134 CONCLUSIONS 



of the existence of life on earth are older, so we do have evidence 

 of the existence of early, anoxygenic life on earth. 



Our present oxygenic atmosphere was already established about 

 one billion years ago. It was somewhere between the two dates of 

 two billion years — for which we have the as yet youngest indication 

 of an anoxygenic atmosphere — and one billion years — that of the 

 earliest deposits known up to now to have originated under an 

 oxygenic atmosphere — that the transition from early anoxygenic 

 to the present oxygenic atmosphere toolc place. 



This transition is in all probability due to organic activity. Free 

 oxygen was formed by organisms which had aquired the ability of 

 assimilating carbon and producing free oxygen through the dissocia- 

 tion of carbon dioxide by organic photosynthesis. It was only after 

 primitive life had aquired this new kind of metabolism that anything 

 like our present plant kingdom could become established on earth. 

 The animal kingdom is, of course, still younger, although perhaps 

 only slightly so, because it scavenges the carbon transformed to or- 

 ganic substances by plants by organic photosynthesis, a process of 

 which animals are incapable. 



The animal kingdom was, however, already well established in 

 late pre-Cambrian times. In a general way, consequently, not only 

 the transition from the primeval anoxygenic atmosphere to the actual 

 oxygenic one took place somewhere in that period between two 

 billion and one bilHon years, but also the subsequent development of 

 the early ancestors of both our present plant and animal kingdoms. 



WHAT WE SHALL NEVER KNOW 



After this enumeration of the facts we have about the early history 

 of life on earth, we may well inquire what type of question we shall 

 presumably never be able to answer. We shall, for instance, never 

 know with any certainty the exact nature of these early inorganic 

 processes of photosyntlicsis which are thought to have produced the 

 organic material in the 'thin soup'. These compounds will not have 

 had any distinctive morphological form, and it is only by their mor- 

 phology, by their outhne, that fossils can be recognized as such. 



Nor can we ever hope to have a detailed account of the evolution 

 of that early life which followed upon, and most probably was even 

 contemporaneous with, the later part of the purely inorganic growth 



