Ecology: Biology of Interactions. II-13. (supplement) The Origin of Life. Pre-living Systems
We can conclude that three conditions are necessary and sufficient for the origin of life. These are: — the possibility of the full spectrum of transitional states between non-living and living systems; — the possibility of spontaneous transitions from one state to another, adjacent; — the action of selection that prev...
II-13. (supplement) The Origin of Life. Pre-living Systems
Mythological consciousness regarded the origin of life as the result of the work of various creators. Different religions developed these ideas, which can be called creationism — the teaching about the creation of life by a Creator. At the same time, in antiquity and the Middle Ages it seemed that the spontaneous generation of living organisms was a common occurrence. With the development of biology through the works of F. Redi (mid-17th century) and L. Pasteur (mid-19th century), it was shown that living organisms arise only from their own kind. Ideas emerged that an abyss separates inorganic and organic substances, to overcome which the action of "vis vitalis" — the vital force — was necessary. The artificial synthesis of urea, carried out in 1828 by F. Wöhler, began the refutation of such views.
In the 20th century, scientific (different from the naive ideas characteristic of antiquity) concepts about the possibility of the origin of organic substances from inorganic ones gained development. They formed the basis of various theories of abiogenesis, which considered possible ways for the first living organisms to arise from non-living matter. Unfortunately, among the general public and even among specialists in other fields of biology, incorrect ideas spread that life arose thanks to an incredible coincidence.
In the origin of life, random events play an important role, but this does not mean that a living thing can randomly arise from non-living matter. The probability of such an event is astronomically small. For chance to lead to self-organization, a mechanism for selecting certain random deviations is needed. Imagine a ball (say, a soccer ball) bouncing in place, using energy from some source. Its movements are analogous to the random changes in developing systems. Is it possible, through small jumps in random directions, to end up on the roof of a 16-story building? The answer seems to be "no," but it is wrong. It is impossible to fly up to the roof in one jump. However, if a staircase leads to the roof — many small steps — such a climb becomes possible. But having jumped onto one step, the ball can slide back down! Therefore, a mechanism is needed that "filters" changes leading in a certain direction. Such a filtering mechanism is natural selection.
Based on this metaphor, we can conclude that three conditions are necessary and sufficient for the origin of life. These are:
— the possibility of the full spectrum of transitional states between non-living and living systems;
— the possibility of spontaneous transitions from one state to another, adjacent;
— the action of selection that predominantly preserves and reproduces "more living" systems.
As far as can be judged on the basis of modern scientific data, all three of these conditions are met. Talking about the random origin of life belongs to the realm of scientific mythology; the task that is relevant today is to study the conditions for the transition of chemical evolution into biological evolution.
One of the important problems from this perspective is clarifying the mechanism of synthesis of organic substances. Without going into unnecessary details, we note that various organic molecules appear naturally under conditions corresponding to the early Earth, and even in outer space!
Earth is a planet with active and mobile shells: the lithosphere, hydrosphere, and atmosphere. Together with the activity of living organisms, their activity maintains biogeochemical cycles in the biosphere. It might seem that these cycles are the result of the existence of life, although in reality they are its cause.
The temperature of open space is 4°K (Kelvin degrees) above absolute zero, while the surface of a star like the Sun is 6,000°K. Planets are in a flow of energy dissipated by the central star. Due to the shape of planets, their surface is heated unevenly; since they rotate, this leads to cyclic changes in the amount of energy falling on their areas. If a planet has an atmosphere or hydrosphere, the unevenness of heating causes their circulation. If the temperature range is such that phase transitions of common substances occur within it (like water on Earth or methane on Titan, a moon of Jupiter), cycles on the surface of such a celestial body become particularly complex. The movement of the atmosphere and hydrosphere draws the surface of the lithosphere into itself. On large rocky planets that have a hot core, mantle, and crust, processes in the lithosphere are complicated by plate tectonics.
In addition to the movement of matter, various chemical reactions begin to proceed on the surface of such planets. Their prerequisite is the chemical complexity of the planetary surface, the presence of organic compounds on it. Cyclic changes in conditions provide a cyclic character to chemical reactions. The same transformations of substances can be provided by different competing reactions. Those reactions that prove to be most effective and stable (for example, due to autocatalytic effects) transform the greater part of the available resources and displace less effective reactions. Thus, even at the level of chemical reactions, the mechanism of natural selection is triggered.
Through natural selection at the level of autocatalytic chemical reactions, their improvement occurred, and mechanisms for storing energy emerged.
In the course of the origin of life on Earth or on another planet, systems must have existed that had an intermediate character between living and non-living. With the appearance of more efficient mechanisms for transforming matter and energy and, ultimately, of modern life, such systems must have disappeared. Our current knowledge about them is mostly hypothetical in nature but is constantly being supplemented through the study of the capacity of non-living systems for self-organization.
There are substantial grounds for the hypothesis that the modern biosphere was preceded by the so-called "RNA World." As is known, proteins (due to enzymatic activity) perform all the main biological functions, with the exception of encoding hereditary information — the function of DNA. In the organisms known to us, these two classes of polymers (DNA and proteins) are inextricably linked to each other. However, the polymer that ensures their interaction (RNA) is capable of performing both functions. The catalytic center of ribosomes, responsible for protein synthesis, is entirely built from RNA. Today, many RNA molecules with enzymatic activity — ribozymes — are known. At the same time, in a solution containing the necessary nucleotides, copies of RNA can form even in the absence of proteins. This gives reason to suppose that one of the stages in the origin of life was the "RNA World" — a world of primitive living (or pre-living) systems based on RNA. If life originated on Earth, the "RNA World" existed on it; and if it was brought to Earth from space — somewhere else. With the improvement of the pre-living systems of the "RNA World," catalytic functions could have passed to proteins, and the functions of storing genetic information — to DNA, a more stable and less chemically active polymer.
Additional materials:
Column: Pre-biological selection
Column: Pre-life