Attempts have been made to simulate and approximate models of primitive
Earth conditions for abiogenic synthesis, and successful synthesis of
essential biochemical constituents necessary for maintaining life has
been partly accomplished.
Urey ([ref.11]) has clearly pointed out the possible role of a reducing
atmosphere in the synthesis of prebiological organic molecules. Miller
([ref.12]) synthesized a variety of amino acids in a reducing atmosphere
by means of an electrical discharge. A variety of organic compounds have
been synthesized by the action of various energy sources upon reducing
atmospheres, and several investigators have extended the
Urey-Miller-type reactions to synthesize nucleic acid components
([ref.13]), adenosine triphosphate ([ref.14]), and a host of
biologically essential organic compounds.
It is likely that in the synthesis of organic moieties, simple and
specific molecules were first produced when the planets had a reducing
atmosphere. Further complexity or degradation of the organic compounds
produced varied, depending on the geochemical changes of the planet’s
surface, the atmospheric constituents, the degree of interaction between
surface and atmosphere, and the rate of the organic synthesis. Oparin
([ref.15]) presented the most detailed mechanisms for the spontaneous
generation of the first living organism arising in a sea of organic
compounds synthesized in a reducing atmosphere on Earth.
It is generally accepted that, under favorable conditions, life can
arise by spontaneous generation. A primary requirement for this
initiation is that there be abundant organic compounds concentrated in
one or more specific zones. These simple organic molecules would undergo
modification to develop a greater structural complexity and specificity,
finally giving rise to a "living" organism. Therefore, because of the
ease with which organic compounds can be synthesized under reducing
conditions, planetary surfaces may contain an abundant source of similar
organic matter. However, difficulties arise in postulating steps for
further organization or modification of the above synthesized organic
matter into a living state. Most of the original organic matter produced
in the primary reducing atmospheres of the various planets may have been
quite similar. However, major variations between planets, in chemical
evolution beyond the prebiotic stage, must have been the rule rather
than the exception.
The primary interest in this area of research has been the realization
of the possible existence of organic molecules on planetary surfaces
and, particularly, Mars. Pertinent synthesis may be either biological or
abiological. Research conducted in the simulation of cosmochemical
synthesis has used most of the available solar spectrum. Simulation
experiments devised to study the effects of these energies on the
assumed early atmosphere of the Earth have yielded products that play a
dominant role in molecular and biochemical organization of the cell.
Calvin ([ref.16]) irradiated water and carbon dioxide in a cyclotron,
obtaining formaldehyde and formic acid. Miller ([ref.17]) found that
when methane, ammonia, water, and hydrogen were subjected to a
high-frequency electrical discharge, several amino acids were produced
along with a variety of other organic compounds.
Corroborating experiments established that the synthesis of amino acids
occurred readily. The apparent mechanism for the production of amino
acids is as follows: aldehydes and hydrogen cyanide are synthesized in
the gas phase by the electrical discharge. These substances react
together and also together with ammonia in the water phase of the system
to give hydroxy and amino nitriles, which are then hydrolyzed to hydroxy
and amino acids. Among the major constituents were aspartic acid,
glutamic acid, glycine, α-alanine, and β-alanine.
The "Miller-Urey" reaction mixture has been extended and several
modifications introduced. Oró ([ref.18]) introduced hydrogen cyanide
into the system as the primary gas component. Adenine was obtained when
Oró heated a concentrated solution of hydrogen cyanide in aqueous
ammonia for several days at temperatures up to 100° C. Adenine is an
essential component of nucleic acids and of several important coenzymes.
Guanine and urea were the two other products identified in the hydrogen
cyanide reaction. Oró further obtained guanine and uracil as products of
nonenzymatic reactions by using certain purine intermediates as starting
materials.
Ponnamperuma ([ref.19]) also obtained adenine upon irradiation of
methane, ammonia, hydrogen, and water, using a high-energy electron beam
as the source of energy of irradiation. These results indicate that
adenine is very readily synthesized under abiotic conditions. Adenine,
among the biologically important purines and pyrimidines, has the
greatest resonance energy, thus making its synthesis more likely and
imparting greater radiation stability to the molecule.
The formation of adenine and guanine, the purines in RNA and DNA, by a
relatively simple abiological process lends further support to the
hypothesis that essential biochemical constituents of life may have
originated on Earth by a gradual chemical evolution and selection. In
this respect, the examination of planetary surfaces—specifically
Mars—presents practical implications for current research on the problem
of chemical evolution.
When Ponnamperuma et al. ([ref.14]) exposed adenine and ribose to
ultraviolet light in the presence of phosphate, adenosine was produced.
When the adenine and ribose were similarly exposed in the presence of
the ethyl ester of polyphosphoric acid, adenosine diphosphate (ADP) and
adenosine triphosphate (ATP) were produced. The abiological formation of
ATP was a major stride along the path of chemical evolution, since ATP
is the principal free energy source of living organisms.
Oparin ([ref.15]) postulated that α-amino acids could have been formed
nonbiologically from hydrocarbons, ammonia, and hydrogen cyanide at a
time when the Earth’s atmosphere contained these substances in high
concentrations. Oparin’s hypothesis has received strong experimental
support, as evidenced by the work of Miller ([ref.12]). Bernal
([ref.20]) has emphasized the role played by ultraviolet light in the
formation of organic compounds at a certain stage of the Earth’s
evolution.
Generally it has been believed that the first proteins or foreprotein
were nonbiologically formed by the polycondensation of preformed free
amino acids ([ref.21]). Akabori ([ref.22]) proposed a hypothesis for the
origin of the foreprotein and speculated that it must have been produced
through reactions consisting of the following three steps.
The first step is the formation of aminoacetonitrile from formaldehyde,
ammonia, and hydrogen cyanide.