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{{pp-semi-protected|small=yes}}
{{lowercase|title=Death}}
{{two other uses||life on Earth|Organism}}
{{dablink|For other uses, see ] and ].}}
{{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'')
**?]
**]
**]
**]
***]
***]
****]
*****]
*****]
******]
******]
*****]
******]
*******]
*******]
*******''']'''
******]
*******]
*******]
*******]
*******]
****]
*****]
*****]
******]
******]
******''']'''
*] (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>
]
'''Death''' is the permanent end of the ] of a biological ]. Death may refer to the end of life as either an event or condition.<ref name='Kastenbaum'>{{cite encyclopedia
| last = Kastenbaum
| first = Robert
| title = Definitions of Death
| encyclopedia = Encyclopedia of Death and Dying
| date = 2006
| url = http://www.deathreference.com/Da-Em/Definitions-of-Death.html
| accessdate = 2007-03-31}}</ref> Many factors can cause or contribute to an organism's death, including ], ], ], ], ] and ]s. The principal causes of death in modern human societies are diseases related to ].<ref name=Kastenbaum /> Traditions and beliefs related to death are an important part of human ], and central to many ]. In ], biological details and definitions of death have become increasingly complicated as ] advances.
==Biology==
===Fate of dead organisms===
]s begin the decomposition of a dead ].]]
In animals, small movements of the limbs (for example twitching legs or wings) known as a ] can sometimes be observed following death. '']'' is a postmortem paleness which accompanies death due to a lack of ] circulation throughout the body. '']'' describes the predictable decline in ] until ] is reached. Within a few hours of death '']'' is observed with a ] in the ]s, causing the limbs of the ] to become stiff (] ''rigor'') and difficult to move or manipulate. Assuming mild temperatures, full rigor occurs at about 12 hours, eventually subsiding to relaxation at about 36 hours. Decomposition isn't always a slow process however - for example ] is the primary mode of decomposition in most grassland ]s.<ref>DeBano, L.F., D.G. Neary, P.F. Ffolliot (1998) ''Fire’s Effects on Ecosystems''. ], Inc., ].</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.
After death an organism's remains become part of the ]. Animals may be ] by a ] or ]. ] may then be further decomposed by ]s, organisms which recycle ], returning it to the environment for reuse in the ]. Examples include ]s, ] and ]. ]s also play a vital role, raising the temperature of the decomposing material as they break it down into simpler molecules. Not all material need be decomposed fully however; for example ] is a ] formed in ] ecosystems where ] remains were saved by water and mud from ] and ].


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.
Some organisms have hard parts such as ]s or ]s which may not decompose and become fossilized. ]s are the ]ized or otherwise preserved remains or traces (such as footprints) of animals, plants, and other organisms. The totality of fossils, both discovered and undiscovered, and their placement in fossiliferous (fossil-containing) ] formations and ]ary layers (]) is known as the fossil record. The study of fossils across ], how they were formed, and the evolutionary relationships between ] (]) are some of the most important functions of the science of ].


==Definitions==
Using ] techniques, geologists have determined most fossils to be several thousands to several billions of years old. Yet there is no minimum age for a fossil. Fossils vary in size from ], such as single cells, to gigantic, such as ]s. A fossil normally preserves only a portion of the deceased organism, usually that portion that was partially mineralized during life, such as the bones and teeth of ]s, or the ]ous ]s of ]s. Preservation of ]s is extremely rare in the fossil record.
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:
===Extinction===
{{Main|Extinction}}
], shown here in illustration, is an often-cited<ref name="Diamond">{{cite book | last = Diamond | first = Jared | authorlink = Jared Diamond | title = ] | publisher = ] | date = 1999| id = ISBN 0-393-31755-2 | pages=43-44 |chapter=Up to the Starting Line }}</ref> example of ].]]


#''']''': Regulation of the internal environment to maintain a constant state; for example, sweating to reduce temperature.
In ] and ], extinction is the cessation of existence of a ] or group of ], reducing ]. The moment of extinction is generally considered to be the death of the last individual of that species (although the ] may have been lost before this point). Because a species' potential ] may be very large, determining this moment is difficult, and is usually done retrospectively. This difficulty leads to phenomena such as ], where a species presumed extinct abruptly "re-appears" (typically in the ]) after a period of apparent absence.
#'''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 ].
Through ], new species arise through the process of ] &mdash; where new varieties of organisms arise and thrive when they are able to find and exploit an ] &mdash; and species become extinct when they are no longer able to survive in changing conditions or against superior competition. A typical species becomes extinct within 10 million years of its first appearance,<ref name="Newman">Newman, Mark. "". ]. ] ]. URL accessed ] ].</ref> although some species, called ]s, survive virtually unchanged for hundreds of millions of years. Only one in a thousand species that have existed remain today.<ref name="Newman" /><ref name="Raup"> Raup, David M. ''Extinction: Bad Genes or Bad Luck?'' W.W. Norton and Company. New York. 1991. pp.3-6 ISBN 978-0393309270 </ref>


