The definition of life in the context of its origin

Abstract

Current life is a complex, multi-level phenomenon that is so diverse in its manifestations that a short and exhaustive definition of life is hardly possible. The high complexity of life, as well as a poor understanding of what life is in essence, are the obstacles to the elaboration of such a definition. Important characteristics of life, such as whole system-, ecosystem-, and information-defined characteristics, have been included in the definition of life only recently. Ecosystem-defined characteristics have been absent in models of the pre-biotic state for a long time. However, without an ecosystem context, the concept of the emergence of life cannot be complete. Interconnections between living and non-living components of a primordial evolving system are decisive for the period of transition from chemical to biological evolution. Information-defined characteristics of life are often reduced to storage and the expression of genetic information, yet the operation of such perfect processes in prebiotic and transitional systems is unlikely. Genetic information, as defined in terms of the Shannon theory of communication, represents only a certain "informational channel" specified with respect to the expression of the structural genes. However, recent findings concerning the molecular mechanisms of the differential regulation of gene activity, and in the genomics, postgenomics and proteomics control mechanisms, suppose a richer diversity of informational flows in the organism. Moreover, considering life in a more general context, other types of related, informational channels, in particular, regarding the differentiation of higher taxa, hiatus, and expansion processes, should be kept in mind. In many publications devoted to the origin of life, the terms "living", "life", and "living organism" are freely interchanged which proves the vagueness of insights about the different levels of the living system. This report considers some variants of the definition of life that have been recently suggested and are based on presentday knowledge of the structures and functions of life. The contradictory demands of a definition, which needs to be complete and short at the same time, are emphasized. A definition characterizing life as a state, a structure, and a process, is proposed.