The Process of Evolution 



trophic organisms even if the environment were favorable. The 

 situation was quite difiFerent under the conditions that were probable 

 on the earth billions of years ago. 



THE EARLY STAGES 



The sine qua non of the production of life as we know it is the de- 

 velopment of certain organic compounds— compounds built around 

 carbon and consisting, in the main, of this element joined in diverse 

 configurations with nitrogen, oxygen, hydrogen, phosphorus, and 

 sulfur. The early stages in the chemical evolution of the earth's 

 surface must have been characterized by the presence of much 

 simpler inorganic molecules. The questions that immediately arise 

 are how these were combined to produce the more complex com- 

 pounds found in living systems, and what the source of energy for 

 such transformations may have been. 



There is considerable evidence to support the thesis of Oparin 

 that the atmosphere of the early earth was reducing in character, 

 being made up principally of methane, water vapor, ammonia, and 

 hydrogen. The behavior of these substances under a variety of con- 

 ditions has been studied. For example. Miller placed mixtures of 

 these gases in an apparatus (Fig. 1.1) in which they could be ex- 

 posed to electrical discharges. Circulation was produced by boiling 

 water on one side of the apparatus and condensing it on the other. 

 Chromatographic analysis at the end of the experiments revealed 

 the presence of amino, hydroxy, and aliphatic acids— three basic 

 types of organic molecules, including the unit of protein structure. 

 The amino acids included glycine and alanine (the most common 

 amino acids in proteins), aspartic acid, and glutamic acid. It is 

 interesting to note that «-alanine predominated over ^-alanine 

 in these experiments; modern proteins contain only a-amino acids, 

 (a-amino acids have the NH2 and COOH groups both attached to 

 the same carbon atom.) Miller argues that the same types of com- 

 pounds would have been produced under the influence of ultra- 

 violet light and electrical discharges if the primitive earth had had 

 a reducing atmosphere. He further contends that organic compounds 

 would not be produced if oxidizing conditions were present and 

 points out that, if amino acids (and other organic compounds) are 

 necessary for life, the presence of life on earth is evidence for a 

 primitive reducing atmosphere. 



The theory that the early atmosphere was reducing in character 

 now seems widely accepted. Free oxygen, which first appeared some 

 800 million to 2 billion or more years ago and which gives the 



