Polymers, Evolution, and the Origin of Life

The origin of life probably occurred in three phases (fig. 1.23) (1) The earliest phase was a period of chemical evolution during which the compounds needed for the nu-cleation of life must have been formed. These compounds include the most important class of biological macromolecules, the nucleic acids. In this phase of evolution, the synthesis of nucleic acids was noninstructed. (2) As soon as some nucleic acids were present, physical forces between them must have led to an instructed synthesis, in which the already formed molecules served as templates for the synthesis of new polymers. It seems likely that feedback loops selected out certain nucleic acids for preferential synthesis. At some point during this period of instructed synthesis more nucleic acids and possibly protein macromolecules were formed. The products of this phase of mo-... 

A large number of successful experimental studies which tried to work out plausible chemical scenarios for the origin of life have been conducted in the past (Mason, 1991). A sketch of a possible sequence of events in prebiotic evolution is shown in Figure 3. Most of the building blocks of present day biomolecules are available from different prebiotic sources, from extraterrestrial origins as well as from processes taking place in the primordial atmosphere or near hot vents in deep oceans. Condensation reactions and polymerization reactions formed non-in-structed polymers, for example random oligopeptides of the protenoid type (Fox... 

Whereas most of the work discussed so far in this essay has dealt with the synthesis of well-defined biochemical species supporting the theory of chemical evolution as first proposed by A. I. Oparin, one of Oparin s major concerns has been to develop a hypothesis of precellular evolution and to experimentally demonstrate that specific biochemical reactions can occur within simulated precellular entities (coacervates). In an elegant experiment, using polynucleotide phosphorylase in coacervate droplets and the appropriate substrate in the external medium, he showed a continuous uptake of the substrate, a rapid internal synthesis of polynucleotides and a continuous release of phosphate to the external environment. His more recent concepts on evolution of probionts and the origin of cells were presented at the 4th International Conference on the Origin of Life held in Barcelona, Spain, in 1973. Experimental models involving microspheres made of polymers of amino acids have been developed by S. W. Fox and coworkers > and other investigators. 

I. S. Kulaev and K. G. Skryabin (1971). Reactions of abiogenic transphosphorylation involving high-polymer polyphosphates and their role in the phosphorus metabolism evolution. In Abstracts of the Symposium on Origin of Life and Evolutionary Biochemisty, Varna, Bulgaria, p. 22. 

The class of polyamino acids includes both the thermal polymers and contemporary proteins (enzymes). The basis for this inclusive class is that both types of the polymer contain a variety of chemically functional groups. Because of their simultaneous occurrence in the same macromolecules, these functional groups provide a yet larger array of chemical reactivity. The association within a macromolecule provides a degree of fixed three-dimensional relationship, with some flexibility within this constraint. One of the crucial requirements of life, its origin, and evolution may well be, accordingly, the chemical polyfunctionality of polymers of amino acids. Selection and specialization was able to proceed from polymers composed of approximately 20 kinds of monomer. Special evolutionary advantage of catalytic polymers would result... 

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