Astrobiological Questions

We do not know as yet how life came to be. On the other hand, we have learned a great deal of the chemical processes that followed and preceded the origin of life and we know that both were evolutionary processes. We have learned from paleontological records and comparative studies of macromolecules that life started much simpler, evolved through the ages and is still evolving today. We have also come to know that the biogenic elements have a long cosmic history of chemical evolution, throughout which they were often present in molecular forms sometimes quite complex. It is reasonable, therefore, to ask whether these evolutionary processes were ever linked. Had this been the case, the meteorites would have been the last recipients of cosmic chemical evolution prior to the onset of life and are most suitable to answer the many questions we may have about its "chemical" origin.

What does the chemistry of meteorites tell us? As it can be easily seen by comparing the data in Tables 6.1-6.3 with even elementary biochemistry, there seems to be a basic difference between the distributions of organic compounds in meteorite and the biosphere. Meteoritic compounds are characterized by large diversity (all isomeric compounds often being present) and appear to be products of random syntheses; biomolecules, instead, are the result of strict compositional selection (e.g., as mentioned, only twenty amino acids make up the whole of terrestrial proteins) and display functional specificity. It is hard to propose that such a varied "soup" of organics could have had any advantage in prebiotic chemical evolution without "some" aid or induction toward molecular selectivity.

The possibility of forming cell-like enclosures may lead to containment ofuseful molecules and is the basis for several hypotheses of molecular evolution.6 Quite possibly, however, the finding of ee in meteoritic amino acids represents the most desirable prebiotic trait ever found in any sample of abiotic materials, be it extraterrestrial or the product of laboratory syntheses. This is because chiral homogeneity of biopolymers is an essential property of extant life (Chapter 7) and it is sensible to suggest that its development was important to the origin and/or evolution of life. The finding of ee is far from definitive, however, and itself leads to several questions: what is the ee scope in meteorites ? How were they formed ? How could the delivery of asymmetric molecules have helped in molecular evolution?

All are the basis of current astrobiological investigations and the subject ofsuggested reading.7 The first two questions have been addressed in part in the previous section. As for the third, several points have been made on the possible advantage of the nonrace-mic compounds in meteorites regarding the molecular evolution pathways on early Earth.

1. The delivery of extraterrestrial organic material by meteorites, micrometeorites and cometary fragments is abundant today (107 kg carbon year-1) and was even larger during the period of early Earth bombardment; it is expected that a significant amount ofthese compounds could reach suitable Earth environments.

2. The ee-carrying amino acids of meteorites do not racemize in water (Chapter 7) and one ofthe difficulties in foreseeing the development of chiral homogeneity in prebiotic scenarios is the ease with which some essential molecules, such as protein amino acids and sugars, racemize in water. Thus these meteoritic compounds would have had an advantage in the early phases of molecular evolution by preserving their chiral asymmetry.

3. Amino acids are known catalysts and have been shown to transfer their asymmetry to sugars during syntheses from simple precursor molecules; 8this effect is suggestive of possible prebiotic pathways toward the dissemination and amplification of chirality.

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