from North to South America. Dramatic 

 climatic fluctuations within the past mil- 

 Uon years and the arrival of humans in the 

 New World during the late Pleistocene 

 contributed to the extinction of Equus in 

 the Americas. In the Old World, the range 

 of Equus became restricted to portions of 

 Africa, where it gave rise to modem ze- 



bras and their relatives, and to the dry 

 steppes of central Asia. The Asian equids, 

 including the now-endangered Przewal- 

 skii's horse, apparently provided the stock 

 from which the horse was domesticated 

 five to six thousand years ago. 



Over the past several million years, spe- 

 cies of Equus, both extinct and extant, 



adapted to a wide variety of ecological sit- 

 uations and successfully spread through- 

 out the Old and the New Worlds. Yet the 

 familiar horses, zebras, asses, and onagers 

 that share our modern world represent but 

 a single surviving branch on a once luxuri- 

 ant equid family tree that reached its full 

 glory during the Miocene. 



■-3^&I/^. 



'^ 



65 



