• That Killer InstinctNorth America and Australia were both menageries of gi- ant vertebrates—until human hunters arrived. Mastodons, woolly mammoths, ground sloths, giant armadillos, and saber-toothed tigers roamed North America 14,000 years ago. Yet within 1,000 years after people showed up, all of these large mammals disappeared. Likewise, before humans arrived in Australia 50,000 years ago, the outback was home to 660-pound claw-footed kangaroos, giant wombats, and the 220-pound flightless Genyornis, the heaviest bird ever known. Some 4,000 years later, the creatures were extinct.
Two studies published this past spring pin the blame squarely on humans. John Alroy of the University of California at Santa Barbara designed a computer model to simulate the actions of "virtual hunters" and found that the arrival of humans in North America correlated with the demise of some 30 species of giant fauna. Geochronologist Richard Roberts of the University of Melbourne used a sophisticated optical ...
