860 MILLER [chap. 32 



4. The Origin of Life 



In order to consider the problem of the origin of hfe, it is necessary to define 

 life. There appears to be no general agreement on how to define life, and it has 

 been asserted that life is incapable of definition. While it may be difficult to 

 find a single definition that will cover all aspects of life, we will define life as 

 follows: A living organism is an entity that is both (a) capable of making a 

 reasonably accurate reproduction of itself, with the copy being able to accom- 

 plish the same task; and (b) is subject to a low rate of alterations (mutations) 

 that are transmitted to its progeny. 



Using this definition, one would say that the development of polynucleotides 

 and associated enzymes capable of self-duplication would be the origin of life. 

 The enzymes, polynucleotides and other factors required for this process might 

 require a membrane to hold them together, or they might be held together on 

 mineral surfaces. 



Romberg's enzyme appears to be able to duplicate the DNA used as a primer 

 for the reaction (Romberg, 1960). This enzyme is synthesized by bacteria, but 

 if this system could be synthesized outside an organism and held together under 

 conditions that might have been present on the primitive Earth, this would 

 probably be considered the synthesis of life. 



Oparin does not view the first organism as a polynucleotide capable of self- 

 duplication, but rather as a coacervate colloid which accumulates proteins and 

 other compounds from the environment, grows in size and then sj^lits into two 

 or more fragments which repeat the process. The coacervate would presumably 

 develop the ability to split into fragments which were similar in composition 

 and structure, and eventually a genetic apparatus would be incorporated which 

 could make very accurate duplicates. 



These two hypotheses for the stef)s in the formation of the first living organism 

 differ mainly in whether the duplication first involved the relatively accurate 

 duplication of nucleic acids followed by the development of cytoplasm duplica- 

 tion, or in the reverse order. Other sequences could easily be enumerated, but 

 it seems clear that any significant hypothesis on this process requires a much 

 greater knowledge of the preliminary steps. 



5. Evolution of Early Organisms 



While little can be said about the development of the first living organisms, 

 reasonable hypotheses can be made for the early evolution of the most primitive 

 organisms. The theory that the primitive oceans contained large quantities of 

 organic compounds implies that the first organisms were heterotrophic. Hetero- 

 trophic organisms do not synthesize their basic constituents such as amino 

 acids, nucleotides, carbohydrates, vitamins, etc., but obtain them from the 

 environment. Autotrophic organisms synthesize all their cell constituents from 

 CO 2, H2O and other inorganic materials. Heterotrophic organisms are simpler 

 than autotrophic organisms in that they contain fewer enzymes and specialized 



