For more than a century, scientists have debated what killed off the big animals in Australia and the Americas. Two new studies place the blame on squarely on ancient human hunters equipped with fire, spears and an appetite for meat.
The studies, appearing today in the journal Science, conclude that after early humans migrated into Australia and the Americas, the heavyweight animals of these new continents were driven to extinction within a few thousand years. In the Americas, 73 percent of the large plant-eaters, along with the saber-toothed cat, were gone within 1,200 years after humans migrated to the continents about 13,600 years ago. Wiped out were animals like mammoths, camels, mastodons, large ground sloths and the glyptodont, a strange armored creature the size of a small car and weighing more than 1,400 pounds. In Australia, researchers precisely dated bone specimens of elephant-sized marsupials, giant snakes, huge lizards and other extinct animals. They found that the wildlife disappeared within 10,000 years after humans arrived at the down-under continent.
The research contributes powerful new evidence to a century-old debate among scientists intrigued by the question: What killed off the big animals in 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. Linda 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 large animals there disappeared around 46,000 years ago, or about 10,000 years or so after the arrival of humans. The rapid demise during that time of 55 species — every land animal, reptile and bird in Australia weighing more than 220 pounds — 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. Ayliffe said the dating is significant because some researchers have blamed the extinctions on extended droughts that occurred later. But she noted that the animals 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 disappeared at about the same time. Diseases are unlikely to affect all species the same way.
Largest Known Bird
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, which would make it difficult for large animals that required plenty of forage to survive, she said. In his study, Alroy created a computer model that factored in such elements as the number of hunters, the number of animals, distribution of prey species and competition among prey for food. He found that with man in the equation, virtually every combination was 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. Alroy said since the animals evolved in the Americas before human habitation, they probably had no fear after the hunters came and were easy prey. "Their strategy for dealing with predators was to stand and fight, and that is the last thing they should do when dealing with humans," Alroy said. Bison, elk and moose probably escaped extinction because they lived in areas, such as the central plains, with fewer humans and vast tracts of open land, he said. Paul 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."