Science

Egg-eating humans helped drive Australia’s ‘thunder bird’ to extinction

Fifty thousand years ago, Australia was populated by big birds—really big birds. One of them, known as mihirunga, or the “thunder bird,” was six times the size of a modern emu; it may have weighed in at 250 kilograms and stood more than 2 meters tall. But the giant Genyornis newtoni disappeared 45,000 years ago, and researchers have long puzzled over whether human hunters or climate change was the culprit. Now, a new analysis of ancient eggshells—the leftovers of a prehistoric feast—suggests “humans were responsible,” says Trevor Worthy, a paleozoologist at Flinders University.

People arrived in Australia about 55,000 years ago; by 45,000 years ago, the Genyornis bird was extinct, along with dozens of other giant species, including marsupial lions and giant kangaroos. But evidence tying their extinction to the arrival of humans was circumstantial at best. Although North American peoples left clear evidence of hunting and butchering of large animals—bones with cutmarks, or stone projectile points embedded in mammoth remains, for example—none of this existed in Australia.

A possible smoking gun appeared in 2016, when researchers linked burned eggshells at sites near Australia’s southern and western coasts to Genyornis. At the time, they argued the shells were evidence of omelet making on a large enough scale to push the thunder bird over the brink. “A lot [of shells] had been burned, which implies human consumption,” says Gifford Miller, a geoscientist at the University of Colorado, Boulder, and a co-author on the paper. “That would have been the first secure evidence of direct predation.”

But other researchers argued the shell pieces were too small and thin to belong to Genyornis, and suggested they belonged to Megapodius, a genus of smaller bird distantly related to chickens and turkeys. To prove that the eggs, about the size of an emu or small ostrich egg, belonged to Genyornis, “we needed some independent way to demonstrate the shells belonged to a giant bird,” Miller says.

The team tried extracting ancient DNA from the fossilized shells, but their efforts came up empty. “The shells were too old, and the climate is too hot,” says University of Turin proteomics expert Beatrice Demarchi, who worked with Miller to identify the eggshells. Instead, the team turned to eggshell proteins.

Eggshells form quickly—within 24 hours inside the bird’s oviduct—and promptly trap proteins inside the calcium and mineral crystals that form the shell. These proteins are “not affected by contamination from the environment—just by temperature and time,” Demarchi says. She was able to recover remnants of proteins related to egg formation.

When the team compared the protein sequences with those found in modern megapode eggs, they were completely different, even falling outside the group that connects all living land birds, Demarchi says. That left Genyornis, thought to be a distant duck relative, as the only possibility, the researchers write this week in the Proceedings of the National Academy of Sciences.

Worthy, who argued against earlier analyses that tied the eggshells to Genyornis and was not involved with the research, says he’s impressed by the results. “They’ve risen to the challenge quite well—the protein evidence seems quite strong,” he says.

But thanks to the scanty skeletal evidence, mysteries remain: Why would such a big bird lay relatively small, thin-shelled eggs? “If they’re right, we’ve got a very large bird with the smallest eggs known for a bird with its mass,” Worthy says. What we might need to solidify that link is an eggshell next to a set of thunder bird remains, he says.

The burned shells suggest the first humans to arrive in Australia were stealing and eating eggs—each of which would have been a family-size meal—rather than taking on the big birds directly. “It’s quite possible humans were successful at chasing birds off the nest,” Miller says. “The most efficient way to cause an extinction is to capture the young.”

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