Evolution protocell formation

Szostak et al. worked on the basis of a simple cellular system which can replicate itself autonomously and which is subject to Darwinian evolution. This simple protocell consists of an RNA replicase, which replicates in a self-replicating vesicle. If this system can take up small molecules from its environment (a type of feeding ), i.e., precursors which are required for membrane construction and RNA synthesis, the protocells will grow and divide. The result should be the formation of improved replicases. Improved chances of survival are only likely if a sequence, coded by RNA, leads to better growth or replication of membrane components, e.g., by means of a ribozyme which catalyses the synthesis of amphiphilic lipids (Figs. 10.8 and 10.9). We can expect further important advances in the near future from this combination ( RNA + lipid world ). 

Without membranes there are no protocells. When discussing their role in early evolution, one should tackle the following issues formation of mebra-nogenic molecules, membrane growth and inheritance, microsphere division, and membrane permeability. 

Chemical evolution concerns the chemical processes that occurred on the ancient Earth about 4.5-3.5 million years ago. It is important to note that it preceded biological evolution that resulted in the formation of protocells. These forerunners of today s living cells were capable of self-reproduction at the expense of some protometabolism. After Oparin s ideas of the origin of life became widely known and especially after Stanley Miller reported his prebiotic soup experiments in 1953, the concept of chemical evolution became accepted. 

The formation of the encapsulating membranes is discussed by Turk-MacLeod et al. the operative strictures of thermodynamics in these processes and in the functional role of cell membranes are elaborated. The competition between vesicles that encapsulate RNA and those incapable of doing so, considered as model protocells, and its relation to the evolutionary fitness of replicator functions, is considered at length in terms of the driving forces of thermodynamics. It is noted that membrane stabilization is a key objective in this competition but this results also in a reduction of permeability, thus diminishing the ability of the protocell to use nutrients. Further evolution of the membrane and its constituents is necessary to overcome this restriction in function. In this respect it is of interest that model protocell membranes composed of particular mixtures of amphiphiles have superior... 

Fig. 19. A scheme for the logical sequence of steps in prebiotic evolution leading from unorganized macromolecules to protocells. The stream on the left-hand side of the diagram represents the course of evolution instructed by polynucleotides. At the beginning instruction is limited to polynucleotide replication. Later on polynucleotides brought polypeptides and membranes under their control through the development of mechanisms for translation and instructed compartment formation or cell division.

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