|Life on a rocky peak in the Waitakere Ranges|
|Domains and Kingdoms|
Life (cf. biota) is a characteristic that distinguishes objects that have self-sustaining biological processes from those that do not–either because such functions have ceased (death), or else because they lack such functions and are classified as inanimate.
In biology, the science of living organisms, life is the condition which distinguishes active organisms from inorganic matter, including the capacity for growth, functional activity and the continual change preceding death. A diverse array of living organisms (life forms) can be found in the biosphere on Earth, and properties common to these organisms—plants, animals, fungi, protists, archaea, and bacteria—are a carbon- and water-based cellular form with complex organization and heritable genetic information. Living organisms undergo metabolism, maintain homeostasis, possess a capacity to grow, respond to stimuli, reproduce and, through natural selection, adapt to their environment in successive generations. More complex living organisms can communicate through various means.
In philosophy and religion, the conception of life and its nature varies. Both offer interpretations as to how life relates to existence and consciousness, and both touch on many related issues, including life stance, purpose, conception of a god or gods, a soul or an afterlife.
Some of the earliest theories of life were materialist, holding that all that exists is matter, and that all life is merely a complex form or arrangement of matter. Empedocles (430 B.C.) argued that every thing in the universe is made up of a combination of four eternal 'elements' or 'roots of all': earth, water, air, and fire. All change is explained by the arrangement and rearrangement of these four elements. The various forms of life are caused by an appropriate mixture of elements. For example, growth in plants is explained by the natural downward movement of earth and the natural upward movement of fire.
Democritus (460 B.C.), the disciple of Leucippus, thought that the essential characteristic of life is having a soul (psychê). In common with other ancient writers, he used the term to mean the principle of living things that causes them to function as a living thing. He thought the soul was composed of fire atoms, because of the apparent connection between life and heat, and because fire moves. He also suggested that humans originally lived like animals, gradually developing communities to help one another, originating language, and developing crafts and agriculture.
Hylomorphism is the theory (originating with Aristotle (322 BC)) that all things are a combination of matter and form. Aristotle was one of the first ancient writers to approach the subject of life in a scientific way. Biology was one of his main interests, and there is extensive biological material in his extant writings. According to him, all things in the material universe have both matter and form. The form of a living thing is its soul (Greek 'psyche', Latin 'anima'). There are three kinds of souls: the 'vegetative soul' of plants, which causes them to grow and decay and nourish themselves, but does not cause motion and sensation; the 'animal soul' which causes animals to move and feel; and the rational soul which is the source of consciousness and reasoning which (Aristotle believed) is found only in man. Each higher soul has all the attributes of the lower one. Aristotle believed that while matter can exist without form, form cannot exist without matter, and therefore the soul cannot exist without the body.
Consistent with this account is a teleological explanation of life. A teleological explanation accounts for phenomena in terms of their purpose or goal-directedness. Thus, the whiteness of the polar bear's coat is explained by its purpose of camouflage. The direction of causality is the other way round from materialistic science, which explains the consequence in terms of a prior cause. Modern biologists now reject this functional view in terms of a material and causal one: biological features are to be explained not by looking forward to future optimal results, but by looking backwards to the past evolutionary history of a species, which led to the natural selection of the features in question.
Vitalism is the belief that the life-principle is essentially immaterial. This originated with Stahl (17th century), and held sway until the middle of the nineteenth century. It appealed to philosophers such as Henri Bergson, Nietzsche, Wilhelm Dilthey, anatomists like Bichat, and chemists like Liebig.
Vitalism underpinned the idea of a fundamental separation of organic and inorganic material, and the belief that organic material can only be derived from living things. This was disproved in 1828 when Wöhler prepared urea from inorganic materials. This so-called Wöhler synthesis is considered the starting point of modern organic chemistry. It is of great historical significance because for the first time an organic compound was produced from inorganic reactants.
Later, Helmholtz, anticipated by Mayer, demonstrated that no energy is lost in muscle movement, suggesting that there were no vital forces necessary to move a muscle. These empirical results led to the abandonment of scientific interest in vitalistic theories, although the belief lingered on in non-scientific theories such as homeopathy, which interprets diseases and sickness as caused by disturbances in a hypothetical vital force or life force.
It is still a challenge for scientists and philosophers to define life in unequivocal terms. Defining life is difficult —in part— because life is a process, not a pure substance. Any definition must be sufficiently broad to encompass all life with which we are familiar, and it should be sufficiently general that, with it, scientists would not miss life that may be fundamentally different from earthly life.
