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Throughout recorded history human beings have recognized the qualitative difference between the living and non-living worlds, the animate and inanimate. Placing that recognition on solid, rational footing or giving it a quantitative basis has remained a major challenge, however. What exactly makes a living being so different from one that is nonliving? Living organisms carry out oxidation, for example, but so does a candle when it burns; living organisms grow from some sort of seedlike beginning to a larger form, but so does a crystal in a supersaturated solution; some organisms move, but others, like plants, do not; organisms reproduce (that is, make copies of themselves), but so do some molecules in the chemical process known as autocatalysis; viruses, probably the most confusing of all forms of matter in this regard, can enter living cells, reproduce themselves, break out of the cell, and infect other cells, yet they can also be crystallized and placed on a shelf for decades, only to become reactivated when placed in a solution in contact with host cells. In short, no single criterion or set of behaviors can unequivocally be said to distinguish living from nonliving matter. Yet there is also little doubt in most cases when we encounter a living organism. Life is characterized by a whole set of activities or functions, no one of which is unique but which collectively set living organisms apart from other physical entities.

In the history of Western thought, attempts to define life have been characterized by a series of alternative or dialectically opposed approaches that have reflected changing philosophical, cultural, and economic conditions. These alternative approaches will be described briefly and then applied to various aspects of the characterization of life.

Idealist versus Materialist Conceptions of Life

One of the oldest debates about the nature of life centered on whether living organisms functioned by means of a nonphysical process that lay outside material nature and therefore could not be fully understood by rational investigation or whether they could be understood in terms of everyday natural processes. The view that dominated the ancient and medieval worlds, known philosophically as idealism, claimed that living beings were qualitatively different from nonliving, representing a special set of categories whose "essence" existed only in the mind of the Creator. Associated particularly with the philosophy of Plato (c. 428348 or 347 b.c.e.), this idealistic perspective claims that rational understanding of the essence of life is philosophically impossible, since by definition the categories of each unique species exist not in the material world but only in the nonmaterial, essentialist categories conceived by the Creator. Idealists did not deny the material reality of living organisms but only claimed that the essence of living organisms could never be understood by human investigation. Most idealists saw life as originating from a special, supernatural process of creation by a nonmaterial being.

The diversity of living organisms observed in the world was always viewed as a product of the creation of separate essences known as species, which were absolute and immutable. The biologist's role was to try to understand the essence as much as possible by examining individual representatives of the species and determining their common or essential features. Variation among individual members of a species was recognized of course but was viewed as natural deviations from the "essence" in the same way that any given piece of pottery can be viewed as a deviation from the potter's mold. The Platonic tradition thus became the basis for the Western idealistic view of "life" in the biological sense, informing questions not only about the functionality of organisms but also about their origin.

Idealism continued to form a backdrop to discussions of the nature and origin of species in the eighteenth and nineteenth centuries in the works of the taxonomist Carolus Linnaeus (Carl von Linné; the so-called "father" of taxonomy), the anatomist and paleontologist Georges Cuvier, and others who continued to see species as fixed entities formed by special creation. The "scientific creation" movement in the United States in the 1970s and 1980s and "intelligent design" arguments in the early twenty-first century are yet more manifestations of idealistic thinking, because they are based on the claim that creation by supernatural (nonmaterial) processes has occurred and is as theoretically valid as theories of descent with modification by material processes, such as gene mutation, selective agents of the environment, and differential fertility. "Intelligent design" is idealistic in that it postulates a supernatural, nonmaterial "designer" to explain the structure, function (adaptation), and diversity of organisms.

A second approach to understanding life, known as materialism, denies that living organisms have any special status in the physical world, maintaining that they are material beings, more complex than other entities in the universe but not immune to rational study. To materialists, all aspects of living organisms can potentially be understood by the same processesknown at present or knowable in the futurethat govern all physical systems. Materialists have generally rejected all accounts of the origin of life by supernatural processes or nonmaterial "Creators." Historically the study of living systems has been characterized by the gradual retreat of idealistic in favor of materialistic approaches to understanding the nature of life.

Mechanistic materialism.

