CHAPTER 8 



EVOLUTION AS SEEN IN THE 



GEOLOGIC RECORD: PRE-CAM- 



BRIAN AND PALEOZOIC ERAS 



Since so little is known of Archeozoic and Proterozoic 

 life, it is sometimes convenient to refer to these eras collectively by the 

 term "Pre-Cambrian." Cambrian rocks are the earliest ones in which 

 abundant fossils are found (Table 7.1, p. 137). 



Beginnings 



In a sense the geologic time scale may be thought of as starting with the 

 beginning of the earth. How long ago did that event occur? The question 

 cannot be answered with exactness, though recent estimates indicate that 

 the earth may be in the vicinity of 4500 million years old (p. 138 ). 



Although it is not within our province to discuss the various theories of 

 the earth's origin, we may state that at one time the planet was evidently 

 extremely hot, with temperatures above the melting point of rock. As the 

 earth cooled the molten materials solidified into the original igneous 

 rocks. During all this time conditions on the planet were unsuitable for life. 

 Accordingly a very long period must have elapsed between the beginning 

 of the earth and the beginning of life on the earth. 



Conditions Necessary for Life 



Both the time and the manner of the beginning of life are shrouded in 

 mystery. We may be reasonably sure that life did not originate until cli- 

 matic conditions arose bearing resemblance to those pertaining today. 



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