Since there is no unequivocal definition of life, the current understanding is descriptive, where life is a characteristic of organisms that exhibit all or most of the following phenomena:
To reflect the minimum phenomena required, some have proposed other biological definitions of life:
Viruses are most often considered replicators rather than forms of life. They have been described as "organisms at the edge of life", since they possess genes, evolve by natural selection, and replicate by creating multiple copies of themselves through self-assembly. However, viruses do not metabolise and require a host cell to make new products. Virus self-assembly within host cells has implications for the study of the origin of life, as it may support the hypothesis that life could have started as self-assembling organic molecules.
Biophysicists have also commented on the nature and qualities of life forms—notably that they function on negative entropy. In more detail, according to physicists such as John Bernal, Erwin Schrödinger, Eugene Wigner, and John Avery, life is a member of the class of phenomena which are open or continuous systems able to decrease their internal entropy at the expense of substances or free energy taken in from the environment and subsequently rejected in a degraded form (see: entropy and life).
Some scientists have proposed in the last few decades that a general living systems theory is required to explain the nature of life. Such general theory, arising out of the ecological and biological sciences, attempts to map general principles for how all living systems work. Instead of examining phenomena by attempting to break things down into component parts, a general living systems theory explores phenomena in terms of dynamic patterns of the relationships of organisms with their environment.
The idea that the Earth is alive is probably as old as humankind, but the first public expression of it as a fact of science was by a Scottish scientist, James Hutton. In 1785 he stated that the Earth was a superorganism and that its proper study should be physiology. Hutton is rightly remembered as the father of geology, but his idea of a living Earth was forgotten in the intense reductionism of the nineteenth century. The Gaia hypothesis, originally proposed in the 1960s by scientist James Lovelock, explores the idea that the life on Earth functions as a single organism which actually defines and maintains environmental conditions necessary for its survival.
Robert Rosen (1991) built on the assumption that the explanatory powers of the mechanistic worldview cannot help understand the realm of living systems. One of several important clarifications he made was to define a system component as "a unit of organization; a part with a function, i.e., a definite relation between part and whole." From this and other starting concepts, he developed a "relational theory of systems" that attempts to explain the special properties of life. Specifically, he identified the "nonfractionability of components in an organism" as the fundamental difference between living systems and 'biological machines.'
A systems view of life treats environmental fluxes and biological fluxes together as a "reciprocity of influence", and a reciprocal relation with environment is arguably as important for understanding life as it is for understanding ecosystems. As Harold J. Morowitz (1992) explains it, life is a property of an ecological system rather than a single organism or species. He argues that an ecosystemic definition of life is preferable to a strictly biochemical or physical one. Robert Ulanowicz (2009) also highlights mutualism as the key to understand the systemic, order-generating behavior of life and ecosystems.
Evidence suggests that life on Earth has existed for about 3.7 billion years. All known life forms share fundamental molecular mechanisms, and based on these observations, theories on the origin of life attempt to find a mechanism explaining the formation of a primordial single cell organism from which all life originates. There are many different hypotheses regarding the path that might have been taken from simple organic molecules via pre-cellular life to protocells and metabolism. Many models fall into the "genes-first" category or the "metabolism-first" category, but a recent trend is the emergence of hybrid models that combine both categories.
There is no scientific consensus as to how life originated and all proposed theories are highly speculative. However, most currently accepted scientific models build in one way or another on the following hypotheses:
Life as we know it today synthesizes proteins, which are polymers of amino acids using instructions encoded by cellular genes—which are polymers of deoxyribonucleic acid (DNA). Protein synthesis also entails intermediary ribonucleic acid (RNA) polymers. One possibility is that genes came first and then proteins. Another possibility is that proteins came first and then genes. However, because genes are required to make proteins, and proteins are required to make genes, the problem of considering which came first is like that of the chicken or the egg. Most scientists have adopted the hypothesis that because DNA and proteins function together so intimately, it's unlikely that they arose independently. Therefore, many scientists consider the possibility, apparently first suggested by Francis Crick, that the first life was based on the DNA-protein intermediary: RNA. In fact, RNA has the DNA-like properties of information storage and replication and the catalytic properties of some proteins. Crick and others actually favored the RNA-first hypothesis even before the catalytic properties of RNA had been demonstrated by Thomas Cech.
