MEXICO CITY – Although it’s hard to imagine in this age of urban sprawl and automobiles, North America once belonged to mammoths, camels, ground sloths as large as cows, bear-sized beavers, and other formidable beasts. Some 11,000 years ago, however, these large-bodied mammals and others – about 70 species in all – disappeared. Their demise coincided roughly with humans’ arrival in the New World and dramatic climatic change – factors that have inspired several theories about the die-off. Yet despite decades of scientific investigation, the exact cause remains a mystery. Now new findings offer support to one of these controversial hypotheses: that human hunting drove this megafaunal menagerie to extinction. The overkill model emerged in the 1960s when it was put forth by Paul S. Martin of the University of Arizona. Since then, critics have charged that no evidence exists to support the idea that the first Americans hunted to the extent necessary to cause these extinctions. But at the annual meeting of the Society of Vertebrate Paleontology in Mexico City last October, paleo-ecologist John Alroy of the University of California at Santa Barbara argued that, in fact, hunting-driven extinction is not only plausible, but it was also unavoidable. Using a computer simulation, he has determined that even a very modest amount of hunting would have wiped these animals out.