The biggest bird ever known, Genyornis newtoni roamed the fertile grasslands of Australia until it went extinct 50,000 years ago, along with a variety of large mammals. Experts have debated for over a century whether climate change or human activity was to blame. Now, new research into the diets of creatures that lived during this period indicates that humans were indeed responsible for the die-off.
Humans first came to Australia about 55,000 years ago and soon began to impact their environment with fire, hunting, and disease. But while some researchers claim that these activities led to mass animal extinctions, others blame dramatic changes in climate. Resolving the issue has been difficult because of the lack of well-dated environmental records covering the period.
The new support for the human causation hypothesis came about quite by accident. Gifford Miller, a geologist at the University of Boulder, Colorado, was trying to establish the date of Australian eggshell fragments belonging to G. newtoni and Dromaius novaehollandiae, an emu that survived the mass extinction. To date the eggs, Miller and colleagues had to quantify the amount of two isotopes of carbon, 12C and 13C. But because grass tends to be high in 13C, and trees and shrubs tend to be high in 12C, the team realized they could also use the data to determine what the birds ate.
The carbon isotope composition of the emu eggs changed dramatically after humans settled Australia about 55,000 years ago, the team reports 8 July in Science. Prior to human settlement, the emus appear to have subsided on both shrubs and grass, whereas G. newtoni stuck to an almost exclusive grass diet. But once humans arrived, much of the grassland appears to have vanished. Emu eggs dated after this period contain a much higher ratio of 12C, while G. newtoni eggs begin to disappear. As a result, the researchers suspect G. newtoni's specialized diet doomed it once humans altered the landscape.
Miller's team suspected that the dietary shift seen in the large birds should also be reflected in the mammals that lived during this period. And indeed, analysis of carbon isotope ratios in wombat teeth reflected the same dietary adaptation and timing as the emus. Like the emu, the wombat can subsist on a variety of vegetation, but Miller suspects other contemporary mammals were more finicky.
"This was a good study for climate change versus human impact in Australia," says Christopher Johnson, an ecologist at James Cook University in Australia. However, not all researchers agree on the timing of human settlement, the basis of Miller's conclusions. "Conservative estimates of human arrival in Australia are 42 to 45,000 years ago, after the period of 'extinctions' that they are proposing", says archaeologist Judith Field from the University of Sydney.