A significant issue with the RNA-first hypothesis is that experiments designed to synthesize RNA from simple precursors have not been nearly as successful as the Miller-Urey experiments that synthesized other organic molecules from inorganic precursors. One reason for the failure to create RNA in the laboratory is that RNA precursors are very stable and don't react with each other under ambient conditions. However, the successful synthesis of certain RNA molecules under conditions hypothesized to exist prior to life on Earth has been achieved by adding alternative precursors in a specified order with the precursor phosphate present throughout the reaction. This study makes the RNA-first hypothesis more plausible to many scientists.
Recent experiments have demonstrated true Darwinian evolution of unique RNA enzymes (ribozymes) made up of two separate catalytic components that replicate each other in vitro. In describing this work from his laboratory, Gerald Joyce stated: "This is the first example, outside of biology, of evolutionary adaptation in a molecular genetic system." Such experiments make the possibility of a primordial RNA World even more attractive to many scientists.
The diversity of life on Earth today is a result of the dynamic interplay between genetic opportunity, metabolic capability, environmental challenges, and symbiosis. For most of its existence, Earth's habitable environment has been dominated by microorganisms and subjected to their metabolism and evolution. As a consequence of such microbial activities on a geologic time scale, the physical-chemical environment on Earth has been changing, thereby determining the path of evolution of subsequent life. For example, the release of molecular oxygen by cyanobacteria as a by-product of photosynthesis induced fundamental, global changes in the Earth's environment. The altered environment, in turn, posed novel evolutionary challenges to the organisms present, which ultimately resulted in the formation of our planet's major animal and plant species. Therefore this 'co-evolution' between organisms and their environment is apparently an inherent feature of living systems.
The inert components of an ecosystem are the physical and chemical factors necessary for life – energy (sunlight or chemical energy), water, temperature, atmosphere, gravity, nutrients, and ultraviolet solar radiation protection. In most ecosystems the conditions vary during the day and often shift from one season to the next. To live in most ecosystems, then, organisms must be able to survive a range of conditions, called 'range of tolerance'. Outside of that are the 'zones of physiological stress', where the survival and reproduction are possible but not optimal. Outside of these zones are the 'zones of intolerance', where life for that organism is implausible. It has been determined that organisms that have a wide range of tolerance are more widely distributed than organisms with a narrow range of tolerance.
Life has evolved strategies that allow it to survive even beyond the physical and chemical limits to which it has adapted to grow. To survive, some microorganisms can assume forms that enable them to withstand freezing, complete desiccation, starvation, high-levels of radiation exposure, and other physical or chemical challenges. Furthermore, some microorganisms can survive exposure to such conditions for weeks, months, years, or even centuries. Extremophiles are microbial life forms that thrive outside the ranges life is commonly found in. They also excel at exploiting uncommon sources of energy. While all organisms are composed of nearly identical molecules, evolution has enabled such microbes to cope with this wide range of physical and chemical conditions. Characterization of the structure and metabolic diversity of microbial communities in such extreme environments is ongoing. An understanding of the tenacity and versatility of life on Earth, as well as an understanding of the molecular systems that some organisms utilize to survive such extremes, will provide a critical foundation for the search for life beyond Earth.
Traditionally, people have divided organisms into the classes of plants and animals, based mainly on their ability of movement. The first known attempt to classify organisms was conducted by the Greek philosopher Aristotle (384-322 BC). He classified all living organisms known at that time as either a plant or an animal. Aristotle distinguished animals with blood from animals without blood (or at least without red blood), which can be compared with the concepts of vertebrates and invertebrates respectively. He divided the blooded animals into five groups: viviparous quadrupeds (mammals), birds, oviparous quadrupeds (reptiles and amphibians), fishes and whales. The bloodless animals were also divided into five groups: cephalopods, crustaceans, insects (which also included the spiders, scorpions, and centipedes, in addition to what we now define as insects), shelled animals (such as most molluscs and echinoderms) and "zoophytes". Though Aristotle's work in zoology was not without errors, it was the grandest biological synthesis of the time and remained the ultimate authority for many centuries after his death.
The exploration of the American continent revealed large numbers of new plants and animals that needed descriptions and classification. In the latter part of the 16th century and the beginning of the 17th, careful study of animals commenced and was gradually extended until it formed a sufficient body of knowledge to serve as an anatomical basis for classification.
In the late 1740s, Carolus Linnaeus introduced his method, still used, to formulate the scientific name of every species. Linnaeus took every effort to improve the composition and reduce the length of the many-worded names by abolishing unnecessary rhetoric, introducing new descriptive terms and defining their meaning with an unprecedented precision. By consistently using his system, Linnaeus separated nomenclature from taxonomy. This convention for naming species is referred to as binomial nomenclature.
