THE ORIGIN OF MAN 15 



When the earth grew to about half of its pres- 

 ent size or about 4200 miles in diameter, the in- 

 organic matter deposited on the shores and bot- 

 toms of bodies of water, gradually came to life. 

 This animal life which consists entirely of inver- 

 tebrates, contains representatives of all the more 

 fundamental types, ranging from simple sponges 

 to complex forms of the Crustacea. The com- 

 mon animals are almost wholly trilobites and 

 brachiopods. It is thought that life first came 

 into being during the Archeozoic and early Pro- 

 terozoic eras, but as fossils of those eras are un- 

 known, it is safe to say that life came into exist- 

 ence in the Cambrian period of the early Paleo- 

 zoic eras in the form of marine faunas. Then 

 came the dominancy of the trilobites, which was 

 followed by the rise of shelled animals, and which 

 concludes the Cambrian period. 



The word trilobites means three-lobe-like, and 

 has reference to the three longitudinal lobes seen 

 on the dorsal or upper side of most trilobites. 



