Chemical evolution is the process of the synthesis of biochemically important molecules starting from simple molecular building blocks, such as water (H2O), nitrogen (N2), carbon dioxide (CO2), and hydrogen sulfide (H2S), under plausible primordial conditions that governed the prebiotic Earth. It describes a process of increasing complexity from simple inorganic compounds toward first simple organic compounds that in turn formed biochemically important structures for a first living system. First life may have started up as a final product of chemical evolution. This event is probably the result of a multitude of processes, most of them not very likely, that led to a proper arrangement and activation of complex molecular building blocks.
Chemical evolution on a timescale is preceded by the formation of elements in stars in consecutive nucleosynthetic processes and by the formation of first simple compounds in space at a later stage. Eventually, interstellar matter aggregated to form the solar system and the terrestrial planets, Earth, Mercury, Venus, and Mars, about 4.6 billion years ago.
Chemical evolution on Earth is a process that probably started later than about 3.8 billion years ago. It is difficult to date the single stages even roughly, as little geological and chemical record remains. During the following millions of years chemical evolution must have led to the origin of life on Earth, as the oldest- known fossil records stem from a period about 3.5 billion years ago.
Chemical evolution is a development that is strongly influenced by the environment of the terrestrial planet, such as composition of the atmosphere, types of radiation, and geology. Any development is determined by the laws of nature and statistics. Therefore, it should be possible to simulate basic chemical processes that occurred several billion years ago. A fundamental problem, however, is the lack of knowledge about the exact environment on planet Earth at that time. Because of the lack of knowledge, the possibility to plausibly reconstruct the process of chemical evolution is very limited.
Not only does the environment affect chemical evolution, but chemical evolution changes the environment. Molecules are formed and altered. There will be a kind of competition between molecules for nutrients. The rate of formation as well the rate of destruction is decisive for an enrichment of certain types of molecules.
Chemical evolutionary processes, in general, should not be limited to Earth but could also proceed on any terrestrial planet because principal chemical laws should be valid throughout the whole universe.
Origin of Elements
Until the beginning of the 19th century, there had been only little knowledge about the abundance and the origin of elements in the universe. The development of spectrochemical analysis in 1860 made it possible to identify elements by their wavelength characteristics of emitted light. By investigating the light of stars and other celestial bodies, for example, nebulae and dust clouds, optical spectroscopy proved the existence of the same chemical elements in the sun and throughout the universe. Hence, there is only one kind of chemistry in the universe.
In the 1950s, after scientists had discovered that hydrogen is the main constituent of the sun, a theory was developed that explained the formation of practically all elements in the core of stars. At extremely high temperatures of at least 107 K (kelvin) and extreme pressures, the elements—more precisely the nuclei of the ele- ments—are built up in consecutive thermonuclear fusion processes starting from hydrogen. As a primary process, hydrogen is converted to helium (He) by the combination of four hydrogen nuclei, simple protons, into one helium nucleus. This process, which can take up to some 10 billion years, goes on until practically all hydrogen in the star is exhausted. In secondary processes and at even higher temperatures, He can be converted to heavier elements that in turn can be converted or combine in complex fusion or secondary processes to form all other elements. Uranium is the heaviest element being formed in stars.
Besides hydrogen and He, which amount to 99% of the elements in the universe, carbon, nitrogen, and oxygen are the next most abundant elements. Beyond these elements, abundance decreases somewhat irregularly with increasing atomic weight, with the exception of elements around iron. The relative abundance of elements in the universe is controlled by the rates of thermonuclear fusion and secondary processes of the nuclei in stars. It is probably decisive for further chemosynthetic processes how the relative abundance of elements is at an actual place of chemical evolution.
Once liberated from a star, the generated elements can readily condense and undergo reactions with each other at lower temperatures in space. Accumulation processes of this material will give rise to nebulae with dust and molecular clouds. Dust in the interstellar matter consists mainly of carbon and different oxides. From such a kind of nebular system that began to condense, our solar system started about 5 billion years ago. That means that practically all atoms that built up the solar system originated from former nucleosyn- thetic processes in other stars.
