"We don't have to be responsible," said John Alroy, author of the study in the current journal Science. But "The mass extinction is probably the responsibility of humans."  The quest to understand extinction's in North America, and similar ones on other continents, has drawn interests in the scientific community because it is filled with modern resonance.  Each of the three rival theories, climate change, over hunting, and a disease spread by humans, holds potential lessons for a world increasingly concerned about global warming, vanishing species and frightening new contagion's like AIDS and EBOLA.

THe computer simulation, scientist said, does not prove that hunting was the culprit.,. But the simulation shows that hunters could easily have caused the population crash, and the idea fits well with mounting evidence. In a companion piece, a team of scientist in Australia were of scientists in Australia ere able to determine when many of that continent's large animals, including a terrifying claw footed kangaroo, met the same fate.  Their estimate of then the animals dies, about 46,000 years ago, implicates humans, who probably arrived several thousand years earlier, which was even more lush at the time than it is today.

However, neither study directly refutes the disease hypothesis, proposed by Ross MacPhee of the American Museum of Natural History in New York. According to that theory, the extinction was caused by a disease, as the flu virus does. He believes the disease was carried by humans, or perhaps dogs traveling with them.  Alroy, though, said that the more than 1,000 year time lag between the arrival of the first humans and the disappearance of the big animals "does not look like an epidemic."

To do the siumulations, Alroy created computer version of a strange, lost world in which camels, sloths, and giant antelope munch lazily on the NOrth American vegetation, and sabre tooth cats flourished. The computer divided the continent into 754 grid squares and took in estimates for a range of ecological parameters, such as the size of each of 41 mammals, how quickly different species reproduced, and how quickly human populations would have moved and grown after arriving from Asia about 13,800 years ago.

WHen Alroy unleashed his virtual Pleistocene, the computer predicted dramatic die offs. THe computer was even able to predict, reasonably accurate, which species went extinct, and which survived, and the timing of the extinction.