THE ORIGIN OF LIFE 



the origin of life at all, but merely changes the scene of origin from the 

 earth to some remote and undefined part of the universe. 



Viruses and the Origin of Life. The discovery of the viruses has placed 

 the problem of the origin of life in a new light. The higher forms of life 

 cannot be descended from viruses as the latter are now known, for they 

 are all parasitic, and parasites must always be descended from free-living 

 ancestors. But the viruses show a unique combination of characters of 

 living and nonliving systems. They reproduce and metabolize, as un- 

 doubted organisms do. That they reproduce is shown by the fact that 

 inoculation of a susceptible organism with a very small amount of virus- 

 containing solution gives rise to a heavy infection. That they metabolize 

 is shown by the production of disease symptoms in the host by the prod- 

 ucts of metabolism. The chemical nature of viruses is that of the nucleo- 

 proteins, and it has been suggested that they are "escaped" genes. Like 

 the genes, viruses are ordinarily reproduced without change, but they can 

 mutate as genes do, that is, they can undergo an inheritable change which 

 does not interfere with their capacity for self-reproduction. Such a muta- 

 tion is detectable by a change in the disease symptoms produced by the 

 virus or by a change in the degree of toxicity. Unlike undoubted organ- 

 isms, however, the viruses do not respire. But the most striking property 

 which viruses share with nonliving systems is the fact that they can be 

 crystallized and stored indefinitely without loss of infective powers. This 

 was first demonstrated in 1935 by the chemist W. M. Stanley, who suc- 

 ceeded in crystallizing the virus which causes tobacco mosaic disease. The 

 crystals turned out to be a nucleoprotein (Figure 30). Chemical purity is 

 indicated not only by crystallization but by a sharp sedimentation bound- 

 ary when a virus suspension is ultracentrifuged. Finally, viruses can be 

 broken down to a protein and a nucleic acid, both of which are inactive, 

 then they can be recombined to form infective virus again. And so the 

 viruses appear to be homogeneous, or nearly so, in contrast to all un- 

 doubted organisms. 



Thus viruses are on the border line between the living and the non- 

 living, even though their parasitism cannot be primitive. The possibility 

 cannot be excluded that there may be free-living viruses, for we know the 

 viruses primarily by their effects ( disease production ) , and no one knows 

 what type of eftect should be looked for from a free-living virus. The ex- 

 istence of these bodies which are intermediate between the living and the 

 nonliving and which have fairly simple chemical properties suggests the 

 possibility that something like a free-living virus may have been produced 

 by chemical evolution under the influence of the unique conditions which 

 prevailed when the earth was a young planet just cooling down toward 

 a temperatiu-e range which could support life. Such a free-living, self- 

 reproducing unit might be regarded as a single gene. Mutation could then 

 lead to formation of gene-aggregates, with differentiation among the mem- 

 bers of each aggregate. Such gene-aggregates could be regarded as inde- 

 pendently existing chromosomes, and it has been suggested that some of 

 the smallest bacteria represent such a stage in the evolution of life. Fur- 



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