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.
Prior to the dispersion of humans across the earth, extinction generally occurred at a continuous low rate, interspersed with rare ] events. Starting approximately 100,000 years ago, and coinciding with an ], species extinctions have increased to a rate unprecedented<ref name="MSNBC">. ]. URL accessed ] ].</ref> since the ]. This is known as the ] and is at least the sixth such ]. Some experts have estimated that up to half of presently existing species may become extinct by 2100.<ref name="Wilson">], ''The Future of Life'' (2002) (ISBN 0-679-76811-4). See also: ]. ''The Sixth Extinction: Patterns of Life and the Future of Humankind'' (ISBN 0-385-46809-1 ).</ref>


]s resting in the sun]]
===Competition and natural selection===
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}}
{{section stub}}] used tree diagrams like this to represent diversification and extinction of species.]]
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.
{{Further|]}}
Death is an important part of the process of ]. Organisms that are less ] for their current environment than others are more likely to die having produced fewer offspring than their more fit cousins, differentially reducing their contribution to the ] of succeeding generations. Weaker genes are thus eventually bred out of a population, leading to processes such as ] and extinction. It should be noted however that ] also plays an equally important role in determining survival, for example an organism that dies young but leaves many offspring will be much fitter than a long lived one which leaves only one.


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).
===Evolution of aging===
{{section stub}}
{{main|Evolution of aging}}
Enquiry into the evolution of ageing aims to explain why almost all living things weaken and die with age. There is not yet agreement in the ] on a single answer. The evolutionary origin of senescence remains one of the fundamental, ].


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 ].
==In medicine==
===Definition===
<!-- This section is a little large, it may need trimming down, though there is no specific article on the subject to move material to. Perhaps a 'Medical definition of death' article should be created. -->
Historically, attempts to define the exact moment of death have been problematic. Death was once defined as the cessation of ]beat (]) and of ]ing, but the development of ] and prompt ] have rendered the previous definition inadequate because breathing and heartbeat can sometimes be restarted. This is now called "]". Events which were ] linked to death in the past no longer kill in all circumstances; without a functioning heart or lungs, life can sometimes be sustained with a combination of ] devices, ] and ]s.


]]]
Today, where a definition of the moment of death is required, doctors and coroners usually turn to "]" or "biological death": People are considered dead when the electrical activity in their brain ceases (cf. ]). It is presumed that a stoppage of electrical activity indicates the end of ]. However, suspension of consciousness must be permanent, and not transient, as occurs during ], and especially a ]. In the case of sleep, ] can easily tell the difference. Identifying the moment of death is important in cases of ], as organs for transplant must be harvested as quickly as possible after the death of the body.
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}}
The possession of brain activity, or ability to resume brain activity, is a ] to legal personhood in the United States. "''It appears that once brain death has been determined … no criminal or civil liability will result from disconnecting the life-support devices.''" (Dority v. Superior Court of ] County, 193 Cal.Rptr. 288, 291 (1983))
#Life is a characteristic of self-organizing, cannibalistic ] consisting of a population of replicators that are capable of mutation, around most of which homeostatic, metabolizing organisms evolve. This definition does not include flames, but does include worker ], ] and ]. Self reproduction and energy consumption is only one means for a system to promote its own continuation. This explains why bees can be alive and yet commit suicide in defending their hive. In this case the whole colony works as such a 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 adaptative benefits.
#Life is a potentially self-perpetuating open system of linked organic reactions, catalyzed simultaneously and almost isothermally by complex chemicals (enzymes) that are themselved produced by the open system.


==Origin of life==
Those maintaining that only the ] of the brain is necessary for consciousness sometimes argue that only electrical activity there should be considered when defining death. Eventually it is possible that the criterion for death will be the permanent and irreversible loss of ] function, as evidenced by the death of the ]. All hope of recovering human thought and ] is then gone. However, at present, in most places the more conservative definition of death — irreversible cessation of electrical activity in the whole brain, as opposed to just in the neo-cortex — has been adopted (for example the ] in the ]). In 2005, the case of ] brought the question of brain death and artificial sustenance to the front of ].
{{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:
Even by whole-brain criteria, the determination of brain death can be complicated. EEGs can detect spurious electrical impulses, while certain ], ], ], or ] can suppress or even stop brain activity on a temporary basis. Because of this, hospitals have protocols for determining brain death involving EEGs at widely separated intervals under defined conditions.


#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 ].
====Misdiagnosed death====
#]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>
There are many anecdotal references to people being declared dead by physicians and then coming back to life, sometimes days later in their own coffin, or when ] procedures are just about to begin. Owing to significant scientific advancements in the ], some people in ] became obsessively worried about living after being declared dead.


==Extraterrestrial life==
A ] is not authorized to pronounce a patient dead. Some ] ]s specifically state that a person is not to be assumed dead unless there are clear and obvious indications that death has occurred.<ref name=Limmer> Limmer, D. et al. (2006). Emergency care (AHA update, Ed. 10e). ].</ref> These indications include mortal ], ] (rigidity of the body), ] (blood pooling in the part of the body at lowest elevation), decomposition, incineration, or other bodily damage that is clearly inconsistent with life. If there is any possibility of life and in the absence of a ] (DNR) order, ] are instructed to begin rescue and not end it until a patient has been brought to a hospital to be examined by a physician. This frequently leads to situation of a patient being pronounced ] (DOA). However, some states allow ] to pronounce death. This is usually based on specific criteria. Aside from the above mentioned conditions include advanced measures including CPR, ], IV access, and administiring medicines without regaining a pulse for at least 20 minutes.
:''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.
In cases of ], CPR for an hour or longer can allow stunned ]s to recover, allowing an apparently-dead person to survive. People found unconscious under icy water may survive if their ]s are kept continuously cold until they arrive at an ].<ref name=Limmer /> This "diving response", in which ] and oxygen requirements are minimal, is something humans share with ]ns called the ].<ref name=Limmer />


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.
As medical technologies advance, ideas about when death occurs may have to be re-evaluated in light of the ability to restore a person to vitality after longer periods of apparent death (as happened when CPR and defibrillation showed that cessation of heartbeat is inadequate as a decisive indicator of death). The lack of electrical brain activity may not be enough to consider someone scientifically dead. Therefore, the concept of ] has been suggested as a better means of defining when true death actually occurs, though the concept has few practical applications outside of the field of ].


