A huge flightless bird known as Genyornis newtoni once roamed the Australian Outback along with a host of other giant animals. Now scientists have unearthed what may be the first reliable evidence that humans contributed to the extinction of these gigantic birds: Fossilized, cooked eggs.

Genyornis was a 2-meter bird that some estimates suggest could reach up to 200 kilograms in weight. It went extinct, along with more than 85 percent of Australia’s other megafauna — mammals, birds and reptiles weighing more than rougshly 45 kilograms — shortly after humans first came to the island continent. Previous research suggests that happened about 50,000 years ago on rafts launched from Indonesian islands several hundred kilometers away.

Scientists have debated the cause of the demise of many of Australia’s largest animals for more than a century, with two potential explanations coming to the fore — human impact or climate change. The lack of clear evidence for humans preying on megafauna despite roughly 50 years of archaeological fieldwork has led some to question a human role in their extinction in Australia. However, while Australia did see gradual drying between 60,000 and 40,000 years ago, megafauna there survived more extreme previous climate shifts, which suggests that aridity alone could not explain their extinction.

Now study lead author Gifford Miller at the University of Colorado, Boulder, and his colleagues suggest they have found the first reliable evidence that people once hunted the extinct Australian megafauna — cooked Genyornis eggs. The findings appear in the Jan 29 issue of Nature Communications.

The scientists analyzed blackened Genyornis eggshell fragments from more than 200 sites across most of the arid zone of Australia, from its west coast to its central deserts, mostly from sand dunes where the birds nested. Previous research suggested the eggs were about the size of cantaloupes, weighing about 1.6 kilograms.

“I started research in Australia to study variations in monsoon circulation on millennial timescales,” Miller explains. The scientists used the eggshell of Genyornis and Dromaius (the emu) to date ancient shorelines and sand dunes tied to expanded lakes. But in the process, they accidentally dated when the big bird became extinct. Later they discovered the evidence that humans had eaten the eggs.

The eggshell fragments were frequently blackened at only one end. Moreover, amino acid decomposition decreased away from the blackened ends. Both these findings suggest the eggs were in contact with a very hot, concentrated source of heat, such as an ember, instead of wildfires that would likely cook them all over.

“We conclude that the only explanation is that humans harvested the giant eggs, built a fire and cooked them, which would not blacken them, then discarded the fragments in and around their fire as they ate the contents,” Miller says.

In addition, the researchers found that many of the burnt eggshell fragments in tight clusters less than 3 meters in diameter. Some clusters had both burnt and unburnt eggshell fragments, a situation that would almost certainly never happen with natural wildfires.

“The case for a human role in megafauna extinction in Australia has become more likely,” Miller says. “What it suggests is not that humans deliberately set about exterminating the giant bird, but that even modest harvesting of eggs would have reduced reproductive success.”

Using three different dating techniques, the scientists found that burnt Genyornis eggshells only occurred during a window between 53,900 and 43,400 years ago, about when humans first arrived in Australia. The researchers suggest these fragments are evidence that humans preyed on the bird as they dispersed across the continent.

These new findings provide “clear evidence that early humans in Australia hunted extinct animals that were part of the Pleistocene megafauna,” says ecologist Christopher Johnson at the University of Tasmania, who did not take part in the research.

Johnson notes that a lack of evidence as to whether or not humans hunted extinct megafauna has frustrated efforts to explain Australia’s megafaunal extinction. Miller suspects that this paucity of fossil clues stems from not only the rarity of well-preserved kill sites, but the chemistry of Australian soils.