Life, Biological Aspects
Life, Biological Aspects
Biologically, life, as contrasted with death or with nonliving objects, is an evident fact but difficult to characterize precisely. Living organisms are self-maintaining systems; they grow and are irritable in response to stimuli. They resist dying. They reproduce. The developing embryo is especially impressive. Organisms post a defended, semipermeable boundary between themselves and the outside world; they assimilate environmental materials to their own needs. They can be healthy or diseased. Some accounts claim that the minimal form of autonomy necessary and sufficient for characterizing biological life is what is termed autopoiesis, literally self-making. Some defense of a "self" (a somatic self, having to do with the body, rather than a psychological self) is thus required.
Living organisms gain and maintain internal order against the disordering tendencies of external nature. They keep recomposing themselves, while inanimate things run down, erode, and decompose. Organisms pump out disorder. Life, as physicist Erwin Schrödinger notes in his 1945 work, What is Life?, is a local countercurrent to entropy, an energetic fight uphill in a world that overall moves thermodynamically downhill.
The organism as system
The constellation of these characteristics is nowhere found outside living organisms, although some of them can be mimicked or analogically extended to products designed by living systems, such as computers, and some are found in spontaneous abiotic nature. A crystal reproduces a pattern and may restore a damaged surface; a planetary system continues in an equilibrium; a volcano may grow in countercurrent to entropy. A lenticular altocumulus cloud, formed as a standing wave over a mountain range, is steadily recomposed by input and output of air flow. A target-seeking missile adjusts its course by environmental feedback. Computers are cognitive processors and can be running well or poorly.
The know-how for life is coded into genetic sets, which are missing in minerals, volcanoes, clouds, computers, and target-seeking missiles. An organism is thus a spontaneous cybernetic system, self-maintaining with a control center, sustaining and reproducing itself on the basis of information about how to make its way through the world. There is some internal representation that is symbolically mediated in the coded "program" and metabolism executing this goal, a checking against performance in the world, using some sentient, perceptive, or other responsive capacities by which to compare match and mismatch. On the basis of information received, the cybernetic system can reckon with vicissitudes, opportunities, and adversities that the world presents.
Organisms employ physical and chemical causes, but, distinctive to life, there is "information" superintending the causes. This information is a modern equivalent of what Aristotle (384–322 b.c.e.) called formal and final causes; it gives the organism a telos, or end, but not always a felt or conscious end-in-view. Formerly, biologists looked for entelechy, some distinctive component in organisms not found in merely physicochemical systems. Although entelechy was never found, the major discovery of biologists in the last half century has been massive amounts of information coded in DNA, a sort of linguistic molecule.
Living organisms impose a code on four nucleotide bases strung as cross links on a double helix. A triplet of bases stands for one of the twenty amino acids, and by a serial "reading" of the DNA, "translated" by messenger RNA, a long polypeptide chain is synthesized, such that its sequential structure predetermines the bioform into which it will fold. Ever-lengthening chains are organized into genes. Diverse proteins, lipids, carbohydrates, enzymes—all the life structures—are thus "written into" the genetic library.
The DNA representing life is thus, to continue analogies, a "cognitive set," not less than a biological set. Organisms use these molecular positions to code the information necessary for life. In this sense, the genome is a set of conservation molecules. The novel resourcefulness lies in the epistemic content conserved, developed, and thrown forward to make biological resources out of the physicochemical sources. The presence of this executive program is often said to be cybernetic, a word recalling a governor or helmsman. An open cybernetic system is partly a special kind of cause and effect system, partly a historical information system discovering and evaluating ends so as to map and make a way through the world, and partly a system of significances attached to operations, pursuits, resources.
Threshold of life
DNA codes a life that is carried on not merely at the level described above, but at the environmental, phenotypical level. What occurs at the level of molecular biology manifests itself, via a complicated translation and interaction from genotypic to phenotypic levels (i.e., from the microscopic level of the genes to the macro level of the whole organism). This translation occurs at the native ranges, where such life is selected for or against as it is defended in its environment. With this process in mind some analysts to define as alive whatever is subject to natural selection, thus presuming also mutation. These features typically do characterize life. Critics of this definition respond that some things (such as viruses or groups of organisms) are subject to natural selection but are not alive. Also, life sometimes continues with much reduced natural selection. This is seen in human in their cultural environment. This phenomenon is also witnessed in clonal organisms that are all genetically identical or in relatively constant environments where most genetic changes result from mutations that are categorized as drift (i.e., functional changes that neutral to survival, neither beneficial nor detrimental).
Various thresholds or borderlines of life are disputed. A person may be considered "brain dead," although somatically the heart is still beating (often with a mechanical respirator). Many biologists hold that viruses are not (fully) alive, but are anomalous self-reproducing DNA fragments that parasitize living cells, largely borrowing most of their vital metabolisms from the host cell. Viruses are not self-contained, not cellular, but must be contained within other selves and cells. Computer advances have raised the possibility of "artificial life," with debates about what would count as a living computer, or perhaps as a living program, within a computer. Some organic molecules are known from space, but no extraterrestrial life is yet known. Scientists, philosophers, and theologians speculate, often intensely, about whether such life is likely to be present.
See also Life, Religious and Philosophical Aspects; Life Sciences
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maturana, humberto r., and varela, francisco j. autopoiesis and cognition: the realization of the living. dordrecht, netherlands, and boston: d. reidel, 1980.
maynard smith, john, and szathmáry, eörs. the origins of life. oxford: oxford university press, 1999.
rosen, robert. life itself: a comprehensive inquiry into the nature, origin, and fabrication of life. new york: columbia university press, 1991.
schrödinger, erwin. what is life? cambridge, uk: cambridge university press, 1945.
holmes rolston, iii