120 MISCELLANEOUS GEOLOGICAL CONSIDERATIONS 



elusion that its order of magnitude is compatible with the assumption 

 that all free oxygen found now is biogenic in origin. 



The answers from geochemistry, it must be concluded, are ex- 

 tremely disappointing and not of a hight degree of certainty. They 

 are, however, the best we can offer at present, and they have been 

 very carefully considered. It is a comforting thought that at least 

 they do not contradict the modem theory on the biogenic origin of 

 free oxygen, which is one of the pivots on which the modern view 

 on the origin of life on earth rests. As was the case with other points 

 mentioned earlier in this book, we have no proof, but we are fortu- 

 nate enough not to be flatly contradicted. 



UNIFORMITY OF SURFACE TEMPERATURE OF THE EARTH 



We now come to yet another aspect in relation to the history of life 

 on earth; namely, the remarkable uniformity in temperature on the 

 surface of the earth over 2 or 3 billion years at least during which 

 there was well developed life. This is not so much a question directly 

 related to the origin of life, but it has made possible its conservation, 

 and its evolution. 



It is, nevertheless, vital to us. For if at some moment during these 

 2 to 3 billion years a catastrophe had occurred exterminating life on 

 earth, the whole process would have to have been repeated, before 

 there arose anew the possibility of life on earth. That is to say, that 

 all early processes of inorganic photosynthesis, plus those of subse- 

 quent selective ascendancy and so forth would have to develop once 

 more. The various selective processes, however, are the result of so 

 many independent variables, such as mutations and environmental 

 factors, that the eventual outcome of such a repetition would cer- 

 tainly have been quite different from the former pattern. Even if it 

 so happened that the same types of biochemistry would become 

 established, the morphological expression of such a second cycle of 

 development of life would certainly have been quite different from 

 the present one. 



Consequently this question of a constant temperature of the sur- 

 face of the earth is of great moment. If this had not been so, we 

 would not have been here to contemplate our distant origin. Apart 

 from that, it is, of course, important also in relation to speculations 

 about life on other planets, either belonging to our own or to other 



