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EVOLUTION AND CONSERVATION 



Part VI 



LIFE 

 BEGINS 



THE 

 FUTURE 



THE 

 PRESENT 



Fishes 

 Amphibians 

 Reptiles 

 Dinosaurs 

 Mammals 

 Birds 

 Homo sapiens- I '/a Sees. 

 Historic man- y^j second 



LIFE CLOCK 



ONE HOUR= 100,000,000 YEARS 

 ONE MINUTE= 1,660,000 YEARS 



If life's past, present and future ore plotted on a 24-t)our 

 clock, modern men oppeared in ttie world about I '/j seconds ago. 



Fig. 38.1. Life clock scaled to 12 hours showing the first appearance of various 

 vertebrates in the history of life on the earth. Only invertebrates existed in the 

 earlier three-fourths of the 12-hour day which represents time from the beginning 

 of life to the present. New estimates ( 1954) of the age of the earth place its begin- 

 ning at 5,000,000,000 years and the beginning of life at 3,500,000 to 4,000,000 

 years. (Redrawn after Ritchie, New York Times, Sept. 29, 1940.) 



existed. There are no fossils of the first soft bits of living matter whose de- 

 velopment must have taken eons of time. The oldest fossils are those of simple 

 water plants that are more than one billion years old. After they appeared there 

 seems to have been a tremendously long period before living organisms were 

 numerous and varied enough to leave a continuous fossil record. 



Evolution is an unimaginably long process that includes periods of pro- 

 found geologic change. In some of these, the currents of life seem to have 

 moved more rapidly; in others, they flowed slowly and evolutionary changes 

 were slight. Fossils in the Cambrian Period mark the beginning of an upswing 

 of change (Table 38.1). 



The earth's past history has been divided into eras according to the evo- 



