Scientists continue to debate the causal factors involved in the extinction of large land mammals at the end of the last Ice Age, about 11,000 years ago. There are three leading schools of thought on this debate: human hunting ("Kill"), climate change ("Chill") or plague ("Ill"). A fourth school of thought holds that many factors converged to cause mass extinction during the last ice age ("Combination"). (BBC Evolution Weekend).
The proponents of the "Kill" theory contend that over-hunting directly caused the mass extinction of the large Pleistocene mammals in North America or that perhaps over-hunting eliminated a "keystone species" and this ultimately led to extinction. This theory proposes that humans migrated into North America across the Bering Land Bridge where they encountered favorable conditions (space and game) that allowed the human population to increase exponentially. Paul McNeil, a Ph.D. candidate in paleontology at the University of Calgary and supporter of this theory, said: "Environment and climate change were definitely factors in the extinction event, but there had been numerous instances of glaciers advancing and retreating during the Pleistocene, and this is the only time we see a megafaunal extinction. The arrival of humans is the only real new factor" (Mayell, Hillary. "Remains Show Ancient Horses Were Hunted for Their Meat"). David Burney, associate professor in the Department of Biological Sciences at Fordham University in New York, "New World animals had never seen a human before, and probably, like the penguins of Antarctica or the tortoises of the Galapagos Islands, had no fear of this new, unlikely-looking predator. That made the human hunter's job much easier" (BBC Evolution Weekend). .
An opponent of the kill theory, Donald Grayson, archaeologist at the University of Washington, states that the theory is based on five tenets: human colonization can lead to the extinction of island species; humans first arrived in North America around 11,000 years ago; humans hunted a wide range of megafauna; the extinction of many species of North American megafauna coincided with the arrival of humans; and therefore, human hunting caused those extinctions.
