The Origin of the Biosphere 



27 



At this time, not only do the morphological signs of organisms disappear, but 

 even the sedimentary deposits themselves have vanished owing to assimilation. 

 As a result of this the physical and chemical aspects of our investigations assume 

 particular importance. 



CONCLUSIONS 



If we can observe signs of a biosphere in geological strata dating from more 

 than 2 X 10^ years ago, then, naturally, the origins of the biosphere must have 

 been earlier than this. In considering what conditions were pecuUar to these 



^ 



Fig. 



3. Radiogenic heat formed in the Earth by U, Th and ^"K separately 

 and together plotted against time. 



times and are not now present, we may also mention some, of which, perhaps, I 

 have not spoken in sufficient detail. It is impossible to make any, even remotely 

 plausible hypothesis as to the amount of change which has taken place in, for 

 example, the intensity of ultraviolet or cosmic radiation, etc. It may be that the 

 most important factor Umiting the development of hving things was the tem- 

 perature of the rocks, waters and gases on the surface of the Earth. From the 

 graph shown in Fig. 3 it may be seen that, as early as 3 x lo^ years after the 

 origin of the Earth, the total heat derived from the decomposition of radioactive 

 elements had begim to abate at a less catastrophic rate than it had done previously. 

 From that time until the present it diminished by less than a factor of 2. The 



