Hydrothermal biogenesis

Cairns-Smith, as the leading proponent of the mineral theory, has also shown interest in both the hydrothermal biogenesis theory (Cairns-Smith, 1992) and the iron-sulphur hypothesis proposed by G. Wachtershauser (see Sect. 7.3). 

The discovery of hydrothermal vents on the ocean floor has led some biogenesis researchers to turn their attention to the hydrosphere (see Sect. 7.2) and to the processes occurring there at a depth of 2-3 km. 

The same problem, the stability of the nucleobases, was taken up by Levi and Miller (1998). They wanted to show that a synthesis of these compounds at high temperatures is unrealistic, and thus they took a critical look at the high temperature biogenesis theories, such as the formation of biomolecules at hydrothermal vents (see Sect. 7.2). The half-life of adenine and guanine at 373 K is about a year, that of uracil about 12 years and of the labile cytosine only 19 days. Such temperatures could have easily been reached when planetoids impacted the primeval ocean. 

These four arguments have led scientists from various disciplines to look more closely at the theory of a possible biogenesis in hydrothermal systems in the deep sea. 

Holm and Andersson have provided an up-to-date survey of simulation experiments on the synthesis under hydrothermal conditions of molecules important for biogenesis (Holm and Andersson, 2005). It is clear that several research groups have been able to show in the meantime, using simulation experiments, that the conditions present at deep sea vents appear suitable for the synthesis of very different groups of substances. However, it remains unclear how these compounds could have been stabilized and protected against rapid decomposition. At present, metal ions (as complexing agents) and mineral surfaces are the subject of discussion and experiment. 

The first indication of a possible connection between geological processes occurring at the boundaries between tectonic plates of the mid-oceanic ridges and the biogenesis problem was provided by J. B. Corliss (1981). He considered the hydrothermal conditions to be ideal reactors for abiotic synthesis these ideal conditions were the water temperature gradients, the pH, and the concentrations of solutes in the hot springs. The presence of certain minerals which could act as catalysts, such as montmorillonite, clay minerals, iron oxide, sulphides etc., was also very important. The initial model presented for the hydrothermal synthesis of biomolecules (Corliss, 1981) was modified, particularly by Russell (1989) and Wachtershauser (see Sect. 7.3). 

Miller and Bada (1988) and Miller et al. (1989) attempted to answer the question as to whether biogenesis processes can occur at hydrothermal vents in regions of the deep sea. Miller states clearly that biomolecules could not have been formed under such conditions. Holm (1992) sums up Miller s arguments in the following four points ... 

In the same year, Miller and the biologist Antonio Lazcano (National Autonomous University of Mexico) spoke out against hypotheses that life could have originated at hydrothermal vents. They believe that the presence of thermophilic bacteria (the oldest life forms) does not prove that biogenesis occurred in the depths of the oceans. Stanley Miller sees a greater chance for successful pre-biotic chemistry under the conditions of a cold primeval Earth rather than at high temperatures in hydrothermal regions (Miller and Lazcano, 1995). 

Further experiments by Huber and Wachtershauser on chemoautotrophic biogenesis under hydrothermal conditions have shown that a number of a-amino acids and a-hydroxyacids could have been formed, subsequent to the binding of carbon (in the form of CO and CN ) to catalytically active transition metal precipitates. The general structure of such compounds is R-CHA-COOH, with R = H, CH3, C2H5 or HOCH2 and A = OH or NH2. 

The discovery of the deep sea hydrothermal systems, and the sulphur-metabolising bacteria which live in them, caused some researchers to look more closely at the element sulphur. It seemed obvious to consider a link between sulphur bacteria— primitive life forms—and the emergence of the simplest forms of life, de Duve, 1974 Nobel Prize winner for medicine, joined the ranks of the biogenesis researchers in the 1980s. 

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