The Primitive Earth
About 4.6 billion years ago, the Sun and planets were formed within the protostar disc, which mainly consisted of gas and dust. At that time meteorites also were formed that still preserve some of the original material of the solar system. It is feasible to date meteoric material found on Earth by analyzing the content of specific radioisotopes, for example, potassium or uranium, that suffer radioactive decay. This gives an estimated age of the solar system, and hence the Earth, of about 4.6 billion years.
An immense heat and strong solar winds were emitted from the contracting protostar in the center of the dust disc. These radiations blew away all volatile substances like hydrogen (H2), He, and N2 from the inner zone, the terrestrial planet zone. Only dust grains and aggregates of nonvolatile, dense matter were left over in that region. Consecutive collisions and accumulation of these materials finally led to the inner protoplanets. In the case of the earth, accretion led to a metal core of mainly iron, which was surrounded by a mantle and a crust of moderately volatile compounds like oxides that also contained volatiles like H2O (water). At the end of this accretion process, which took about 100 to 200 million years, the planet Earth presumably was covered by magma oceans due to an enormous release of energy from core formation and radioactive decay of instable isotopes. How and how long the cooling of the earth took place is still discussed.
An accepted model of the primitive atmosphere does not exist because air records are missing and geological records are incomplete. Anyhow, it is decisive for the plausibility of any hypothesis regarding chemical evolution to know which components were the major and minor constituents of the primitive earth’s atmosphere.
Because most gases from the original dust cloud were swept away by solar winds during the formation of Earth, there was no or only a thin atmosphere at the beginning. It is assumed that intense volcanic activity led to the degassing of occluded and chemically combined volatiles that formed a secondary atmosphere. Some components like methane (CH4), N2, H2, and ammonia (NH3) are liberated if meteoric material, assumed to be similar to the material of the primitive earth, is heated to about 1000 K. Hence, these gases could contribute to a primitive, reducing atmosphere. Such an atmosphere is indeed crucial for one major scientific theory about the origin of life, the so-called soup theory. Nevertheless, it is doubtful whether such an atmosphere could persist for a long time. Hydrogen could escape easily from the weak gravitational field of the earth, and NH3 as well as CH4 would have been rapidly destroyed by the intense solar ultraviolet radiation.
Today it is supposed that the outgassing from intense volcanic activities provided the compounds for a secondary, only weakly reducing atmosphere consisting of mainly steam and CO2. These compounds are the major constituents of extant volcanic exhalations, too, and it is reasonable to assume a similar composition for the extant ones as for those on the primitive Earth. Besides, there was no oxygen (O2) present in the atmosphere at that time, as could be concluded by the abundance of reducing ferrous ions in geological material. Recent studies hint at a higher proportion of H2 in the primitive atmosphere.
One has to regard possible energy sources for the formation and conversion of first organic molecules as one prerequisite for chemical evolution. On the primitive Earth that was already cooled down, the three main sources of energy were solar irradiation, electric discharges, and energy from geochemical reactions, beside minor contributions from radioactivity, volcanoes, and shock waves as a result of impacts. Solar radiation provided the main source of energy, although the relevant wavelength range for organic synthesis, the ultraviolet (UV) radiation, contributes only a small fraction to the total irradiation. Many simple gaseous molecules like CO2, carbon monoxide (CO), H2O, and CH4 absorb UV radiation only below 190 nanometers, and very simple organic molecules can be generated as well as destroyed by that short-wave UV radiation.
Electric discharges are an efficient source of energy for the production of biomolecules from gas molecules, but it is doubtful whether this source contributed remarkably to the generation of organic substances. Energy and catalytic activation could also be provided by geochemical reactions that could have led to the selective generation and further modification of organic compounds from simple inorganic precursors.
Any just-formed organic compound was immediately endangered by decomposition due to UV radiation or other secondary processes. Only molecules that found a sheltered place or could react to form more resistant molecules could persist for some time.