A long-standing debate among materialists has concerned whether and to what degree it is possible to treat organisms as simply special, complex kinds of machines or whether they are qualitatively different from machines, due to characteristics such as the ability to self-replicate or repair themselves, control their internal environment by self-regulating feedback loops, and so on. Mechanists argue that the basic principles on which machines functionmatter in motion, transformation of energy, chemical reactionsare also at work in living organisms and provide a way of understanding life in accordance with the same laws of physics that govern nonliving systems.

Proponents of mechanistic thinking advocate the idea that complex entities are composed of separate, dissociable parts; that each part has its own characteristics that can only be investigated separately from other parts; that the functioning of the whole organism or machine is a result of the sum of its interacting parts and nothing more; and finally, that changes in the state of a machine or organism are the result of factors impinging on it from the outside (for example, machines and organisms decline in function due to physical wear and tear over time).

With the advent of the scientific revolution in the sixteenth and seventeenth centuries, living organisms came to be seen for the first time as truly mechanical entities functioning physically like machines and chemically like alchemical retorts. The "mechanical philosophy," as it was called, was a version of mechanistic materialism, describing organisms in terms of levers, pulleys, and chemical combustions. William Harvey (15781657) compared the animal heart to a pump, with valves to insure one-way flow; Giovanni Borelli (16081679) described flight in birds as the compression of a "wedge" of air between the wings as they moved upward; and René Descartes (15961650) described the contraction of muscles as due to a hydraulic flow of "nervous fluid" down the nerves into the muscle tissue. This view persisted through the Enlightenment, which made the mechanical analogy explicit in its obsession with "automata," models of birds, insects, and humans that moved by a series of windup gears and levers, drank from dishes of water, flapped their wings, or crowed.

In the nineteenth and twentieth centuries mechanistic views again gained considerable support with the school of Berlin medical materialists, spearheaded by the physicist Hermann von Helmholtz (18211894). In a famous manifesto of 1847, Helmholtz and his colleagues Ernst Brücke and Emil Du Bois-Reymond stated emphatically that living organisms have no special "vital force," and thus research on organisms should be based only on the known laws of physics and chemistry. Life was, to the medical materialists, a manifestation of matter in motion. Their successor in the next generation, the German-born physiologist Jacques Loeb, after moving to the United States published a new version of the materialists' manifesto as the widely read book The Mechanistic Conception of Life (1912). With a blatant mechanistic, materialist bias, Loeb declared that organisms moving unconsciously toward a light source were "photochemical machines enslaved to the light" and that life could ultimately be explained in terms of the physical chemistry of colloidal compounds. Though somewhat extreme, such claims emphasized that the biologist needed to probe "life" with the tools of physics and chemistry, not abstract or metaphysical conceptions.

Holistic materialism.

Opposed to the mechanistic view is a philosophy known as holism. While some forms of holism, especially in the early twentieth century, had a mystical, idealistic quality about them (associated in particular with Ludwig von Bertalanffy [19011971] and Jakob von Uexküll [18641944] in Germany and Pierre Teilhard de Chardin in France), holistic views within a materialist framework have become more and more common since the 1960s. The holistic materialist view maintains that while organisms (or any complex systems, for that matter) are indeed only material entities, they acquire special properties by virtue of their multiple levels of organization (from the atomic and molecular to the organismic and populational) and through the interactions of their parts.

What is missing from the mechanistic view, to holistic thinkers, is the description of each component of a system in terms not only of its isolated properties but also of its interactions with others. The characteristics derived from such interactions are known as "emergent properties" and function at a higher level of organization (including the parts and their interactions) than the individual parts alone. A cell can carry out certain functions in isolation (in a culture dish), but the many different functions it carries out as part of a tissue (a group of like cells) represent a higher level of organization. Individual nerve cells, for example, can depolarize when stimulated and release neurotransmitters at their terminal ends, thus acting like neurons; but when integrated into a nerve network, they function to stimulate a whole set of other neurons that can lead to complex outcomes, such as coordinated muscle contraction or thought, which would be emergent properties of the complex, integrated system. Holistic materialists do not admit supernatural or metaphysical explanations, only the insistence that complex systems are more than the sum of their individual parts.

