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life, although there is no universal agreement as to a definition of life, its biological manifestations are generally considered to be organization, metabolism, growth, irritability, adaptation, and reproduction. Protozoa perform, in a single cell, the same life functions as those carried on by the complex tissues and organs of humans and other highly developed organisms. The attributes of life are inherent in such minute structures as viruses, bacteria, and genes, just as they are in the whale and the giant sequoia. In seeking an understanding of life, scientists have broken down many barriers that once separated the physical sciences from the biological sciences; a result of the growth of biochemistry, biophysics, and other interrelated fields of study has been a better understanding of the composition and functioning of living tissues of all kinds.

Characteristics of Life

Organization is found in the basic living unit, the cell, and in the organized groupings of cells into organs and organisms. Metabolism includes the conversion of nonliving material into cellular components (synthesis) and the decomposition of organic matter (catalysis), producing energy. Growth in living matter is an increase in size of all parts, as distinguished from simple addition of material; it results from a higher rate of synthesis than catalysis. Irritability, or response to stimuli, takes many forms, from the contraction of a unicellular organism when touched to complex reactions involving all the senses of higher animals; in plants response is usually much different than in animals but is nonetheless present. Adaptation, the accommodation of a living organism to its present or to a new environment, is fundamental to the process of evolution and is determined by the individual's heredity. The division of one cell to form two new cells is reproduction; usually the term is applied to the production of a new individual (either asexually, from a single parent organism, or sexually, from two differing parent organisms), although strictly speaking it also describes the production of new cells in the process of growth.

The Basis of Life

Much of the history of biology and of philosophy as related to biology has been marked by a division of thought between vitalistic (or animistic) and mechanistic (or materialistic) concepts. In the most antithetic interpretations of these concepts, the vitalistic school maintains that there is a vital force that distinguishes the living from the nonliving and the mechanistic school holds that there is no essential difference between the animate and inanimate and that all life can be explained by physical and chemical laws. Such diametrically opposed views have actually seldom been held by investigators of either school; elements of both are usually involved. The animistic school, largely predicated on the inexplicability of the basic phenomena of life, has been greatly overshadowed by the accumulating weight of scientific data. As more and more is learned of the minute details of the structure and composition of the substances that make up the cell (to the extent that some have been synthesized chemically), it has become increasingly apparent that living matter is made up of the same (and only those) elements found in inorganic material, except that they are differently organized.

The Origin of Life

Fundamental religious concepts center around special creation and belief in the infusion of life into inanimate substance by God or another superhuman entity. On the other hand, many scientists have hypothesized that during an early geological period there gradually formed in the atmosphere increasingly complex organic substances composed of available inorganic compounds and water, utilizing ultraviolet rays and electrical discharges as energy sources. At a certain stage they formed a diffuse solution of "nutrient broth." Then in some way they were drawn together and developed the capacity for self-renewal and self-reproduction. In 1953, S. L. Miller synthesized several of the most basic amino acids in a glass flask by introducing an electrical discharge into an atmosphere of water vapor and some simple compounds thought to have been present naturally at the time when life first developed on earth. A more recent theory now widely held is that life originated in a volcanic setting more than 3.5 billion years ago, perhaps in hot deep-sea vents, utilizing a biochemistry based largely on sulfur and iron. The theory that life on earth came in a simple form from another planet has had small currency, although the discovery by Melvin Calvin of molecules resembling genetic material in meteors has given it some force.

Bibliography

See M. Calvin, Chemical Evolution (1969); E. Borek, The Sculpture of Life (1973); N. D. Newell, Creation and Evolution (1985); S. W. Fox and K. Dose, Molecular Evolution and the Origins of Life (3d ed. 1990); R. Fortey, Life (1998).

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life

State characterized by the ability to metabolize nutrients (process materials for energy and tissue building), grow, reproduce, and respond and adapt to environmental stimuli. Fossil evidence suggests that earth's first living organisms, bacteria and cyanobacteria, arose about 3.5 billion years ago. All known life-forms possess either DNA or RNA. Viruses, which possess DNA and RNA, cannot reproduce without a host cell and do not metabolize nutrients, and it is uncertain whether they should be classified as living or nonliving. Scientists disagree on the likelihood of extraterrestrial life. See also Drake equation.

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Life

U.S. picture magazine published weekly in New York City from 1936 to 1972 and in special editions thereafter. One of the most popular and widely imitated of U.S. magazines, it was founded by Henry R. Luce and quickly became a cornerstone of Time-Life Publications. From the start it emphasized photography, with gripping, superbly chosen news photographs, photographic features, and photo-essays by the best photographers; gradually more writing was added. Its war coverage—particularly that of World War II—was notably vivid, authentic, and moving. Life ceased publication largely because its costs outstripped revenues. It reappeared in special issues and then, from 1978 to 2000, as a monthly.

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life

1. the state or quality that distinguishes living beings or organisms from dead ones and from inorganic matter, characterized chiefly by metabolism, growth, and the ability to reproduce and respond to stimuli

2. 

a. a biography

b. (as modifier): a life story

3. all living things, taken as a whole

4. sparkle, as of wines

5. Arts drawn or taken from a living model

6. (in certain games) one of a number of opportunities of participation

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Life

Clotho

one of the three Fates, spins the thread that represents the life of each individual. [Gk. Myth.: NCE, 927]

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1.	(language)	LIFE - Logic of Inheritance, Functions and Equations. An object-oriented, functional, constraint-based language by Hassan Ait-Kacy <hak@prl.dec.com> et al of MCC, Austin TX, 1987. LIFE integrates ideas from LOGIN and LeFun. Mailing list: life-users@prl.dec.com. See also Wild_LIFE. ["Is There a Meaning to LIFE?", H. Ait-Kacy et al, Intl Conf on Logic Prog, 1991].
2.	(games)	Life - The first popular cellular automata based artificial life "game". Life was invented by British mathematician John Horton Conway in 1970 and was first introduced publicly in "Scientific American" later that year. Conway first devised what he called "The Game of Life" and "ran" it using plates placed on floor tiles in his house. Because of he ran out of floor space and kept stepping on the plates, he later moved to doing it on paper or on a checkerboard, and then moved to running Life as a computer program on a PDP-7. That first implementation of Life as a computer program was written by M. J. T. Guy and S. R. Bourne (the author of Unix's Bourne shell). Life uses a rectangular grid of binary (live or dead) cells each of which is updated at each step according to the previous state of its eight neighbours as follows: a live cell with less than two, or more than three, live neighbours dies. A dead cell with exactly three neighbours becomes alive. Other cells do not change. While the rules are fairly simple, the patterns that can arise are of a complexity resembling that of organic systems -- hence the name "Life". Many hackers pass through a stage of fascination with Life, and hackers at various places contributed heavily to the mathematical analysis of this game (most notably Bill Gosper at MIT, who even implemented Life in TECO!; see Gosperism). When a hacker mentions "life", he is more likely to mean this game than the magazine, the breakfast cereal, the 1950s-era board game or the human state of existence. Yahoo!. Demonstration. ["Scientific American" 223, October 1970, p120-123, 224; February 1971 p121-117, Martin Gardner]. ["The Garden in The Machine: the Emerging Science of Artificial Life", Claus Emmeche, 1994]. ["Winning Ways, For Your Mathematical Plays", Elwyn R. Berlekamp, John Horton Conway and Richard K. Guy, 1982]. ["The Recursive Universe: Cosmic Complexity and the Limits of Scientific Knowledge", William Poundstone, 1985].
3.	(jargon)	life - The opposite of Usenet. As in "Get a life!"