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Home , Early Earth, Organic matter, Paleoatmosphere

CHEMICAL IN THE EARLY

A.NEGRON-MENDOZA AND S. RAMOS-BERNAL Instituto de Ciencias Nucleares, U.N.A.M. Circuito Exterior ,CO U. 04510, Mexico D.F., Mexico.

1. Introduction

How did begin? It has been a mystery for humankind during the past and present. One of the few things that it is possible to affirm is that the complexity of the problem requires a mayor contribution from very many disciplines. However, this is subject of very many speculations that has been presented in all possible and different ways. As Bernal (1951) pointed out, a general solution to a problem of cosmic proportions demands a multidisciplinary approach. Oparin and Haldane, in an independent way, proposed during the 1920's, the hypothesis that life was originated from its abiotic surroundings. It presupposes a long chemical evolution period in which the synthesis of bioorganic compounds was carried out from simple inorganic compounds under the influence of natural sources of energy. Therefore, a main objective in the chemical approach to this problem has been to disentangle the path by which organic compounds of biological importance could have been formed before the emergence of life. Chemical evolution of organic matter is a part of an integral evolution of the planet and the . Thus, the chemistry involved on the prebiotic Earth must have been constrained by global physical chemistry of the planet. These constrains may be determined the temperature, pressure and chemical composition of the environment. These considerations need to be taken into account for possible pathways for prebiotic synthesis of organic compounds. The accumulation of organic matter on the primitive Earth and the generation of replicating molecules are two factors of prime importance in chemical evolution. This process may be considered to have taken place in three stages: the inorganic, the organic and the biological. From the scientific point of view, the problem of the origin of life can be studied from two approaches: A) The analytical approach, in which imormation from , paleontology, , and other disciplines give us a panorama of the conditions that were probably present in the primitive Earth. B) By simulating in the laboratory the conditions proposed for the primitive scenario. These experiments are known as "prebiotic synthesis" or "simulated experiments" and their purpose is to synthesize compounds of biological significance. In this review our aim will be to resume the major events that were the preamble for the emergence of life on Earth. Thus, it is important for us to learn from the proposed physical and chemical environments of the primitive Earth in order to proposed reactions that could conduce towards the so call event that is call life.

71 J. Chela-Flores et al. (eds.), , 71-84. © 2000 Kluwer Academic Publishers. 72 NEGRON-MENDOZA & RAMOS-BERNAL

2. Analytical Approach

To account for the major event that led to the origin of life, an extensive study from several disciplines has been made by: examining sediments, looking for fossils that give information about the antiquity and evolution of life, from extraterrestrial sources as and lunar samples, by space missions, etc. In this part we will discuss briefly some environmental conditions about the early Earth. Geological time encompasses the total history of the Earth. It is divided in two eons of markedly unequal duration. The earlier and longer is Precambrian Eon that extents from 4550 to 550 ago, the later and shorter, is Phanerozoic Eon that expands from 550 million years to the present. These eons are divided in eras. Precambrian eon is divided in two eras: Achaean Era from 4550 to 2500 Ma ago and Proterozoic Era spanning from the end of the Achean to 550 Ma ago (Schopf, 1992). When we study history at school, very often we study the development of history, which is of about 10,000 years. But, in relation with the age of our planet about 4750 Ma, this period is incredible small, What happened before the event of life appeared? Was the early Earth similar to the present? How did life appear? How far back into the geological past can biology be traced? To answer these questions we can go back to the history of the Earth. Because it is on Earth where we have a' great deal of information on the early stages still preserved in the rock record. We need to look for the fossilization process, dating techniques and a variety of other information to help understand this amassed, interesting field of the origin of life. Today, we can observe, that ; and specially change drastically the environment. This supports the idea that the ! lithosphere of our planet has been changed by biological activity. Many of these changes occurred in the Precambrian. The oldest rock is 3800 Ma from Isua, . Evidence of the first 600 million years ofthe Earth's history has not yet been found in the rock record.

