Earths hypothetical

All the models of the chemical composition of the atmosphere of primeval Earth are hypothetical. Samples from this period of development of the Earth are not available And the oldest rocks give us only a limited amount of information. 

The term chemical evolution" was introduced by the Nobel Prize winner Melvin Calvin and refers to the process of the synthesis of biochemically important molecules from small molecules and certain chemical elements under the (hypothetical) conditions present on prebiotic Earth. It is assumed that the smaller building block molecules such as amino acids, fatty acids or nucleobases were formed initially, and that these underwent polycondensation to give macromolecules in later stages of development. 

Shapiro published a critical analysis of the availability of the nucleobase cytosine on the primeval Earth in the highly-regarded Proceedings of the National Academy of Science (Shapiro, 1999). Some biogeneticists still believe that all the substances necessary for the synthesis of a nucleic acid were available in the much-cited (but hypothetical) primeval soup . Shapiro directs these optimists to the following problems ... 

The synthesis of dipeptides under the conditions found on primeval Earth appears highly likely. The discovery that these small molecules can act as catalysts makes it possible to discuss their being involved in basic synthetic reactions occurring in an (as yet hypothetical) RNA world (Weber and Pizzarello, 2006). 

The still hypothetical RNA world must fill the gap. Orgel suspects that, in this still dark phase of chemical and molecular evolution, a class of molecules was present which could have been relatively easily formed under the conditions present on the young Earth, and which was found in rocks from meteorites perhaps amino acids. These need not to have been the 20 proteinogenic amino acids which we know today, and which are in part quite complex, such as phenylalanine, histidine and tryptophane simple examples would have sufficed. 

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 . 

A detailed theoretical study of the properties of the redox system FeS/FeS2 was carried out in the Department of Geosciences of SUNY Stony Brook (Schoonen et al., 1999). The authors conclude that the hypothetical reduction of CO2 (by the FeS/FeS2 redox pair) formulated in Wachtershauser s early work, and the carbon fixation cycle on the primeval Earth associated with it, probably could not have occurred. This judgement is made on the basis of a theoretical analysis of thermodynamic data other conditions would naturally have been involved if CO had reacted rather than C02. It is not known whether free CO existed in the hydrosphere, or if so, at what concentrations. 

The hypercycle models developed later by Eigen were much more complex. Since both protein enzymes and nucleic acids contribute to hypercycles, the latter could only have come into operation at a later stage of the (hypothetical) RNA world. It seems possible that the protein enzymes on the primeval Earth could have been replaced by ribozymes. 

UV radiation hypothetical, but so is the transport of molecules from outer space to Earth. Recent analyses of the Murchison meteorite by two scientists from the University of Arizona, Tucson (Cronin and Pizzarello, 1997 Cronin, 1998) have shown it to contain the four stereoisomeric amino acids DL-a-methylisoleucine and DL-a-methylalloisoleucine. In both cases, the L-enantiomer is present in a clear excess (7.0 and 9.1%). Similar results were obtained for two other a-methyl amino acids, isovaline and a-methylvaline. Contamination by terrestrial proteins can be ruled out, since these amino acids are either not found in nature or are present in only very small amounts. Since the carbonaceous chondrites are thought to have been formed around 4.5 billion years ago (see Sect. 3.3.2), the amino acids referred to above must have been subject to one or more asymmetric effects prior to biogenesis. 

With reference to the R (rare earth) metals, the existence of various hypothetical polymorphs, that are now known not to exist in high purity R metals, was discussed at length in the past and is now only of historical interest (see a summary in Gschneidner 1993). 

In 1717 Louis Lemery stated that saltpeter was usually obtained from tile earth and refuse piles near old lime-plastered walls and in stables and churchyards. To explain its origin, John Mayow postulated the existence of a hypothetical saltpeter m the atmosphere. When Mariotte exposed to the air of an upper room some saltpeter earth (earth from which all the saltpeter had previously been leached out), however, he was unable to prepare even a gram of saltpeter. When he placed the same earth in the cellar, it soon became covered with salt-petei. Lemery placed three earthen vessels containing respectively lime, potassium carbonate, and leached saltpeter earth on pedestals, and exposed them to the moist air of a dark cellar whose walls and floor were covered widi saltpeter. Even after two years, however, he found not a trace of saltpeter in any of the three vessels. By frequently moistening the contents with animal substances, however, he soon prepared a considerable quantity of it (42). 

Figure 8.2 Logarithmic spiral with superimposed mean planetary orbits. The circles in blue define the orbits of inner planets on a larger (self-similarj scale. The divergence angle of 108° causes those planets at angles of 5 x 108° apart to lie on opposite sides of the spiral origin. These pairs are Neptune-Mars, Uranus-Earth, Saturn-Venus and Jupiter-Mercury. The hypothetical antipode of the asteroid belt, a second, unobserved group of unagglomerated fragments, has been swallowed up by the sun...

An analogous series of experiments has shown that unnatural amino acids function in proteins as well as the standard 20 amino acids found globally in terran proteins. The natural ribosome found globally on Earth was able to incorporate these into proteins. In that way, synthetic biologists have expanded the amino acid repertoire of proteins.13 The experiments showed no reason to exclude alternative sets of amino acids from hypothetical proteins in hypothetical alien life forms. 

Those facts mean that alternative nucleotides and amino acids could have been accessible to terran life. The observation that terran life did not exploit them can be explained in various ways. It may be that some terran life forms do exploit alternative nucleotides or amino acids but have not yet been discovered indeed, hypothetical terran life that uses alternative nucleotides or amino acids would not necessarily be detected by probes designed to look for terran life. It is conceivable that some terran life forms did use alternative nucleotides or amino acids but were less fit than life forms that used the standard set and thus did not survive in Darwinian competition for us to discover them. Alternatively, we might assume that chemical processes needed to make the extra nucleotides or amino acids did not happen in the 4 billion years of life on Earth. 

The atmospheres of these planets convect, of course. To survive on Jupiter, any hypothetical life based on molecules containing carbon-carbon covalent bonds would have to avoid being moved by convection to positions in the atmosphere where they are not stable. This is, of course, not impossible. Even on Earth, life in the oceans must avoid being moved by convection from its particular habitable zone. Sagan and Salpeter presented a detailed discussion of what might be necessary for a floater to remain stable in the jovian atmosphere.25... 

Figure 10.5 Hypothetical scheme for the formation and reactions of high-molecular-weight PolyPs, pyrophosphate and ATP at different stages of appearance of life on Earth (Kulaev and Skryabin, 1974).

Stable phases in the rare earth oxide systems are tabulated and discussed. New data on the structure of sesquioxides quenched from the melt are reported. The structural interrelations between the A, B, and C type sesquioxides and the fiuorite dioxides are pointed out. The sequences of several intermediate oxides in the CeO, PrO., and TbO, systems are observed to be related to the fluorite structure and the C form sesquioxide with respect to the metal atom positions. A hypothetical homologous series of the general formula Mn02n i, related to the fluorite structure and the A form sesquioxide with a more or less fixed oxygen lattice, is suggested. 

Gaia A hypothetical superorganism composed of the Earth s four spheres the biosphere, hydrosphere, lithosphere, and atmosphere. 

Fig. 4. Calculated temperature at the earth s surface (T.) and in three atmospheric layers (Ti, Tj and T3) as function of the atmospheric smoke content. A relative smoke content of unity corresponds to 1.2 g of smoke per m surface area. Two sets of calculations are presented for hypothetical surface albedos of 30% and 50%.

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