CHAP. 32] THE OKIGIN OF LIFE 851 



which is valid for pH's less than 9. At pH = 8 and Pk^= ^.^ x IQ-^, we have 



rNH4+]/P^^^ = 47, 



which shows that most of the ammonia would be in the ocean instead of in the 

 atmosphere. The ammonia in the ocean would be largely decomposed when the 

 pressure of hydrogen fell below 10"^ atm, assuming that the pH of the ocean 

 was the present value of 8. A higher pH would make the ammonia less stable 

 and conversely for a lower pH. 



The presence of large quantities of ammonia in the ocean does not necessarily 

 imply that the oceans had a pH greater than at present. The NH4+ would 

 weather acidic rocks, thereby lowering the pH. It is even possible that the 

 oceans under reducing conditions were more acidic by one or two units of pH. 



It is unlikely that there exist any effective inorganic syntheses of nitrogenous 

 organic compounds of biological interest which could use nitrogen in a higher 

 oxidation state than ammonia, unless strong reducing agents Avere present. It 

 seems almost necessary that ammonia was present in oceans of the primitive 

 Earth but not necessarily in the atmosphere. This point alone is a strong argu- 

 ment for the presence of reducing conditions on the primitive Earth. 



All the oxides of nitrogen would have been unstable and should have been 

 rare. Hydrogen sulfide would have been present in the atmosphere only as a 

 trace constituent because it would precipitate as ferrous and other sulfides. 

 Sulfur would be reduced to hydrogen sulfide by the reaction, 



H2 + S = H2S, ^^25° = 6x104. 



It is evident that the calculations do not have a quantitative validity because 

 of many uncertainties, such as temperature, processes by which equilibrium 

 could be approached, the atmosj)heric level where such processes were effective, 

 the partial pressure of hydrogen required to provide the necessary rate of 

 escape, etc. 



It should also be noted that these thermodynamic calculations do not mean 

 that compounds which are highly unstable in a reducing atmosphere — such as 

 oxygen, oxides of nitrogen, oxides of sulfur, etc. — were entirely absent, but 

 that they were present in no more than a few parts per million. In the present 

 atmosphere, hydrogen, ozone, methane and nitrous oxide are highly unstable 

 thermodynamically, but they are present only in a few parts per million. 



The above discussion does not prove that the primitive atmosphere contained 

 low partial jDressures of hydrogen and ammonia and moderate partial pressures 

 of methane and nitrogen. However, this atmosphere is stable and can account 

 for the loss of hydrogen from the Earth, w^hich is not the case for atmospheres 

 containing large quantities of carbon dioxide and carbon monoxide. The 

 reducing atmosphere also provides favorable conditions for the origin of life. 



D. Sources of Energy 



At the present time the direct or indirect source of free energy for all living 

 organisms is the sunlight utilized by photosynthetic organisms. But before the 