2.1 EARLIEST EVIDENCE FOR LIFE

The earliest life forms can be traced by the study offossil records. Fossils are remained or trace of prehistoric plants or animals, buried and preserved in , or trapped in organic matter. Fossils represent most living groups we have discovered; also many fossils represent groups that are now extinct. Many factors can influence how fossils are preserved. Remains of an may be replaced by minerals, dissolved by an acidic solution to leave only their impression, or simply reduced to a more stable form. The fossilization of an organism depends on the chemistry of the environment and on the biochemical makeup of the organism. Fossils are most commonly found in limestone, sandstone, and shale (sedimentary rock). The rocks that contain the fossils are pressure• cooked, squeezed, heated (that is, metamorphosed) under conditions that can wipe out any evidence of life that the rocks originally contained (Schoft, 1992). Once the fossil is found, it is necessary to determine bow old it is. For these purposes, there are dating techniques, which enable us to determine the absolute ages of rocks. In a journey into the Precambrian Eon, the first evidence for life is found in the relatively well preserved rock 3500 to 3300 Ma in age. These are stromatolites (macroscopic structures formed by sediment-trapping algae), microfossils, and Rubuisco• type isotopic values, dated about 3500' Ma found in the Warrawoona formation (Schopf, 1983, 1993). By this stage in the evolution of life, complex microbial CHEMICAL EVOLUTION IN THE EARLY EARTH 73 stromatolitic , possible -producing , had already become established. Both microscopic fossils and fossil stromatolites require life to have originated on Earth prior to 3500 Ma ago. Controversial evidence for biologically mediated carbon isotope fractionation (Schidlowski, 1988) suggests that life may have exist~ by 3800 Ma ago. The origin of life coincided with the last stage of the heavy bombardment. By 3000 Ma stromatolitic communities dominated by photosynthetic oxygen• producing cyanobacteria (Blue-green algae) were abundant and widespread. Between 2000 and 1000 Ma ago, the eucariotic with their nuclei, complex system of organelles and membranes developed and began to experiment with multicelled body structures. The evolution of the plants and animals most familiar to us occurred only in the last 570 million years. The principal Precambrian rocks that have been examined for their content of microfossils and organic compounds are 1) Bitter Spring Chert (900 Ma), Central Australia. 2) Gunflint chert (1900 Ma), Southern Ontario. 3) Sudan shale (2700 Ma), Northeastern Minnesota. 4) Bulawayan limestone (2700 Ma), Rhodesia. 5) Fig-tree chert (3100 Ma), Transvaal, South Africa. 6) Onverwacht (3200 Ma), Transvaal, South Africa.

2.1.1. Molecular Fossils. Molecular or chemicals fossils are compounds found in ancient rocks. Analytical techniques allow us to distinguish if these chemical compounds have a non-biological origin, they are preserved products of prebiotic synthesis or they are conserved remains of living . The biochemical organization of living systems provides chemical compounds 'like the enzyme ribulosa bisphosphate carboxylase/oxigenase, in shorthand Rubisco, which catalyzes the fixation of . This Rubisco has unique properties, and it leaves a telltale isotopic signature in its products, a signature that can be decoded in organic matter having an age even as great as 3500 Ma (Schopf, 1992). The most important information derived from these geological data is the time scale from the origin of life. The time available has been considerably compressed. The organisms in the Warrawoona Formation (3500 Ma ago) and the end of the heavy bombardment (4100 Ma) leave a short time for going from the primitive soup to the cyanobacteria.

2.2. PRIMITIVE SCENARIO

2.2.1. Time Scale That life did evolve on the Earth, is a fact, but the question to answer is what was the planet like when the process began? Evidences suggest that the Earth was formed about 4.75 billions years ago (Tilton and Steiger, 1965). The evolutionary sequence presumable began with the origin of the universe and the formation of the elements from the primeval cloud of gas. Since the Earth was formed many changes were produced and establish the conditions for the emergence of life. The earliest forms of life are dated at 3500 Ma. Between the formation ofthe Earth and the development of life's forms was a very crucial period in that the evolution of the molecules arrived to a critical point were everything about life began. In this period many changes, and unknown phenomena took place and originated the transformation from inorganic matter, to organic matter to life. This whole pattern introduces the idea that modem biological molecules may have had non-biological 74 NEGRON-MENDOZA & RAMOS-BERNAL origins in the past. Therefore molecules that are important for the present organism, also were important at the time of the origin of life.

