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. 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. Ms. Ayliffe said the dating is significant because some researchers have blamed the extinctions on extended droughts that occurred later. 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.