==Classification of life==
There have been some scientific attempts to bring dead organisms back to life, but with limited success . In ] ]s where such technology is readily available, ] is distinguished from reversible death.
{{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.
===Causes of human death===


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 ].
{{See also|List of causes of death by rate}}


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.
Death can be caused by ], ], ], or ]. The leading cause of death in ] is ]. The leading causes of death in ] are ] (] and ]), ], and other diseases related to ] and ]. These conditions cause loss of ], leading to ], causing loss of ] and nutrient supply, causing irreversible deterioration of the ] and other ]. With improved medical capability, dying has become ]. Home deaths, once the norm, are now rare in the first world.


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.
In ] countries, inferior sanitary conditions and lack of access to ] makes death from ] more common than in ]. One such disease is ], a bacterial disease which killed 1.7 million people in 2004.<ref name="WHO2004data">] (WHO). March 2006, Retrieved on 6 October 2006.</ref>


And thus the primary ] ]s were established: ], ], ], ], ], ], ], ]
Many leading first world causes of death can be postponed by ] and ], but the accelerating incidence of disease with age still imposes limits on human ]. The ] is, at best, only just beginning to be understood. It has been suggested that direct intervention in the aging process may now be the most effective intervention against major causes of death.<ref>{{cite journal
| last = SJ Olshanksy et al
| title = Longevity dividend: What should we be doing to prepare for the unprecedented aging of humanity?
| journal = The Scientist
| volume = 20
| pages = 28-36
| year = 2006
| publisher = Scientist (The), Philadelphia
| url = http://www.grg.org/resources/TheScientist.htm
| accessdate = 2007-03-31}}</ref>


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 ].
====Autopsy====
]'', by ], depicts an autopsy]]
{{main|Autopsy}}
An autopsy, also known as a ''postmortem examination'' or an ''obduction'', is a ] that consists of a thorough ] of a ] ] to determine the cause and manner of a person's death and to evaluate any ] or ] that may be present. It is usually performed by a specialized ] called a ].


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.
Autopsies are either performed for legal or medical purposes. A forensic autopsy is carried out when the cause of death may be a criminal matter, while a clinical or academic autopsy is performed to find the medical cause of death and is used in cases of unknown or uncertain death, or for research purposes. Autopsies can be further classified into cases where external examination suffices, and those where the body is dissected and an internal examination is conducted. Permission from ] may be required for internal autopsy in some cases. Once an internal autopsy is complete the body is reconstituted by sewing it back together. Autopsy is important in a medical environment and may shed light on mistakes and help improve practices.


{{Biological systems}}
A necropsy is a postmortem examination performed on a non-human animal, such as a pet.

===Life extension===
{{main|Life extension}}
] refers to an increase in ] or ], especially in humans, by slowing down or reversing the ]. Average lifespan is determined by vulnerability to ]s and age-related afflictions such as ] or ]. Extension of average lifespan can be achieved by good ], ] and avoidance of hazards such as ] and excessive eating of ]-containing foods. ] is determined by the rate of ] for a species inherent in its ]s. Currently, the only widely recognized method of extending maximum lifespan is ]. Theoretically, extension of maximum lifespan can be achieved by reducing the rate of aging damage, by ], or by ] or ] of deteriorated cells and tissues.

Researchers of life extension are a subclass of ] known as "biomedical ]". They seek to understand the nature of aging and they develop treatments to reverse aging processes or to at least slow them down, for the improvement of health and the maintenance of youthful vigor at every stage of life. Those who take advantage of life extension findings and seek to apply them upon themselves are called "life extensionists" or "longevists". The primary life extension strategy currently is to apply available anti-aging methods in the hope of living long enough to benefit from a complete cure to aging once it is developed, which given the rapidly advancing state of biogenetic and general medical technology, could conceivably occur within the lifetimes of people living today.

Many biomedical gerontologists and ]s believe that future breakthroughs in tissue ] with ]s, ] replacement (with artificial ] or ]s) and ] repair will eliminate all aging and disease as well as allow for complete rejuvenation to a youthful condition. Whether such breakthroughs can occur within the next few decades is impossible to predict. Some life extensionists arrange to be ] upon legal death so that they can await the time when future ] can eliminate ], ] them to a lasting youthful condition and repair damage caused by the ] process. Whether the maximum human lifespan should be extended is the subject of much ] debate amongst politicians and scientists.

