Ecology: The Biology of Interactions. 2.11. (Supplement) What Is Life?

Ukrainian (latest version) / Russian language (updates stopped)

2.10. (addendum) Evolution of the Universe, the Solar System, and Earth

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

2.12. (addendum) The Origin of Life. Prebiotic Systems

2.11. (addendum) What is life? For representatives of the exact sciences, it seems impossible that in biology, key concepts lack unambiguous definitions. These include the concepts of 'species', 'organism', 'adaptation', 'ecological niche', 'evolution', and others, including 'life'. The question of the essence of life belongs to the eternal questions that have no exhaustive answer. Some definitions of life are more like aphorisms: — we call life any nourishment, growth, and decay of the body, which has its basis within itself (Aristotle); — life is expansion (Andrei Dmitrievich Sakharov, Soviet physicist, creator of the hydrogen bomb). A common approach to developing a definition of life is to search for the most characteristic features of the living systems known to us. For example, Friedrich Engels, a classic of scientific communism, formulated a well-known definition in the 19th century, which fully corresponded to the level of knowledge at that time. Life is the mode of existence of protein bodies, the essential moment of which is the constant exchange of substances with external nature. The following definitions better correspond to modern understandings: — systems are called living if they contain nucleic acids and proteins and are capable of synthesizing these substances themselves; — systems are called living if they have a genotype encoded in nucleic acids. Viruses and viroids do not fit the first definition, and prions fit neither the first nor the second, if we consider them living systems. What if life is possible on a different chemical basis than what we know? It is unclear whether these definitions are suitable for biological systems at the early stages of the origin of life and for extraterrestrial living systems, whose organization is unknown to us. Many other definitions of life can be given. They will better or worse point to some feature of biosystems, but at the same time, they will refer to its partial, non-essential qualities and, moreover, may have many exceptions. Among the characteristics of living systems, there is no single one that would clearly distinguish living from non-living nature. For example, a mixture of proteins and nucleic acids is not alive. The implementation of any of the reactions occurring in a cell (or even their entire complex) in a chemical reactor will not make this reactor alive. Processes in an ordinary puddle or the flame of a burning candle have much in common with metabolism in an organism. A dead organism has a characteristic structure, is the result of ontogeny and phylogeny, and even carries out a certain metabolism, but it is still not alive. Not only living organisms are capable of reproducing themselves, but also salt crystallization nuclei in a saturated solution, as well as computer viruses in a personal computer operating system. The fact that biological viruses (and even more so viroids and prions) are traditionally considered part of the living world significantly complicates the distinction between living and non-living. In the cells of other organisms, viruses exhibit many properties of the living, but outside the cells, they are deprived of these properties. It would be logical to decide to consider viruses, viroids, and prions as molecular-genetic infectious systems and not consider them alive. Life, in this case, would be associated with cells — structures that have a whole complex of common features. However, this decision is contradicted not only by scientific tradition but also by the existence of life forms that have a transitional character between viruses and cells (very complex viruses and very simple bacteria). The most general approach to finding a definition of life is related to the thermodynamic properties of living systems. This is primarily due to the fact that living organisms are dissipative structures (Latin dissipatio — scattering), which increase their own orderliness by increasing the disorderliness of the environment. Accordingly, we propose the following definition of the phenomenon of life. Life is the maintenance and reproduction of characteristic highly ordered structures, which improves in the course of evolution and is carried out according to an internal program through external sources of matter and energy. This definition emphasizes the peculiarities of structure (but without detailing them: we do not know with what structures life can and cannot be associated), the presence of an internal program (without specifying what substances and how it functions), the ability to evolve, and the use of matter and energy. By adopting such a definition of life, one can understand why molecular-genetic infectious systems, which are not characterized by life in its manifestations known to us at the organismal level, can be classified as living systems. An essential feature of these systems is their ability to evolve. No matter how simple a virus is, it is the result of developing adaptations dependent on the features of the environment and the results of its interaction with it. To explain a virus, it is necessary to consider its entire history, and even then, it cannot be exhaustively and deterministically predicted. In contrast, artificial life, when created in a laboratory, will be subject to deterministic description. It will have precisely those properties that its creators deem necessary, considering certain regularities.

2.10. (addendum) Evolution of the Universe, the Solar System, and Earth

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

2.12. (addendum) The Origin of Life. Prebiotic Systems