92 THE FOSSILS 



suite as the Cambrian. According to this position, the Pharusian is 

 said to belong to the Middle pre-Cambrian. But it is clear that this 

 is a very vague classification only, based exclusively on relative 

 techniques of correlation and dating, which do not supply a basis 

 for absolute dating. 



We consequently cannot agree with Furon (1960), who gives an 

 age of between 3 billion and 2 billion years for the Middle pre- 

 Cambrian of the Sahara. This age is not based on a sufficient num- 

 ber of absolute datings. Even if some members of the series of very 

 different rocks grouped together as Pharusian should really be that 

 old, this age certainly does not apply to the recrystallized limestones 

 with reefs of Collenia or Conophyton. These must be younger than 

 2 billion years, because they are representatives of modem, actualistic 

 life. 



To conclude, the pre-Cambrian biogenic deposits of Collenia and 

 Conophyton in the Sahara are younger than 2 billion years, but we 

 cannot even assess a minimum age, beyond the fact that they are 

 older than late pre-Cambrian. Their probable age falls somewhere 

 between 2 billion and 1 billion years. In view of the many localities 

 in Central Africa where such remains of Stromatolites are found, it 

 would be very important to have some more dependable figures for 

 their absolute age. 



OPTIMISTIC OUTLOOK 



After having reviewed in some detail the three most important re- 

 mains of early life on earth, I think it is appropriate to end this 

 chapter on an optimistic note. Although geologic remains of early 

 life are scanty indeed, they leave no doubt that there was life on 

 earth in the dawn of geologic history. Moreover, we have seen that 

 there was already life on earth under the early anoxygenic, pre- 

 actualistic atmosphere. This lends weight to the speculative theories 

 of biologists and astronomers according to which all present free 

 oxygen is biogenic. 



So, although the remains of early life found up to now are scanty, 

 they are very revealing. Even these scanty finds have taught us a 

 great deal about early life on earth. We havT advanced very much 

 in our knowledge during the last two decades. 



The scarcity of remains of early life on earth is to be expected. 

 Early life will have been largely microbic, for one thing, and thus 