===The physician's perspective===
A qualitative survey of ] doctors in the United States found three sources of satisfaction from medical practice:
# realizing a fundamental change in perspective via an experience with a patient
# making a difference made in someone's life
# connecting with patients
The authors of the survey noted how often the meaningful events, such as connecting with patients, occurred at events, such as death, that normally suggest a failure of ].<ref name="pmid12729445">{{cite journal |author=Horowitz C, Suchman A, Branch W, Frankel R |title=What do doctors find meaningful about their work? |journal=Ann Intern Med |volume=138 |issue=9 |pages=772-5 |year=2003 |pmid=12729445 | url=http://www.annals.org/cgi/content/full/138/9/772}}</ref> The following research suggests factors associated with a meaningful death.

A qualitative study using ]s that consisted of "physicians, nurses, ]ers, chaplains, hospice volunteers, patients, and recently bereaved family members". The groups identified the following themes associated with a 'good death'.<ref name="pmid10819707">{{cite journal |author=Steinhauser K, Clipp E, McNeilly M, Christakis N, McIntyre L, Tulsky J |title=In search of a good death: observations of patients, families, and providers |journal=Ann Intern Med |volume=132 |issue=10 |pages=825-32 |year=2000 |pmid=10819707 | url=http://www.annals.org/cgi/content/full/132/10/825}}</ref> The article is freely available and provides much more detail.
# '''Pain and Symptom Management'''. Patients want reassurance that symptoms, such as pain or ] that may occur at death, will be well treated.
# '''Clear Decision Making'''. According to the study, 'participants stated that fear of pain and inadequate symptom management could be reduced through communication and clear decision making with physicians. Patients felt empowered by participating in treatment decisions'.
# '''Preparation for Death'''. Patients wanted to know what to expect near death and to be able to plan for the events that would follow death.
# '''Completion'''. 'Completion includes not only faith issues but also ], resolving conflicts, spending time with family and friends, and saying good-bye.'
# '''Contributing to Others'''. A family member noted, "I guess it was really poignant for me when a nurse or new resident came into his room, and the first thing he'd say would be, ‘Take care of your wife’ or ‘Take care of your husband. Spend time with your children.’ He wanted to make sure he imparted that there's a purpose for life."
# '''Affirmation of the Whole Person'''. 'They didn't come in and say, "I'm Doctor so and so." There wasn't any kind of separation or aloofness. They would sit right on his bed, hold his hand, talk about their families, his family, golf, and sports.'
# '''Distinctions in Perspectives of a Good Death'''

In an essay, 'On Saying Goodbye: Acknowledging the End of the Patient–Physician Relationship with Patients Who Are Near Death' suggestions are made to health care providers for saying good-bye to patients near death.<ref name="pmid15838086">{{cite journal |author=Back A, Arnold R, Tulsky J, Baile W, Fryer-Edwards K |title=On saying goodbye: acknowledging the end of the patient-physician relationship with patients who are near death |journal=Ann Intern Med |volume=142 |issue=8 |pages=682-5 |year=2005 |pmid=15838086 | url=http://www.annals.org/cgi/content/full/142/8/682}}</ref> The quotes below are from the article. The article is freely available and provides much more detail.

# '''Choose an Appropriate Time and Place'''
# '''Acknowledge the End of Your Routine Contact and the Uncertainty about Future Contact''' The doctor could say, "You know, I'm not sure if we will see each other again in person, so while we are with each other now I want to say something about our relationship."
# '''Invite the Patient To Respond, and Use That Response as a Piece of Data about the Patient's State of Mind''' The authors suggest saying "Would that be okay?" or "how would you feel about that?"
# '''Frame the Goodbye as an Appreciation''' The authors suggest examples such as "I just wanted to say how much I've enjoyed you and how much I've appreciated your flexibility and your good humor ."
# '''Give Space for the Patient to Reciprocate, and Respond Empathically to the Patient's Emotion''' If the patients becomes tearful, the doctor can provide silence to allow the patient to respond, or the doctor may ask about what the patient is feeling.
# '''Articulate an Ongoing Commitment to the Patient's Care''' Do not make the patient feel abandoned, "Of course you know I remain available to you and that you can still call me".
# '''Later, Reflect on Your Work with This Patient'''

A ] of communication between health care providers and family members at the time of death reported that the intervention decreased the burden of bereavement.<ref name="pmid17267907">Lautrette A, Darmon M, Megarbane B, Joly LM, Chevret S, Adrie C et al. A communication strategy and brochure for relatives of patients dying in the ICU. N Engl J Med. 2007 February 1;356(5):469-78. PMID 17267907</ref> The intervention consisted of a brochure and family conference that focused on the following items that are remembered with the mnemonic '''value''':
* to '''V'''alue and appreciate what the family members said
* to '''A'''cknowledge the family members' emotions
* to '''L'''isten
* to ask questions that would allow the caregiver to '''U'''nderstand who the patient was as a person
* to '''E'''licit questions from the family members. Each investigator received a detailed description of the conference procedure

