THE EVOLUTION OF MAN 83 



This antiquity vastly exceeds that of the nations of Europe or of the 

 Americans or of Africa. Fourth, central Asia is the source of almost 

 all of our domestic animals, many of which have been subjected to 

 human will and control for thousands of years, and this is equally true 

 of many of our domestic plants. This is not due to the fact that man 

 first reached civilization in Asia, but rather that he chose for his com- 

 panions the highest and best of their several evolutionary lines, and 

 Asia was the place of all others upon earth where the evolution in 

 general of organic life reached its highest development in late Cenozoic 

 time (Williston). Fifth, climatic conditions in Asia in the Miocene 

 or early Pliocene were such as to compel the descent of the prehuman 

 ancestor from the trees, a step which was absolutely essential to 

 further human development. 



Impelling cause. We look for a geologic cause back of this most 

 momentous crisis in the evolution of humanity and we find it in conti- 

 nental elevation and consequent increasing aridity of climate, espe- 

 cially to the northward of the Himalayas. With this increased aridity 

 and tempering of tropical heat came the dwindling of the forested 

 areas suitable to primate occupancy. Barrell has suggested that this 

 diminution left residual forests comparable to the diminishing lakes 

 and ponds of the Devonian, which upon final desiccation compelled 

 their denizens to become terrestrial or perish. The dwindling of the 

 residual forests would have an effect upon the tree-dwellers which may 

 be expressed in precisely the same words. Once upon the ground the 

 effect upon even a conservative type and the primates in general, 

 where constant conditions prevail, are slow of change would be the 

 rapid acquisition of such adaptations as were necessary to insure sur- 

 vival under the new conditions. The other man-like apes had, 

 unfortunately for their further evolution, reached a region where 

 tropical forests continued to be available and hence have retained their 

 arboreal life and with it a stagnation of progress. The result has been, 

 at any rate on the part of the three larger forms, a degeneracy from 

 the estate of their common ancestry with mankind; the gibbons seem 

 to have deteriorated less, while terrestrial man has risen to the summit 

 of primate evolution. 



Time. The time of the descent is not later than early Pliocene 

 nor earlier than Miocene time; when the terrestrial ape-man became 

 what we would call human was perhaps later, but certainly during the 

 Pliocene, which makes the age of man as such measurable in terms of 

 hundreds of thousands of years! 



