The Evolution of the Universe 27 



postulated conditions of early earth history can account for the 

 escape of all its original atmospheric gases. He believes that the 

 progenitor of our present atmosphere was composed chiefly of 

 water vapor, nitrogen, and carbon dioxide and agrees with Urey 

 that the present high content of atmospheric oxygen is the result 

 of biological activity. Bernal (1951) believes that some methane 

 and ammonia were present, and Revelle (1955) would consider 

 likely the presence of some carbon monoxide. Rubey (1955) ad- 

 duces evidence that the chemical composition of the atmosphere 

 and oceans has varied little since early in geologic time. 



A review of this chapter emphasizes the sketchy and controversial 

 nature of our knowledge and opinions concerning many facets of 

 the origin and evolution of the present universe and particularly 

 of the earth. Yet a general theme can be discerned even if the 

 details are obscure. Whatever the start of the process, laws of 

 physics and chemistry coupled with laws of probability have led, 

 step by step, from gas masses to stars and planets. Under the con- 

 ditions set by one situation, the next result occurred automatically 

 and in turn created the conditions which led to the next evolutionary 

 development. When we finally learn the truth about the evolution 

 of the earth, we will unquestionably find that its present properties 

 are the result of changes set in motion by its original chemical 

 composition and physical characteristics. 



Two items concerning the earth are of unusual evolutionary 

 interest. In the first place, the earth's crust, and probably its interior 

 as well, is not static but highly dynamic. As will be seen later, this 

 dynamic aspect of the earth's crust has been, and is, one of the most 

 important features in shaping the course of biological evolution. 

 Secondly, from early in the earth's history to the present, the gases 

 occluded in its formative materials have leaked through the crustal 

 layers, bringing reactive gaseous mixtures containing the elements 

 carbon, hydrogen, oxygen, nitrogen, and sulphur and discharging 

 these and their products above the crust to form or influence our 

 oceans and atmosphere. These masses of escaping gases likely 

 played a responsible part in the initial formation of life on this 

 planet. 



