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| AHAHA TANNERPK WHIPES OUT ALL LIFE ON WIKIPEDIA! | |||
| {{pp-semi-protected|small=yes}} | |||
| {{two other uses||life on Earth|Organism}} | |||
| {{Taxobox | color = limegreen | |||
| | name = Life | |||
| | image = Waitakere Piha n.jpg | |||
| | image_width = 240px | |||
| | image_caption = Life colonizing a rocky peak | |||
| | unranked_classis = '''Life ('']'')''' | |||
| |subdivision_ranks = ]s and ]s | |||
| | subdivision = | |||
| *] (''Gaeabionta'') | |||
| **?] | |||
| **] | |||
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| **] | |||
| ***] | |||
| ***] | |||
| ****] | |||
| *****] | |||
| *****] | |||
| ******] | |||
| ******] | |||
| *****] | |||
| ******] | |||
| *******] | |||
| *******] | |||
| *******''']''' | |||
| ******] | |||
| *******] | |||
| *******] | |||
| *******] | |||
| *******] | |||
| ****] | |||
| *****] | |||
| *****] | |||
| ******] | |||
| ******] | |||
| ******''']''' | |||
| *] (hypothetical) | |||
| }} | |||
| {{wiktionarypar2|life|living}} | |||
| '''Life''' is a condition that distinguishes ] from ] objects, i.e. ], and ] organisms, being manifested by growth through ], ], and the power of ] to environment through changes originating internally. In ] terms, life is an organism that feeds on ].<ref>{{cite book | last = Schrödinger | first = Erwin | title = What is Life? | publisher = Cambridge University Press | year = 1944 | id = ISBN 0-521-42708-8}}</ref><ref>{{cite book | last = Margulis | first = Lynn | coauthors = Sagan, Dorion | title = What is Life? | publisher = University of California Press | year = 1995 | id = ISBN 0-520-22021-8}}</ref> In more detail, according to physicists such as ], ], ], and ], life is a member of the class of phenomena which are open or continuous systems able to decrease their internal ] at the expense of substances or ] taken in from the environment and subsequently rejected in a degraded form (see: ]).<ref>{{cite book | last = Lovelock | first = James | title = Gaia – a New Look at Life on Earth | publisher = Oxford University Press | year = 2000 | id = ISBN 0-19-286218-9}}</ref><ref>{{cite book | last = Avery | first = John | title = Information Theory and Evolution | publisher = World Scientific | year = 2003 | id = ISBN 9812383999}}</ref> | |||
| A diverse array of living organisms can be found in the ] on Earth. Properties common to these organisms – ]s, ]s, ], ]s, ] and ] – are a ] and ] ] form with complex ] and ]tic information. They undergo ], possess a capacity to grow, respond to ], ] and, through ], adapt to their environment in successive generations. | |||
| An entity with the above properties is considered to be a ''living'' ], that is an organism that is alive hence can be called a life form. However, not every definition of life considers all of these properties to be essential. For example, the capacity for descent with modification is often taken as the only essential property of life. This definition notably includes ]es, which do not qualify under narrower definitions as they are ] and do not metabolise. Broader definitions of life may also include theoretical ] and other ]. Some forms of ], however, especially ], might alternatively be classified as real life. | |||
| ==Definitions== | |||
| There is no universal definition of life; there are a variety of definitions proposed by different scientists.To define life in unequivocal terms is still a challenge for scientists<ref>http://www.astrobio.net/news/article226</ref><ref>http://www.nbi.dk/~emmeche/cePubl/97e.defLife.v3f.html</ref>. | |||
| '''Conventional definition''': Often scientists say that life is a characteristic of organisms that exhibit the following phenomena: | |||
| #''']''': Regulation of the internal environment to maintain a constant state; for example, sweating to reduce temperature. | |||
| #'''Organization''': Being composed of one or more ]s, which are the basic units of life. | |||
| #'''Metabolism''': Consumption of ] by converting nonliving material into cellular components (]) and decomposing organic matter (]). Living things require energy to maintain internal organization (homeostasis) and to produce the other phenomena associated with life. | |||
| #''']''': Maintenance of a higher rate of synthesis than catalysis. 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. | |||
| #'''Adaptation''': The ability to change over a period of time in response to the environment. This ability is fundamental to the process of ] and is determined by the organism's ] as well as the composition of metabolized substances, and external factors present. | |||
| #'''Response to stimuli''': A response can take many forms, from the contraction of a unicellular organism when touched 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 or an animal chasing its prey. | |||
| #'''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 ], from a single parent organism, or ], from at least two differing parent organisms), although strictly speaking it also describes the production of new cells in the process of growth. | |||
| ] life]] | |||
| However, others cite several limitations of this ]<ref>http://forums.hypography.com/biology/6702-what-exactly-constitutes-life.html</ref>. Thus, many members of several species do not reproduce, possibly because they belong to specialized sterile castes (such as ant workers), these are still considered forms of life. One could say that the property of life is inherited; hence, sterile or hybrid organisms such as the ], ] or ]s are alive although they are not capable of self reproduction. However, non-reproducing organisms may still propagate through mechanisms such as ]. | |||
