50 



THE ORIGIN AND EVOLUTION OF LIFE 



space instead of resolutely seeking it within the forces and 

 elements of our own humble planet. 



The thermal conditions of living matter point to the prob- 

 ability that life originated at a time when portions at least 



Fig. 5. The Earliest Phyla of Plant and Animal Life. 



Chart showing the theoretic derivation of chordates and vertebrates from some inverte- 

 brate stock, and of the invertebrates from some of the protozoa. The diagonal lines 

 indicate the geologic date of the earliest known fossil forms in the middle Algonkian. 

 The earliest well-known invertebrate fauna is in the Middle Cambrian (see pp. 

 118-134; and Figs. 20-27). Although diatoms are among the simplest known liv- 

 ing forms and probably represent a very early stage in the evolution of life, no fossil 

 forms are known earlier than two species from the Lias, while all the rest date 

 from the Cretaceous. 



of the earth's surface and waters had temperatures of between 

 89° C. and 6° C; and also to the possibility of the origin of 

 life before the atmospheric vapors admitted a regular supply 

 of sunlight. 



