84 



Nonreducing Atmosphere 



If the early Earth's atmosphere were dominated by the gases, 

 carbon dioxide, nitrogen, and water, then the Miller-Urey experiment 

 would not be relevant to the origin of terrestrial life. The reduction 

 of carbon dioxide and nitrogen would have to have taken place by 

 means other than molecular hydrogen. Recall that reduction means 

 the acceptance of electrons by a molecule or ion. Thus, an organic 

 molecule in water, upon accepting an electron, gains a net negative 

 charge which is neutralized by a proton donated by the water 

 molecule itself. 



If the early atmosphere were not hydrogen-rich, the reduction 

 of carbon dioxide could only be carried out if another supply of elec- 

 tron donors were available. A conceivable source of electron donors 

 on the early Earth would be iron — the ferrous ions. 



In 1960, Getoff irradiated an aqueous solution of ferrous sul- 

 fate and carbon dioxide with light (~2600 A) and got a yield of 

 formaldehyde of approximately 1%. These observations should be 

 considered in the light of recent proposals that the reducing condi- 

 tions on a primitive Earth were to be sought in the abundance of 

 ferrous iron in the crust rather than in the amount of hydrogen in 

 the atmosphere. The problem of photochemically reducing N 2 and 

 NH 3 is currently under active investigation. Low yields of NH 3 form 

 from N 2 in the presence of the metals Mo, Fe, and Ti when irradi- 

 ated by long-wavelength UV light. The reduction of carbon dioxide 

 and nitrogen may have been possible on a primitive Earth if the right 

 electron donors were present. This opens up new experimental vistas 

 in the studies of the origin of life, especially in the synthesis of the 

 amino acids. While the above discussion has dealt predominantly 

 with the synthesis of amino acids, we would be remiss if we 

 neglected the laboratory synthesis of the other organic species we 

 have encountered in this text. 



As pointed out many times in the text, the nucleic acids play a 

 fundamental role in terrestrial biology. In addition to phosphorous 

 in the form of phosphate, the nucleic acids require both sugars and 

 certain nitrogenous molecules, the purines and pyrimidines. There is 

 ample laboratory evidence to show that these molecules are pro- 

 duced from the reactions of the simple molecule HCN in water. Of 

 the purines, adenine (see Appendix) is the major one produced from 