The fungi were originally treated as plants. For a short period Linnaeus had placed them in the taxon Vermes in Animalia. He later placed them back in Plantae. Copeland classified the Fungi in his Protoctista, thus partially avoiding the problem but acknowledged their special status. The problem was eventually solved by Whittaker, when he gave them their own kingdom in his five-kingdom system. As it turned out, the fungi are more closely related to animals than to plants.
As new discoveries enabled us to study cells and microorganisms, new groups of life were revealed, and the fields of cell biology and microbiology were created. These new organisms were originally described separately in protozoa as animals and protophyta/thallophyta as plants, but were united by Haeckel in his kingdom protista, later the group of prokaryotes were split off in the kingdom Monera, eventually this kingdom would be divided in two separate groups, the Bacteria and the Archaea, leading to the six-kingdom system and eventually to the current three-domain system. The classification of eukaryotes is still controversial, with protist taxonomy especially problematic.
As microbiology, molecular biology and virology developed, non-cellular reproducing agents were discovered, such as viruses and viroids. Sometimes these entities are considered to be alive but others argue that viruses are not living organisms since they lack characteristics such as cell membrane, metabolism and do not grow or respond to their environments. Viruses can however be classed into "species" based on their biology and genetics but many aspects of such a classification remain controversial.
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|Woese et al.
Earth is the only planet in the universe known to harbour life. The Drake equation, which relates the number of extraterrestrial civilizations in our galaxy with which we might come in contact, has been used to discuss the probability of life elsewhere, but scientists disagree on many of the values of variables in this equation. Depending on those values, the equation may either suggest that life arises frequently or infrequently.
Panspermia, also called exogenesis, is a hypothesis proposing that life originated elsewhere in the universe and was subsequently transferred to Earth in the form of spores perhaps via meteorites, comets or cosmic dust. However, this hypothesis does not help explain the ultimate origin of life.
Death is the permanent termination of all vital functions or life processes in an organism or cell. After death, the remains of an organism become part of the biogeochemical cycle. Organisms may be consumed by a predator or a scavenger and leftover organic material may then be further decomposed by detritivores, organisms which recycle detritus, returning it to the environment for reuse in the food chain.
One of the challenges in defining death is in distinguishing it from life. Death would seem to refer to either the moment at which life ends, or when the state that follows life begins. However, determining when death has occurred requires drawing precise conceptual boundaries between life and death. This is problematic, however, because there is little consensus over how to define life. The nature of death has for millennia been a central concern of the world's religious traditions and of philosophical inquiry. Many religions maintain faith in either some kind of afterlife, reincarnation, or resurrection.
Extinction is the gradual process by which a group of taxa or species dies out, reducing biodiversity. The moment of extinction is generally considered to be the death of the last individual of that species. Because a species' potential range may be very large, determining this moment is difficult, and is usually done retrospectively after a period of apparent absence. Species become extinct when they are no longer able to survive in changing habitat or against superior competition. Over the history of the Earth, over 99% of all the species that have ever lived have gone extinct.
Fossils are the preserved remains or traces of animals, plants, and other organisms from the remote past. The totality of fossils, both discovered and undiscovered, and their placement in fossil-containing rock formations and sedimentary layers (strata) is known as the fossil record. Such a preserved specimen is called a "fossil" if it is older than the arbitrary date of 10,000 years ago. Hence, fossils range in age from the youngest at the start of the Holocene Epoch to the oldest from the Archaean Eon, a few billion years old.
Reported in Josiah Hotchkiss Gilbert, Dictionary of Burning Words of Brilliant Writers (1895).
'From Wikipedia, the free encyclopedia
Life is a characteristic of organisms that exhibit certain biological processes such as chemical reactions or other events that results in a transformation. Living organisms are capable of growth and reproduction, some can communicate and many can adapt to their environment through changes originating internally. A physical characteristic of life is that it feeds on negative entropy. In more detail, according to physicists such as John Bernal, Erwin Schrödinger, Eugene Wigner, and John Avery, life is a member of the class of phenomena which are open or continuous systems able to decrease their internal entropy at the expense of substances or free energy taken in from the environment and subsequently rejected in a degraded form (see: entropy and life).
An entity with the above properties is considered to be a living organism, hence, a 'life form'. However, not every definition of life considers all of these properties to be essential. For example, the capacity for evolution is sometimes taken as the only essential property of life; this definition notably includes viruses, which do not qualify under narrower definitions as they are cellular and do not metabolize.
