Exobiology 



diameter) cometary and asteroidal objects. These studies should also reveal the 

 nature and intensity of the fluctuations in Earth's environments caused by such 

 impacts, which may have had a bearing on the ecological niches available over 

 time for the origin and early evolution of life. 



The emergence of models of a prebiological environment rich in carbon dioxide 

 and poor in organic compounds raises the possibility that the first organisms may 

 not have been limited to those depending solely on heterotrophic metabolism, the 

 use of preformed organic compounds for energy and for cellular biosynthesis. 

 Permissive evidence of anaerobic photosynthetic microorganisms dates back to the 

 earliest known fossil record, and sulfide oxidation is a capability that appears 

 among prokaryotic organisms with the most ancient phylogenetic lineage. These 

 considerations suggest that the nature of the earliest bioenergetic and biosynthetic 

 pathways remains an open issue, and both autotrophic and heterotrophic organ- 

 isms could have coexisted in Earth's earliest biosphere. 



Although many studies have been directed toward the synthesis of monomers and 

 oligomers of amino acids and nucleic acids under putative prebiological conditions, 

 relatively few investigations have been carried out to understand how the 

 metabolic function itself arose in the prebiological environment. Research on 

 chemical models of metabolic systems should be intensified, and efforts should be 

 made to develop photo- or chemo-autotiophic systems. The roles of peptides, 

 minerals, and membrane-forming organic compounds in these models should be 

 investigated. The photochemical or geochemical oxidation-reduction reactions that 

 provide the energy sources in these models should be consistent with environ- 

 mental constraints. 



Understanding how a self-replicating system with a genetic code arose on Earth is 

 arguably the central problem in the origin of life (3). The theory that nucleic acids 

 were the first replicating systems has gained considerable strength from the 

 revolutionary discovery that ribonucleic acids (RNA's) are capable of splitting and 

 joining oligonucleotides. In principle, primitive RNA's could have been capable of 

 catalyzing rudimentary metabolic reactions as well as replication. Recent advances 

 in RNA technology make it possible to synthesize sequence-specific RNA's for the 

 purpose of assessing this possibility. Studies on the reactions and catalytic 

 properties of RNA's and RNA-like compounds aimed at development models for 

 molecular self-replication should be intensified; they should include assessments of 

 the limitations placed on these systems by environmental conditions consistent 

 with early Earth models. 



Clay minerals have also been proposed as the first replicating systems, but this 

 alternative has received little experimental study. Criteria need to be established to 

 distinguish replicating clays from nonreplicating systems, and laboratory investi- 

 gations of clay syntheses at low temperatures and the role of organic chelating 

 agents in the syntheses should be initiated to test the clay theory. 



The complex contemporary apparatus for translation of the genetic information 

 stored in nucleic acids into protein biosynthesis must have had its beginnings in 



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