The Primitive Soup
How life could develop on Earth has been the subject of religious and philosophical discussions for centuries. The first approach to explaining the generation of organic matter scientifically and a proposal for how first life originated on Earth were done independently by Alexander Oparin and J. B. S. Haldane in the 1920s.
Their hypothesis was based on the assumption that Earth’s primitive atmosphere had reducing properties and consisted mainly of CH4, NH3, H2, and H2O. The atmosphere was exposed to various energy sources, for example, lightning, volcanic heat, and solar radiation, leading to the formation of organic compounds. These compounds accumulated in the primitive oceans to form a “hot dilute soup.” Because the compounds would remain there for long periods of time, first life would ultimately develop in a spontaneous process of organization.
Today, the primitive soup theory is seriously questioned although it is still strongly propagated. The primitive atmosphere assumed to be reducing, according to the theory, was evidently rather neutral, lacking significant amounts of CH4 and NH3. Moreover, these compounds would have suffered rapid decay by photolysis. Furthermore, most organic molecules would not persist for a long time in the postulated primitive soup but would be destroyed quickly so that high concentrations of organic compounds are not likely. Additionally, a spontaneous linkage of many building blocks (monomers) to polymers that exist in extant life cannot happen as easily as proposed, for energetic reasons. Thus, the origin of a living system based on biopolymers is not plausibly explained.
Precursors to Organic Evolution
Since 1953, laboratory scientists have attempted to produce all biologically relevant compounds, such as amino acids, nucleobases, and sugars, under prebiotic conditions, thus mimicking the environmental conditions of the primeval Earth. These investigations were motivated by the assumption that the same kinds of biochemical substances as extant ones were the basis for the development of first life.
In 1953, Stanley Miller and his supervisor Harold Urey set up an experiment designed to simulate the atmospheric conditions of the primeval Earth. It was intended to produce possible precursors of chemical evolution. A flask was filled with H2O and the reducing gases NH3, CH4, and H2. After closing, the flask was heated and exposed to electric discharge for 1 week. The obtained reddish solution proved to be rich in some sort of tar as well as simple amino acids (a total of about 1%) and some small reactive intermediates like hydrogen cyanide (HCN) and formic acid. The energy input into the experiment is comparable to that of about 50 million years ago on the primitive Earth. Experiments using another input of energy like heat and solar radiation generated yields of products that were consistent with the Miller-Urey experiment. Further experiments that simulated the interaction of the primitive atmosphere’s gases with lava flows at temperatures of up to 900°C also showed a complex variety of organic molecules. All these experiments support the idea of chemical evolution in a way that the interplay of gaseous carbon, N2, H2O, and H2 and an energy input leads first to the synthesis of reactive intermediates that in turn generate biologically related molecules. Some of the formed intermediates were used in further prebiotic experiments. In such experiments, usually rather high starting concentrations lead to the formation of the nucleobases, from HCN, and mixtures of sugars, synthesized from formaldehyde.
Many critical remarks were addressed to these results. First, experiments in which a mixture of N2, CO2, and some H2O was exposed to electric discharges and heat did not yield amino acids, although such atmospheric conditions seem more realistic today. Besides, peptides (proteins)—as crucial compounds in extant life—could never be built up selectively from amino acids under prebiotic conditions. It is in general not very likely that larger molecules form spontaneously from building blocks in solution. Even if they do form, the sequence will be random and not specific; that is, no two identical copies will occur in billions of possible polymers. Moreover, many of the produced compounds are reactive, especially the nucleobases and sugars, and therefore not stable for many years under pre- biotic conditions. Hence, higher concentrations and stability for a longer time as required for organization processes need more complicated constructions for a plausible primeval enrichment.
To summarize, the experiments provide a possible explanation for the formation of building blocks for extant biopolymers. Further organic evolution pathways leading to the specific buildup of biopolymers are highly uncertain and exhibit several principal problems that still need to be solved by scientists.
Theories Involving Minerals
One major question in the field of chemical evolution is how a replicating system, a system that could generate and transfer information, could have evolved. The modern genetic machinery is much too complicated and was probably preceded by other systems.
