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Debate competition:humans to blame for animal extinction
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First proposed in 1966 by paleontologist Paul Martin, this “overkill hypothesis” stated that the arrival of modern humans in each new part of the world brought with it the extinction of all those huge animals, whether through hunting them or outcompeting them. The hypothesis paints humans as a potent force of destruction and was highly controversial when Martin first proposed it. But over time it gained traction—though never full consensus—in the archaeological community.
Today, some archaeologists continue to fight back. For many of these Pleistocene extinctions, humans probably aren’t to blame, says archaeologist Ben Marwick. The key to his argument is timing: Marwick and other researchers recently found human artifacts in the Madjedbebe rock shelter in northern Australian that indicate humans came to the island 65,000 years ago, 10,000 years earlier than previously believed. That’s critical, because Australian megafauna didn’t start going extinct until sometime between 60,000 and 40,000 years ago.
The new data “puts people on the landscape well before megafauna started suffering population stress and showing signs of extinction,” Markwick says. “It’s like the alibi for humans: It absolves them of central responsibility.”
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Two predators, Smilodon californicus (saber-tooth cat) and Canis dirus (dire wolf) fight over a carcass in an illustration of the megafauna of North America. (Wikimedia Commons)
Marwick believes his team’s find in Australia may be close to a deathblow for the hypothesis. “In archaeology we rarely have such decisive finds that shift the argument from an ongoing debate to a fact, but my feeling is that this could be one of those moments,” he says. Indeed, recent finds have threatened to upend the conventional timeline of human migration. Marwick pointed to a controversial study from April that suggests humans arrived in North America 100,000 years earlier than previously believed. “It looks as if the whole global debate of megafauna extinction is getting a simultaneous revision,” he says.
If that’s true, the consequences would go beyond the scientific. “If it turns out we didn’t kill the megafauna,” says Marwick, “that might suggest our relationship is more one of being just another species on the landscape, rather than a total domination and inflicting environmental violence.”
But for two scientists, the same data can tell vastly different stories. Geologist and paleontologist Gifford Miller of University of Colorado at Boulder thinks Marwick’s study proves the exact opposite of what Marwick claims. “One of the previous arguments against a human role in the Australian megafaunal extinction was that humans first appeared there 50,000 years ago and animals were gone almost immediately after, which doesn’t given them enough time to build a population size sufficient to have any kind of impact," he says.
Marwick’s data, he says, helps solve this discrepancy. The earlier arrival date gives humans time to grow in number over generations, spreading across the landscape, eating whatever they came across and transforming the environment. “It’s undeniable that humans are preying on some of these large animals,” says Miller, “and undeniably something happens to the ecosystem structure and function at about the same time.”
Miller knows the signs of human hunting better than most. He has spent years studying the burnt remains of eggs laid by Australian thunder birds (Genyornis newtoni), giant flightless avians that went extinct approximately 47,000 years ago. In 2015 Miller and others published a paper in Nature Communications arguing the burn patterns on these eggshells, which have been found in more than 200 hearth sites across Australia, were different than what would be seen from natural wildfires.
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