Dialectical materialism.

A particular version of holistic materialism known as dialectical materialism emerged in the later nineteenth century in the work of Karl Marx (18181883) and Friedrich Engels (18201895) and was further developed in the twentieth century by Karl Kautsky (18541938) and Gyorgy Plekhanov (18561918) in the Soviet Union and J. B. S. Haldane (18921964) and others in Britain and subsequently by Richard Lewontin and Richard Levins (The Dialectical Biologist; 1985) in the United States. Dialectical materialists maintain not only that the whole is greater than the sum of its parts and that complex systems have various levels of organization, each with its own emergent properties, but also that such systems are always in flux, changing dynamically due to the interaction of opposing forces within them. Thus organisms move developmentally through their life cycle in a constant struggle between the opposing forces of anabolism (building up of molecules, tissues) and catabolism (the breaking down of molecules and tissues). Ultimately the forces of catabolism win out, and death follows. Similarly evolution can be seen as change in a species over time due to the interaction of the opposing forces of heredity (faithful replication) and variation (unfaithful replication). Constant temperature in homoeothermic organisms is maintained by the interaction of heat-generating and heat-dissipating processes. A dialectical materialist view of life particularly emphasizes the dynamic, ever-changing nature of living systems.

Holistic approaches to "life"dialectical or otherwisehave become increasingly prominent in certain areas of the life sciences since the 1980s, for example, in physiology (especially the study of homeostatic feedback systems), in neurobiology (brain and behavior in particular), and in population biology and ecosystems work, where any useful understanding of the system must take into account numerous variables and their interactions. The advent of the computer in the study of such systems has aided greatly in providing ways of handling the immense amount of data that such investigations must utilize. Growing out of this revived holistic movement is an increasingly prominent field known as "systems science" or in some quarters as "the study of complexity."

Methodological Debates about the Study of Life

A corollary of the philosophical contrast between mechanistic and holistic materialist conceptions of life is the distinction between reductionist and integrative methodologies. Reductionism, closely allied to mechanistic materialism, is the view that the proper way to study organisms is to take them apart and examine and characterize their individual components in isolation under strictly controlled external conditions. For example, to study the way in which the heart functions, a reductionist would remove the organ from the body and place it in a chamber where temperature, pH, and concentration of other ions could be held constant. Integrative biologists argue that the reductionist approach is a necessary if insufficient approach to understanding complex systems. The heart in the intact animal, they point out, is connected to nerves, blood vessels, and other organs and thus is subject to neural and hormonal influences that cannot be understood from investigation of the heart in an isolated chamber. According to proponents of holism, it is necessary to devise methods for studying component parts in the living state, in the context of the whole organism of which they are a part (in vivo), as opposed to studying them only as isolated entities (in vitro).

A component of the methodological debate between reductionism and holism is the debate between the strictly observational and the experimental approach to living systems. Proponents of strictly observational studies, such as the Austrian animal behaviorist Konrad Lorenz (19031989) from the 1930s to the 1960s, argue that living systems must be studied in their natural context and that when experimenters bring organisms into the laboratory under highly artificial (controlled) conditions, they create an environment so foreign to the organisms that the information obtained is an artifact and of limited use. In contrast, proponents of experimentation, such as the behaviorist Daniel Lehrman, point out that restricting investigations to only what can be observed under "natural" conditions limits the kinds of questions the investigator can ask and the kinds of information that he or she can obtain. Such debates have surfaced in fields such as ecology, evolution, and animal behavior, where field investigators have often claimed that laboratory conditions are so different from those the organisms experience in the wild that the information obtained can have little relevance to how the organism functions in its natural habitat. Experimentalists argue that those who limit their work strictly to field observation have no ways to test their theories and consequently can never develop a rigorous, scientific explanation. Of course as many scientists and philosophers have pointed out, the approaches, like those of reductionism and holism, are actually complementary. Nonetheless, debates on reductionism and holism, observation and experimentation, have continued to resurface and influence the development of biology down to the twenty-first century.