Graham Cairns-Smith proposes that clay minerals were the first species that were able to reproduce. Clay minerals, being ubiquitous on the primitive Earth, are composed of many parallel layers that are stacked on each other. Information could be generated by stacking defects during crystallization that produced many identical layers until further modification, that is, occurrence of further mutation. Separation of the layers could spread new forms. Various types of surfaces formed and showed different catalytic activity for organic synthesis. Gradually, more and more complex organic substances were built up that finally started a more sophisticated replicating system replacing the clay replication system in a process termed genetic takeover.
Apart from a few controversial experimental investigations that show a conservation of layer structures on crystal growth, there is not much experimental evidence for the hypothesis. However, experimental investigations show the ability of clay minerals to attach organic molecules and to act as catalysts in organic syntheses.
Iron-Sulfur World Theory
Günther Wachtershauser introduced an alternative hypothesis that suggests a process of chemical evolution in the environment of hydrothermal systems. A geochemical reaction plays the central role, namely, the reaction of the mineral iron sulfide FeS with the gas H2S that provides the energy (reducing power) for the conversion of inorganic building blocks like CO2 and N2 to simple organic molecules. This reaction is driven by the formation of the mineral pyrite, commonly known as fool’s gold. Any generated organic material becomes electrostatic when attached to and accumulated at the surface of pyrite. Further reactions at the surface could lead to the development of a surface reaction system and finally a true chemo- autotrophic surface metabolism.
Proponents of this hypothesis rejected the idea that a situation like the primitive soup that led to a first heterotrophic organism ever existed on the primitive Earth. Instead, the first organism is supposed to have originated from the postulated surface metabolism.
The hypothesis gains attention because of the importance of iron-sulfur clusters in extant enzymes that convert simple molecules to complex molecules, for example, N2, H2, and CO, which also existed on the primitive Earth. Furthermore, one can find a rich fauna around hydrothermal vents (black smokers) at the bottom of the sea where hot fluxes rich in metal ions and sulfur species mix with cold sea H2O. Similar places presumably existed on the primeval Earth, too. Today, they host several Archaebacteria, some of the most ancient living species on Earth, which feed on chemical reactions. Hence, these bacteria may be successors of first living species in that habitat.
First experimental studies did prove the reducing power of the proposed energy source, for example, showing the reduction of N2 to NH3. Critics remark that a direct reduction of CO2 could not be observed in several studies. Moreover, it seems to be very difficult to verify experimentally whether the postulated complex metabolism would operate and how a replicating living system could finally emerge.
Precursors to Chemical Evolution on Earth
Comets, asteroids, and meteorites have always been deliverers of extraterrestrial material to Earth. Especially the primitive Earth was exposed to heavy bombardments, the last intense one 3.8 to 4 billion years ago. These impacts provided large amounts of H2O and carbon-containing substances probably available for the process of chemical evolution.
Asteroids and their collision fragments, meteorites, stem from the asteroid belt that is situated between the orbits of the planets Mars and Jupiter. Meteorites contain ancient material from the time of the formation of the solar system that resembles the material of the accreting Earth. Beside stony- iron and iron meteorites as two main groups, there are carbonaceous meteorites representing about 3% of the meteoric material on Earth. The carbonaceous meteorites contain substantial amounts of carbon, including small fractions of simple organic molecules such as hydrocarbons, fatty acids, and amino acids. Meteorites of Martian material that were flung into space during impact events on Mars have also been found on Earth.
Comets originate from the outer solar system. Hence, they are rich in volatiles like water ice and simple carbon-containing substances that gathered together with dust grains around 4.6 billion years ago. They contain the oldest, mainly unchanged material within the solar system. It is assumed that comets carried large amounts of H2O as well as some organic material to the primitive Earth, as simple amino acids could be detected in experimental studies simulating the behavior of cometlike material in space.
Although the impact of larger bodies led rather to a net loss of H2O and destruction of organic material, certain amounts of organic molecules still arrived on the primitive Earth carried by smaller bodies. It remains uncertain to which extent the extraterrestrial material contributed to the chemical evolution on the primitive Earth.