2.2.2. Sources ofEnergy It is obvious that a variety of energy sources must have been available for the formation of organic compounds upon the primitive Earth. Some of these may have been useful for particular aspects of this synthesis. The variety and intensity of the action may have caused different products. In the laboratory most of these forms of energy are used to simulate the primitive conditions. To consider an energy source as a potential source for prebiotic synthesis, it needs to be available, abundant and efficient to induce chemical reactions. Different sources of energy have been proposed to contribute to the synthesis of organic matter. Ultraviolet light from the Sun, which produces a total energy of 260000 cal cm-2 -I. is by far the most important. This is not surprising since life in our planet today is dependent on the Sun for its energy. The sunlight, which is most effective in the synthesis of organic compounds, is in the region that the atmospheric components could be excited. From the study of the photochemistry of these compounds it is showed that these regions are between 1200 and 2200 A. Faithful simulation of geologically relevant prebiotic conditions would require the consideration of multi phase systems; chemistry occurring at atmospheric/lithospheric, atmosphericlhydrospheric, and hydrosphericllithospheric interfaces; and fluctuating climatic conditions. Under these conditions, natural energy sources not standardly considered in laboratory investigations, such as natural radioactive decay, and triboelectric energy (i.e., energy of mechanical stress) may have a great importance, because these natural scenarios, beside the requirements already mentioned, need a penetrating source of energy. To estimate the potential contribution from these energy sources it is necessary to evaluate the chemical effective energy dose, the energy transductor mechanism and the prebiotic dose rate.

Why ionizing radiation? Primordial radionuclides were made in the stars, by violent nuclear processes. Because of subsequent circulation of matter, they became incorporated into the Solar system. Since its formation our planet has been permanently exposed to ionizing radiation of both terrestrial and extraterrestrial origin. In this context, the application of ionizing radiation to prebiotic synthesis is based on its unique qualities: its omnipresence, its specific way of energy deposition, and its distribution. Reactions are through free radicals and the radiation furnishes pathways in the in which chemical reactions occur. The use of this source of energy is substantiated by calculations of the energy available from the decay of radioactive elements like potassium-40, -235, uranium-238 and thorium-232. It is also possible to have other sources of energy like heat from volcanoes and hot springs, electric discharges from the atmosphere, cosmic rays, sonic energy generated from ocean waves; shock waves from thunderstorms or entering in the Earth's atmosphere. The problem of evaluating these energy sources is difficult, and in general entails non-equilibrium conditions, product quenching and protection.

2. 2. 3.Environmenta/ Conditions Atmosphere. The environmental conditions in the primitive Earth are a matter of relevant importance. The starting point of the discussion must be around the of the Earth's CHEMICAL EVOLUTION IN THE EARLY EARTH 75 primitive atmosphere, which supplies the raw material for the synthesis. While it is generally agreed that the primitive atmosphere did not contain more than a trace of free oxygen, widely different views have been expressed in relation to its constitution and character. When and how did the atmosphere start to accumulate molecular oxygen? , The Earth's atmosphere I and oceans were formed, along with the planet from impact degassing of in-falling planetesimals. The ocean was completely vaporized during at least part of the period, creating a dense steam atmosphere that was hot enough to melt partially the Earth's surface. Evidence of the existence of this steam atmosphere is provided by neon isotopes (Kasting, 1983). The steam atmosphere collapsed once main accretion process was over. Models of atmospheric evolution suggest that the Archean atmosphere was composed mainly of CO:!, H20 and N2 (Kasting, 1986, Walker, 1990). A higher atmospheric CO2 level was particularly important. This is because carbon dioxide is an effective infrared absorber that could have warmed the Earth's surface by contributing to the greenhouse effect (Kasting, 1986). This provided surface temperatures of 30-50 Cat 3500-3200 Ma. (Lowe, 1992). Exactly how much CO:! was present in the early atmosphere is uncertain. The estimate the abundance of carbon, in the crust, much of which stored in carbonate rocks, is about 1023g. (Ronov and Yaroshevsky, 1967). Walker (1986) proposed that a lO-bar CO2 atmosphere could have persisted for several hundred millions years if the early Earth was entirely covered by oceans (Kasting, 1983). This composition is consistent with the inventory and probable evolution of terrestrial volatiles. Also it is Consistent with the inference that, because of the Archean Sun's lower luminosity, above the freezing temperatures were maintained by this COrrich atmosphere on the early Earth by an enhanced greenhouse effect (Lowe, 1992; Kasting, 1992). The existence of a global greenhouse effect is also consistent with the absence of unambiguous glacial deposits until 2400-2200 Ma (Lowe, 1992).