Other difficult issues for physicians include providing sedation for a patient at death and discontinuing life support. The following case reports detail these experiences from the physician's perspective.<ref name="pmid1616221">{{cite journal |author=Edwards M, Tolle S |title=Disconnecting a ventilator at the request of a patient who knows he will then die: the doctor's anguish |journal=Ann Intern Med |volume=117 |issue=3 |pages=254-6 |year=1992 |pmid=1616221|url=http://openurl.ebscohost.com/linksvc/linking.aspx?genre=article&sid=PubMed&issn=0003-4819&title=Ann%20Intern%20Med&volume=117&issue=3&spage=254&atitle=Disconnecting%20a%20ventilator%20at%20the%20request%20of%20a%20patient%20who%20knows%20he%20will%20then%20die:%20the%20doctor's%20anguish.&aulast=Edwards&date=1992}}</ref><ref name="pmid10744597">{{cite journal |author=Petty T |title=Technology transfer and continuity of care by a "consultant" |journal=Ann Intern Med |volume=132 |issue=7 |pages=587-8 |year=2000 |pmid=10744597 | url=http://www.annals.org/cgi/content/full/132/7/587}}</ref>

==Death in culture==
{{split2|Death in culture}}
<!-- This section was previously a small separate article which was merged with this, but it has probably reached a stage where it should be broken out again as there is still more to be added and it contains the vast majority of the text. To keep the article balanced, this section should just summarize the main points of death's cultural aspects, allowing a larger focus on medical and scientific perspectives. -->

], suggesting an intertwinement between life and death.]]
===Settlement of dead bodies===
{{Main|Disposal of human corpses}}
In most cultures, before the onset of significant decay, the body undergoes some type of ritual disposal, usually either ] or internment in a ]. Cremation is a very old and quite common custom, if one takes into account the sheer numbers of next of kin (of dead) practicing it. The act of cremation exemplifies the belief of the concept of "ashes to ashes". The other modes of disposal include interment in a ], but may also be a ], ], ], or ], a mound or barrow, or a monumental ] such as a ] (exemplified by the ]) or a ] (as exemplified by the ]).

In ], one method of corpse disposal is ], which involves placing the body of the deceased on ] (a ]) and leaving it for ] of prey to dispose of. Sometimes this is because in some religious views, ] are carriers of the ] to the heavens, but at other times this simply reflects the fact that when terrain (as in ]) makes the ground too hard to dig, there are few trees around to burn and the local religion (]) believes that the body after death is only an empty shell, there are more practical ways of disposing of a body, such as leaving it for animals to consume.

In certain cultures, efforts are made to retard the decay processes before burial (resulting even in the retardation of decay processes after the burial), as in ] or ]. This happens during or after a ] ceremony. Many ] exist in different cultures. In some fishing or naval communities, the body is sent into the water, in what is known as ]. Several mountain villages have a tradition of hanging the ] in woods.

A new alternative is ]. This is a sequence of deep-freezing, pulverisation by vibration, ], removing ]s, and burying the resulting powder, which has 30% of the body mass.

] of ] bringing the ], in Le Petit Journal]]
] is the process of ] of a body to liquid ] ] to stop the natural decay processes that occur after death. Those practicing cryonics hope that future technology will allow the ] person to be restored to life when and if science is able to cure all disease, ] people to a youthful condition and repair damage from the cryopreservation process itself. As of 2007, there were over 150 people in some form of cryopreservation at one of the two largest cryonics organizations, ] and the ].

] uses a rocket to launch the cremated remains of a body into orbit. This has been done at least 150 times.

Graves are usually grouped together in a plot of land called a '']'' or ''graveyard'', and burials can be arranged by a ], ], ] or by a religious body such as a ] or (for some ]s) the community's ], a charitable or voluntary body charged with these duties.

], made by the donor while living (or by a family member in some cases), are an important source of human cadavers used in ] and similar training, and in research. In the United States, these gifts, along with organ donations, are governed by the ]. In addition to wishing to benefit others, individuals might choose to donate their bodies to avoid the cost of funeral arrangements; however, willed body programs often encourage families to make alternative arrangements for burial if the body is not accepted.

===Grief and mourning===
{{Main|Grief|mourning}}
] in ]]]
Grief is a multi-faceted response to loss. Although conventionally focused on the emotional response to loss, it also has a physical, cognitive, behavioural, social and philosophical dimensions. Common to human experience is the death of a loved one, be they friend, family, or other. While the terms are often used interchangeably, bereavement often refers to the state of loss, and grief to the reaction to loss. Response to loss is varied and researchers have moved away from conventional views of grief (that is, that people move through an orderly and predictable series of responses to loss) to one that considers the wide variety of responses that are influenced by ], ], ], and ] and ] beliefs and practices.

Bereavement, while a normal part of life for most people, carries a degree of risk when limited support is available. Severe reactions to loss may carry over into familial relations and cause trauma for children, spouses and any other family members. Many forms of what are termed ']' have loss as their root, but covered by many years and circumstances this often goes unnoticed. Issues of personal faith and beliefs may also face challenge, as bereaved persons reassess personal definitions in the face of great pain. While many who grieve are able to work through their loss independently, accessing additional support from bereavement professionals may promote the process of healing. Individual counseling, professional ]s or educational classes, and peer-lead support groups are primary resources available to the bereaved. In some regions local hospice agencies may be an important first contact for those seeking bereavement support.
], a mythical place where beloved pets wait in health and happiness for their owners to arrive, after they die.]]
Mourning is the process of and practices surrounding death related grief. The word is also used to describe a cultural complex of behaviours in which the bereaved participate or are expected to participate. Customs vary between different cultures and evolve over time, though many core behaviors remain constant. Wearing dark, sombre clothes is one practice followed in many countries, though other forms of dress are also seen. Those most affected by the loss of a loved one often observe a period of grieving, marked by withdrawal from social events and quiet, respectful behavior. People may also follow certain religious traditions for such occasions.