A diverse array of living organisms can be found in the biosphere on Earth. Properties common to these organisms—plants, animals, fungi, protists, archaea and bacteria—are a carbon- and water-based cellular form with complex organization and heritable genetic information. They undergo metabolism, possess a capacity to grow, respond to stimuli, reproduce and, through natural selection, adapt to their environment in successive generations. So far, there is no evidence of extraterrestrial life.
There is no universal definition of life. To define life in unequivocal terms is still a challenge for scientists, as the definition must be sufficiently broad that would encompass all life with which we are familiar. It should be sufficiently general that, with it, scientists would not miss life that may be fundamentally different from earthly life. In addition, defining life requires measurable terms, and when derived from analysis of known organisms, life is usually defined at the cellular level.
Conventional definition: The consensus is that life is a characteristic of organisms that exhibit all or most of the following phenomena:
1. Homeostasis: Regulation of the internal environment to maintain a constant state; for example, electrolyte concentration or sweating to reduce temperature. 2. Organization: Being structurally composed of one or more cells, which are the basic units of life. 3. Metabolism: Consumption of energy by converting chemicals and energy into cellular components (anabolism) and decomposing organic matter (catabolism). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life. 4. Growth: Maintenance of a higher rate of synthesis than catabolism. A growing organism increases in size in all of its parts, rather than simply accumulating matter. The particular species begins to multiply and expand as the evolution continues to flourish. 5. Adaptation: The ability to change over a period of time in response to the environment. This ability is fundamental to the process of evolution and is determined by the organism's heredity as well as the composition of metabolized substances, and external factors present. 6. Response to stimuli: A response can take many forms, from the contraction of a unicellular organism to external chemicals, to complex reactions involving all the senses of higher animals. A response is often expressed by motion, for example, the leaves of a plant turning toward the sun (phototropism) and chemotaxis. 7. Reproduction: The ability to produce new organisms. Reproduction can be the division of one cell to form two new cells. Usually the term is applied to the production of a new individual (either asexually, from a single parent organism, or sexually, from at least two differing parent organisms), although strictly speaking it also describes the production of new cells in the process of growth.
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LIFE, the popular name for the activity peculiar to protoplasm. This conception has been extended by analogy to phenomena different in kind, such as the activities of masses of water or of air, or of machinery, or by another analogy, to the duration of a composite structure, and by imagination to real or supposed phenomena such as the manifestations of incorporeal entities. From the point of view of exact science life is associated with matter, is displayed only by living bodies, by all living bodies, and is what distinguishes living bodies from bodies that are not alive. Herbert Spencer's formula that life is "the continuous adjustment of internal relations to external relations" was the result of a profound and subtle analysis, but omits the fundamental consideration that we know life only as a quality of and in association with living matter.
In developing our conception we must discard from consideration the complexities that arise from the organization of the higher living bodies, the differences between one living animal and another, or between plant and animal. Such differentiations and integrations of living bodies are the subject-matter of discussions on evolution; some will see in the play of circumambient media, natural or supernatural, on the simplest forms of living matter, sufficient explanation of the development of such matter into the highest forms of living organisms; others will regard the potency of such living matter so to develop as a mysterious and peculiar quality that must be added to the conception of life. Choice amongst these alternatives need not complicate investigation of the nature of life. The explanation that serves for the evolution of living matter, the vehicle of life, will serve for the evolution of life. What we have to deal with here is life in its simplest form.
The definition of life must really be a description of the essential characters of life, and we must set out with an investigation of the characters of living substance with the special object of detecting the differences between organisms and unorganized matter, and the differences between dead and living organized matter.
Living substance (see Protoplasm), as it now exists in all animals and plants, is particulate, consisting of elementary organisms living independently, or grouped in communities, the communities forming the bodies of the higher animals and plants. These small particles or larger communities are subject to accidents, internal or external, which destroy them, immediately or slowly, and thus life ceases; or they may wear out, or become clogged by the products of their own activity. There is no reason to regard the mortality of protoplasm and the consequent limited duration of life as more than the necessary consequence of particulate character of living matter (see Longevity).
Protoplasm, the living material, contains only a few elements, all of which are extremely common and none of which is peculiar to it. These elements, however, form compounds characteristic of living substance and for the most part peculiar to it. Proteid, which consists of carbon, hydrogen, nitrogen, oxygen and sulphur, is present in all protoplasm, is the most complex of all organic bodies, and, so far, is known only from organic bodies. A multitude of minor and simpler organic compounds, of which carbohydrates and fats are the best known, occur in different protoplasm in varying forms and proportions, and are much less isolated from the inorganic world. They may be stages in the elaboration or disintegration of protoplasm, and although they were at one time believed to occur only as products of living matter, are gradually being conquered by the synthetic chemist. Finally, protoplasm contains various inorganic substances, such as salts and water, the latter giving it its varying degrees of liquid consistency.
