Life, Origins of

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Life, Origins of

Fossilized microbes or their chemical traces have been found in the oldest rocks on Earth. These rocks, which are about 3.8 billion years old, draw a picture of the structurally complex and metabolically sophisticated microbiota that already existed at that time. This leaves for the emergence of living things on Earth only a relatively short period of less than three hundred million years. During that period, complicated metabolic pathways must have developed. How?

Many religious as well as philosophical ideas assume that the interference with matter by a creative force or a creator results in the appearance of self-replicating life. Often the teleological character of living things in combination with a lack of understanding of natural processes leads to the conclusion that life is, in principle, "not of this world." In contract, the materialistic philosophical tradition interprets life as the most refined form of self-organizing matter.

It is an enormous intellectual challenge to explain the transition from lifeless chemistry to biology. The possible scenarios for this transition push scientific theories, worldviews, and the imagination to their limits. The following questions need to be answered. How is it possible that the beautifully well-balanced and encapsulated web of interaction that constitutes a living cell arose unintended from abiotic building blocks? How did its metabolism evolve? How could it happen that all the information that a parent cell needs to reproduce itself in the form of progeny was written in the chemical letters of the DNA molecule? And what was the first step that linked information (DNA) to function (proteins)? None of these questions has yet been answered by experimental evidence, however, numerous theories propose plausible scenarios that could result in the appearance of all living things. In addition, chemists have recreated chemical reactions that result in the formation of numerous of the most essential organic building blocks, such as amino acids and nucleic acids. Chemists have so far failed, however, to reconstruct the abiological formation under prebiotic conditions of the molecule ribose, which is essential to life as the backbone of DNA and RNA molecules. It has also been shown that amphiphilic compounds, such as fatty acids, spontaneously self-assemble and form encapsulated spheres, which separate into an "inside" from an "outside," and thus represent the origin of compartimentation. The most prominent of theories about the origin of life will be presented in this entry, and some of the problems related to the models will be discussed.

The Oparin-Haldane model

The Oparin-Haldane model of prebiotic evolution and cell formation was developed independently during the 1920s by British biologist J. B. S. Hal-dane and Soviet biochemist Aleksandr Oparin. According to the Oparin-Haldane model, organic molecules were formed in the reducing atmosphere of the early Earth and then accumulated in the oceans, where a thin organic solution, the socalled primordial soup, formed. In addition to the atmospheric source of organics, the theory also considers input from comets and certain types of meteorites as an important source of organic building blocks.

In the primordial soup, amphiphilic molecules, such as fatty acids, continuously formed small fatty droplets called coacervates. During self-assembly these coacervates encapsulated a small amount of the soup, which supplied them with building blocks and energy sources. Further coacervates grew by incorporation of more amphiphilic compounds until they became large enough to be unstable, resulting in coacervate division. Even during the coacervate state, competition and selection among these structures were driving a progressive evolution. The Oparin-Hal-dane model assumed that the original way of feeding was heterotrophic, which means that the cellular structures grew by assimilation of prefabricated organic building blocks that also served as energy sources. Overall, the model supposes that cells came first, proteins second, and genes third.

The major contribution of the Oparin-Haldane model to the scientific origin of life debate was to link abiotic chemistry to the history of life. Haldane and Oparin broke with the powerful and experimentally supported paradigm of the French chemist Louis Pasteur that only life can be the source of new life and that life can not arise spontaneously from a nonliving material. Despite its narrative eloquence, the Oparin-Haldane model has serious scientific shortcomings: (1) The atmosphere of the early Earth was most likely different and less reduced than the Oparin-Haldane model requires for the synthesis of all the molecules necessary for prebiotic synthesis; (2) The intensity of ultraviolet radiation on the surface of the ocean would have constrained the accumulation of prebiotic molecules. In addition, complex molecules are less stable when dissolved in water, which seriously limits the formation of information and function-carrying macromolecules like RNA, DNA, and proteins; and (3) Prebiotic synthesis produces only small amounts of the desired organic compounds.

Template-based models

Clay-based template. Some scientists have proposed alternatives to the primeval soup model that can be summarized as template-based origin of life models. One of them, the clay-based origin of life theory, attributes to microcrystals of clay both an informational and a catalytic function. According to the theory, the matrix of clay contains a regular array of ionic sites, which are occupied by irregularly alternating patterns of metal ions such as magnesium or aluminum. The pattern of alternating metal ions contains information similar to the patterns of nucleic acids in DNA or RNA molecules. Organic molecules, which are present in the environment of clay crystals, may have been attracted by the electrostatic potential of the metal ions and positioned themselves in a way that facilitates a chemical reaction between the adsorbed molecules (i.e., those attached to the surface). In this scenario, clay surfaces have a catalytic function comparable to those of proteins. The information content of clay crystals was transferred to a new generation of crystals by accreting silicate and metals from the surrounding water and by reproducing the original pattern of metal ions associated with the new clay matrix. It may be that clay-based life existed for millions of years but was finally out-competed by RNA-based life, which had much better chemical properties, and all traces of these original clay-based life forms disappeared.