Mourning may also apply to the death of, or anniversary of the passing of, an important individual like a local leader, monarch, religious figure etc. State mourning may occur on such an occasion. In recent years some traditions have given way to less strict practices, though many customs and traditions continue to be followed.

====Animal loss====
] is the loss of a ] or a non-human animal to which one has become ]. Though sometimes trivialized by those who have not experienced it themselves, it can be an intense loss, comparable with the death of a loved one.

===Settlement of legal entity===
{{section stub}}
Aside from the physical disposition of the corpse, the ] of a person must be settled. This includes attributes such as assets and debts. Depending on the ], ] or a ] may determine the final disposition of the ]. A ], or ], will guide these proceedings.

===Capital punishment===
{{Main|Capital punishment}}
], also known as the death penalty, is the execution of a convicted criminal by the state as punishment for crimes known as ''capital crimes'' or ''capital offences''.
Historically, the execution of ]s and political opponents was used by nearly all societies—both to punish crime and to suppress ]. Among democratic countries around the world, all ] (except ]) and ]n states, many Pacific Area states (including ], ] and ]), and ] have abolished capital punishment, while the ], ], and most of the ] as well as some democracies in ] (''e.g.'', ] and ]) and ] (''e.g.'', ] and ]) retain it. Among nondemocratic countries, the use of the death penalty is common but not universal.

In most places that practice capital punishment today, the death penalty is reserved as punishment for premeditated ], ], ], or as part of ]. In some countries, sexual crimes, such as ] and ], carry the death penalty, as do religious crimes such as ], the formal renunciation of one's religion. In many ] countries, ] is also a capital offense. In China ] and serious cases of ] are also punished by the death penalty. In militaries around the world ] have imposed death sentences for offenses such as ], ], ], and ].<ref>{{cite web|url=http://www.shotatdawn.org.uk/ |title= Shot at Dawn, campaign for pardons for British and Commonwealth soldiers executed in World War I|accessdate= 2006-07-20||publisher= Shot at Dawn Pardons Campaign}}</ref>

Capital punishment is a very contentious issue. Supporters of capital punishment argue that it ]s crime, prevents ], and is an appropriate form of punishment for the crime of ]. Opponents of capital punishment argue that it does not deter criminals more than life imprisonment, violates ], leads to executions of some who are ], and discriminates against minorities and the poor.

===Warfare===
{{Main|War}}
].]]
War is a prolonged state of violent, large scale conflict involving two or more groups of people. When and how war originated is a highly controversial topic. Some think war has existed as long as ]s, while others believe it began only about 5000 years ago with the rise of the first states; afterward war "spread to peaceful ]s and agriculturalists."<ref>Otterbein, Keith, 2004, How War Began. ].</ref>

Often opposing leaders or governing bodies get other people to fight for them, even if those fighting have no ] in the issues fought over. In time it became practical for some people to have warfare as their sole occupation, either as a member of a ] or mercenary. The original cause of war is not always known. Wars may be prosecuted simultaneously in one or more different ]. Within each theatre, there may be one or more consecutive ]s. Individual actions of war within a specific campaign are traditionally called ]s, although this terminology is not always applied to contentions in modernity involving aircraft, missiles or bombs alone in the absence of ground troops or ].

The factors leading to war are often complicated and due to a range of issues. Where disputes arise over issues such as ], ], ], ] and a ]able resolution is not sought, fails, or is thwarted, war often results.

A war may begin following an official ] in the case of international war, although this has not always been observed either historically or currently. ] and ] are not usually initiated by a formal declaration of war, but sometimes a statement about the purposes of the fighting is made. Such statements may be interpreted to be declarations of war, or at least a willingness to fight for a cause.

====Military suicide====
{{Main|Suicide attack}}
]]]

A suicide attack occurs when an individual or group violently sacrifice their own lives for the benefit of their side. In the desperate final days of ], many ]ese pilots volunteered for ] missions in an attempt to forestall defeat for the Empire. In ], ] squadrons were formed to smash into ] ]s during daylight bombing missions, in order to delay the highly-probable ] victory, although in this case, inspiration was primarily the Soviet and Polish ''taran'' ] attacks, and death of the pilot was not a desired outcome. The degree to which such a pilot was engaging in a heroic, selfless action or whether they faced immense ] is a matter of historical debate. The Japanese also built one-man "]" suicide ]s.

However, suicide has been fairly common in warfare throughout history. Soldiers and civilians committed suicide to avoid capture and slavery (including the wave of German and Japanese suicides in the last days of World War II). Commanders committed suicide rather than accept defeat. Spies and officers have committed suicide to avoid revealing secrets under ] and/or ]. Behaviour that could be seen as suicidal occurred often in battle. Japanese ]men usually fought to the last man, launched ], and committed suicide during the ] battles in World War II. In ] and ], civilians joined in the suicides. Suicidal attacks by pilots were common in the ]: the attack by U.S. ] at the ] was very similar to a kamikaze attack.