There are supporters of the idea that germs of life were brought to the primitive Earth by comets or other carriers. This hypothesis lacks evidence, and it just shifts the location but does not provide a plausible explanation for how chemical evolutionary processes proceeded.
Is Chemical Evolution the Pathway to Life?
It is crucial to first determine the driving forces of chemical evolutionary processes. The main parameters are energy and kinetics, which determine whether and how fast reactions can proceed. If there are several parallel reactions that can occur, there will be, under thermodynamic equilibrium conditions, a statistical distribution of the possible products at the end, depending on the relative energy of the products. Thus, it may take time until energetically only less-favored products reach a significant quantity to establish new reaction pathways. Furthermore, any energetically less- favored product tends to convert to another, more stable product. Thus, rather simple and low- molecular-weight compounds (monomers) that are energetically favored will be the main products of reactions under thermodynamic equilibrium conditions.
For the formation of more complex systems, statistically not favored conditions or nonequilibrium conditions are required that favor rather complex and high-molecular-weight compounds (polymers). The latter case is only attainable if a continual energy source is present that can sustain such a nonequilibrium system. High-molecular- weight compounds can form from a complex reaction system either by a series of chance events with almost zero probability or by sequential processes driven by a suitable energy input that keeps the system away from thermodynamic equilibrium. Such nonequilibrium conditions may be essential for the generation of more complex compounds in a reaction system directed by catalytically active substances.
Even if for some reason high-molecular-weight compounds form that contain some kind of inheritable information, there will be still many other factors that have to occur simultaneously to start up a living system. Due to the complexity of life and its emergent property, it is difficult to imagine a process that will inevitably lead to life. The process of chemical evolution toward the origin of life, because it is partly due to nonequilibrium conditions, can be better described as a contingent process. This means that more or less probable processes like pre-organization have to be interpreted as embedded in other independent, external processes whose probability of occurring cannot easily be estimated. So the origin of life as a result of chemical evolution is neither exclusively directed by chance (consequently, unique) nor completely deterministically explainable and inevitable.
Any molecular process cannot be purposive in the sense of ultimately producing life, because molecules are not dead or alive and therefore do not show the complex characteristics of living systems. Likewise there is no indication that natural laws possess a preference that favors the production of chemicals important for biochemistry and ultimately will lead to life.
Is Chemical Evolution a Repetitive Process?
Chemical evolution is a process driven by one or multiple energy sources. A conversion of available inorganic precursors to organic molecules is followed by further steps that produce more complex molecules until, finally, biochemical compounds start up life. Such a development cannot proceed in the presence of O2, it being harmful to organic molecules. Thus, sites that were exposed to the atmosphere could provide a possible environment only until about 2 billion years ago when the O2 content in the atmosphere significantly increased. Despite the deterioration of the atmospheric conditions, there probably existed niches devoid of O2 where chemical evolution still could take place.
A more serious problem was the heavy impact of asteroids or comets on the primitive Earth during the so-called late heavy bombardment around 3.8 billion years ago. At that time, Earth was regularly, in periods of every 10 to 100 million years, sterilized by heavy impacts whose violence could lead to the evaporation of the oceanic water and the melting of the earth’s crust down to 1 kilometer. One can imagine that all life having arisen between two such impacts suffered complete extinction, and a new development had to start from the beginning. The picture of production-extinction cycles may describe aptly the situation at that time. As long as the basic environmental conditions of the primitive Earth did not change dramatically for a long time, chemical evolution could repeatedly take place. It is possible that different kinds of biochemical systems developed and disappeared during that rough period and that the last one was chosen by chance as the basis for further evolution.
In addition, such differently distinctive biochemical systems may also have developed simultaneously or successively in different environments as the result of similar chemical evolutionary processes. Nevertheless, ultimately all but one became extinct due to competition or accidents, because all extant life is based on a common biochemistry.
