10 F. M. BURNET 



IV. Evolution of the Animal Viruses 



Any thoughts about the evolution of the animal viruses must depend on 

 our understanding of their present ecological and functional attributes, and 

 they must be subject to modification with every increase in knowledge. 

 Their only importance is to provide an occasional hint that may point toward 

 an area that has been inadequately studied. 



For many years the Green-Laidlaw theory of parasitic degeneration has 

 been the only hypothesis of viral evolution that has been seriously discussed. 

 This does not necessarily mean that it is correct. Broadly, there are three 

 possible hypotheses: (1) that the specific patterns characteristic of viral 

 protein and nucleic acid are determined by a genetic mechanism which can 

 be traced back to that of some autonomous group of microorganisms; or (2) 

 that the genetic mechanism was at some stage in its descent part of the 

 genome of an organism related to the present host or possibly to an insect 

 vector. The third possibility is that viruses descend from an extremely 

 primitive precellular stock of proto organisms which could only survive as 

 intracellular parasites once modern organisms had appeared and which have 

 been viruses since the pre-Cambrian era, changing host and habit as the 

 course of evolution dictated. 



On any of these concepts, once a virus species has found it possible to 

 survive by transfer from one host individual to another, the possibilities of 

 change by mutation and selective survival are introduced. Having regard to 

 the genetic lability of such viruses as have been studied in the laboratory and 

 the rapidity of their generation time, one would expect (a) that, on transfer 

 to a new environment, adaptation to the form optimal for survival would 

 take place rapidly, and (b) that once a satisfactory host-parasite relationship 

 had been evolved the situation will remain virtually unchanged indefinitely. 



The only real virtue of the Green-Laidlaw theory is that it allows the 

 logical inclusion of psittacosis and poxgroup viruses among the animal 

 viruses. Neither of the other two hypotheses makes sense for these. The small 

 RNA viruses which have no other components than nucleic acid and protein 

 can be interpreted either as the limiting stage of parasitic degeneration or as 

 equivalent to, and perhaps developed from, the smallest protein synthetic 

 units found in all types of cells. The nature of the microorganisms from which 

 viruses are presumed to have descended can only be guessed at. It has already 

 been suggested that the poxviruses and psittacosis group viruses may be 

 descended from bacteria not unlike brucella, with rickettsiae as an inter- 

 mediate step. Andre wes (1957) has suggested that the rickettsiae are pri- 

 marily parasites of insects and that much of the evolution of animal viruses 

 may have taken place in the terrestrial arthropods. The arborviruses could 

 also be regarded primarily as having evolved in insects, but more probably 

 from protozoal than from bacterial ancestors. This concept may be found 



