Ecology: Biology of Interactions. II-12. (supplement) What is Life?

II-12. (supplement) What is Life?
Life is not about finding yourself. Life is about creating yourself.
George Bernard Shaw
Representatives of exact sciences find it impossible that key concepts in biology have no unambiguous definitions. These include the concepts of "species", "organism", "adaptation", "ecological niche", "evolution", and others, including "life". The question of the nature of life belongs to the category of eternal questions without exhaustive answers. Some definitions of life are more like aphorisms (but, in the view of the authors of this textbook, — brilliant aphorisms):
— "by life we call any nutrition, growth, and decay of the body that have their foundation in themselves" (Aristotle);
— "life is expansion" (Andrei Dmitrievich Sakharov, Soviet physicist, co-creator of the hydrogen bomb).
A common approach to developing a definition of life is to search for the most characteristic features of living systems known to us. For example, Friedrich Engels, a classic of "scientific communism," formulated in the 19th century a well-known definition that fully corresponded to the level of knowledge of his time. "Life is the mode of existence of protein bodies, the essential moment of which is the constant exchange of substances with the surrounding external nature."
Modern understanding is better served by the following definitions:
— living systems are those that contain nucleic acids and proteins and are capable of synthesizing these substances themselves;
— living systems are those that have a genotype encoded in nucleic acids.
The first of these definitions is not met by viruses and viroids, and neither the first nor the second — by prions, if they are considered living systems. What if life is possible on a different chemical basis than what is known to us? It is unclear whether these definitions are applicable to biological systems at the early stages of the origin of life and to extraterrestrial living systems whose organization is unknown to us.
Many other definitions of life can be given. They will point better or worse to some feature of biosystems, but at the same time will touch on their individual, non-essential qualities, and moreover may have many exceptions. Among the characteristics of living systems, there is not a single one by which living and non-living nature could be clearly distinguished.
For example, a mixture of proteins and nucleic acids is not alive. Carrying out in a chemical reactor any of the reactions that occur in a cell (or even their entire complex) will not make this reactor alive. Processes in an ordinary puddle or in a candle flame have much in common with metabolism in an organism. A dead organism has a characteristic structure, is the result of onto- and phylogenesis, even carries out a certain exchange of substances, but is still not alive. The ability to reproduce themselves is not only possessed by living organisms, but also by crystallization nuclei of salt in its saturated solution and computer viruses in the operating system of a personal computer.
The fact that biological viruses (and even more so viroids and prions, i.e., infectious agents consisting of a single molecule of nucleic acid or protein) are traditionally considered part of living nature significantly complicates the distinction between the living and the non-living. In cells of other organisms, viruses exhibit many properties of the living, but outside cells they are deprived of these properties. It would be logical to consider viruses, viroids, and prions as molecular-genetic infectious systems and not regard them as living. Life would then be associated with cells — structures having a whole complex of common features. However, such a decision is contradicted not only by scientific tradition, but also by the existence of life forms that have a character intermediate between viruses and cells (very complex viruses and very simple bacteria).
The most general approach to finding a definition of life is connected with the thermodynamic properties of living systems. First of all, with the fact that living organisms are dissipative structures (Latin dissipatio — dissipation), which increase their own order as a result of the growth of disorder in the environment. Accordingly, we propose the following definition of the life phenomenon.
Life is the maintenance and reproduction of characteristic highly ordered structures, which are perfected through selection for reproductive efficiency and are carried out according to an internal program through external sources of matter and energy.
Note: the given definition in a certain sense follows the aphoristic definitions of Aristotle and Sakharov and combines their approaches. In this definition, the emphasis is on the peculiarities of structure (but without detailing them: we do not know which structures life can and cannot be associated with), the presence of an internal program (without specifying which substances enable it and how exactly it functions), the ability to evolve as a result of selection for reproducibility, as well as the use of matter and energy. By agreeing with such a definition of life, one can understand why molecular-genetic infectious systems can be classified as living systems, for which life in those manifestations at the organismal level that we know is not characteristic. An essential feature of these systems is their capacity for evolution. Simple as a virus is, it is the result of the development of adaptations that depend on the characteristics of the environment and the results of interaction with it by this molecular-genetic system. To explain a virus, it is necessary to consider its entire prior history, and, even so, it cannot be exhaustively, deterministically predicted.
In contrast to this, artificial life, when it is created in a laboratory, will be subject to deterministic description. It will have properties that, depending on certain patterns, its creators will consider necessary to give it.