| Viruses and aberrant ] proteins are often considered replicators rather than forms of life, a distinction warranted because they cannot reproduce without very specialized substrates such as host cells or proteins, respectively. Also, the ] and ] are examples of ] that cannot independently fulfill many vital biochemical processes, and depend on entry, growth, and replication within the ] of ] host cells. However, most forms of life rely on foods produced by other species, or at least the specific chemistry of Earth's environment. | |||
| ]s resting in the sun]] | |||
| Still others contest such definitions of life on philosophical grounds. They offer the following as examples of life: viruses which reproduce; storms or flames which "burn"; certain computer software programs which are programmed to mutate and evolve; future software programs which may evince (even high-order) behavior; machines which can move; and some forms of proto-life consisting of metabolizing cells without the ability to reproduce. {{Fact|date=February 2007}} | |||
| Still, most scientists would not call such phenomena expressive of life. Generally all seven characteristics are required for a population to be considered a life form. | |||
| The ] definition of life is that living things are self-organizing and ] (self-producing). These objects are not to be confused with ] (e.g. fire). | |||
| Variations of this definition include ]'s definition of life as an ] or a ] capable of reproducing itself or themselves, and of completing at least one ]. | |||
| ]]] | |||
| Yet other definitions of life are: | |||
| #Living things are systems that tend to respond to changes in their environment, and inside themselves, in such a way as to promote their own continuation.{{Fact|date=February 2007}} | |||
| #Life is a ] of ], self-recycling ] consisting of ]s of ]s that are capable of ], around most of which ], ] organisms evolve. This definition includes ] ], ] and ], and precludes ]s. It also explains why ]s can be alive and yet commit suicide in defending their hive. In this case the ], not the ], is the living system. | |||
| #Type of organization of matter producing various interacting forms of variable complexity, whose main property is to replicate ''almost perfectly'' by using matter and energy available in their environment to which they may adapt. In this definition "almost perfectly" relates to mutations happening during replication of organisms that may have adaptive benefits. | |||
| #Life is a potentially self-perpetuating open system of linked organic reactions, catalyzed simultaneously and almost isothermally by complex chemicals (enzymes) that are themselves produced by the open system. | |||
| ==Origin of life== | |||
| {{Main|Origin of life}} | |||
| ] of ]]] | |||
| Although it cannot be pinpointed exactly, evidence suggests that ] has existed for about 3.7 ] years <ref>http://www.ucmp.berkeley.edu/exhibits/historyoflife.php</ref>. | |||
| There is no truly "standard" model for the origin of life, but most currently accepted scientific models<ref>http://www.scribd.com/doc/1569/Origin-of-Life-in-Universe</ref> build in one way or another on the following discoveries, which are listed roughly in order of postulated emergence: | |||
| #Plausible pre-biotic conditions result in the creation of the basic small molecules of life. This was demonstrated in the ], and in the work of ]. | |||
| #]s spontaneously form ]s, the basic structure of a ]. | |||
| #Procedures for producing random ] molecules can produce ]s, which are able to produce more of themselves under very specific conditions. | |||
| There are many different hypotheses regarding the path that might have been taken from simple ]s to protocells and metabolism. Many models fall into the "]s-first" category or the "]-first" category, but a recent trend is the emergence of hybrid models that do not fit into either of these categories.<ref>http://www.journals.royalsoc.ac.uk/openurl.asp?genre=article&id=doi:10.1098/rsif.2005.0045</ref> | |||
| ==Extraterrestrial life== | |||
| :''Main articles: ], ]'' | |||
| ] is the only planet in the ] ''known'' to harbour life. The ] has been used to estimate the probability of life elsewhere, but scientists disagree on many of the values of variables in this equation (although strictly speaking Drake equation estimates the number of extraterrestrial civilizations in our galaxy with which we might come in contact - not probability of life elsewhere). Depending on those values, the equation may either suggest that life arises frequently or infrequently. Drake himself estimated the number of civilizations in our galaxy with which we might expect to be able to communicate at any given time as equal to one. | |||
| Relating to the origin of life on Earth, ] and exogenesis are theories proposing that life originated elsewhere in the universe and was subsequently transferred to Earth perhaps via ]s, ]s or ]. However those theories do not help explain the origin of this extraterrestrial life. | |||
| ==Classification of life== | |||
| {{Main|Scientific classification}} | |||
| ] | |||
| Traditionally people divided living things into ]s and ]s, this was mainly based upon whether they had the ability to move or not: plants couldn't move, animals could. Originally humans were not considered to be animals, but they treated themselves as a 'higher' form of life, this still survives in common use of the word "animals" which refers to non-human animals. The first known attempt of a real classification of life came from the Greek philosopher ], who classified all living organisms known at that time as either a plant or an animal. He further classified animals based on their means of transportation (air, land, or water). | |||
