96 S. L. MILLER 



primitive Earth, then ammonia must have been present in the ocean even though 

 N2 could be the principal nitrogen species in the atmosphere*. 



This impUes that the Earth must have been rather reducing, with a pressure 

 of H2 of at least io~^ atmospheres, unless one is to assimie that the amino acids 

 were formed in limited areas containing reducing conditions. 



This argiunent would not be valid if there are other reasonable syntheses of 

 amino acids. One possibility would be the reductive amination of any a-keto 

 acids present in the ocean, although decarboxylation of the keto acid would be 

 a competing reaction. Another reaction would be synthesis of amino acids from 

 a-keto aldehydes and ammonia catalysed by mercaptans [33, 34]. A possible 

 source of the a-keto aldehydes would be from the oxidation of polyhydroxyl 

 compounds obtained from aldehyde condensations. These two syntheses require 

 ammonia, however. It is very difficult to see how an amino group can be syn- 

 thesized directly from N2 by any reasonable process except under reducing 

 conditions. Reasonable syntheses of amino acids involving hydroxy lamine, 

 nitrites or nitrates would require strong reducing agents to convert the nitrogen 

 to an amino group. A direct synthesis of the amino acids in an electric discharge, 

 if possible, would probably require reducing conditions. 



On the basis of primarily geochemical arguments Rubey [35] has argued that 

 the primitive Earth had an atmosphere of carbon dioxide, nitrogen, carbon 

 monoxide and water. This atmosphere would come mainly from the interior of 

 the Earth instead of being the residual gases of the cosmic dust cloud. Abelson 

 [36] has examined the action of a spark discharge on this mixture of gases and 

 found that good yields of amino acids could be obtained as long as some hydrogen 

 was presentf. If no hydrogen was present, then no amino acids were obtained. 

 The amino acid production was more rapid if CO2, H2O and NH3 (instead of 

 N2) were used. The mechanism of the reaction was not investigated, but it may 

 well be a Strecker synthesis as in the methane, ammonia and water case. 



Because of the presence of hydrogen in the gas mixtures used by Abelson, 

 the mixtures were reducing although not as reducing as the methane, am- 

 monia, water and hydrogen mixture. Therefore, the argument that reducing 

 conditions are necessary to synthesize organic compounds is not altered, but 

 whether the atmosphere was strongly reducing or only weakly reducing carmot 

 be decided on the basis of ability to synthesize organic compounds. 



As hydrogen escapes into outer space from a strongly reducing atmosphere, 

 it becomes less reducing and finally becomes oxidizing. Thus the atmosphere 

 proposed by Urey would be converted in the course of time to the atmosphere 



* Ammonia is quite soluble in water. The vapour pressure in atmospheres is given 

 by PNH3 = a [(NH4OH) + (NH4+)], where the concentrations of NH4OH and NH4 

 are in moles/litre. For 25 °C a is 9-3 X iQ-^ at pH = 7; 8-8 x lO"^ at pH = 8, and 

 6-1 X lO"^ at pH = 9. Thus unless the temperature of the ocean was rather high (70- 

 100°) most of the ammonia would be in the ocean. The cyanide could be formed from 

 the N2 in the atmosphere (as in Run 6). 



t The equilibrium constant for the reaction CO2 + 4H2 = CH4 + 2H2O is 7 X lo^i 

 at 25 'C, so that the mixture of gases used by Abelson is thermodynamically unstable. 

 Whether this equilibrium would be attained on the Earth is not predicted by thermo- 

 dynamics, but carbon dioxide in the presence of hydrogen can be reduced to carbon 

 monoxide and methane in electric discharges and probably by ultraviolet light. 



