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system without interfering with each other. Also, can relevant 

 reactions carried out with optically pure starting materials be done 

 with racemic starting materials? The study of pure systems is cer- 

 tainly easier, but the study of mixed systems should be encouraged 

 due to the important unanswered questions in this area. These 

 questions are among the more important ones facing workers in this 

 field at the present time, since they bear on the order of events in 

 the origin of life; e.g., whether polypeptides or polynucleotides 

 appeared first, as well as whether laboratory experiments are relevant 

 to more complex situations on the prebiotic Earth. 



MODELS FOR EARLY LIFE FORMS 



Since we define life in terms of its genetic properties, and since 

 the only known system possessing these properties is the protein- 

 nucleic acid system, the most easily defended position holds that the 

 first living things were based on this system. However, the sponta- 

 neous origin of such a complex mechanism poses great conceptual 

 difficulties. Therefore, other possibilities should be considered. An 

 important constraint is that the original self-replicating system, 

 whatever it may have been, must have had the capability of evolving 

 into the protein-nucleic acid system. Possibilities that are worth 

 exporing include: (1) polynucleotides with some catalytic capability 

 and (2) polypeptides with some replicative capability. Obviously, 

 these systems would be extremely inefficient in comparison with the 

 highly evolved modern mechanism, but they might have been suffi- 

 ciently accurate to survive and evolve under the benign conditions of 

 the primordial Earth. Either one might have been capable of develop- 

 ing into the modern cell. 



The question of whether polynucleotides alone can constitute a 

 self-replicating system can in part be answered by the development 

 of experimental models for nonenzymatic replication. Some pro- 

 gress has been made in understanding how preformed polynucleotide 

 chains can function as templates to direct the synthesis of their 

 complements in nonenzymatic reactions. However, much remains to 

 be learned about the catalytic effect of peptides, metal ions, etc. 



