Abstract
To approach the question of what life is, we first have to state that life exists exclusively as the “being-alive” of discrete spatio-temporal entities. The simplest “unit” that can legitimately be considered to be alive is an intact prokaryotic cell as a whole. In this review, I discuss critically various aspects of the nature and singularity of living beings from the biologist’s point of view. In spite of the enormous richness of forms and performances in the biotic realm, there is a considerable uniformity in the chemical “machinery of life,” which powers all organisms. Life represents a dynamic state; it is performance of a system of singular kind: “life-as-action” approach. All “life-as-things” hypotheses are wrong from the beginning. Life is conditioned by certain substances but not defined by them. Living systems are endowed with a power to maintain their inherent functional order (organization) permanently against disruptive influences. The term organization inherently involves the aspect of functionality, the teleonomic, purposeful cooperation of structural and functional elements. Structures in turn require information for their specification, and information presupposes a source. This source is constituted in living systems by the nucleic acids. Organisms are unique in having a capacity to use, maintain, and replicate internal information, which yields the basis for their specific organization in its perpetuation. The existence of a genome is a necessary condition for life and one of the absolute differences between living and non-living matter. Organization includes both what makes life possible and what is determined by it. It is not something “implanted” into the living beings but has its origin and capacity for maintenance within the system itself. It is the essence of life. The property of being alive we can consider as an emergent property of cells that corresponds to a certain level of self-maintained complex order or organization.
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Notes
This cell-as-elementary-organism theorem is not inconsistent with the well-known fact that the totipotency of the zygote gets lost in the descendants of the zygote during the ontogenesis of a multicellular organism. The cells become determined and differentiate into specific types of cells. This differentiation usually results from the differential expression of genes in the cell, i.e., from the differential regulation of transcription, posttranscriptional events, or translation but not from a loss of DNA or irreversible changes in the genome. It is only irreversible in certain types of cells. In many cases, differentiation is reversible under the right environmental circumstances. Transdifferentiation of one differentiated cell type into another type has been shown to occur for instance during regeneration and in cells in tissue culture.
Some living entities—spores of microorganisms, seeds of plants, some lower invertebrates (tardigrades, nematodes etc.)—are able to enter into an ametabolic state of “cryptobiosis” or “latent life” by undergoing a phase of liquid water loss. This process may result directly from evaporation or arise through vitrification promoted by the synthesis of a carbohydrate matrix accompanied by metabolic depression. Finally, metabolism comes close to being fully arrested. In this actively caused state of “frozen life,” the organisms are able to resist extreme environmental condition. In the literature, we find several reports of microorganisms being revived after millions of years from Precambrian or Silurian rocks or salt deposits (Dombrowski 1963; Vreeland et al. 2000). It is, however, still an open question whether those microorganisms are truly long-term survivors, or whether artifacts (contamination) could explain the revivals (Kennedy et al. 1994).
Moreno and Umerez (2000) consider downward causation in terms of formal causation, which they relate to the concept of information. In Aristotelian terms—so the authors—we can say “that DNA molecules are the formal cause of proteins ..., because their specific sequence of nucleotides convey the ìdea or form of the latter.” The informational components of the DNA constraint the lower-level chemical reactions that constitute the cell.
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Penzlin, H. The riddle of “life,” a biologist’s critical view. Naturwissenschaften 96, 1–23 (2009). https://doi.org/10.1007/s00114-008-0422-8
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DOI: https://doi.org/10.1007/s00114-008-0422-8