Unity and Diversity in Living Organisms

Another important aspect of life is its vast diversity built on a base of underlying unity. For example, organisms as outwardly dissimilar as a bacterium, a human, and an oak tree are all composed of the same basic structural element, the cell, which in turn have many similar subcellular and molecular components. All eukaryotic cells (those cells of higher organisms that have a membrane-bound nucleus) contain mitochondria (organelles that carry out oxidation and thus provide energy), Golgi apparatus (a membrane system involved in packaging newly synthesized proteins), endoplasmic reticulum (a complex system of internal membranes), and ribosomes (small structures that form the site of protein synthesis). Simpler cells known as prokaryotes (bacteria, blue-green algae, and so forth), while lacking mitochondria and other organelles, contain ribosomes and share all the basic molecular infrastructure with eukaryotes. For example, both prokaryotes and eukaryotes have their hereditary information encoded in the molecule of deoxyribonucleic acid (DNA), transcribe that message into messenger RNA (mRNA), and translate that message into proteins in exactly the same ways. Furthermore the language in which the DNA code is written is the same in all organisms: as a triplet in which specific sequences of three out of four possible bases (adenine, thymine, guanine, and cytosine) specify each of the twenty amino acids that make up all proteins in the living world. Thus beneath apparent diversity lies a major infrastructure of unity. How to interpret this obvious contradiction has motivated a wide variety of views of the nature of life since at least the early twentieth century.

The Molecular and Biochemical View of Life

Deriving from the reductionism-holism debate, an important issue from the 1930s onward has been the extent to which living systems are ultimately reducible to molecules and chemical reactions. Biochemical definitions of life surfaced in the late nineteenth century with the discovery of enzymes as "living ferments" and became particularly prominent during the heyday of biochemical work (in England and Germany from 1920 to 1939) on enzyme-catalyzed pathways for synthesizing or degrading the major molecules in living systems. Many biochemists, flushed with success in elucidating the multistep pathways for fermentation or oxidation, attempted to define life in terms of enzyme catalysis. They held that what differentiated living from nonliving systems was the rapidity with which enzyme-catalyzed reactions and energy conversions could take place and the precision with which they could be controlled. The cell, one biochemist argued, is nothing more than a bag of enzymes. The biochemical view of life paid little if any attention to cell structure and organization, focusing almost exclusively on metabolic pathways, their interconnections, reaction kinetics, and energetics.

In the decades following the working out of the double helical structure of DNA by James D. Watson and Francis Crick in 1953, the biochemical definition of life was replaced by, or encompassed within, what came to be called the molecular view of life. The molecular view was more comprehensive than the biochemical, including the study of the three-dimensional structure of molecules, such as hemoglobin and myoglobin, and attention to cell structure and its relation to function, using techniques such as electron microscopy, ultracentrifugation, electrophoresis, fluorescence dyeing, and later confocal microscopy. Paradigmatic along these lines were detailed investigation of the structure of hemoglobin, the oxygen-carrying molecule in animal blood, and the discovery of its allosteric changes (positional shifts) in structure as it alternately bonds to and releases oxygen. The molecular and biochemical views of life tended to be highly reductionist, seeing life as merely a manifestation of molecular structure. Nonetheless, the molecular view did emphasize the importance of understanding life in terms of molecular configurations and the ways various molecules interacted chemically in such living processes as respiration, photosynthesis, protein synthesis, cell-to-cell communication, and signal transduction (the way a cell responds internally to receiving a specific message from the outside).

A particularly prominent aspect of the biochemical and molecular views of life has been the field of abiogenesis or the origin of life. Beginning with the work of the Russian biochemist A. I. Oparin (18941980) in the 1930s through that of Sidney Fox (19122001) from the 1950s to the 1990s, Stanley Miller in the 1950s, and Cyril Ponnamperuma in the 1970s, investigations as to how living systems might have originated on the primitive earth (or other extraterrestrial bodies) have gained considerable attention. Oparin showed that simple globular formulations that he called coacervates (formed from gum arabic and other organic substances in an aqueous medium) could perform simple functions analogous to living cells (movement, fission). Miller's experiments in the early 1950s demonstrated that basic amino acids, sugars, and other organic compounds (formic acid, urea) could be produced from components of what was hypothesized to have been the earth's early atmosphere (ammonia, carbon dioxide, water vapor, and hydrogen), thus giving credence to the view that life could indeed have originated on earth by simple biochemical processes. Later work of Fox and others on how the basic building blocks of organic matter (amino acids, simple sugars, nucleotides, and glycerides) could have become organized into macromolecules and basic cell structures showed that the origin of the next level of organization up from the molecule, the cell and its components, could be studied by experimental means. These investigations gave considerable support to the view that life is truly an expression, though an emergent one, of the basic properties of all matter, as understood through the analysis of atomic and molecular structure.

