Discussion
The hypothesized process can explain features associated with RNA, homochirality [13] and heredity. Homochirality was established during the nucleotide formation and the polymerization. Canonical subunits and other subunits involved in formation of proto-nucleotides. These were formed and dissolved many times and the canonical nucleotides eventually dominated. During the polymerization, if remaining non-canonical nucleotides were attached, the growth stopped, therefore long polymers were exclusively made from the canonical nucleotides, thus homochirality was established. In other words, the homochirality progressively started in the nucleotide formation and completed in the polymerization by crowding out less advantaged nucleotides and polymers. Since the polymerization proceeded by the self-replication, heredity was naturally established. Double strands created by the self-replication was inherently more stable than the corresponding single strand because the hydrogen bonding contributed for stability, thus extending the length of the polymers.
With the advance of RNA molecule replication, evolution became possible due to occasional copying mistakes. Various variants were formed from the original molecules and some of them were turned out be more stable and gradually dominated. This was the molecular level Darwinian evolution where the most fit molecule survived. A significant change was replacement of uracil (U) with thymine (T) that formed DNA which was much more stable than RNA and further advanced the evolution.
The valid clay minerals for the formation of nucleotides from the subunits have not been found yet. Selective adsorption of prebiotic molecules in oceans depends on atomic structure and composition of the minerals. For RNA nucleotides, montmorillonite seems a good candidate [14]. It has been shown to catalyze the formation of RNA polymers. Experimental investigations on montmorillonite and similar minerals may be the first step to identify right clay minerals for the formation of the nucleotides.
The formation of nucleotides from the subunits is a condensation process where two water molecules are released on the reaction. The self-replication reaction starts with bonding of a complimentary pair base, and then D-ribose is joined to the base to form a nucleoside. Next, a phosphate is joined to the nucleoside. Bonding is possible due to activation energy modification by the catalytic minerals. After the complemental nucleotide is replicated, the two nucleotides undergo separation. The breaking up at the site of the hydrogen bond requires energy input that exceeds the energy of hydrogen bond which is about 10 ~ 30 kJ/mole. Thermal energy at RT is approximately 2.5kJ/mole, which is not sufficient. Potential sources of the energy to surmount the hydrogen bond energy are UV energy when the surface of the mineral is directly exposed to the sun light, and flow of water molecules around the mineral surface due to tide cycle. The strength of UV rapidly weakens as the depth of sea water increases due to absorption. On the other hand, covalent bond is much stronger (400 ~ 500 kJ/mole) compared with hydrogen bond, thus it is unlikely to easily break up due to tidal cycle. Higher energy source like lightning is required.
Growth rate of single strand of RNA polymer is linearly proportional to the concentration of the nucleotides, so if the concentration is high and a suitable abiotic catalyst is available, it may grow to a functional RNA. However, it is statistically unlikely to have the second polymer with the same sequence nearby. On the other hand, if a functional RNA undergoes the self-replication, multiple copies of the polymer are available for assisting chemical reactions. For the functional RNA to take hold, many copies of the polymer are needed.
The first functional RNAs were likely to be biotic catalysts (ribozymes) that engaged in accelerating the formation of themselves through formation of nucleotides from the subunits and self-replication of the nucleotides and the polymers. These catalysts replaced the abiotic catalysts such as minerals, were much more efficient and allowed the reactions to take place at various places besides tideland and estuary and were different from enzymes that were proteins and formed with help of RNA in a later stage of evolution. Over many years, some RNAs self-replicated and evolved to a variety of functional RNAs. At some later stages of evolution, DNA and proteins were formed with help of RNA and took over the jobs of storing genetic information and driving chemical reactions, respectively.
There are competitive hypotheses of the origin of life in literature [15]. Hydro vent in deep ocean bed where life started as a simple metabolic process and hot spring where fresh water contributed for the formation of original life form of proto cells. These hypotheses are plausible in some respects but not in others [16]. We think that these places are occupied later stage of evolution of life by adaptation.