isotopes) to analyze the birds' fossilized eggshells and set the date of Genyornis' sudden disappearance at 50,000 years. This date loosely matches the time of the Aborigines' arrival to the continent as indicated by the most reliable evidence yet available.
This was also a time of moderate climate change, making it unlikely that climate played a role in Genyornis' extinction. The authors bolstered this conclusion by comparing eggshell data (which spanned 50,000 years of Genyornis presence in ancient Australia) collected from different regions with distinct climates. It appears that the birds survived through a range of natural environmental changes with no ill effect before their sudden disappearance.
How might humans have caused Genyornis' extinction? Probably not by hunting, as many researchers have proposed. The authors conducted carbon isotope studies on the eggshell fossils to reconstruct Genyornis' diet. The bird appears to have been a relatively picky eater who depended on extensive shrubland for food. In fact, most of the extinct megafauna were browsers of trees and shrubs rather than grazers. The authors speculate that humans disrupted the natural fire cycle by burning the landscape periodically. This, in turn, may have decreased the vegetation necessary for animals like Genyornis. The findings imply that even low-technology human impacts can have large-scale environmental effects.
More work is needed to prove that Genyornis disappeared at the same time-for the same reasons-as the rest of the extinct Australian megafauna. But recently Miller's group has begun to investigate sites that contain both Genyornis and other megafaunal fossils at what seems to have been the eve of their existence--younger deposits contain no trace of the extinct megafauna, only fossilized eggshells of the still-living emu. "We're starting to be able to put together a composite picture of the fossils in this age group," Miller said.
Order Article # 11 "Pleistocene Extinction of Genyornis newtoni: Human Impact on Australian Megafauna," by G.H. Miller and B.J. Johnson, at U. of Colorado in Boulder, CO; J.W. Magee, L.K. Ayliffe, and M.T. McCulloch, at Australian National U., in Canberra, Australia; M.L. Fogel, at Carnegie Institution of Washington, in Washinton, D.C. B.J. Johnson is currently at U. of Washington, in Seattle, Washington. L.K. Ayliffe is currently at Laboratoire des Sciences du Climatet et de l'Environnement, in Gif-sur-Yvette, France. Contact: Gifford H. Miller in Australia, at 61-2-6279-8099 (phone), or gmiller@colorado.edu (email).
Order Article #3 "Debating Extinction," by T.F. Flannery at Harvard U. in Cambridge, MA. Contact: Tim Flannery at 617-496-4786 (phone) or tflannery@oeb.harvard.edu (email).
For copies of these articles please email scipak@aaas.org, call 202-326-6440, or fax the form below to 202-789-0455. For copies of the related cover art contact Heather Singmaster at 202-326-6414 or hsingmas@aaas.org.
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