See also Behaviorism ; Biology ; Creationism ; Determinism ; Development ; Ecology ; Evolution ; Historical and Dialectical Materialism ; Life Cycle ; Materialism in Eighteenth-Century European Thought ; Natural History ; Nature ; Naturphilosophie ; Organicism ; Science, History of ; Sexuality ; Suicide .


Allen, Garland E. "Dialectical Materialism in Modern Biology." Science and Nature 3 (1980): 4357.

. Life Science in the Twentieth Century. New York: Wiley, 1975.

Bertalanffy, Ludwig von. Problems of Life. New York: Harper, 1960. Translation of vol. 1 of Das Biologische Weltbild (1949).

Coleman, William. Biology in the Nineteenth Century: Problems of Form, Function, and Transformation. New York: Wiley, 1971.

Fruton, Joseph S. Proteins, Enzymes, Genes: The Interplay of Chemistry and Biology. New Haven, Conn.: Yale University Press, 1999.

Hall, Thomas S. Ideas of Life and Matter. 2 vols. Chicago: University of Chicago Press, 1969.

Harrington, Anne. Reenchanted Science. Princeton, N.J.: Princeton University Press, 1996.

Lenoir, Timothy. The Strategy of Life: Teleology and Mechanism in Nineteenth-Century German Biology. Dordrecht, Netherlands: Reidel, 1982.

Mayr, Ernst. The Growth of Biological Thought. Cambridge, Mass.: Belknap Press, 1982.

Oparin, A. I. Life: Its Nature, Origin, and Development. Translated by Ann Synge. New York: Academic Press, 1961.

