space resources WILLIAM H. BOWMAN' RICHARD M. LAWRENCE
for teachers
Ball State Univenity Muncie, Indiana 47306
For centuries men have asked the questions: How did life originate on Earth? Does life exist on other planets? For most of us these questions have represented honest but speculative thinking. Acquiring more definitive answers to these and similar questions represents one of the objectives of our Nation's space program. Darwin's Theory of Biological Evolution states, in effect, that the diverse forms of life present on Earth today have evolved, over long periods of geologic time, from a common ancestry by the processes of mutation and natural selection. This theory begins with the earliest living system2 (or systems) and attempts to account for the elaboration and diversification of that "simple" system into the many 'Lcomplex" systems present today. The theory does not, however, attempt to account for the origin of the first living system. It is to this latter point that the theory of chemical evolution addresses itself. The theory of chemical evolution proposes that life originated spontaneously on Earth as a result of processes in which biological molecules were elaborated by chemical reactions and then were organized through chemical and physical processes into living systems. Although a great diversity of morphological forms exists on Earth today, a t the molecular and macromolecular levels all living systems are quite similar. They all contain water as the principal liquid component; proteins composed of the same twenty ba-amino acids; nucleic acidscomposed of the same purine and pyrimidine bases; polysaccharides composed, in many cases, of Dglucose; and lipids of relatively limited structural diversity. The relative proportions of these components and the manner in which they are organized serve to differentiate the variety of morphological forms. The purpose of studies in chemical evolution is to account first for the formation of these biological molecules and secondly for their organization into living systems. This article is one of theseries of articles based on resource units R. M., AND BOWMAN, W. H., "Space Resources in LAWRENCE, for Teachers: Chemistry," NASA EP-87, 1971, available through the Superintendent of Documents, Government Printing Office,Washington, D. C. 20402 ($2.50). Request reprints of this srticle from Dr. Lawrence. Present address: Laboratory of Biochemistry, Metropolitan Life. One Madison Avenue. New York. N . Y. 10010.
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Chemical Evolution
The major assumptions upon which experiments in chemical'evolutionare predicated have been summarized by Kenyou and Steinman (8) as follows: (1) the physical and chemical laws governing our Universe today were essentially the same during the period of chemical evolution; (2) only relatively probable chemical and physical events were important to the origin of life; and (3) the compounds found universally in living systems today were also essential t o the origin of life. The formation of biological molecules on the primitive Earth may thus be regarded as a complex problem in organic chemical synthesis (3). The Primitive Environment
Although there is some disagreement concerning the physical processes involved in the formation of the Earth, it is generally agreed that the nature of the primitive atmosphere was highly reducing (4, 5). Under these conditions the elements of major importance in biological molecules-carbon, hydrogen, nitrogen, and oxygen-would most likely be present as methane, ammonia, water, and hydrogen. Therefore, it is probably through initial interactions of these substances that biological molecules were formed. Probable sources of energy that could have initiated reactions among these substances include ultraviolet radiation, electrical discharge, radioactivity, and heat. Of these, ultraviolet radiation and electrical discharge were probably the most important (4). The relative absence of oxygen and ozone-good absorbers of ultraviolet radiation-from the primitive atmosphere would have permitted much larger quantities of ultraviolet radiation to reach the Earth's surface than at the present time. In addition, there is no reason to doubt that electrical discharges occurred in the primitive atmosphere in much the same way as today. There is then the question of whether these conditions were sufficient to form biological molecules. Testing the hypothesis is confined mainly to laboratory experiments in which a reasonable approximation of the primitive atmosphere is reproduced. Such experiments have become quite numerous during recent years and only a few examples will be m e ~ t i o n e d . ~ Experiments in Chemical Evolution
One of the earliest laboratory experiments was that of S. L. Miller in which an electric discharge was passed through a mixture of methane, ammonia, water vapor, and hydrogen. Analysis of the reaction products
revealed the presence of a variety of compounds, including formaldehyde, hydrogen cyanide, and at least four different amino acids commonly found in proteins. A Strecker-type synthesis, illustrated below for the simplest amino acid, glycine, has been proposed as the route by which the amino %ids were formed (7) Gas phase CH, NH,
+
Aqueous phase HCHO HCN
+
-
+ HpO + HI + NHI
HCHO
+ HCN +
NHnCHlCN
other products
+ HzO
-
+
NHzCH~COOH NH1 glycine
This mechanism is consistent with the observed time sequence of product formation (Fig. 1). These results
Research Center (9) using ultraviolet light as the energy source. Additional experiments by this group have established the synthesis of ribose and deoxyribose, the sugar moieties of the nucleic acids, by the action of ultraviolet radiation on formaldehyde. Analysis of the results of chemical evolution experiments reveals several important features (6) (1) Numerous combinations of reactants and energy sources (within the general framework of maintaining reducing conditions) will give rise to biological molecules. This reduces the need of being able to define precisely the conditions existing on the primitive Earth. (2) Nitriles and aldehydes, relatively reactive chemical species, are found in many of these experiments, suggesting that they may have played important roles in chemical evolut,i""~
(3) Essentially all of the major classes of molecules common to
living systems could have been formed under the conditions postulated for the primitive Earth.
