144 INTRODUCTION TO EVOLUTION 



Suitable conditions for life are rather narrowly delimited. Extreme cold 

 and extreme heat render life impossible. Abundant water is necessary to 

 life. An abundant supply of carbon in usable form is also essential. As 

 noted previously (Chap. 5), carbon is the fundamental structural element 

 in all living things. Animals are dependent upon plants for their supply of 

 carbon. Plants are dependent upon the carbon dioxide in the atmosphere 

 for their carbon. Animals obtain their carbon by eating the plants, or by 

 eating other animals which have eaten plants. Thus life as we know it 

 would be impossible without carbon dioxide in the atmosphere, and with- 

 out sunlight. 



With very few exceptions all living things depend upon oxygen in the 

 atmosphere, including atmosphere dissolved in the water of streams, lakes, 

 and oceans. The oxygen combines with (oxidizes) carbon-containing com- 

 pounds, a chemical reaction which releases heat and energy. An analogy is 

 the burning (oxidizing) of coal, relatively pure carbon, under the boiler 

 of a steam engine to produce heat and energy. Almost all of the processes 

 within the bodies of animals, as well as the outward activities of these 

 animals, are entirely dependent upon this source of energy. Without it they 

 would be as dead as a steam engine without a fire. Thus life would be im- 

 possible in the absence of an atmosphere containing oxygen. 



Again, if pressure of the atmosphere and force of gravitation diverged 

 widely from those actually found to prevail on the earth, life as we know 

 it could not exist. Our discussion of the requirements for life might be 

 greatly expanded, but the points enumerated will suffice to demonstrate 

 that conditions on the earth must have approached those prevailing in mod- 

 ern oceans before life could have come into existence at all. 



How did life originate on this planet? There is a certain fascination in 

 speculating on this perhaps forever unsolvable mystery. In Chapter 5 we 

 discussed attributes which first living things must have possessed, together 

 with some ideas concerning the form taken by these first possessors of life. 

 Whatever the sequence of events leading to its origin, life did originate; 

 that is evident. Hence its subsequent evolution may be traced, despite the 

 fact that the earliest chapters of the story are unknown and the later ones 

 are incomplete. 



ARCHEOZOIC ERA 



There are at least two reasons why the earliest chap- 

 ters of the history of life on this planet are unknown. In the first place, the 

 earliest forms of life probably lacked those "hard parts" which we have 

 seen to be most readily preserved as fossils (Chap. 7). Viruses, bacteria, 



