PREHISTORIC MAN 



491 



American animals that died out were mammoths of several kinds, some of them 

 enormously large, and the wide-ranging mastodon. The once abundant saber- 

 toothed tiger (Fig. 28.30) disappeared with the mastodons which may have been 

 its chief prey. Camels and horses, giant bison and lions vanished from the plains 

 and savannas. The giant beaver became extinct, as well as certain bizarre immi- 

 grants from South America — ground sloths as heavy as elephants that reached 

 Alaska during the interglacials and huge armadillolike glyptodonts with tails 

 like spike-knobbed war clubs that lived in the southern states. Man reached the 

 New World in time to encounter some at least of these now extinct species, for 

 remains of ground sloths and giant bison have been found with his campfires and 

 weapons. 



Fig. 30.3. Giant ground sloths and dub-tailed glyptodonts lived in North America until 

 late in the Pleistocene. (By Charles R. Knight, courtesy Chicago Natural History Museum.) 



Pleistocene chronology. Whether the Pleistocene lasted 1 million 

 years, as most geologists think, or was only two-thirds that long, it repre- 

 sents only an insignificant fraction of earth history. Brief as it was in 

 terms of geological time, it covers the whole period of man's known evolu- 

 tion, and from his viewpoint it stretches interminably into the past. For 

 our account of human origins we must change time scales. To appreciate 

 the vast duration of the Pleistocene we need only make some simple 

 calculations. If we let the whole epoch be represented by one 24-hour 

 day, then Christ was born about 5 minutes ago, history began about 10 

 minutes earlier, and the last climax of the fourth glaciation occurred 

 about an hour past. According to one theory of glacial climates the ice 

 should return in about 2 hours. Again, figuring 20 years to an average 

 human generation among primitive peoples, Julius Caesar lived 100 

 generations ago, history began 250 generations back, and there are more 

 than 30,000 generations between us and the men who lived early in the 

 ice age. 



