Planet was too small for man and beasts, study finds


Associated Press

 WASHINGTON For more than a century, scientists have debated what killed off the big animals in Australia and the Americas. Now, two studies blame ancient human hunters equipped with fire, spears and an appetite for meat. 

The studies, appearing Friday 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 such animals as 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 at Santa Barbara, author of the study of the American extinctions. 

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 after the arrival of humans. 

The rapid demise during that time of 55 species including every land animal, reptile and bird in Australia weighing more than 220 pounds is strong evidence of human involvement in the extinctions, said Ms. Ayliffe. 

"It is clear that the downward spiral of these animals was after the arrival of humans," she said. 

Ms. Ayliffe said the dating is significant because some researchers have blamed the extinctions on extended droughts that occurred later. But the animals had withstood climate changes previously; so she concluded that it is unlikely that 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. 

Ms. 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, Mr. 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. 

Mr. Alroy said that since the animals evolved in the Americas before human habitation, they probably had no fear and became easy prey when the hunters came. 

Paul Martin of the University of Arizona at Tucson, a leading authority on extinctions, said the two papers "strengthen the case for human involvement in all these extinctions."