The clay model has two major strengths: (1) A variety of clays do in fact catalyze the polymerization of organic compounds under conditions that are realistic for the early Earth; (2) Stereospecific amino acid binding and polymerization have been demonstrated. Among several weaknesses of the clay model are: (1) a lack of environmental settings that support clay evolution and stability; (2) the late development of cellularization ; and (3) the inability of the model to explain the relation between microorganisms and the presumed traits of early life.

Pyrite-based template. While the clay-model, like the Oparin-Haldane model, assumes that the organic molecules necessary for cell formation and polymerization were provided by a thin soup in the surroundings of the clay crystals, the pyrite-based template model rejects this concept and postulates the inorganic origin of life instead. In this scenario, organic molecules were synthesized in high temperature environments comparable to hydrothermal vents on the surface of growing pyrite crystals, which form from the reaction between ferrous sulfide and hydrogen sulfide. During the process of pyrite formation, electrons are released, which, due to the catalytic properties of the pyrite crystal, can be transferred directly to carbon dioxide. (In a later version of the theory based on experimental results, carbon monoxide replaced carbon dioxide.) In this process, simple reduced compounds like formic and acetic acid were formed. In addition to their catalytic properties, the positive charge of pyrite crystals allows them to retain the newly-synthesized negatively-charged organic molecules. Consequently, organic molecules accumulated on the pyrite surface, steadily coating it with an organic layer in a process of cellularization. It can thus also be attributed a selective property of the surface charge of the pyrite crystal: Molecules that do not stick to the crystal are lost by diffusion and do not participate in further processes. As simple molecules polymerized on the surface of the pyrite crystal, gradually more complex molecules were formed, including a membrane-like layer, which enclosed the pyrite crystal. In the pyrite model, metabolism came first. For a long time, competing cell-like structures resulted exclusively from the structuring properties of the pyrite crystal, which both served as the energy and electron source for carbon fixation, and as a cellularization nucleus without the involvement of information carriers such as RNA and DNA.

Unlike the other models, it is a major concern of the proponents of the pyrite model that all specific predictions, at least in principle, stand up to experimental investigation. Hitherto, several predictions deduced from the pyrite model have been verified experimentally. In addition, the presence of iron sulfur clusters in the catalytic centers of ancient enzymes has been interpreted as the remains of a pyrite past. Still the model is not without problems. Most of the criticism concerns the environmental sites where pyrite life potentially could have occurred. Hydrothermal vents are relatively short-lived structures, which limits the time available to pyrite-based life formation at a particular site. In addition, the high temperature, which may be required for some types of synthesis, may effect other processes negatively. Another serious concern addresses the phosphate necessary for the initial surface binding of organic compounds to pyrite. Because of precipitation, typical vent environments are strongly depleted in phosphate. If the same holds true for ancient vent systems, it is difficult to explain how life could have originated there.

RNA-world model. The most popular scenario of the origin of life is the so-called RNA-world model, which elaborates on the Haldane-Oparin theory. The RNA-world model proposes that RNA molecules were the precursors of proteins as catalysts and of DNA as information carriers. This concept gained support when the catalytic properties of modern RNA molecules were demonstrated. Proponents of this theory introduced the term ribozyme, which stresses the functional similarity of RNA molecules to protein molecules, in addition to their already established role as informational molecules. The following scenario has been outlined for the development of the RNA world: (1) Short RNA molecules formed from random combination of mononucleotides; (2) Some oligonucleotide structurally include the potential of catalyzing the synthesis of complementary copies of themselves, with the chemical energy for the process provided by reactive molecules combined with the mononucleotides; (3) RNA molecules developed that catalyzed their own synthesis, and as a consequence evolution by natural selection became possible.

Stage three in the RNA-world scenario can be summarized by the so-called hypercycle model, which links related RNA molecules in the form of a catalytic cycle. The interaction between RNA molecules was already characterized by a selection-constrained evolution potential. Steps one and two of the RNA-world model have some shortcomings which have yet to be overcome. These shortcomings include the synthesis of important components of the RNA molecule such as sugar ribose and reactive phosphate molecules. It has been proposed that not RNA itself but a simpler RNA-like molecule was at the origin of life.

Self-organizing models

As an alternative to the template-based scenarios described above, scientists have developed a theoretical framework that is based on the concept of catalytic closure. Here life started as autocatalytic sets of molecules, which means that all the molecules within the set catalyze their formation, and also catalyze the formation of their catalysts. A hypercycle is based on the interaction between RNA molecules. In principle, protein-based cycles or cycles which combine different types of macromolecules may also exist.

The scientific theories of the origin of life presented here are only preliminary and require numerous experiments and sophisticate modeling before they can gain general acceptance. At best, researchers will find answers to most of the open questions. At worst, they will end up producing more questions than they answer. The consequences of such scientific concepts for philosophical and religious traditions are likely to be marginal as the latter address mainly phenotypical expressions of living processes rather than "happenings" in the distant past.

See also Autopoeisis; Biology; Emergence; Evolution, Biological; Life, Religious and Philosophical Aspects; Life Sciences


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kai finster

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Life, Origins of

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