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.