We attain, therefore, our first generalized description of life as the property or peculiar quality of a substance composed of none but the more common elements, but of these elements grouped in various ways to form compounds ranging from proteid, the most complex of known substances to the simplest salts. The living substance, moreover, has its mixture of elaborate and simple compounds associated in a fashion that is peculiar. The older writers have spoken of protoplasm or the cell as being in a sense "manufactured articles"; in the more modern view such a conception is replaced by the statement that protoplasm and the cell have behind them a long historical architecture. Both ideas, or both modes of expressing what is fundamentally the same idea, have this in common, that life is not a sum of the qualities of the chemical elements contained in protoplasm, but a function first of the peculiar architecture of the mixture, and then of the high complexity of the compounds contained in the mixture. The qualities of water are no sum of the qualities of oxygen and hydrogen, and still less can we expect to explain the qualities of life without regard to the immense complexity of the living substance.
We must now examine in more detail the differences which exist or have been alleged to exist between living organisms and inorganic bodies. There is no essential difference in structure. Confusion has arisen in regard to this point from attempts to compare organized bodies with crystals, the comparison having been suggested by the view that as crystals present the highest type of inorganic structure, it was reasonable to compare them with organic matter. Differences between crystals and organized bodies have no bearing on the problem of life, for organic substance must be compared with a liquid rather than with a crystal, and differs in structure no more from inorganic liquids than these do amongst themselves, and less than they differ from crystals. Living matter is a mixture of substances chiefly dissolved in water; the comparison with the crystals has led to a supposed distinction in the mode of growth, crystals growing by the superficial apposition of new particles and living substance by intussusception. But inorganic liquids also grow in the latter mode, as when a soluble substance is added to them.
The phenomena of movement do not supply any absolute distinction. Although these are the most obvious characters of life, they cannot be detected in quiescent seeds, which we know to be alive, and they are displayed in a fashion very like life by inorganic foams brought in contact with liquids of different composition. Irritability, again, although a notable quality of living substance, is not peculiar to it, for many inorganic substances respond to external stimulation by definite changes. Instability, again, which lies at the root of Spencer's definition "continuous adjustment of internal relations to external relations" is displayed by living matter in very varying degrees from the apparent absolute quiescence of frozen seeds to the activity of the central nervous system, whilst there is a similar range amongst inorganic substances.
The phenomena of reproduction present no fundamental distinction. Most living bodies, it is true, are capable of reproduction, but there are many without this capacity, whilst, on the other hand, it would be difficult to draw an effective distinction between that reproduction of simple organisms which consists of a sub-division of their substance with consequent resumption of symmetry by the separate pieces, and the breaking up of a drop of mercury into a number of droplets.
Consideration of the mode of origin reveals a more real if not an absolute distinction. All living substance so far as is known at present (see Biogenesis) arises only from already existing living substance. It is to be noticed, however, that green plants have the power of building up living substance from inorganic material, and there is a certain analogy between the building up of new living material only in association with pre-existing living material, and the greater readiness with which certain inorganic reactions take place if there already be present some trace of the result of the reaction.
The real distinction between living matter and inorganic matter is chemical. Living substance always contains proteid, and although we know that proteid contains only common inorganic elements, we know neither how these are combined to form proteid, nor any way in which proteid can be brought into existence except in the presence of previously existing proteid. The central position of the problem of life lies in the chemistry of proteid, and until that has been fully explored, we are unable to say that there is any problem of life behind the problem of proteid.
Comparison of living and lifeless organic matter presents the initial difficulty that we cannot draw an exact line between a living and a dead organism. The higher "warm-blooded" creatures appear to present the simplest case and in their lifehistory there seems to be a point at which we can say "that which was alive is now dead." We judge from some major arrest of activity, as when the heart ceases to beat. Long after this, however, various tissues remain alive and active, and the event to which we give the name of death is no more than a superficially visible stage in a series of changes. In less highly integrated organisms, such as "cold-blooded" vertebrates, the point of death is less conspicuous, and when we carry our observations further down the scale of animal life, there ceases to be any salient phase in the slow transition from life to death.