Condensotion and Organization Processes
Figure 1. Changes in consenhation of NHs, HCN, aldehydes, and amino acids during sporting experiment (71. The concentrotions of H C N and aldehydes ore noted to Rrst increase and then decrease as these reastion intermediates combine with ommonio to give omino acids.
have been confirmed and extended by Miller and numerous other workers using various starting materials and energy sources. In all successful experiments, reducing conditions prevailed in the reaction mixture. Synthesis of amino acids preceded by several years the first demonstrated synthesis of a nucleic acid component under possible primitive Earth conditions. In 1960, J. Or6 reported that upon heating an aqueous solution of ammonia and hydrogen cyanide the compound adenine is formed. The reaction probably proceeds with ammonium cyanide as an intermediate
A more detailed mechanism has been proposed (8). Of all the nitrogen bases commonly found in nucleic acids, adenine is very likely the most important. Not only is it found as a constituent of nucleic acids hut also in numerous other biological molecules, including the energy-transfer compound adenosine triphosphate (ATP). Confirmation of the synthesis of adenine from hydrogen cyanide as well as evidence for the synthesis of guanine was established by workers at NASA Ames
Synthesis of the necessary biological molecules is only the first step, however, in the formation of a living system. The processes by which these molecules could have condensed into macromolecules and become organized into simple cell-like structures allow even more latitude for speculation than do the processes of biomolecule formation. It is generally postulated that biological molecules tended to accumulate in bodies of water on the primitive Earth. Thus it would seem natural to expect that the condensation reactions would have occurred in an aqueous environment. Most condensation reactions between biological molecules, however, involve elimination of water and would not be favored in aqueous medium under ordinary conditions. Condensation under anhydrous conditions circumvents this problem but requires postulating a process for accumulation of reactants in a nonaqueous environment. Representative of the efforts being made along these lines is the work of Fox and his associates (10, 11) on the thermal polymerization of amino acids. These workers have shown that moderate heating
Figure 2. "Cell-like" structures found omong microspheres formed b y heating proleinoid in rater.
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(170°C) of free a-amino acids, which could have occurred as a result of volcanic activity on the primitive Earth, yields a protein-like material which they have termed "proteinoid." Among the protein-like properties of this material is a minimal catalytic activity-an ability to promote simple metabolic reactions. In addition, this proteinoid can, through contact with water, organize itself into spherical particles (microspheres) which resemble simple cellular structures (Fig. 2) (1% I n summary, experiments of the types described demonstrate the feasibility of the synthesis of biological molecules and their self-organization under primitive Earth conditions into structures resembling primitive cells. It may also may be logically inferred that similar syntheses could have occurred or could be occurring on other planets having conditions analogous to those postulated for the primitive Earth. A widely held theory of the origin of the Solar System suggests that in their early stages the atmospheres of all the planets may have been essentially the same (IS). If so, one might logically expect to find biological molecules on all planets;
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the degree of organization attained by these molecules will be a function of the environmental changes that subsequently occurred on the various planets. Such findingd would do much to substantiate the theory of chemical evolution. Literature Cited (1) Yoma. R. 8.. in "Bioastron.utics," NASA 8P-18. 1962, p. 29. (2) KeNron. D. W., Awn S T ~ X N W A N . G., "Bioohemical Predestination," MoGraw-Hill Book Ca.. New York. 1969, pp. 3C-32. (3) Fox. 8. W.. in "Collosuium on Elementary Biological Systama and Abiogsnasia." NASA CR-74819, 1965, pp. 1-34. . L., AND UBEY.H. G.. Science. 130,245 (1959). (4) M m e ~ S. (5) K m r o n , D. W., AND ST&INW*N, G.. "Biochemicd Predestination;' MoGraa-Hill Book Co.. New York. 1969. Chapter 2. G., "Biaohemioal Predsatination." (6) K r ; n r o ~ ,D. ,W.. A N D STEINWAN, MoGraw-H111 Book Co.. New York. 1969, Chapter 4. (7) MILLER, S. L.. Biochini. Biophys. Ada, 23,484 (1957). (8) 0x6, J.. in "The Origin of Prebiologieal Systems and of Their Moleaular Matrices" (Editor: Fox. S. W.) Aoadamio Press, New Yark, 1965, p p 137-61. ~ . in "The Origin of Prebiologioal Srstsma and of (9) P o n m u ~ s n r n C., Their Molaoular Matrices" (Edrtor: Fox. 8. W.) Academic press. New York, 1965, pp. 221-235. (10) Fox, S. W.. AND HARADA, X.,Sciance, 128, 214 (1958). (11) Fox, 8. W., A N D HARADA, K., J. Amer. Cham. Soc., 82,3745 (1980). (12) Fox, S. W., HARADA, K., AND KENOBIOH, J., Scieme, 129, 1221 (1959). (13) "Significant Achievements in Spacs Bioscianaa 1958 to 1984." NASA SP-92, 1966, p. 8. . "The Book of Mars." NASA SP-179. 1968. Chapters (14) G u s s ~ o w r 8.. 8 and 9.
