Organisms are usually being characterized as the systems with irritability andmetabolism. However, theirritability, i.e. the ability to receive signalsfrom the external environment, is exhibited by a great many nonliving systems, such as the security devices in cars or a central heating regulator. Again, the ability of metabolism, material conversion, is exhibited by a great many chemical dissipation systems. The only actually unique property of living systems, i.e. organisms (including the viruses), remains the capability of biological evolution.At the same time, it is quite probable that this is simultaneously a necessary and sufficient condition. It can be assumed that any system capable of undergoing biological evolution, whatever its physical nature, will sooner or later develop into a living system, i.e. also acquire the other types of traits encountered in contemporary organisms.
The capability of “biological evolution” must be defined in a manner so that we will avoid unacceptable circular definitions. Actually, the ability to undergo biological evolution overlaps to a certain degree with the ability to undergo natural selection (the source of usefulness). The ability to undergo biological evolution is a property consisting of a number of individual components. Only sufficiently complex systems, capable of undergoing natural selection, i.e. containing mutually competing elements capable of reproduction, variability and inheritance, can (and probably will) become a subject of biological evolution.