276 OR 1(1 IK OF A\.\i:ii'OlHOShS 



Since Snyder's work was published new geophysical 

 theories have been evolved and various other possible 

 sources have been suggested for the origin of the atmos- 

 pheric oxygen. According to Nutting (1926) the at- 

 mosphere (when the earth had a temperature of 5000" 

 or above) contained up to 90 per cent oxygen. Enor- 

 mous amounts of this gas were removed by oxidations 

 as the temperature fell. But the protective action of the 

 superficial layer of oxides thus formed prevented a com- 

 plete depletion of the oxygen. According to other theo- 

 ries (Chamberlin and Salisbury, 1906; Nichols, 1941), 

 water vapor may become dissociated in the upper at- 

 mosphere by various mechanisms, e.g., by the impact of 

 white-hot planetesimals, or by the radiation emitted from 

 the sun. The hydrogen so formed would escape the grav- 

 itational control of the earth while the oxygen would 

 remain in the atmosphere. Nichols (1941) finally i:)oints 

 out that even some volcanic gases may liberate molecu- 

 lar oxygen from w^ater, while other gases would tend to 

 remove it from the atmosphere. 



Thus the geologists are evidently not in agreement on 

 this question. The present writer does not feel compe- 

 tent to judge the relative merit of these various theories 

 and h(^ doubts whether the point can be used at all as 

 an argument pro or contra in this discussion. 



Weighing all the evidence one finds, then, that the 

 only strong argument in favor of the idea that the most 

 primitive form of life was anaerobic is based on the 

 ubiquity of reducing processes and on the fact that these 

 anaerobic reactions are the first in the chain of events; 

 the reactions involving molecular oxygen occur only 

 later. From this viewpoint a fairly strong case for the 

 theory of anaerobic precedence as suck can be made. 

 Whether invertebrate life was originally purely anaer- 

 obic, however, is quite a different question. 



