cies of saber-toothed cats. They had a 

 tough time making a Hving by huding 

 themselves against America's giant ungu- 

 lates; the predators show an unexpectedly 

 high percentage of fractured teeth and 

 partly healed breaks in bones. 



These giant, ever-hungry predators 

 would have made short shrift of any hunter 

 so bold or foolish as to confront them with 

 the puny weapons of the time. My guess is 

 that humans could only colonize North 

 America late in the Pleistocene because 



the ungulate-hunting predators formed a 

 barrier until relatively recently. We don't 

 know why, but by 12,000 years ago, the 

 largest of these predators, the giant short- 

 faced bear, had died out. 



According to recent studies by paleon- 

 tologist Jerry McDonald, who examined 

 remains of North American hoofed ani- 

 mals going back to 20,000 years ago, the 

 number of ungulate fossils dramatically 

 increases after the bear's extinction, sug- 

 gesting a much greater abundance of large 



herbivores after about 12,000 years ago. 

 That is also the date of the Folsom stone 

 tools, the first major evidence of humans 

 in North America. Perhaps only with the 

 disappearance of the short-faced bear — 

 humankind's .single most ferocious preda- 

 tor — could New World hunters live off un- 

 gulates like the proverbial mice in cheese. 

 Eventually, human dependence on the un- 

 gulates that sustained them may well have 

 contributed to the extinction of the great 

 Pleistocene herds. 