===Martyrdom===
{{Main|Martyr}}
], by ]]]
A martyr is a person who is put to death or endures suffering for their beliefs, principles or ideology. The death of a martyr or the value attributed to it is called martyrdom. In different belief systems, the criteria for being considered a martyr is different. In the ] context, a martyr is an innocent person who, without seeking death, is murdered or put to death for his or her religious faith or convictions. An example is the persecution of ] in the ]. ] sometimes decline to defend themselves at all, in what they see as an imitation of ]' willing sacrifice.

Islam accepts a broader view of what constitutes a ''martyr'', including anyone who dies in the struggle between ] and ]. Generally, some seek to include ] as a "martyr" of Islam, however, this is widely disputed in mainstream ], which argues that a martyr may not commit suicide.

Though often religious in nature, martyrdom can be applied to a ] context as well. The term is sometimes applied to those who die or are otherwise severely affected in support of a cause, such as soldiers fighting in a war, doctors fighting an epidemic, or people leading ]. Proclaiming martyrdom is a common way to draw attention to a cause and garner support.

===Suicide===
{{Main|Suicide}}
Suicide is the willful act of taking one's own life, and may be caused by psychological factors such as the difficulty of coping with ] or other ]s. It may also stem from social and cultural pressures.

Nearly a million people worldwide commit suicide annually. While completed suicides are higher in men, women have higher rates for ]s. Elderly males have the highest suicide rate, although rates for ]s have been increasing in recent years.<ref>{{cite web |url=http://www.who.int/features/qa/24/en/index.html |title=How can suicide be prevented? |date=2005-09-09 |accessdate=2007-04-13}}</ref>

Views toward suicide have varied in history and society. Buddhism, Christianity, Hinduism, Islam, ] generally condemn suicide as a dishonorable act and some countries have made it a ] to attempt to kill oneself. In some cultures committing suicide may be accepted under some circumstances, such as Japanese committing ] for ], Islamic ]s, or the ] of Buddhist monks as a form of protest.

===Euthanasia===
{{main|Euthanasia|animal euthanasia}}
Euthanasia is the practice of terminating the life of a person or animal in a ] or minimally painful way in order to prevent suffering or other undesired conditions in life. This may be voluntary or involuntary, and carried out with or without a physician. In a medical environment, it is normally carried out by oral, intravenous or intramuscular drug administration.

Laws around the world vary greatly with regard to euthanasia and are subject to change as people's values shift and better ] or treatments become available. It is legal in some nations, while in others it may be criminalized. Due to the gravity of the issue, strict restrictions and proceedings are enforced regardless of ]. Euthanasia is a ] because of conflicting moral feelings both within a person's own beliefs and between different cultures, ethnicities, religions and other groups. The subject is explored by the mass media, authors, film makers and philosophers, and is the source of ongoing debate and emotion.

===Customs and superstitions===
Death's finality and the relative lack of firm scientific understanding of its processes for most of ] have led to many different traditions and cultural rituals for dealing with death.

====Sacrifices====
{{Main|Sacrifice}}
Sacrifice ("to make sacred") includes the practice of offering the lives of animals or people to the ], as an act of ] or ]. The practice of sacrifice is found in the ] records, and the ] record finds corpses, both animal and human, that show marks of having been sacrificed and have been dated to long before any records. ] was practiced in many ancient ]s. The practice has varied between different civilizations, with some like the ] being notorious for their ritual killings, while others have looked down on the practice. Victims ranging from prisoners to infants to ]s were killed to please their ]s, suffering such fates as burning, beheading and being buried alive.

] is the ritual killing of an animal as practised by many religions as a means of appeasing a god or ], changing the course of nature or divining the future. Animal sacrifice has occurred in almost all cultures, from the ] to the ] and ] to the ]. Over time human and animal sacrifices have become less common in the world, such that modern sacrifices are rare. Most religions condemn the practice of human sacrifices, and present day laws generally treat them as a criminal matter. Nonetheless traditional sacrifice rituals are still seen in less developed areas of the world where traditional beliefs and superstitions linger, including the sacrifice of human beings.

====Religion and mythology====
]
{{See also|Afterlife|death deity}}
Faith in some form of ] is an important aspect of many people's beliefs. Such beliefs are usually manifested as part of a religion, as they pertain to phenomena beyond the ordinary experience of the natural world. For example, one aspect of ] involves belief in a continuing cycle of birth, life, death and rebirth (]) and the liberation from the cycle (]). Though various evidence has been advanced in attempts to demonstrate the reality of an afterlife, these claims have never been validated. For this reason, the material or metaphysical existence of an afterlife remains a matter of ] more than a matter of ].

]

Many ]s have incorporated a ] of death into their ] or ]. As death, along with ], is among the major parts of human life, these deities may often be one of the most important deities of a religion. In some religions with a single powerful deity as the source of worship, the ] is an antagonistic deity against which the primary deity struggles.

In ] religions or mythologies which have a ] of deities governing various natural phenomena and aspects of human life, it is common to have a deity who is assigned the function of presiding over death. The inclusion of such a "departmental" deity of death in a religion's pantheon is not necessarily the same as the glorification of death which is commonly condemned by the use of the term "death-worship" in modern political ].

In the theology of ] religion, the one god governs both life and death. However, in practice this manifests in different rituals and traditions and varies according to a number of factors including geography, politics, traditions and the influence of other religions.

