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. 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.  A more specific criticism comes from mammalogist Ross D. E. MacPhee of the American Museum of Natural History in New York City, who points out that the relevant archaeological record contains barely a dozen examples of stone points embedded in mammoth bones (and none, it should be noted, are known from other megafaunal remains) – hardly what one might expect if hunting drove these animals to extinction. Furthermore, some of these species had huge ranges – the giant Jefferson’s ground sloth, for example, lived as far north as the Yukon and as far south as Mexico – which would have made slaughtering them in numbers sufficient to cause their extinction rather implausible, he says.

Rather he suggests that people may have introduced hyper lethal disease, perhaps through their dogs or hitchhiking vermin, which then spread widely among the immunologically naive species of the New World. Repeated outbreaks of a hyper disease could thus quickly drive them to the point of no return. So far MacPhee does not have empirical evidence for the hyper disease hypothesis, and it won’t be easy to come by hyper lethal disease that would kill far too quickly to leave its signature on the bones themselves.  The third explanation blames the loss on the weather. The Pleistocene epoch witnessed considerable climatic instability, explains palaeontology Russell W. Graham of the Denver Museum of Nature and Science. As a result, certain habitats disappeared, and species that had once formed communities split apart. For much of the megafauna, however, the increasingly homogeneous environment left them with shrinking geographical ranges – a death sentence for large animals, which need large ranges. Although these creatures managed to maintain viable populations through most of the Pleistocene, the final major fluctuation – the so-called Younger Dryas event – pushed them over the edge, Graham says.