| The exploration of parts of the ] produced large numbers of new plants and animals that needed descriptions and classification. The old systems made it difficult to study and locate all these new specimens within a collection and often the same plants or animals were given different names because the number of specimens were too large to memorize. A system was needed that could group these specimens together so they could be found, the binomial system was developed based on ] with groups having similar appearances. In the latter part of the 16th century and the beginning of the 17th, careful study of animals commenced, which, directed first to familiar kinds, was gradually extended until it formed a sufficient body of knowledge to serve as an anatomical basis for classification. | |||
| Linnaeus is best known for his introduction of the method still used to formulate the ] of every species. Before Linnaeus, long many-worded names (composed of a generic name and a ''differentia specifica'') had been used, but as these names gave a description of the species, they were not fixed. In his ''Philosophia Botanica'' (1751) Linnaeus took every effort to improve the composition and reduce the length of the many-worded names by abolishing unnecessary rhetorics, introducing new descriptive terms and defining their meaning with an unprecedented precision. In the late 1740s Linnaeus began to use a parallel system of naming species with ''nomina trivialia.'' ''Nomen triviale'', a trivial name, was a single- or two-word epithet placed on the margin of the page next to the many-worded "scientific" name. The only rules Linnaeus applied to them was that the trivial names should be short, unique within a given genus, and that they should not be changed. Linnaeus consistently applied ''nomina trivialia'' to the species of plants in '']'' (1st edn. 1753) and to the species of animals in the 10th edition of '']'' (1758). By consistently using these specific epithets, Linnaeus separated ] from ]. Even though the parallel use of ''nomina trivialia'' and many-worded descriptive names continued until late in the eighteenth century, it was gradually replaced by the practice of using shorter proper names combined of the generic name and the trivial name of the species. In the nineteenth century, this new practice was codified in the first Rules and Laws of Nomenclature, and the 1st edn. of '']'' and the 10th edn. of '']'' were chosen as starting points for the ] and ] respectively. This convention for naming species is referred to as ]. Today, nomenclature is regulated by ], which allows names divided into ranks; separately ] and ]. Whereas Linnaeus classified for ease of identification, it is now generally accepted that classification should reflect the Darwinian principle of ]. | |||
| The ] have long been a problematic group in the biological classification: Originally, they were treated as plants. For a short period Linnaeus had placed them in the taxon ] in Animalia because he was misinformed: the ]e were said to have been ]s. He later placed them back in Plantae. ] classified the Fungi in his Protoctista, thus partially avoiding the problem but acknowledging their special status. The problem was eventually solved by ], when he gave them their own kingdom in his ]. As it turned out, the fungi are more closely related to animals than to plants. | |||
| As new discoveries enabled us to study ] and ]s, new groups of life where revealed, and the fields of ] and ] were created. These new organisms were originally described separately in ] as animals and ] as plants, but were united by ] in his kingdom ], later the group of ]s were split of in the kingdom ], eventually this kingdom would be divided in two separate groups, the ] and the ]. The 'remaining' protists would later be divided into smaller groups in clades in relation to more complex organisms. As the ] developed, non-cellular reproducing agents were discovered, sometimes these are considered to be alive and are treated in the domain of ] named Acytota or Aphanobionta. | |||
| And thus the primary ] ]s were established: ], ], ], ], ], ], ], ] | |||
| Since the 1960s a trend called ] has emerged, arranging taxa in an ]. If a ] includes all the descendants of some ancestral form, it is called ], as opposed to ]. Other groups, where neither the most recent common ancestor nor all the descendants are included, are called ]. | |||
| A new formal code of nomenclature, the ], is currently under development, intended to deal with clades rather than taxa. It is unclear, should this be implemented, how the different codes will coexist. | |||
| {{Biological systems}} | |||
| ==See also== | |||
| * ], the scientific study of life | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ] | |||
| * ], a discrete model of an infinite, regular grid of ''cells'' | |||
| * ] | |||
| * ], organisms that live in so called 'extreme' conditions e.g. ] | |||
| * ] | |||
| * ] | |||
| * ], life from before the human history started on Earth | |||
| * ], the opposite of life, and also more specifically the end of life | |||
| * ], opposed to life | |||
| * ] | |||
| * ], the science of describing, categorising and naming organisms | |||
| * ], is the study of evolutionary relatedness among ] | |||
| * ], simple mathematical 'cellular automaton' that mimicks the dynamics of an ecosystem. | |||
| * ], in the original meaning, it is strongly associated with life. | |||
| ==References== | |||
| <div class="references-small"> | |||
| <references/> | |||
| </div> | |||
| ==Further reading== | |||
| *Kauffman, Stuart. The Adjacent Possible: A Talk with Stuart Kauffman. Retrieved Nov. 30, 2003 from | |||
| * Walker, Martin G. ''LIFE! Why We Exist...And What We Must Do to Survive'' ( Wiki Book Page) ( Web Site), Dog Ear Publishing, 2006, ISBN 1-59858-243-7 | |||
| ==External links== | |||
| {{wikiquote}} | |||
| * | |||
| * - a free directory of life | |||
| * | |||
| * | |||
| * - life's origin and trajectory through the fundamental philosophy of existence | |||
| * | |||
| *: Biology | |||
| * | |||
| * An in depth look at how life can form under the most extreme conditions. | |||
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Revision as of 16:15, 25 June 2007
AHAHA TANNERPK WHIPES OUT ALL LIFE ON WIKIPEDIA!