6 The Nature of Biological Diversity 



differs with time of day and the latitude. If this rate continued 

 throughout 4.5 X 10 9 years, the loss would amount to the hydrogen 

 from only 20 g cm -2 of water. In order to account for the oxidation of 

 carhon, ammonia, sulfide, and some ferrous oxide, Miller and Urey 

 estimated that prohahly 1,500 times as much hydrogen must have 

 escaped and hence that the hydrogen pressure must have heen some 

 1.5 X 10 -3 atmospheres during much of geologic time and the total 

 water decomposed ahout 3 X 10 4 g cm -2 or ahout one-tenth of the 

 present surface water." It is reasonahle to suppose that hydrogen would 

 he more ahundant in an atmosphere that did not contain free oxygen 

 and that it was the appearance of this oxygen which decreased the 

 hydrogen ahundance and initiated the present rate of loss. 



If hydrogen were present at these comparatively high pressures, it 

 is necessary to ask whether it would interfere with other chemical 

 facts. Carhon dioxide reacts with silicates to give limestone and silicon 

 dioxide, and at the same time hydrogen reacts with carhon dioxide to 

 form methane and water. If the carhon dioxide is destroyed by the 

 second reaction until its pressure becomes lower than that required 

 to precipitate calcium carbonate, the first reaction cannot proceed. 

 Since very ancient limestones are known, this would be impossible. 

 Miller and Urey (1959) showed that these conditions can be met satis- 

 factorily. They also found that nitrogen would be present as NH 4 + 

 ion if the hydrogen pressure were the assumed value. This study 

 showed that the necessary escape of hydrogen was possible, that cal- 

 cium carbonate could be formed, and that reduced nitrogen could be 

 present in the oceans. Such a study does not prove that the conditions 

 did exist but only that they are not inconsistent with observations. 



The production of carbon compounds 



As a first approximation, it is reasonable to assume that the chem- 

 ical elements will be present in nature in chemical compounds which 

 are thermodynamically stable under the existing conditions. It is well 

 known also that deviations from this condition are so very common 

 as to be almost universal, though such deviations are usually not large. 

 But we may well review briefly some general facts about the stability 

 of carbon-hydrogen-oxygen compounds. The simpler compounds of 



* There are indications that deuterium is more ahundant in the ocean than in 

 juvenile water by some 4 per cent. This indicates that hydrogen has been lost to a 

 larger extent proportionally than has deuterium. The observations are not incon- 

 sistent with the assumptions of Miller and Urey. See N. Kokubu, T. Mayeda, and 

 H. C. Urey (1961), Geochim. et Cosmochim. Acta, 21 :247. 



