Scientists have long argued about the mysterious extinct creature that laid cantaloupe-size eggs in Australia’s arid expanses until about 45,000 years ago. Some contended it was an enormous, stubby-winged bird known as Genyornis newtoni. Others pointed to a turkey-like bird weighing a mere 10 pounds.
Using a novel technique called paleoproteomics, an international team of researchers studied proteins extracted from shards of fossilized eggshell. They compared those to proteins coded for in the DNA of living birds and found that they were unlike any in the genomes of modern-day species closely related to the smaller extinct fowl.
The shells, they decided, were most likely from the 500-pound Genyornis. Their conclusion is “pivotal for understanding how Australia’s first people interacted with their new environment,” the paper notes. Yet it also may shed light on an even greater scientific mystery about the disappearance of the planet’s megafauna — the gigantic animals that once roamed the continents.
“It's one of the few science questions that pretty much everybody knows something about,” said Gifford Miller, a professor of geological sciences at the University of Colorado and the senior member of the research team.
The Earth once was traversed by scores of megafauna, including mastodons in North America and armadillo-like Glyptodonts in South America. Then they vanished. Some scientists say climate events wiped them out. Others blame early humans and the hunting that sustained their growing numbers.
Miller is in the second camp, and the ancient, cream-colored eggshell fragments he began gathering in 1992 are key to his conviction. An Australian paleontologist had previously deemed such shell notable for the huge eggs that were its source and hypothesized that only one bird, known by its fossilized bones, was big enough to have laid them.
Miller wanted to determine when the continent’s megafauna menagerie — the flightless Genyornis as well as a 7-foot-tall kangaroo, 23-foot-long reptile and marsupial the size of a pickup truck — had disappeared. He hoped to answer whether humans had overlapped with them and possibly caused their demise.
But dating the death of these and other exotic species had been impossible. Not only are megafauna bones relatively rare there, but carbon-14 dating could only show that the animals had been gone at least 40,000 years.
Eggshells of the kind identified as Genyornis eggs are common, though, and Miller, a geological-dating specialist, realized that durable proteins in the shell could serve as clocks. Amino acids, the building blocks of proteins, have structures that may be configured with a twist to the left or the right. After living things die, the ratio between the two configurations changes at a predictable rate as part of a process called racemization. Measuring that ratio can reveal how much time has elapsed since death.
To date Miller has gathered about 150,000 shell pieces presumed to be from Genyornis birds and has shown that no egg was laid more recently than 45,000 years ago. Using different methods, other researchers have dated the arrival of people in Australia at no later than around 60,000 years ago. The two time frames signal an overlap of thousands of years between humans and megafauna down under.
In 2016, however, a study directly challenged Miller’s work. A team led by paleontologist Trevor Worthy of Australia’s Flinders University said the shells came not from a Genyornis but from a megapode, a family of stout birds with descendants still found on the continent and on western-Pacific islands. Worthy’s primary argument was that the eggs in question would have been too small for a massive bird like Genyornis. He and two co-authors also asserted that Genyornis eggs would have a thicker shell with more surface texture.
If right, it meant that Miller had spent three decades focused not on one of the largest birds in history — in a futile effort to help investigate an extinction theory — but on one that would barely fill a family bucket of KFC chicken. It was a bona fide scientific debate, though it was also a matter of pride for the American: “I took it as a personal affront if my story all of a sudden isn’t about megafauna anymore [but] some stupid little megapode bird.”
Once again, he turned to molecular techniques to determine where the bird that laid the eggs would be on the tree of life. Establishing that through DNA was not possible; the DNA in the eggshell bits had degraded too much because of their age. Proteins are more durable, and Miller, an Australian molecular biologist and a British biochemist realized they could use them to identify Genyornis.
The trio brought in Beatrice Demarchi, a biomolecular archaeologist, at the University of Turin. She pulverized pieces of shell and analyzed the proteins she extracted in a mass spectrometer, an instrument that sorts molecules by firing them past a powerful magnet. With the machine’s output, she could identify the order in which amino acids had been linked together.
While no Genyornis descendants’ DNA exists — its entire lineage has died out — the genomes of more than 350 modern birds, including one megapode, have recently been catalogued through a collaboration involving the University of Copenhagen, Smithsonian National Museum of Natural History, Rockefeller University and other leading research institutions.
Demarchi and several colleagues compared the amino-acid sequence of the eggshells with sequences inferred from genomes in the database. By then, the team had grown to 14 members from Australia, the United States, several European countries and China. The results, their new paper states, eliminate a megapode as the source.
The protein sequence “is very different from a megapode,” Demarchi said in an interview. “It’s more consistent with Genyornis.”
Worthy is not convinced. He says researchers can’t reliably predict the molecular fingerprint of extinct megapodes based on those alive today, as Miller’s team did. “Thus I do not think they have closed the case,” he noted in an email.
Chris Johnson, an ecologist at the University of Tasmania who wrote a book on Australia’s megafauna, takes a different view. The protein analysis was impressive, he said via email, and such studies “could really change the science of paleontology.”
Still, Johnson is not sure if determining whether a bigger or a smaller bird laid the eggs is so important. “Either way, the implications for the role of people in causing extinction by hunting are the same,” he said.
Miller insists that size matters. He believes the molecular evidence that people killed off a bird as large as the Genyornis has implications for the story of why megafauna went extinct globally.
Though some scientists continue to think that a cold snap about 12,000 years ago exterminated the giants of the northern Hemisphere, the hypothesis that they were hunted to death has much support. Paleontologists in North America have identified sites where early inhabitants butchered mastodons and mammoths, lending credence to that theory.
Miller himself discovered evidence of early Australians pursuing Genyornis: Hundreds of his shell fragments are blackened, with distinct heat spots as if they had been tossed into a campfire after being cooked. An egg burned in a forest fire, an obvious alternative explanation, would look different.
If more primitive peoples on that continent killed the huge animals around them some 45,000 years ago, he reasons, “then it’s even more likely that humans 11,000 years ago could have done the same thing in the Americas.”