WASHINGTON -- With fire, spear and an appetite for meat, ancient human hunters drove to extinction most of the large animals in Australia and the Americas, according to two new studies.
Researchers precisely dating bone specimens of elephant-sized marsupials, giant snakes and other extinct animals in Australia found that the wildlife disappeared within a few thousands years after humans reached the continent.
Another study, using a computer math model, concluded that the arrival of humans on the American continents initiated the decline of mammoths, camels, saber-toothed tigers and other large animals. More than two-thirds of the large animals that evolved in the Americas before humans were gone by 11,000 years ago.
Both studies, appearing Friday in the journal Science, contribute to a debate that has been continuing for more than a century among scientists intrigued by the question: What killed off the big animals in the newly settled continents of the world?
Some have long blamed humans, but other experts say it could have been climate change, disease or a gradual change in habitat.
The two new studies pin the blame firmly on humans.
"Human population growth and hunting almost invariably leads to major mass extinctions," said John Alroy of the University of California, Santa Barbara, author of the study of the American extinctions.
"The results show how much havoc our species can cause, without anyone at the time having the slightest idea of what is going on, much less any intention of causing harm," Alroy said in Science.
Linda K. Ayliffe of the University of Utah in Salt Lake City said precise dating of rocks and fossils from 27 sites in Australia and West Papua New Guinea clearly show that the large animals there disappeared around 46,000 years ago, or about 10,000 years or so after humans arrived.
The rapid demise of every land animal, reptile and bird in Australia weighing more than 220 pounds during that time is strong evidence for human involvement in the extinctions, said Ayliffe.
"It is clear that the downward spiral of these animals was after the arrival of humans," she said.
All of the animals, said Ayliffe, had withstood climate changes previously so it is unlikely they all would have succumbed to natural forces. Also, disease is improbable since so many different species of reptiles, birds and mammals all disappeared at about the same time. Diseases are unlikely to affect all species the same way.
Among the Australian victims was the largest known bird, a flightless, ostrich-like creature that is thought to have weighed about 220 pounds. Another victim was a claw-foot kangaroo that weighed more than 600 pounds, and still another was a 20-foot-long lizard.
Ayliffe said it is unlikely that hunting alone led to the disappearance of so many large animals. She said there is evidence that humans 55,000 years ago used fire as a hunting tool, burning vast areas of Australia.
Such fires would change the habitat, making it difficult for large animals that required lots of forage to survive, she said.
Also, Ayliffe said large animals reproduce less often, making it more difficult for them to recover from the one-two punch of human hunting and habitat change.
In his study, Alroy created a computer model that factored in such things as the number of hunters, the number of animals, distribution of prey species and competition among prey for food.
He found that as long as man was in the equation virtually all combinations were bad news for the big animals of America.
"In fact, it is hard to find a combination of ... values that permits all species to survive," he said in the study.
Paul S. Martin of the University of Arizona, Tucson, a leading authority on extinctions, said the two papers "strengthen the case for human involvement in all these extinctions."
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