7 1 9 GENETICS AND EVOLUTION 



they became autocatalytic. This hypothetical, autocatalytic particle 

 would have some oi the properties of a virus, or perhaps of a free-livhig 

 gene. Ihe next step in the development of a living thing is the addition 

 of the ability of the autocatalytic particle to undergo inherited changes 

 -to mutate. Then, if a number of these free genes had joined to form 

 a single larger unit, the resulting organism would have been similar to 

 certain present-day viruses. All the known viruses are parasites that can 

 live only within the cells of higher animals and plants. However, a 

 little reflection will suggest that free-living viruses, ones which do not 

 produce a disease, would be very difficult to detect; such organisms may 

 intleed exist. 



The first living organisms, having arisen in a sea of organic mole- 

 cules and in contact with an atmosphere free of oxygen, presumably 

 obtained energy by the fermentation of certain of these organic sub- 

 stances. These heterotrophs could survive only as long as the supply of 

 organic molecules in the sea broth, accumulated from the past, lasted. 

 Before the supply was exhausted, however, the heterotrophs evolved 

 further and became autotrophs, able to make their own organic mole- 

 cules by chemosynthesis or photosynthesis. One of the by-products of 

 photosynthesis is gaseous oxygen, and it is likely that all the oxygen in 

 the atmosphere was produced and is still produced in this way. It is 

 estimated that all the oxygen of our atmosphere is renewed by photo- 

 synthesis every 2000 years and all the carbon dioxide molecules pass 

 through the photosynthetic process every 300 years. All the oxygen and 

 carbon dioxide in the earth's atmosphere are the products of living or- 

 ganisms and have passed through living organisms over and over again in 

 times past. 



The explanation of how an autotroph may have evolved from one 

 of these primitive, fermenting heterotrophs was presented by N. H. 

 Horowitz in 1945. According to Horowitz' hypothesis, an organism 

 might acquire, by successive mutations, the enzymes needed to synthesize 

 complex from simple substances, in the reverse order to the sequence 

 in which they are used in normal metabolism. Let us suppose that our 

 first primitive heterotroph required organic compound A for its 

 growth. Substance A, and a variety of other organic compounds, B, C, 

 D, etc., were present in the organic sea broth which was the environ- 

 ment of this heterotroph. They had been synthesized previously by the 

 action of nonliving factors of the environment. The heterotroph would 

 survive nicely as long as the supply of compound A lasted. If a mutation 

 occurred which enabled the heterotroph to synthesize substance A from 

 substance B, the strain of heterotroph with this mutation would be 

 able to survive when the supply of substance A was exhausted. A second 

 mutation, which established an enzyme catalyzing a reaction by which 

 substance B could be made from the simpler substance C, would again 

 have great survival value when the supply of B was exhausted. Similar 

 mutations, setting up enzymes enabling the organism to use success- 

 ively simpler substances, D, E, F, . . . and finally some inorganic sub- 

 stance, Z, would eventually result in an organism able to make sub- 

 stance A, which it needs for growth, out of substance Z by way of all 



