Origin of Life, Theories

The ribosome is where the message carried by the mRNA is translated into the amino sequence of a protein. How it occurs is described in the next section. One of its most noteworthy aspects was discovered only recently. It was formerly believed that the RNA part of the ribosome was a structural component and the protein part was the catalyst for protein biosynthesis. Present thinking tilts toward reversing these two functions by ascribing the structural role to the protein and the catalytic one to rRNA. RNAs that catalyze biological processes are called ribozymes. Catalysis by RNA is an important element in origins of life theories as outlined in the accompanying boxed essay RNA World. ... 

The proteinoid theory] has attracted a number of vehement critics, ranging from chemist Stanley Miller. .. to Creationist Duane Gish. On perhaps no other point in origin-of-life theory could we find such harmony between evolutionists and Creationists as in opposing the relevance of the experiments of Sidney Fox.3... 

Today, Oparin s coacervates are not as favoured as Fox s microspheres or Wachtershausers s vesicles, and RNA replicators are preferred to Haldane s viroids, but these differences have not changed the substance of the original opposition. Between the two fundamental functions of life - metabolism and replication - Oparin gave an evolutionary priority to metabolism, while Haldane gave it to replication, and the choice between these two alternatives is still the key point that divides the origin-of-life theories in two contrasting camps. 

Such a distinction is important because it gives us a criterion for a better evaluation of the origin-of-life theories. The solutions proposed by Sidney Fox or Wachtershauser, for example, are exclusively theories of chemical evolution, and tell us nothing about postchemical evolution. It would be wrong to criticise them for this, but it would also be wrong to say that, if they explain chemical evolution, they also explain postchemical evolution and therefore the origin of the cell. 

A prominent origin of life theory suggests that such a process may have provided energy and reducing equivalents for the earliest biochemistry [10,38]. 

Scientists still do not have a satisfactory explanation for how the first self-replicating living organisms appeared on Earth. Origin-of-life theories continue to be an area of active scientific research. 

Hydrogen cyanide has been discussed as a precursor to amino acids and nucleic acids. It is believed by some, for example, that HCN played a part in the origin of life. Although the relationship of these chemical reactions to the origin of life theory remains speculative, studies in this area have led to discoveries of new pathways to organic compounds derived from condensation of HCN. 

Fe-S complexes have important functions in today s living systems, in enzymes such as the ferredoxins and oxidoreductases, as well as in electron transport proteins. It is striking that these redox reactions mainly involve elements and compounds such as CO, H2 and N2, which were probably also components of the primeval Earth s atmosphere. Thus, the assumption of an active involvement of Fe-S clusters in a (hypothetical) Fe-S world in processes which finally led to biogenesis appears completely reasonable We now have a background to the theory of the chemoau-totrophic origin of life . 

Freeman Dyson considers that any theory on the origin of life which begins with cooperative organisation in a large population of molecules, and makes no provision for short circuits in the metabolic pathways, will be met by the criticism just described (Dyson, 1985). 

But what does all this have to do with the origin of life The answer is simple a great deal The second law, with its consequences, has often been used by the opponents of evolution theory and modern biogenesis research as the basis for their own interpretation of the history of creation. According to them, application of the second law of thermodynamics means that order (i.e., life) could never have emerged from disorder without the help of an almighty creator. 

Thus, the approaches, models, hypotheses, theories, experiments and computer simulations presented in the various chapters, as well as the speculations and lively controversies, have not yet led to a clear picture of the origins of life. While the individual research results discussed above provide important contributions, and individual pieces of the jigsaw puzzle, vital parts of the complete picture are still missing the process of life s emergence from inanimate precursors can still not be described satisfactorily. 

All the models, hypotheses and theories on the origins of life are based on two assumptions ... 

There is no one correct theory for the origin of life on Earth or any habitable planet, although many have been presented. The current set of ideas is summarised in Figure 1.5. Aside from the theory of creation, which seems particularly hard to test, the testable theories of the origins of life divide into two extraterrestrial or panspermia, the theory that life was seeded everywhere somewhat randomly and terrestrial, that life originated de novo on Earth or other habitable planets around other stars. The theories of terrestrial origin are more favoured but the recent discovery of habitable planets and life within any solar system suddenly makes panspermia more likely. 

Figure 1.5 Theories of the origins of life. (Reproduced from Davis and McKay 1966 by courtesy...

Courtina R. and Kimb S. J. (2002). Mapping of Titan s tropopause and surface temperatures from Voyager IRIS spectra, Planetary and Space Science 50 309-321. Davis W. L. and McKay C. P. (1996). Origins of Life a comparison of theories and applications to Mars. Origins of Life and Evolution of the Biosphere 26 61-73. 

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