Ecology: The Biology of Interactions. 2.12. (Supplement) The Origin of Life. Preliving Systems

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2.11. (supplement) What is Life?

D. Shabanov, M. Kravchenko. Ecology: Biology of Interactions Section 2. Biospherology

2.13. (supplement) Geochronological Scale

2.12. (supplement) The Origin of Life. Preliving Systems Mythological consciousness regarded the origin of life as the result of the work of various creators. Different religions developed these notions, which may be called creationism, the doctrine of the creation of life by the Creator. At the same time, in antiquity and the Middle Ages it seemed that spontaneous generation of living organisms was an ordinary occurrence. With the development of biology, through the work of F. Redi in the middle of the seventeenth century and L. Pasteur in the middle of the nineteenth, it was shown that living organisms arise only from their own kind. There arose the notion that inorganic and organic substances are separated by an abyss that can be overcome only by the action of vis vitalis, the vital force. The artificial synthesis of urea, carried out by F. Wöhler in 1828, marked the beginning of the refutation of such views. In the twentieth century, scientific ideas developed concerning the possibility that organic substances could arise from inorganic ones, ideas quite different from the naïve ones characteristic of antiquity. They formed the basis of various theories of abiogenesis, which consider possible pathways for the emergence of the first living organisms from nonliving matter. Unfortunately, among the general public, and even among specialists in other branches of biology, there spread the mistaken idea that life arose thanks to an improbable accident. Chance events do play an important role in the origin of life, but this does not mean that living matter can arise from nonliving matter purely by chance. The probability of such an event is astronomically small. For chance to lead to self-organization, a mechanism is required that selects particular random deviations. Imagine a ball, for example a football, bouncing in place by using energy from some source. Its movements are an analogue of random changes in developing systems. Is it possible, as a result of small jumps in random directions, to end up on the roof of a sixteen-story building? The obvious answer is “no,” but it is incorrect. It is impossible to leap onto the roof in a single jump. However, if a staircase made up of many small steps leads to the roof, such an ascent becomes possible. Yet after jumping onto one step, the ball may come down from it. Therefore a mechanism is needed that filters changes leading in a particular direction. Such a filtering mechanism is natural selection. With the help of this metaphor, we may conclude that for the origin of life three conditions are necessary and sufficient: - the possibility of a full spectrum of transitional states between nonliving and living systems; - the possibility of spontaneous transitions from one state to neighboring ones; - the action of selection that predominantly preserves and reproduces the “more living” systems. As far as one can judge on the basis of current scientific data, all three of these conditions are fulfilled. Talk about the random origin of life belongs to the domain of scientific mythology; now one should investigate the possibility of the transition from chemical evolution to biological evolution. One important problem from this standpoint is the elucidation of the mechanism of the synthesis of organic substances. Without going into excessive detail, let us note that diverse organic molecules arise naturally under conditions corresponding to the early Earth and even to open space. The Earth is a planet possessing active and mobile envelopes: the lithosphere, hydrosphere, and atmosphere. Together with the activity of living organisms, their activity maintains biogeochemical cycles in the biosphere. It may seem that these cycles are the result of the existence of life, whereas in fact they are its cause. The temperature of outer space is 4 K, kelvin, above absolute zero, whereas the surface temperature of a star like the Sun is 6,000 K. Planets are located within the flow of energy radiated by their central luminary. Because of the shape of planets, their surfaces are heated unevenly; if they rotate, this leads to cyclic changes in the amount of energy reaching particular areas. If a planet has an atmosphere or hydrosphere, this uneven heating leads to their circulation. If the range of temperature changes is such that phase transitions of common substances occur within it, as with water on Earth or methane on Titan, a satellite of Jupiter, the cycles on the surface of such a celestial body become especially complex. The movement of the atmosphere and hydrosphere entrains the surface of the lithosphere. On large planets possessing a hot core, mantle, and crust, processes in the lithosphere become more complex thanks to plate tectonics. In addition to the transport of substances, diverse chemical reactions begin on the surfaces of such planets. Their prerequisite is the chemical complexity of the planetary surface and the presence on it of organic compounds. The cyclical alternation of conditions ensures the cyclical character of chemical reactions. The same transformations of substances can be carried out by different competing reactions. Those reactions that prove the most effective and stable, for example thanks to an autocatalytic effect, transform most of the available resources and displace less effective reactions. Thus, already at the level of chemical reactions, the mechanism of natural selection is switched on. Because of natural selection acting at the level of autocatalytic chemical reactions, these reactions were improved, and mechanisms for the storage of energy appeared. In the course of the origin of life on Earth or on another planet, systems intermediate in character between the living and the nonliving must have existed. As more effective mechanisms for the transformation of matter and energy appeared and, finally, modern life emerged, such systems must have disappeared. Our current knowledge of them is largely hypothetical, but it is continually enriched by the study of the capacity of nonliving systems for self-organization. For example, one may assume that the modern biosphere was preceded by the so-called RNA World. As is well known, proteins, thanks to their enzymatic activity, perform all the main biological functions except the encoding of hereditary information, which is the function of DNA. In the organisms known to us, these two classes of polymers, DNA and proteins, are inseparably linked to each other. However, the polymer that ensures their interaction, RNA, is capable of performing both functions. The catalytic center of the ribosome, responsible for protein synthesis, is composed entirely of RNA. Many RNA molecules possessing enzymatic activity, ribozymes, are now known. At the same time, in a solution containing the necessary nucleotides, copies of RNA can also form in the absence of proteins. This gives grounds to suppose that one of the stages in the origin of life was the RNA World, a world of primitive living, or preliving, systems whose basis was RNA. If life originated on Earth, the RNA World existed here; if it was brought to Earth from space, then it existed somewhere else. As the preliving systems of the RNA World became more perfect, catalytic functions could pass to proteins, whereas the functions of storing genetic information could pass to DNA, a more stable and less chemically active polymer. Additional materials: Column: Prebiological Selection Column: Prelife

2.11. (supplement) What is Life?

D. Shabanov, M. Kravchenko. Ecology: Biology of Interactions Section 2. Biospherology

2.13. (supplement) Geochronological Scale