Primitive atmospheric conditions

There is only limited agreement on whether primitive atmospheric conditions were really reductive. However this is not the key point. The key point is the fact that relatively complex biochemicals can be formed from a mixture of very simple gaseous components in a chemical pathway that can indeed be regarded as prebiotic. 

FIGURE 1-33 Abiotic production of biomolecules. Spark-discharge apparatus of the type used by Miller and Urey in experiments demonstrating abiotic formation of organic compounds under primitive atmospheric conditions. After subjection of the gaseous contents of the system to electrical sparks, products were collected by condensation. Biomolecules such as amino acids were among the products. 

In order to estimate the abundance of hydrogen in the primitive atmosphere, we assume steady-state conditions and set the escape flux equal to that resulting from thermal degassing ... 

Figure 12-2 shows the course of hydrogen partial pressure and mixing ratio in the primitive atmosphere during the first 3 billion yr after Earth s formation. The critical parameter is Kz. We have here adopted the value appropriate for present conditions at the homopause, Kz = 102 m2/s, which... 

Fig. 12-2. Partial pressure (in bar, solid line) and mixing ratio (dashed line) of hydrogen in the primitive atmosphere on Earth according to Eqs. (12-7) and (12-8). K2 = 102m2/s, H = 9 km, A = 3 x KT4. The eddy diffusion coefficient corresponds to present conditions at the homopause. If K2 in the primitive atmosphere had been smaller by a certain factor, the hydrogen partial pressure would have been greater by the same factor and vice versa.

Prebiotic peptides, peptides formed before the origin of life. Most likely, amino acids were already present on primitive Earth. They are supposed to have been produced in the primitive atmosphere, in hydrothermal vents, or to have been imported in meterorites. a-Amino acids can undergo peptide formation by activation with carbon monoxide under hot aqueous conditions in the presence of freshly co-precipitated colloidal (Fe,Ni)S. Peptides may have been formed via —>-N-carboxy anhydrides. A replicative synthesis involving aminoacyl-RNA intermediates has also been suggested. The question of whether a peptide/protein world preceded the RNA-driven template synthesis, or whether RNA and proteins should not be viewed as eti-ologically discrete entities in the origin of life, is still under debate [V. Borsenberger et al., Chem. Biodivers. 2004, 1, 203 C. Huber et al., Science 2003, 301, 938 A. Brack, Chem. Biodivers. 2007, 4, 665]. 

The similarities in products and pathways between interstellar molecules and terrestrial laboratory experiments imply a unity of physical and chemical laws in the universe. Given certain conditions and appropriate energy sources, the same chemical pathways will be followed to create certain products from the elements. That is not to say that life, even in primitive form, could be supported in interstellar space. The significant precursor molecules found in interstellar space are at extremely low concentrations, but if they were transported to planetary atmospheres, perhaps by comets, they might then react in the proper environment and evolve into self-replicating systems. 

For these reasons, it is generally felt that the interstellar molecules played at most a minor role in the origin of life. However, the presence of so many molecules of prebiotic importance in interstellar space, combined with the fact that their synthesis must differ from that on the primitive earth where the conditions were very different, indicates that some molecules are particularly easily synthesized when radicals and ions recombine. Another way of saying this is that there appears to be a universal organic chemistry, which shows up in interstellar space, in the atmospheres of the major planets, and in the reducing atmosphere of the primitive earth. 

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