The animals of Australia would have made an intimidating sight 100,000 years ago. Many were giant creatures: a two-ton wombatlike marsupial, a 16-foot snake, and a bigger, fiercer relative of the Komodo dragon, whose Latin name means “ancient giant butcher.” Then suddenly, about 50,000 years later, they were gone. For more than a century, debate raged about whether climate change caused this massive extinction, but now a group of scientists say humans are to blame.
Gifford Miller, a geologist at the University of Colorado at Boulder, and his colleagues believe people systematically burned the nutrient-rich grasslands, inadvertently transforming them into the expanses of fire-tolerant, less nutritious grasses found in Australia today. Animals that couldn’t survive on the remaining alternatives of shrubs and trees were driven to extinction—bringing down the carnivores that ate them as well.
The researchers’ conclusions came from a study of the eggshells of emus and the even larger bird Genyornis newtoni, which disappeared during the die-off. Eggshells contain carbon, which comes from the birds’ diets, and because different plants have different ratios of carbon isotopes, the shells can be used as a rough record of what birds ate. While emus will eat just about anything green,Genyornis preferred grass. The emu eggshells show, though, that after humans arrived, the available vegetation changed. “The specialized diet that Genyornis seemed to prefer ceased to exist,” says John Magee of the Australian National University. And so, therefore, did Genyornis and almost 60 other species.