The distinction between life and death is made more difficult by a consideration of cases of so-called "arrested vitality." If credit can be given to the stories of Indian fakirs, it appears that human beings can pass voluntarily into a state of suspended animation that may last for weeks. The state of involuntary trance, sometimes mistaken for death, is a similar occurrence. A. Leeuwenhoek, in 1719, made the remarkable discovery, since abundantly confirmed, that many animalculae, notably tardigrades and rotifers, may be completely desiccated and remain in that condition for long periods without losing the power of awaking to active life when moistened with water. W. Preyer has more recently investigated the matter and has given it the name "anabiosis." Later observers have found similar occurrences in the cases of small nematodes, rotifers and bacteria. The capacity of plant seeds to remain dry and inactive for very long periods is still better known. It has been supposed that in the case of the plant seeds and still more in that of the animals, the condition of anabiosis was merely one in which the metabolism was too faint to be perceptible by ordinary methods of observation, but the elaborate experiments of W. Kochs would seem to show that a complete arrest of vital activity is compatible with viability. The categories, "alive" and "dead," are not sufficiently distinct for us to add to our conception of life by comparing them. A living organism usually displays active metabolism of proteid, but the metabolism may slow down, actually cease and yet reawaken; a dead organism is one in which the metabolism has ceased and does not reawaken.
It is plain that we cannot discuss adequately the origin of life or the possibility of the artificial construction of living matter (see Abiogenesis and Biogenesis) until the chemistry of protoplasm and specially of proteid is more advanced. The investigations of O. Biitschli have shown how a model of protoplasm can be manufactured. Very finely triturated soluble particles are rubbed into a smooth paste with an oil of the requisite consistency. A fragment of such a paste brought into a liquid in which the solid particles are soluble, slowly expands into a honeycomb like foam, the walls of the minute vesicles being films of oil, and the contents being the soluble particles dissolved in droplets of the circumambient liquid. Such a model, properly constructed, that is to say, with the vesicles of the foam microscopic in size, is a marvellous imitation of the appearance of protoplasm, being distinguishable from it only by a greater symmetry. The nicely balanced conditions of solution produce a state of unstable equilibrium, with the result that internal streaming movements and changes of shape and changes of position in the model simulate closely the corresponding manifestations in real protoplasm. The model has no power of recuperation; in a comparatively short time equilibrium is restored and the resemblance with protoplasm disappears. But it suggests a method by which, when the chemistry of protoplasm and proteid is better known, the proper substances which compose protoplasm may be brought together to form a simple kind of protoplasm.
It has been suggested from time to time that conditions very unlike those now existing were necessary for the first appearance of life, and must be repeated if living matter is to be constructed artificially. No support for such a view can be derived from observations of the existing conditions of life. The chemical elements involved are abundant; the physical conditions of temperature pressure and so forth at which living matter is most active, and within the limits of which it is confined, are familiar and almost constant in the world around us. On the other hand, it may be that the initial conditions for the synthesis of proteid are different from those under which proteid and living matter display their activities. E. Pfliiger has argued that the analogies between living proteid and the compounds of cyanogen are so numerous that they suggest cyanogen as the startingpoint of protoplasm. Cyanogen and its compounds, so far as we know, arise only in a state of incandescent heat. Pfliiger suggests that such compounds arose when the surface of the earth was incandescent, and that in the long process of cooling, compounds of cyanogen and hydrocarbons passed into living protoplasm by such processes of transformation and polymerization as are familiar in the chemical groups in question, and by the acquisition of water and oxygen. His theory is in consonance with the interpretation of the structure of protoplasm as having behind it a long historical architecture and leads to the obvious conclusion that if protoplasm be constructed artificially it will be by a series of stages and that the product will be simpler than any of the existing animals or plants.
Until greater knowledge of protoplasm and particularly of proteid has been acquired, there is no scientific room for the suggestion that there is a mysterious factor differentiating living matter from other matter and life from other activities. We have to scale the walls, open the windows, and explore the castle before crying out that it is so marvellous that it must contain ghosts.
As may be supposed, theories of the origin of life apart from doctrines of special creation or of a primitive and slow spontaneous generation are mere fantastic speculations. The most striking of these suggests an extra-terrestrial origin. H. E. Richter appears to have been the first to propound the idea that life came to this planet as cosmic dust or in meteorites thrown off from stars and planets. Towards the end of the 10th century Lord Kelvin (then Sir W. Thomson) and H. von Helmholtz independently raised and discussed the possibility of such an origin of terrestrial life, laying stress on the presence of hydrocarbons in meteoric stones and on the indications of their presence revealed by the spectra of the tails of comets. W. Preyer has criticized such views, grouping them under the phrase "theory of cosmozoa," and has suggested that living matter preceded inorganic matter. Preyer's view, however, enlarges the conception of life until it can be applied to the phenomena of incandescent gases and has no relation to ideas of life derived from observation of the living matter we know.