The rise of atmospheric oxygen. Where was this oxygen coming from? Two sources of oxygen are considered plausible, one of these is abiotic: the photodissociation of water vapor by solar UV radiation in the upper layers of the atmosphere (Canuto et al., 1983). The second is a biotic source. Cyanobacteria, photosynthetic organisms that produced oxygen as byproduct, had first appeared 3500 Ma ago. They became common and widespread in the Proterozoic. The photosynthetic activity was ,primary responsible for the rise of atmospheric oxygen (Holland, 1994). Another source that recently has been proposed by Draganic et al. (1991) came from the of the ocean by the potassium-40 radiation. Geological evidences, in particular, the distribution of pyritic conglomerates over the time suggests that oxygen production by cyanobacteria did not result immediately in a major increase of atmospheric oxygen. Instead three oxygen sinks consumed the molecular oxygen. 1) Reaction with volcanic gases, 2) Facultative aerobes microbes used oxygen and 3) The oxygen waS burial in the iron-rich layers of banded iron formations (abbreviated BIF). Ultimately, about 2000-1800 Ma, after the oceans had been slowly, but irrevocably, swept free of dissolved iron; atmospheric oxygen levels began to rise. After this, a stable aerobic environment became established. This was the "first pollution crisis" that hit the Earth about 2200Ma. It may seem strange to call this a "pollution crisis, " since most of the organisms that we are familiar with not only tolerate but require oxygen to live. However, oxygen is a powerful degrader of organiC compounds. Even today many 76 NEGRON-MENDOZA & RAMOS-BERNAL are killed by oxygen. Organism had to evolve biochemical methods for rending oxygen harmless. Beginning about 1700 Ma ago, banded iron formations disappeared from the geologic record and red beds (red-colored sediments) began to appear. Red beds are sedimentary rocks made of iron-bearing sand and mud eroded from rock deposited on land. The rock is red because it contains iron that has been oxidized (rusted). Its presence on land means that the eroding rock was exposed to free oxygen in the atmosphere. As it was mentioned earlier in this section this oxygen was produced during of the increasingly abundant microorganisms in the sea. Also, fossil have been found and they contain oxidized iron in their upper layers. This is another indicator of higher levels of oxygen in the atmosphere. While oxygen increased in the atmosphere, carbon dioxide decreased, as more photosynthesis took place. This is shown in the geologic record by the presence of limestone, which is made of calcium carbonate. From evidence found in some Precambrian rocks, it is fairly certain that several periods of widespread glaciation occurred on the continents, which suggests that the atmosphere was cooling during the Proterozoic Eon

Lithosphere. There is considerable agreement that Archean Earth 3500-3200 Ma was dominated by oceanic lithosphere with micro-continents probably constituting less that 5% of the present continental area (Lowe, 1992). The composition of Archean seawater was controlled by its interaction with the oceanic crust and mantle (Lowe, 1992).

Hydrosphere. The chemical composition of the primitive ocean is not known. There is substantial evidence that the Archean oceans were strongly and permanently stratified, including a deep anoxic bottom layer and a thin, wind-mixed upper layer. This interpretation derives from the implications of climating modeling, the distribution and sedimentation of Archean banded iron formation (BIF), and the oxidation state of hallow• water Archean sediments. Although there is geological evidence of liquid water on the Earth after 3800 Ma, similar evidences do not exist from the period from accretion to 3800 Ma, the time of deposition of the oldest preserved sedimentary rocks. There is yet no geological evidences regarding atmospheric CO2 and surface temperatures prior 3800 Ma (Lowe, 1992). Geological evidences establish that the starting material needed for oxygenic photosynthesis water, and carbon dioxide, were present in the environment very early in the Earth history. For example the occurrence ofliquid water is evidenced by ripple marks preserved in the upper units of Swaziland Supergroup, about 3300 Ma in ago. The presence of water is also well evidenced by the occurrence of pillow lavas (pillow-shaped masses of volcanic rock formed when lava flows into a lake or ocean and rapidly cools and solidifies) near the base ofthe Swaziland Supergroup, deposited 3500 Ma ago. Highly evidence of metarnorphophosed lsua rocks· indicates that both water and carbon dioxide were present in the environment as least as early as 3750 Ma ago.

2.2.4 Temperature ofthe Primitive Earth Archean climate was controlled by a steady-state carbon cycle. Higher atmospheric carbon dioxide levels maintain a surface temperature of perhaps 30-50 C at 3500-3200 Ma. Prior 4000 Ma, surface temperatures may have been buffered at 90-100 C. CHEMICAL EVOLUTION IN THE EARLY EARTH 77 Impacts on Earth's surface. There are several evidences that show that the Earth must have been bombarded by large objects until about 3800 million years ago. Larger asteroids would supply enough energy to boil the ocean and kill all but the hardiest thermophilic bacteria. High temperatures can destroy prebiotic compounds dissolved in the ocean. This, any living system that may be originated very early in the history could not have persisted until the end of the major bombardment. This is one of the reasons to think that life must have arisen in ten million years or less (Miller, 1992). After 4.1-3.9 Ga, impacts declined.

