There is an alternative theory for the chemical origin of life. According to this view, the first proto-cells were not heterotrophic scavengers of organic molecules but were autotrophic and fixed carbon dioxide into organic matter themselves. An autotroph is defined as any organism that uses an inorganic source of carbon and makes its own organic matter as opposed to a heterotroph, which uses pre-made organic matter. The most familiar autotrophs are plants that use energy from sunlight to convert carbon dioxide into sugar derivatives. However, a variety of bacteria exist that fix carbon dioxide without light but instead rely on other sources of energy. Furthermore, the pathways of carbon dioxide fixation vary. In particular, some autotrophic bacteria incorporate carbon dioxide into carboxylic acids rather than generating sugar derivatives like plants.
The autotrophic theory of the origin of life postulates the chemical oxidation of readily available iron compounds as the primeval energy source. In particular, the conversion of ferrous sulfide (FeS) to pyrite (FeS2) by hydrogen sulfide (H2S) releases energy and provides H atoms to reduce carbon dioxide to organic matter. [Anaerobic bacteria are found today that generate energy by the oxidation of iron Fe2+ compounds to Fe3+, as well as others that generate energy by oxidizing sulfur compounds. Thus a primeval metabolism based on iron and sulfur seems reasonable.]
Evolution of DNA, RNA and Protein Sequences 545
Organic matter can be generated from simple gas molecules using metallic catalysts.
Several possible schemes have been suggested for the first carbon dioxide fixation reactions. One scheme involves iron catalyzed insertion of CO2 into sulfur derivatives of those carboxylic acids still found today as metabolic intermediates (e.g., acetic acid, pyruvic acid, succinic acid, etc.). These early reactions would have occurred on the surface of iron sulfide minerals buried underground, rather than in a primeval soup. This leaves open the question of where such organic acids came from originally. One possibility is that they resulted from a Miller type synthesis, as described above. More radical is the suggestion that the first organic molecules were derived directly from carbon monoxide plus hydrogen sulfide. It has been demonstrated that a mixed FeS/NiS catalyst can convert carbon monoxide (CO) plus methane thiol (CH3SH) into a thioester (CH3-CO-SCH3), which then hydrolyses into acetic acid. Inclusion of catalytic amounts of selenium allows conversion of CO plus H2S alone to CH3SH (and then to the thioester and acetic acid).
Recently it was shown that carbon monoxide (CO) activated by the same mixed FeS/NiS catalyst can also drive the formation of peptide bonds between alpha-amino acids in hot aqueous solution. Not surprisingly, this system will also hydrolyze polypeptides.
Due to mutation, the sequences of DNA and the encoded proteins will gradually change over long periods of time.
Related organisms contain genes and proteins with related sequences. These may be used to construct evolutionary trees.
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