Garland E. Allen

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life / līf/ • n. (pl. lives / līvz/ ) 1. the condition that distinguishes animals and plants from inorganic matter, including the capacity for growth, reproduction, functional activity, and continual change preceding death: the origins of life. ∎  living things and their activity: some sort of life existed on Mars lower forms of life the ice-cream vendors were the only signs of life. ∎  the state of being alive as a human being: she didn't want to die; she loved life a superficial world where life revolved around the minutiae of outward appearance. ∎  [with adj.] a particular type or aspect of people's existence: an experienced teacher will help you settle into school life revelations about his private life. ∎  vitality, vigor, or energy: she was beautiful and full of life. 2. the existence of an individual human being or animal: a disaster that claimed the lives of 266 Americans. ∎  [often with adj.] a way of living: his father decided to start a new life in California. ∎  a biography: a life of Shelley. ∎  either of the two states of a person's existence separated by death (as in Christianity and some other religious traditions): too much happiness in this life could reduce the chances of salvation in the next. ∎  any of a number of successive existences in which a soul is held to be reincarnated (as in Hinduism and some other religious traditions). ∎  a chance to live after narrowly escaping death (esp. with reference to the nine lives traditionally attributed to cats). 3. (usu. one's life) the period between the birth and death of a living thing, esp. a human being: she has lived all her life in the country I want to be with you for the rest of my life they became friends for life. ∎  the period during which something inanimate or abstract continues to exist, function, or be valid: underlay helps to prolong the life of a carpet. ∎ inf. a sentence of imprisonment for life. 4. (in art) the depiction of a subject from a real model, rather than from an artist's imagination: the pose and clothing were sketched from life [as adj.] life drawing. See also still life. PHRASES: bring (or come) to life regain or cause to regain consciousness or return as if from death: all this was of great interest to her, as if she were coming to life after a long sleep. ∎  (with reference to a fictional character or inanimate object) cause or seem to be alive or real: he brings the character of MacDonald to life with power and precision all the puppets came to life again. ∎  make or become active, lively, or interesting: soon, with the return of the peasants and fishermen, the village comes to life again you can bring any room to life with these coordinating cushions. do anything for a quiet life make any concession to avoid being disturbed. for dear (or one's) life as if or in order to escape death: I clung to the tree for dear life Sue struggled free and ran for her life. for the life of me inf. however hard I try; even if my life depended on it: I can't for the life of me understand what it is you see in that place. frighten the life out of terrify. get a life [often in imperative] inf. start living a fuller or more interesting existence: if he's a lout, then get yourself out of there and get a life. give one's life for die for. (as) large as life inf. used to emphasize that a person is conspicuously present: he was standing nearby, large as life. larger than life (of a person) attracting special attention because of unusual and flamboyant appearance or behavior. ∎  (of a thing) seeming disproportionately important: your problems seem larger than life at that time of night. life and limbsee limb1 . the life of the party a vivacious and sociable person. life in the fast lane inf. an exciting and eventful lifestyle, esp. one bringing wealth and success. one's life's work the work (esp. that of an academic or artistic nature) accomplished in or pursued throughout someone's lifetime. lose one's life be killed: he lost his life in a car accident. a matter of life and death a matter of vital importance. not on your life inf. said to emphasize one's refusal to comply with a request: “I want to see Clare alone.” “Not on your life,” said Buzz. save someone's (or one's own) life prevent someone's (or one's own) death: the driver of the truck managed to save his life by leaping out of the cab. ∎ inf. provide much-needed relief from boredom or a difficult situation. see life gain a wide experience of the world, esp. its more pleasurable aspects. take one's life in one's hands risk being killed. take someone's (or one's own) life kill someone (or oneself). that's life an expression of one's acceptance of a situation, however difficult: we'll miss each other, but still, that's life. this is the life an expression of contentment with one's present circumstances: Ice cubes clinked in crystal glasses. “This is the life,” she said. to the life exactly like the original: there he was, Nathan to the life, sitting at a table. to save one's life [with modal and negative] even if one's life were to depend on it: she couldn't stop crying now to save her life.

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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.


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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244. Life

See also 44. BIOLOGY ; 430. ZOOLOGY

Biology. the production of living organisms from inanimate matter. Also called spontaneous generation . abiogenetic , adj.
a state or condition in which life is absent. abiotic, abiotical , adj.
a revival or return to a living state after apparent death. anabiotic , adj.
the study of the chemical processes that take place in living organisms. biochemist , n. biochemical , adj.
biogenesis, biogeny
1. the process by which living organisms develop from other living organisms.
2. the belief that this process is the only way in which living organisms can develop. biogenetic, biogenic , adj.
the science or study of all manner of life and living organisms. biologist , n. biological , adj.
the destruction of life, as by bacteria. biolytic , adj.
biometrics, biometry.
1. the calculation of the probable extent of human lifespans.
2. the application to biology of mathematical and statistical theory and methods. biometric, biometrical , adj.
that part of the earths surface where most forms of life exist, specifically those parts where there is water or atmosphere.
Philosophy. the theory or doctrine that all the phenomena of the universe, especially life, can ultimately be explained in terms of physics and chemistry and that the difference between organic and inorganic lies only in degree. Cf. vitalism . mechanist , n. mechanistic , adj.
ontogeny. ontogenetic, ontogenetical , adj.
the life cycle, development, or developmental history of an organism. Also called ontogenesis . ontogenic , adj.
Biology. the development of an egg or seed without fertilization. Also called unigenesis . parthenogenetic , adj.
the branch of biology that studies the functions and vital processes of living organisms. physiologist , n. physiologic, physiological , adj.
spontaneous generation
asexual reproduction; parthenogenesis. unigenetic , adj.
1. Philosophy. the doctrine that phenomena are only partly controlled by mechanistic forces and are in some measure self-determining.
2. Biology. the doctrine that the life in living organisms is caused and sustained by a vital principle that is distinct from all physical and chemical forces. Cf. mechanism . vitalist , n. vitalistic , adj.
Phrenology. 1. the love of life and fear of death.
2. the organ serving as the seat of instincts of self-preservation.
1. Philosophy. a doctrine that the phenomena of life are controlled by a vital principle, as Bergsons élan vital.
2. a high regard for animal life.
3. a belief in animal magnetism. zoist , n. zoistic , adj.