3. Synthetic Experiments Approach

In addition to the analytical approach for the study of the origin of life, just briefly described, another approach is used to study chemical evolution by simulating in the laboratory the synthesis of compounds in conditions that are a recreation of the possible conditions of the early Earth. How did life originate on the Earth? How were the complex molecules of life formed in the primeval environment? The exact process is not known, but it probably started by reactions among simple molecules brought to our planet by comets, and meteorites. Also by compounds formed by the action of different sources of energy on the Earth's primitive atmosphere. These simple compounds condensed either in water-or more likely on the surface of sediments. In particular the single units, which constitute the nucleic acids and the , could have been synthesized by the action ofvarlous forms of energy in the primitive atmosphere. These small molecules may have been formed directly from the primordial atmosphere or may have resulted in a stepwise manner from reactive intermediates. In the synthesis of the polymers there is a removal of a molecule of water at every step of the condensation of two units. This process was to take place in an aqueous medium and is not energetically favorable. Such a sequence of events would have been feasible on the primordial Earth. Bernal (1951) was the first to point out that a lagoon when it dried up, would have been an ideal locale for the origin of large molecules. Organic material, adsorbed on the clay at the bottom of the lagoons, would have been exposed to solar radiation or other sources of energy, and dehydration would have taken place. Reactions of this type also may have occurred along the ocean shoreline. The lagoons that are strung along the waterfronts are occasionally found to be dry. At other times, at high tide for example, the water rushes in and fill these areas. This alternate drying and flooding would have been a very useful method for the synthesis of polymers and their removal into the prebiotic ocean. Random chemical reactions could have formed a variety of organic compounds, many of which could not have led to life. Unproductive chemical diversity may have been limited by catalysis that favored the synthesis of specific molecules important for life. Those may have been organic compounds formed on Earth or by accretion from extraterrestrial sources.

3.1 CHEMICAL COMPONENTS IN CONTEMPORARY LIFE.

RNA Word. A fundamental concern in this field is what molecules were essential for the first life. form DNA (deoxyribonucleic acid) and proteins are the central biological polymers in present-day living organisms. DNA stores the genetic information. The DNA 78 NEGRON-MENDOZA & RAMOS-BERNAL information is expressed in proteins. Proteins perform the structural and catalytic functions that are characteristic of a particular specie. RNA (ribonucleic acid) allows link between DNA and proteins. RNA carries the genetic information encoded in DNA to the molecular machinery for the synthesis of proteins. A crucial problem for the studies about the origin of life is to establish whether the first organism contained proteins or RNA or both. The -RNA dilemma has been resolved, at least for the present, in favor of the RNA. It has been discovered the catalytic activity in certain RNA molecules, the ribozymes, (Cech and Bass, 1986). This indicates that RNA could carry out the function of both DNA and proteins, and if this is correct, this interpretation implies that the first organisms were composed of RNA. This hypothetical biosphere is called "RNA word" (Gilbert, 1986). Life eventually switched over from RNA to DNA and proteins, because these molecular specialists are able to perform the task of information storage and catalysis. Other important molecules in present -day organism are the phospholipids, which composed the membrane. The energy that drives the biochemical reactions within a cell is stored in a molecule called adenosine triphosphate (A TP). Other molecules of interest are fatty acids; dicarboxylic acids related with metabolic processes; porphyrins-like pigments; nicotinamide and its derivatives; and sugars (which are also part of the nucleic acid backbone).

