60 THE BIOLOGICAL APPROACH 



However, another, still more important change will have domi- 

 nated gradually. Through the organic photosynthesis, through the 

 assimilation of carbon dioxide, free oxygen will have become a more 

 and more important part of the atmosphere. It is postulated that all 

 free oxygen present now in the atmosphere has been formed through 

 organic photosynthesis. All of our free oxygen thus is biogenic in 

 origin. This oxygen will have gradually shielded the surface of the 

 earth from the shorter ultraviolet rays of the sunlight, and an en- 

 vironment comparable to our present one will have been realized 

 step by step. This in turn will finally have led to the extinction of 

 all earlier processes of life or life-like reactions which were possible 

 in the primeval atmosphere by utilisation of the energy of the shorter 

 ultraviolet radiation from the sun. 



GRADUAL TRANSITION FROM PRIMEVAL TO PRESENT ATMOSPHERE 



This transition from the primeval anoxygenic, reducing atmosphere 

 to the present oxidizing one consequently was one of the prime events 

 in the history of our earth. One is tempted to describe this transition 

 as catastrophic. All earlier life was doomed to extinction, and all 

 later life, up to that of the present, found its origin at that period. 

 But, as we have seen in the preceding chapters, even such a 'catas- 

 trophic' event will have taken its time. It will have occurred very 

 gradually, not only according to human perception of time, but even 

 geologically speaking. The production of free oxygen through organic 

 assimilation was, of course, a slow process. It will have taken quite 

 a long time before free oxygen had been produced in such quantities 

 that a layer of ozone was developed in the higher atmosphere to be 

 capable of effective absorption of the shorter ultraviolet sunlight. 

 So, because the change over from the primeval atmosphere to our 

 present one, although catastrophic in its ultimate results, was a 

 gradual process, both the earlier chemical inorganic reactions pro- 

 ducing 'organic' material, and the earlier forms of life based on 

 anoxygenic metabolism, and the new life capable of organic photo- 

 synthesis, of assimilation of carbon dioxyde, must have been co- 

 existent on earth for a long time; probably for millions of years. The 

 point of intersection between the two cones in the famous Pirie 

 drawing (Fig. 14), will in reality have formed an extensive area 

 in time. 