Chemical Evolution in Extraterrestrial Places
In the 16th century, philosopher Giordano Bruno, at the expense of his life, claimed that many inhabited worlds existed. He might have been right, as a simple count of the number of stars in the universe yields an enormous number. Many stars probably have a planetary system, so that there should be a huge number of planets similar to Earth. Taking into account that the same physicochemical laws and principles are valid throughout the whole universe and with the same elements present, chemical evolutionary processes should be possible at many sites in the universe. Even if extraterrestrial biochemical systems different from the terrestrial ones developed, the crucial bioelements carbon, oxygen, nitrogen, and sulfur, as well as H2O, would have been required to start a chemical evolution.
At this point, one has to distinguish between (a) places of prebiotic synthesis of organic material in general, like dust clouds, meteorites, and comets; and (b) places of prebiotic synthesis that can lead to chemical evolution, such as planets or moons.
Interstellar clouds of dust contain a large variety of simple and reactive organic molecules, such as HCN and formaldehyde. Little is known about the mechanisms that alter the organic precursors, and there is no indication that more complex or even polymeric molecules can form. Nevertheless, the organic compounds in the dust grains will contribute to planetary material if solar systems arise from dust clouds.
Meteorites and comets contain larger and more complex organic molecules than was once thought by scientists. Remains of living systems, however, have not been found in that ancient material, and it is uncertain whether cosmic conditions allow life to arise there.
Terrestrial planets and moons, on the other hand, may have an atmosphere and some liquid solvent at the surface; in other words, there is a true environment for chemical evolutionary processes. Even if extraterrestrial bodies are similar to Earth, chemical evolution may have led to a different outcome. Because extrasolar sites are difficult to access, two prominent examples in our solar system, the planet Mars and Saturn’s largest moon Titan, are briefly discussed.
Mars had a planetary history similar to that of Earth. About 3.8 billion years ago, an atmosphere of CO2 probably caused a greenhouse effect that generated mild conditions with liquid H2O at the surface. This short period was finished by bombardment from space, leading to the loss of the atmosphere and thus the end of the greenhouse effect. The dropping of temperatures on Mars caused water to freeze, being covered by rock gradually; solid CO2 was deposited in the polar regions. It is assumed that geothermal processes releasing frozen water and CO2 from the ground as well as instabilities in the orbit around the sun could have led to repeated short warmer periods. Today, Mars’s surface is very cold and exposed to intense UV radiation so that no life is expected to be found there. It is rather probable that some chemical evolutionary process proceeded during the short warm period in Martian history 3.8 billion years ago and that the chemical record of that time exists deep in the rocks.
Saturn’s moon Titan has a thick N2 atmosphere with some proportions of CH4, a reducing atmosphere as postulated by the “primitive soup” theory. There is evidence for continents covered with frozen water that are surrounded by oceans of liquid hydrocarbons. The visit of the Huygens probe on Titan indicated the occurrence of complex chemical processes at the surface as well as in the atmosphere. However, at temperatures of about -180°C organic reactions will proceed at a very slow rate. Because H2O is solid, only a completely different pathway of chemical evolution using hydrocarbons, such as methane as solvent, could lead to some sort of biochemistry.
Verification Problems of Theories on Chemical Evolution
Any theory that proposes an explanation for chemical evolutionary processes is confined to an incomplete description. This is due, among other reasons, to our fragmentary knowledge of the primitive Earth, its atmosphere, and its detailed geological features, as almost no geological record from that time is conserved. Hence, it is impossible to imagine all possible sites where relevant chemical processes could proceed. Therefore, some local particularities that might have been decisive cannot be taken into consideration.
Nevertheless, this is not absolutely necessary as scientists are searching for plausible environments of chemical evolution. The exact place and the order of the pivotal processes of chemical evolution are and always will be out of the reach of scientific knowledge. Plausible environments are characterized by several features, such as availability of energy, input of molecular building blocks, and possible sites for accumulation and further modification of the generated molecules. Besides, every theory has to focus on a few central aspects of chemical interaction, simplifying other features of the environment, because it is not possible to include all potentially important characteristics within one theory. Thus, every theory may include some crucial features of the then-process.
One basic objection to any hypothesis is that it is not known how first life developed and how it was constituted. Most hypotheses assume a similar but much simpler constitution than extant life, and there are only a few hypotheses that propose first living systems on a completely different basis.