3.2. OUTLINE OF THE OPARIN-HALDANE HYPOTHESIS.

According to the Oparin-Haldane hypothesis living organisms arose naturally on the primitive Earth through a process of chemical evolution of organic matter. In this process the flux of energy through the prebiotic environment transformed simple molecules into complex bio-organic compounds. Eventually they became the precursors of proteins, nucleic acids and other biochemical compounds. One of the major contributions of Oparin was to propose that the first organisms were heterotrophic, that is, they used prebiologically produced organic compounds available in the environment. Haldane (1928) introduced the idea of primordial soup. He suggested that in bodies of water these reactions would be going on under the rather large concentrations of organic substances. In fact we need very concentrated primitive soup. Oparin, as well as Drey proposed that the early Earth had a composed of , ammonia, water vapor and molecular hydrogen. A basic pH and moderated temperatures were needed for the accumulation of organic compounds. Before that Oparin was proposed that the origin and evolution of life involved a stepwise approach, other researchers had dealt with the problem from their own perspectives. Alfonso L. Herrera (1868-1942), was a Mexican scientist of great breath. In 1930s, had easily and quickly produced organized structures that had the look of a great variety of cells. Moreover, he started his experiments with substances such formaldehyde and ammonium thiocyanate. Decades after these experiments, new investigators found that the materials he used were organic substances abundant in the interstellar matter. In this he was far ahead of his time (Fox, 1988). While many features of Oparin-Haldane scenario are untenable in the light of modern theory and knowledge they are still an important cornerstone in the study of the origin of life. CHEMICAL EVOLUTION IN THE EARLY EARTH 79

3.3. SIMULATED EXPERIMENTS

S. Miller, working with H. Urey at the University of Chicago carried out the first experiment in this field in 1953. The apparatus was designed to circulate methaue, ammonia, hydrogen and water, though an electric spark Electrodes were placed in a 5 I flask, and small tesla coil produced the spark (Figure 1). After a week of sparking, the water containing organic compounds was analyzed. The results were beyond belief. A number of life- related substances were synthesized. Among these substances were amino acids, simple fatty acids, and .

Figure 1. Diagram of the Miller experiment. Steam circulates from the -boiling water at lower left, through the "atmosphere" of methane, ammonia and hydrogen.

After this experiment many others were performed using different sources of energy like heat, ionizing radiation, UV light, etc. These different types of experimentation have produced many building blocks of life such as amino acids, purines, pyrimidines, hydrocarbons, etc. Still, there are several compounds whose synthesis present several problems as low yield of formation, unrealistic experimental conditions, the optical activity problem, etc. Much effort should be given to this regard The modeling of the possible pathways for the formation of organic compounds on the primitive Earth is called "prebiotic synthesis" or "simulated experiment" In other words, chemical processes are the first steps in modeling the chemical events in the primitive Earth and other bodies in our solar system. For a "simulated experiment" we need to take into account the following considerations: 1) to choose the raw material, and the energy source. 2) To allow the system to operate for some time, and 3) to perform a variety of analysis of the products. There are several approaches to perfonn simulated experiments: To choose a mixture of gases that constituted the primitive atmosphere or gases detected in extraterrestrial bodies with an atmosphere. The mixture is exposed to an energy source, and the products detected are analyzed. In the case of the simulation of the of extraterrestrial bodies, the results can be compared with the detection of compounds in the atmosphere that was simulated. Since there are several ways to arrive to the same product is necessary to choose a pathway to visualize and simplify the experiment. In particular in synthetic chemistry the synnnetry is of great importance for the simplification of such synthesis. Other two elements that can be very useful to visualize the synthesis are the followings: to observe structural relationships among reac~ 'Ults and products, to use chemical properties. Lets examine the synthesis of succinic acid to illustrate the previous statements. 80 NEGRON-MENDOZA & RAMOS-BERNAL

