130 ORIGIN OF LIFE AND EVOLUTION 



Quite possibly life will have developed in many different ways and 

 at many different dates during this period. But natural selection will 

 have been very strong during the period of transition to the oxygenic 

 atmosphere. It may then have played upon this wide field of possible 

 life forms, eventually narrowing it down to the beginning of our 

 later life. This has been schematized in a re-drawn version of Pirie's 

 double cone (Fig. 34). 



All this, of course, is mere hypothesis. It is, however, the sort of 

 hypothesizing which logically follows from observed facts, and, 

 moreover, it is not in conflict with the known facts of the geological 

 records. It is, on the other hand, to return to the starting point of this 

 chapter, quite a different thing than the study of the later evolution 

 of life on earth, which, for all shell-bearing or skeleton-bearing 

 animals, is based on the facts of the paleontological record. 



