THE EXPERIMENTAL INVESTIGATION OF CHEMICAL EVOLUTION

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.

CH2O + NH3 + HCN ————> H2N—CH2—CN + H2O

The second is the polymerization of aminoacetonitrile on a solid surface, probably absorbed on clay, followed by the hydrolysis of the polymer to polyglycine and ammonia.

x H2N—CH2—CN ————> (—NH—CH2—C—)x

||

NH

|

| + x H2O

V

(—NH—CH2—CO—)x + x NH3

The third step is the introduction of side chains into polyglycine by the reaction with aldehydes or with unsaturated hydrocarbons. Akabori has demonstrated experimentally the formation of cystinyl and cysteinyl residue in his above-postulated mechanism.

Fox's theory of thermal copolymerization ([ref.23]) suggests that proteins or like molecular units could have been formed in the Earth's crust, under geothermal conditions. The accumulated amino acids were heat polymerized and transported into the primary oceans for further modifications. Fox has obtained polymers consisting of all 18 amino acids usually present in proteins. The polymerization is generally done at 160° C to 200° C, although in the presence of polyphosphoric acid it can be accomplished at temperatures below 100° C. Molecular weights increased from 3600 in a proteinoid made at 160° C to 8600 in one made at 190° C.

Fox showed that when hot saturated solutions of thermal copolymers containing the 18 common amino acids were allowed to cool, large numbers of uniform, relatively firm, and elastic spherules separate. These range from 0.2µ to 60µ in diameter and are quite uniform within each preparation. Various chemical observations suggest the presence of peptide bonds in the structural organization of these proteinoids. Continuing observations of these microspheres have established further characteristics that point to the possibility of their interpretation as a kind of primitive protein macromolecule with self-organizing properties, such that a primitive form of cell, with boundary and other properties, might form.

In laboratory experiments the behavior of gram-negative and gram-positive microspheres in dilute alkali parallels that of gram-negative and gram-positive bacteria ([ref.23]). Furthermore, time-lapse studies indicate that the proteinoid microspheres undergo a septate kind of fission, mimicking cell division as shown in figure 1. Cytochemical studies show that the microsphere's boundary is membranelike in having a primitive selectivity. Electron micrographs of sections of stained microspheres also indicate the presence of a boundary.

Oparin ([ref.15]) states that the type of organization peculiar to life could only result from the evolution of a multimolecular organic system separated from its environment by a distinct boundary but constantly interacting with this environment. In his concept of coacervates as precell models, Oparin ([ref.24]) indicates that present-day protoplasm possesses a number of features similar to coacervate structure. These coacervates could represent the starting point for evolution leading to the origin of life. Moreover, in the course of their evolution the initial systems may gradually become more complex. Oparin also showed ([ref.15]) that mixing solutions of different proteins and other substances of high molecular weight produced these coacervate droplets. These droplets are characterized by the formation of a surface layer with altered structure and mechanical properties, thus providing a somewhat selective barrier in which to house a molecular system capable of replication. However, these coacervates are unstable structurally.

Figure 1.—Protenoid microspheres undergoing septate fission. Small microspheres and filamentous associations thereof are also shown ([ref.25]).

The NASA program has further provided considerable impetus for continuing research with respect to the chemical evolution of life, since its life-detection experiments may encounter prebiological molecules in their search for extraterrestrial life on other planetary surfaces.

In the area of exobiological research, the significant accomplishments to date have been—

  1. The reconstruction of some of the pathways which may have led to the origin of life, by means of laboratory simulation of processes yielding prebiological organic molecules
  2. The developments in experimental and theoretical biology; specifically, the role of nucleic acid-protein interactions in storage and transmission of information both within living cells and from generation to generation of cells
  3. The suspected role of DNA in information storage and the development of new concepts of the coding mechanism in DNA that may lead to a universal biological theory embracing evolutionary, as well as homeostatic, adaptation to environment and learned behavioral systems

With the essential biochemical constituents of life and the mechanism of replication beginning to be understood, the challenge for the synthesis of living matter by abiogenic experimental techniques has become to many scientists the ultimate goal of the scientific era.

NASA has established an exobiology laboratory at Ames Research Center in addition to the sizable support of research at various academic centers of excellence for the continuation of abiogenic synthesis.

Although research on organochemical evolution is in its infancy, the data from relatively few experiments have already created an immense enthusiasm for knowledge of the biochemical pathways of evolution. This kind of research will ultimately elucidate the terrestrial evolution of life and, perhaps, the nature of life on other planetary bodies and the distribution of life in our galaxy.

This program, with its vast demands on the scientific community at large, is coordinated with related endeavors of a number of Federal agencies. It is allied with certain biochemical studies at the National Institutes of Health for the eventual elucidation of the dynamic pathways in cosmochemical synthesis of life's essential biochemical constituents.