Succinic acid is an important compound in metabolic patlnvays and it is a precursor molecule in tlle synthesis of porphyrins. The structure of this acid is shown in Figure 2. It has a symmetry plane (axial symmetry) tllat divides in two identical parts tlris molecule. Firstly, this means that if we s}TIiliesize ilie fragment· CH2COOH (since it has a non• share electron, it is a free radical), we have half of the succinic acid molecule. The problem now is to form ilie fragment ·CH2COOH and secondly to put together two of these fragments (tlris reaction is known as dimerization). The synthesis of succinic acid, a molecule of 4 carbon atoms has been reduced, just by symmetry considerations, to the synthesis of- CH2COOH with 2 carbon atoms. How can CH2COOH be synthesized? This fragment is very similar to CH3COOH, acetic acid; the difference is an extra hydrogen atom in the acetic acid molecule. So, if we remove one H from acetic acid molecule, we will have the fragment iliat we were looking for. How can tlris be achieved? It is known in Radiation Chemistry, that the interaction of ionizing radiation wiili an aqueous solution of an (with H atoms next to a functional group, like the COOH group) will react abstracting an H atom. In these conditions the radical' CH2COOH is formed, and later it will dimerize to form succinic acid Since acetic acid was used as raw material for our synthesis, the next question would be may be acetic acid considered as a prebiotic compound? In order to answer tlris, it will be needed 1) to look for its prcbiotic synthesis. The acetic acid molecule can be visualized by a rearrangement of its atoms. It is as formed by CH4 and C02, compounds that we can find in the atmosphere or in other prebiotic synthesis. Would acetic acid be formed from the reaction of those gases? A search in the bibliography shows that this reaction was attempted and acetic acid was indeed formed with a high yield. Another way to synthesize acetic acid is from an electric discharge experiment starting with methane and water (Allen and PonnampefUllla, 1967). Now, for testing tlris scheme it is necessary to look for the synthesis of succinic acid from acetic acid. It has been reported that succinic acid is obtained in a very high yield from the gamma irradiation of dilute solutions of acetic acid (Negron-Mendoza and Ponnamperuma, 1996). Another example of the simplification of a synthesis is showed in figure 3 for uracil. This molecule can be divided in two parts; iliis is shown in ilie Figure 3. It is needed to go backwards to try to get the backbone of those fragments and ilien related them wiili a known molecules. In tlris case, these are ilie urea and cyanoacetylene. In 1974, Ferris et al., reported ilie syniliesis of cytosine and uracil starting wiili cyano• acetaldehyde, from ilie hydratation of cyanoacetilene and guanidine. 2) The second hint to simplify a syniliesis is by structural relationships. For example, aspartic acid is an antino- Figure 2. Synthesis of Succinic acid starting with acetic acid. CHEMICAL EVOLUTION IN THE EARLY EARTH 81

acid compound; and it is fonned readily from many prebiotic experiments. Its structure is presented in Figure 4. This structure is very similar to the succinic acid (structure B) and malic acid (structure C). The difference resides in the NH2, OH and H groups. So, it is very likely that if we have aspartic acid, the amino group can be removed by a simple chemical reaction and it will produce succinic or malic acids. In 1980 Aguilar et al., fonned aspartic acid starting with succinic acid and one N source (ammonium ions), and malic acid from the irradiation of succinic acid. In the same manner irradiation of malic acid yields succinic acid (Castillo et. al., 1984). This is an example of structural relationships for the synthesis of compounds.

Figure 3. Synthesis of uracil from cyanoacetilene and guanidine ..

HzN-CH -COzH CH-CO;.H HO-CH-COzH I I I (A) CHz-CO-.H (B) CHz-CO;.H (C) CHz-CO-.H

Figure 4. A) aspartic acid, B) Succinic acid and C) malic acid

3) The use of chemical reactions. In most of the prebiotic experiments the raw material present chemical groups as C=O, CN or C=C. In an alkaline pH, which was believed to exist on the prebiotic oceans, and with the action of an energy source, condensation reactions are very likely to occur. For example the condensation reaction of formaldehyde in alkaline medium yielded (Gabel and Ponnamperuma, 1967). Another example of these condensations is the Strecker synthesis for amino acids from an aldehyde and ammonia. This is the mechanism that Miller proposed to explain the products of his first experiment, already discussed. All these statements are just a guide for the planning of a prebiotic experiment in relation to the raw material. Another very important aspect to consider in planning a 82 NEGRON-MENDOZA & RAMOS-BERNAL prebiotic synthesis, is to re-create in the laboratory geologically relevant scenarios for the primitive Earth, this is a very difficult task. More realistically simulated environment that could be the resemblance, at the laboratory, of the primitive Earth would require the consideration of multiphase systems Chemical reactions may took place at hydrosphere-lithosphere, atmosphere-lithosphere interfaces. Thus it is relevant to consider the contribution due to the enhancement of chemical reactions by solid surfaces as it was proposed by Bernal (1951).

