baudrunner's space: Two theories of life's origins
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Monday, October 20, 2008

Two theories of life's origins

Two distinct theoretical approaches have emerged in the quest for explanations concerning the origins of life. One school of thought subscribes to the idea that metabolism predated genetics. The main proponent of this theory is Dr. Günter Wächtershäuser, a chemist by training, and now a patent lawyer in Munich.

anaerobic methanogens - archaea

Dr. Wächtershäuser thinks that life on Earth had hydrothermal origins, - i.e., probably near deep hydrothermal vents on the ocean floor. Metabolism by this reckoning involves the evolution of progressively complex chemical reactions ultimately yielding life out of primitive energy exchange mechanisms in a cyclic process. This process of evolutionary development has come to be known as the iron-sulfur world theory.

The iron-sulfur recipe described by Wächtershäuser can be summed up by the following: bring water to a boil and stir in iron sulfide and nickel sulfide. Mix in carbon monoxide and hydrogen sulfide gas, and then wait for the amino acids to convert into peptides.

In 1997, Wächtershäuser and Claudia Huber actually performed the experiments and successfully produced peptides by mixing carbon monoxide, hydrogen sulfide, nickel sulfide, and iron sulfide particles at 100°C. In 2006 the two collaborators produced glycine, alanine and serine from a similar mixture. These are simple amino acids found in proteins. Potassium cyanide was their source of nitrogen.

A clear description of a proto-ecological system exists, from the establishment of the catalytic environment consisting of micro scale caverns coated by thin membranous metal sulfide walls up to the synthesis of the lipid membrane which finalizes the creation of a true living cellular organism capable of escaping the confines of the alkaline hydrothermal vent environment. The condition of the “life form” up to the evolution of the lipid cell walls represents the “Last Universal Common Ancestor” (LUCA).

RNA world hypothesis is the title given to the other explanation for how life began. In this scenario, RNA predates the emergence of DNA-based life – i.e., genetics before metabolism. The logic in this theory lies in the fact that RNA can act as both a storage mechanism and a catalyst. The thinking is that ribosomal RNA, which catalyzes protein production, is the evolutionary remnant of the RNA world.

Some adherents to the theory suggest that pre-RNA nucleic acids, which manifest in the pre-biotic environment, were the first types of nucleic acids to emerge as a self-reproducing molecule. Among the candidates are peptide nucleic acid (PNA), threose nucleic acid (TNA), and glycerol nucleic acid (GNA), all of which are similar to RNA but for their backbones, which are not composed of the ribose sugars that comprise RNA. For example, TNA is composed of repeating threose units linked by phosphodiester bonds. These molecules do not occur in the natural conditions of today's Earth.

What encourages RNA world hypotheses is the fact that short RNA molecules with self-replicating properties have been synthesized in the lab. What discourages the theory is that stable pre-biotic conditions would have to exist which encourage the proliferation of self-reproducing RNA-like analogs. This includes the ability of these molecules to have been capable of independent life in an environment dense with sugar-phosphate molecules.

The iron-sulfur world theory is very compelling. It is likely that both processes occurred in the earliest stages of the emergence of living organisms, with the RNA world only a phase predated by the iron-sulfur world.

That is not to say that other conditions cannot bring about the beginnings of primitive life forms. In an earlier post, I noted Carl Sagan's description of a prebiotic Earth, and that they were very much the same as those which currently exist on the moon Titan, around the planet Saturn. It wouldn't surprise me in the least if some type of methanogen were periodically struggling to emerge there.

Whichever of the two theories of life's origins holds sway, the fact remains that they are both integral to a theory of origins based on anthropic reasoning, which yet remains uncomfortably lodged in the realm of philosophy more so than in true science. However, one can't deny that life is determined to emerge, however it comes about.

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