81 



Because of the uncertainty of the nature of the primitive atmo- 

 sphere, experimentation has explored a range of plausible models 

 from those whose gas composition is strongly reducing to those with 

 a more oxidizing composition. 



Strongly Reducing Atmosphere 



We will now explore that model which takes as its premise that 

 the initial atmosphere of the early Earth was rich in hydrogen com- 

 pounds, the one which has received most attention in the last 

 generation. 



According to the Oparin-Miller-Urey paradigm, a highly reduc- 

 ing atmosphere consisting of methane, ammonia, and water - all 

 hydrogenous - prevailed on the primitive Earth. Passage of energy in 

 various forms through this hypothetical atmosphere produced the 

 reservoir of organic molecules from which life evolved. 



The first successful prebiotic amino acid syntheses were carried 

 out using his reducing gas mixture of CH 4 , NH 3 , H 2 O (or CH 4 , NH 3 , 

 H 2 0, H 2 ) and an electric discharge as an energy source. The result 

 was a large yield of amino acids (the yield of glycine alone was 2.1% 

 based on the amount of carbon present), together with hydroxy 

 acids, short aliphatic acids and urea. One of the surprising results of 

 this experiment was that the products were not a random mixture of 

 organic compounds, but rather a relatively small number of com- 

 pounds were produced in substantial yield. In addition the com- 

 pounds produced were, with a few exceptions, of biological 

 importance. 



The special mechanism of synthesis of the amino and hydroxy 

 acids was further investigated. It was shown that the amino acids 

 were not formed directly in the electric discharge but were the 

 result of solution reactions of smaller molecules produced in the 

 discharge, in particular, hydrogen cyanide and aldehydes. These 

 reactions were studied subsequently in detail and the equilibrium and 

 rate constants of these reactions were measured. These results show 

 that amino and hydroxy acids could have been synthesized at high 

 dilutions of HCN and aldehydes in a primitive ocean. 



Ultraviolet light acting on this mixture of gases is not effective 

 in producing amino acids except at very short wavelengths 