REFERENCES.-O. Biitschli, Investigations on Microscopic Foams and Protoplasm (Eng. trans. by E. A. Minchin, 1894), with a useful list of references; H. von Helmholtz, Vortrcige and Reden, ii. (1884); W. Kochs, Allgemeine Naturkunde, x. 673 (1890); A. Leeuwenhoek, Epistolae ad Societatem regiam Anglicam (1719); E. Pfliiger, "Uber einige Gesetze des Eiweissstoffwechsels," in Archiv. Ges. Physiol. liv. 333 (1893); W. Preyer, Die Hypothesen fiber den Ursprung des Lebens (1880); H. E. Richter, Zur Darwinischen Lehre (1865); Herbert Spencer, Principles of Biology; Max Verworm, General Physiology (English trans. by F. S. Lee, 1899), with a very full literature. (P. C. M.)
generally of physical life (Gen 2:7; Lk 16:25, etc.); also used figuratively (1) for immortality (Heb 7:16); (2) conduct or manner of life (Rom 6:4); (3) spiritual life or salvation (Jn 3:16, 17, 18, 36); (4) eternal life (Mt 19:16, 17; Jn 3:15); of God and Christ as the absolute source and cause of all life (Jn 1:4; 5:26, 39; 11:25; 12:50).
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|Publisher(s)||Milton Bradley Company|
|Number of Players||2-6 players|
|Age Range||9 to Adult|
|Setup Time||5 minutes|
|Playing Time||1 hour|
|Credits | History | Rules|
At the start of the game the player can choose to enter college or immediately start working. There is advantages and disadvantages to both: if you go to college you must borrow money but when you graduate you have a larger choice of careers that potentially pay more.
Players travel the board in a miniature car and eventually must stop to get married and buy a house. Over the course of the game players will have children, change jobs, pay taxes, and encounter pitfalls and windfalls that generally occur in life. Players also have the option of taking out loans which must be repaid with interest, buying home and automobile insurance to minimize the cost of some pitfalls, and buying stocks that pay out when any player's spin points to the same number as their stock card.
Some spaces on the board allow you to collect LIFE tiles that are kept face down until the end of the game. Each tile describes a special achievement such as "Compose a Symphony", and gives a monetary reward. When all the players reach retirement they may choose to retire at either "Countryside Acres" or "Millionaire Estates". Four LIFE tile are reserved at "Millionaire Estates" for the player with the most cash. If more than one player chooses to retire there, the players must add up their cash and the one with the most gets the last four LIFE tiles.
The winner is the player with the highest LIFE tile value and cash value.
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Life is mainly a biological idea that has no simple definition. The study of life is called biology and people who study life are biologists. A lifespan is the average length of life in a species. All life is directly or indirectly powered by solar energy. Without energy from the sun no life could exist.
All known life on Earth is based on the chemistry of carbon compounds. Some say that this must be true for all possible forms of life throughout the Universe; others describe this position as "carbon chauvinism".
Currently, the Earth is the only planet in the Universe known to have things living on it. The question of whether life exists elsewhere in the Universe remains open. There have been a number of false alarms of life elsewhere in the Universe, but none of these apparent discoveries have so far been confirmed. The best evidence of life outside of Earth is fossil evidence of possible bacterial life on Mars.
One explanation of life is called the cell theory. The cell theory has three basic points: All living things are made of cells. The cell is the smallest living thing that can do all the things needed for life. All cells must come from preexisting cells.
Something is often said to be alive if it:
However, there are a few exceptions to these rules. Because this definition is not exact, it could be said that:
Many organisms are not able to reproduce and yet are still generally considered to be "alive"; see mule and ant for examples. However, these exceptions can be covered by defining life at the level of entire species or of individual genes (for example, see kin selection for one way that non-reproducing individuals can still enhance the spread of their genes and the survival of their species).
Death is the end of life in a living system, or in a part of it.
Port Ghalib march
Fish are examples of marine life
Adult citrus root weevil, Diaprepes
An Adult citrus root weevil is an example of an insect
Salmonella typhimurium is an example of bacteria
Chromista is an example of protista (protozoa)
Amanita muscaria tyndrum (Fly agaric) is an example of fungi