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."