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life if life hands you lemons, make lemonade an adjuration to make the best of difficult circumstances; saying recorded from the late 20th century.
life begins at forty proverbial saying, mid 20th century, first recorded as the title of a book (1932) by Walter B. Pitkin.
life in the fast lane an exciting and eventful lifestyle, especially one bringing wealth and success.
life isn't all beer and skittles life is not unalloyed pleasure or relaxation; proverbial saying, mid 19th century.
while there's life there's hope proverbial saying, mid 16th century, often used as encouragement not to despair in an unpromising situation. The saying is found earlier in Greek and Latin, as in the writings of the Greek poet Theocritus (c.310–250 bc), ‘there's hope among the living.’ It is reworked in a modern political quotation by the Labour politician Richard Crossman (1907–74), who is said to have commented on the death of the Labour leader Hugh Gaitskell in 1963, ‘While there is death there is hope.’

See also art is long and life is short, book of life, a dog is for life, large as life, and twice as natural, a slice of life, the staff of life, thread of life, tree of life.

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"life." The Oxford Dictionary of Phrase and Fable. . 16 Dec. 2017 <>.

"life." The Oxford Dictionary of Phrase and Fable. . (December 16, 2017).

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life A state of physical entities that utilize substances derived from outside themselves for the purposes of growth, the repair of their own structure, and the maintenance of their functional systems, and that also reproduce. The oldest known fossils are from rocks about 3300 Ma old and the Earth is estimated to be about 4600 Ma old. If it is accepted that life began on Earth (rather than being introduced from elsewhere), then it must have done so during the intervening 1300 Ma. There is disagreement on the mechanism by which this happened. The classic view is that lightning discharges in a reducing atmosphere produced prebiotic substances which became dissolved in the oceans where, protected from ultra-violet light, they were somehow assembled into the first organisms. Recently, evidence has been advanced to suggest that life may have originated in small freshwater pools in an atmosphere of carbon dioxide with traces of ammonia.

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"life." A Dictionary of Ecology. . 16 Dec. 2017 <>.

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life Feature of organisms that sets them apart from inorganic matter. Life can be regarded as the ability to obtain energy from the Sun or from food, and to use this energy for growth and reproduction. The current theory on life's origin is that giant molecules, similar to proteins and nucleic acids, reacted together in the watery surface environment of the young Earth that is now commonly called the ‘primordial soup’. The evolution and development of cellular life out of this molecular ‘pre-life’ has yet to be explained fully. The various stages through which an organism passes make up its life cycle.

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"life." World Encyclopedia. . 16 Dec. 2017 <>.

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life OE. līf. corr. to OS. līf life, person (Du. lijf body), OHG. līb life (G. leib body), ON. līf life, body :- Gmc. *līƀam(-az), f. *līƀ, the weak grade of which appears in LIVE1.
Hence lifeguard bodyguard of soldiers. XVII. prob. after Du. †lijfgarde, G. leibgarde (in which the first el. means ‘body’), later assoc. with life.

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"life." The Concise Oxford Dictionary of English Etymology. . 16 Dec. 2017 <>.

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407. Life

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

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"Life." Allusions--Cultural, Literary, Biblical, and Historical: A Thematic Dictionary. . 16 Dec. 2017 <>.

"Life." Allusions--Cultural, Literary, Biblical, and Historical: A Thematic Dictionary. . (December 16, 2017).

"Life." Allusions--Cultural, Literary, Biblical, and Historical: A Thematic Dictionary. . Retrieved December 16, 2017 from


lifefife, Fyfe, knife, life, pro-life, rife, still-life, strife, wife •shelf-life • midlife • wildlife •nightlife • lowlife • afterlife •jackknife • penknife • paperknife •spaewife • alewife • midwife •fishwife • housewife • goodwife

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"life." Oxford Dictionary of Rhymes. . 16 Dec. 2017 <>.

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