EVOLUTION AND VIRUSES 



usual amounts of both protein and nucleic acid. Hence nucleic 

 acid seems to be the replicating part, able both to reproduce 

 itself in the right environment and to initiate the synthesis of 

 a specific protein to go with it. This is a still simpler replicating 

 system than virus particles, so have we here the model for the 

 first forms? Again it seems improbable. Not only do electric 

 discharges in the conditions that produce aminoacids fail to 

 produce purines and pyrimidines, the raw materials of nucleic 

 acid, but the nucleic acid of tobacco mosaic virus is so unstable 

 that even in the benign atmosphere of cold distilled water it 

 soon inactivates. Exposure to almost any other environment 

 speeds inactivation and the chances of such a nucleic acid 

 surviving in the inhospitable conditions usually postulated for 

 the pre-biotic era would seem to have been slim indeed. 



It is probably as idle to look at present biological systems 

 for hints about the origin of life as it would be to examine a 

 nuclear-powered submarine to find the first means whereby 

 man crossed the seas. The more efficient systems oust the 

 less efficient, and in time they do so completely. The fact that 

 all reproductive systems seem now to consist of nucleic acids 

 associated with proteins probably goes far to explain the 

 success of Darwin's generalization. But is does not necessarily 

 mean the first systems were of this type; it may mean only 

 that nucleoproteins are the most efficient and adaptable 

 reproduction systems that have yet evolved. 



Intact tobacco mosaic virus is one of the most stable of 

 nucleoproteins. Whatever other part the protein may play in 

 the activities of the virus, it is clearly indispensable now simply 

 to protect the nucleic acid from being destroyed. Can we 

 infer from this that proteins came first? I think not. The first 

 nucleic acids would have had to survive unprotected, but they 

 may well have lost their sturdy independence during continued 

 association with proteins, perhaps substituting other characters 

 for the stability they no longer required. The fact that viruses 

 are associations of two highly complex and mutually dependent 

 substances fits better with the idea that they are relatively 

 late products of evolution rather than primitive forms. 



The Origin of Viruses 



Our immediate subject is the origin of viruses rather than 



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