THE ANIMAL, MAN (ANTHROPOLOGY) 



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The critical emergence from long centuries of cave life, up through 

 the beginnings of agriculture to community life, must have come 

 during the transitional millenniums between 10,000 b.c, after the 

 retreat of the last ice cap at the close of the Pleistocene period, and 

 the earliest known traces of community or city life, around 5000 b.c. 

 There were in the entire Pleistocene period, at least in the northern 

 hemispheres, four great invasions of arctic climates, periods of per- 

 petual winter with unmelted snow and ice, when an extensive gla- 

 cial blanket covered the land the year around. Between these ice 

 ages intervened warmer centuries without perpetual ice, when at 

 times even tropical conditions obtained. It was probably within 

 this span of Pleistocene time, in which there was such a wide range of 

 alternating climates to keep adaptable organisms on the qui vive in 

 order to maintain themselves, that man put in his initial appearance 

 and gradually established himself among the existing forms of life. 

 The Pleistocene period, therefore, is called the Age of Man, in dis- 

 tinction to the Cenozoic era, of which it is a part, and which is 

 designated as the Age of Mammals. 



There have been various attempts to estimate the relative duration 

 of the three great geological eras. Paleozoic, Mesozoic, and Cenozoic, 

 that are represented by sedimentary rocks from which fossil remains 

 of animals and plants have been recovered. The following table, 

 derived from ^-arious sources, shows the guesses made by a dozen 

 investigators, in which the relative duration of the three fossiliferous 

 eras is represented in percentages of the entire time that has elapsed 

 from the beginning of the Paleozoic era down to the present. 



TABLE OF PERCENTAGES OF TIME 