Limitations of Simulated Chemical Evolution
An experiment can only be as good as the hypothesis on which it is based. Experiments cannot take all environmental features into consideration, but some environmental conditions, over a certain period of time, might facilitate a complex process like chemical evolution. A high complexity in the interplay of the contributing factors to chemical evolution would hint at a singular process. If that process was singular, however, then a theoretical interpretation and an experimental repetition would be almost impossible.
Other challenges of experimental simulations are time and the rate of investigated reactions determined by kinetics. Chemical evolution might have required a time span of some 10 million years; thus, slow reactions also could have influenced the process. Such slow reactions proceeding over a period of, for example, 1,000 years may lead to a slow accumulation of certain reaction products that were important for chemical evolution. In the laboratory, such reactions have to be sped up by changing the reaction conditions to be able to detect some products at all. Different or more drastic reaction conditions may generate secondary reactions that change the primary result. Therefore, the outcome of simulation experiments has to be interpreted carefully.
Furthermore, experimenters may encounter other time problems when investigating buildup processes of various monomers, for example, amino acids to peptides. Statistics show that a huge number of possible sequences in a peptide results from the combination of few different amino acids. Hence, it will take many runs to find a desired peptide with a certain sequence.
Finally, it will be always uncertain whether experimental results show one feasible pathway of chemical evolution or the actual process of chemical evolution having proceeded on Earth.
Recent Chemical Evolution on Earth
Hypotheses about recent biogenesis existed until the mid-19th century. Then, Louis Pasteur showed that life is not spontaneously generated in rotten organic material, concluding that all life is generated from other life.
All living species known on Earth are based on the same basic principles. Thus, if there are still places where chemical evolution evolves into living systems, they will be somehow remote. Otherwise they would have already been detected. Because bacteria have been discovered in secluded areas as deep as several kilometers in the rock, such places or recesses at the bottom of the sea may be promising candidates. Energy could be provided by radioactivity or geochemical reactions there. However, it remains uncertain how such processes can be identified, considering that organic material is permanently produced and altered by natural processes and living species.
In general it is unlikely that processes similar to those that led to first life on Earth still occur, because the environmental conditions (e.g., the composition of the atmosphere) have changed much since then. This is due to chemical evolutionary processes that continually change the environment. Besides, there always will be interaction between living species as competitors for energy and nutrients.
Can Simulated Chemical Evolution Create Life?
Laboratory experiments can generate the basic constituents of modern life in chemical evolution simulations. Furthermore, the simulation of basic processes in living systems like replication, in the case of separate ribonucleic acid (RNA) strands and their replication enzyme, and metabolic reactions can be performed in the laboratory.
Nevertheless, these experiments are still remote from the creation of an interaction of the separate and complex replicable and metabolic machinery. These two running systems show a complex interplay so that the mixing of all necessary proteins and the genetic apparatus from dead bacteria in a nutrient-rich environment would probably not start new life easily. It is not sufficient to put all relevant molecules together to start life, as illustrated by the analogy that a heap of bricks is still far away from being a house. At present, the only possibility of simulating life-like systems is by computer modeling.
One can still imagine that simple living systems may arise unexpectedly and with uncommon characteristics in laboratory experiments. Such systems are difficult to predict because they may differ remarkably from known systems. It is uncertain whether such emerging living systems would form at all and whether they would be recognized as life.
Could Chemical Evolution
Occur Again on Earth?
Because there is still disagreement about how chemical evolution, leading to life, once proceeded and how unlikely this process is, it is rather difficult to predict the future. Starting from restrictions on possible present chemical evolution, future processes also will be limited to niches in remote areas. These restrictions seem even more plausible as an extinction of life may be caused or accompanied by some sort of environmental catastrophe that deteriorates the atmospheric situation. Future chemical evolution, if possible, would probably have to choose different pathways due to other environmental conditions.
Theodor Alpermann and Wolfgang Weigand
See also Chemistry; DNA; Evolution, Organic; Geologic Timescale; Life, Origin of
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