3.4. GROWTH OF MOLECULES TO BIOPOLYMERS

A whole set of small molecules upon which living organisms of today depend, have been formed, by a variety of energy inputs and using rather extensive raw materials. For a review of some of the synthesis of these small molecules see Miller (1992) and the references therein. However, there are many compounds that do not yet have adequate prebiotic synthesis (amino acids such as arginine, lysine and histidine). An important step in these chemical sequences to yield compounds of biological relevance is a polymerization reaction. Fundamental molecules for life are biological polymers such as carbohydrates, nucleic acids and proteins. The synthesis of these compounds is a bottleneck in this synthetic approach. Not only because the condensation reaction is energetically unfavorable in an aqueous medium. A more important problem came with the information that these polymers carried out. For example, the word "evolution", it has a defined meaning of something that evolves. The word "voonetiu" has exactly the same letters that the word evolution, but it is meaningless. So, not only the number and kind of letters are important to build a word, but is essential for giving the information, the order in which they are put. The same problem happens with the proteins or nucleic acids. The protein molecule is made up of a string of amino acids. The individual amino acids are linked together by a peptide bond. Proteins perform the structural and catalytic properties, and the order of amino acids in which they are binding is important. Although the problem of dehydration-condensation reactions to form polymers has not yet been solved, many approaches have been proposed. If the removal of water is necessary for the synthesis, perhaps a method of synthesis would involves the removal of that molecule of water by thermal methods. A typical experiment conducted by Fox, was to heat at 180 C a mixture of amino acids, with a large abundance of glutamic acid. This type of condensation yielded polymers of large molecular weight. Another way to attack the problem is with the use of "condensing agents." These are compounds which can stored primary energy input to be used later in the hydration• condensation reaction. These compounds present multiple carbon- bond as Cyanamid, dicyandiamide (see Miller, 1974 and the references therein).

Template polymerization. In present-day organism, by the process call template polymerization the genetic information in DNA is replicated by a complex process. Each of the two strands of DNA directs the synthesis of its complement. There are several very interesting results of potentially prebiotic template polymerization carried out by the group of Orgel. Their results showed that starting with purineribotides on a polypyrimidine template produced oligomers up to 50 units ofnucleosides.·However, the converse reaction of pyrimidine nucleosides on polypurine , does not work. This is a limitation of this model of polymerization. CHEMICAL EVOLUTION IN THE EARLY EARTH 83

3.5.MODELS FOR PRECELLULAR ORGANIZATION

At some stage in chemical evolution it would have been necessary to achieve a separation phase between an evolving organic system and the external environment. One of the factors in establishing an orderly system from a random pool of chemicals was probably the formation of a membrane. The set-up of monomers and biopolymers within such a membrane would have had the effect of both, concentrating and constraining their environment. This allows a faster synthesis of biopolymers within the protocellular structure (Oro, 1995). For this reason models of semipermable membranes are very important in the study of the origin of life. Several theories exist today to explain how a biological entity could have emerged naturally out of the abundance of organic chemicals. Protocell arose as a result of natural physico-chemical interactions. However, there is not yet an answer on how the protocell was formed.

4. Concluding Remarks

The central question about life is how it was originated? Early in the history some system of replication powered by external sources must have been formed. Chemical studies have provided to be fruitful avenues to our understanding for the transition from the non-living to the living. The discovery of amino acids and other compounds in extraterrestrial samples, such as meteorites, has given us some measure of conviction that many of the process we outlined for the infant Earth are commonplace in the universe. They constitute a part of the orderly sequence of cosmic evolution. There is a remarkable progress in the understanding of the origin oflife. We are still a long way from understanding the exact process to lead to the first living cell. Many answers to the questions that have been made are well supported by facts. Some of them are very likely models and others as plausible guesses that remain to be tested by future experiments. Much has been learned over the past two decades about the antiquity and evolutionary history of Precambrian life; a great deal remains unknown. We now know that life existed at least as early as 3500 Ma ago. How much earlier did the origin of life occur? Chemical evolution experiments have suggested that almost all bio-monomers can easily be produced by suitable simulation experiments. Only in the most general sense it is true. There are several difficulties. One crucial problem is the synthesis and stability of sugars. These molecules play a central role in our entire metabolism. However, prebiotic synthesis of them gave low yields. Also, they have very unstable nature. Another difficulty with chemical evolution resides in the "selection problem" What chemical mechanism selects the molecules that constitute a specific polymer? Connected with this selection problem, still there are many bottlenecks such as the origin of the optical activity and the selection of one isomer for biological processes. How did the genetic apparatus evolve? Of paramount importance, because it bridges the gap between chemical evolution and biological evolution, is the problem of precellular organization. The problem is to envision how the biologically important chemicals spontaneously organized themselves 84 NEGRON-MENDOZA & RAMOS-BERNAL into a three dimensional matrix that would eventually acquire the characteristics of life fu~. . Finally, this attempt to dmw from the scientific experience, some generalized concepts in chemical evolution has the intention to illustmte how by various pathways, quite diverse in their origin, gmdually evolved into a common work and scientific interest. Each one of these independent discoveries has given rise to quite effective and profound changes in our environment and helps us to study the origin and evolution of life.

5. References

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