====Personification of death====
{{main|Death (personification)}}
] card from the ].]]
Death has been ] as a figure or ] in ] and ] since the earliest days of storytelling. Because the reality of death has had a substantial influence on the human psyche and the development of ] as a whole, the personification of Death as a living, sentient entity is a concept that has existed in many societies since the beginning of ]. In ], death is usually shown as a skeletal figure carrying a large ], and sometimes wearing a midnight black gown with a hood.

Examples of death personified are:

*In modern-day European-based folklore, Death is known as the "]" or "The grim spectre of death". This form typically wields a ], and is sometimes portrayed riding a white horse.

*In the ], Death was imagined as a decaying or ] human corpse, later becoming the familiar skeleton in a robe.
*Death is sometimes portrayed in fiction and ]ism as ], the ] (note that the name "Azrael" does not appear in any versions of either the ] or the ]).
*] is sometimes said to be Death.
*A ] is a spirit, deity, or other being whose task is to conduct the souls of the recently dead into the afterlife.

====The number 4 in East Asia====
In ], ], and ] the ] is often associated with death because the sound of the Chinese, Japanese, and Korean words for ''four'' and ''death'' are similar (for example, the sound ''sì'' in Chinese is the ] number 4 (四), whereas ''sǐ'' is the word for death (死). For this reason, hospitals and hotels often omit the 4th, 14th, 24th, floors (etc.), or substitute the number '4' with the letter 'F'. Koreans are buried under a mound standing vertical in coffins made from six planks of wood. Four of the planks represent their respective four ] of the compass, while a fifth represents sky and the sixth represents earth. This relates back to the importance that the ] society placed upon the four cardinal points having mystical powers.

====Glorification of and fascination with death====
{{see also|Fascination with death}}
[[Image:CatonWoodvilleLightBrigade.jpeg|thumb|right|200px|'Charge of the Light Brigade'. An
Example of the artistic glorification of death.]]
]]]
Whether because of its very poetic nature or because of the great mystery it presents, or both, death is and has very often been glorified in many cultures through many different means. War, crime, revenge, martyrdom, suicide and many other forms of violence involving death are often glorified by different media, often in modern times being glorified even in spite of the attempts at depicting death meant to be de-glorifying. For example, film critic ] mentions in a number of articles that Francis Truffaut makes the claim it's impossible to make an ], as any depiction of war ends up glorifying it. The most prevalent and permanent form of death's glorification is through artistic expression. Through song, such as ] or ], many artists show death through poetic analogy or even as a poetic analogy, as in the latter mentioned song. Events such as ] and ] have served as inspirations for artistic depictions of and myths regarding death.

Perception of glory in death is subjective and can even differ wildly from one member of a group to another. Religion plays a key role, especially in terms of expectations of an afterlife. Personal and perceptions about mode of death are also important factors. One person's martyr could be another person's waste of life.

<br clear="all">


==See also== ==See also==
* ], the scientific study of life
<!--
* ]
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* ]

* ]
{{col-begin}}
* ]
{{col-1-of-2}}
*] * ]
*] * ]
*] * ]
* ], a discrete model of an infinite, regular grid of ''cells''
*] ("The Art of Dying")
* ]
*]
* ], organisms that live in so called 'extreme' conditions e.g. ]
*]
* ]
*]
* ]
*'']'' by ]
* ], 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.
{{col-2-of-2}}
*]
*]
*], occupational sudden death
*]
*]
*]
*]
*]
*]
*]
*]
*]
*] (death among humans; its causes and social aspects)
*]
*]
{{col-end}}


==References== ==References==
<div class="references-small"><references/></div> <div class="references-small">
<references/>
</div>


==Further reading==
'''Additional references:'''
*Kauffman, Stuart. The Adjacent Possible: A Talk with Stuart Kauffman. Retrieved Nov. 30, 2003 from
*{{cite web| last = Pounder| first = Derrick J.| title = POSTMORTEM CHANGES AND TIME OF DEATH| publisher =University of Dundee|date= ]| url = http://www.dundee.ac.uk/forensicmedicine/notes/timedeath.pdf| accessdate = 2006-12-13 }}
* 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
*Vass AA (2001) Microbiology Today 28: 190-192 at:
*Piepenbrink H (1985) J Archaeolog Sci 13: 417-430
*Piepenbrink H (1989) Applied Geochem 4: 273-280
*Child AM (1995) J Archaeolog Sci 22: 165-174
*Hedges REM & Millard AR (1995) J Archaeolog Sci 22: 155-164
*{{cite book
| last = Cook
| first = C
| title = Death in Ancient China: The Tale of One Man's Journey
| publisher = ]
| date = 2006
| isbn = 9004153128 }}
*Maloney, George, A., S.J. (1980) ''The Everlasting Now: Meditations on the mysteries of life and death as they touch us in our daily lives''. ISBN 0877932018


==External links== ==External links==
{{wikiquote}} {{wikiquote}}
*
{{commonscat}}
* - a free directory of life
*
*
* (])
*
* Source: ], United States, 2001
* - life's origin and trajectory through the fundamental philosophy of existence
*
*
* How the medical profession categorized causes of death a century ago.
*: Biology
* A biologist explains life and death in different kinds of organisms in relation to evolution.
* Informational site about death and dying. *
* An in depth look at how life can form under the most extreme conditions.

]
===Religious views===
]
<!-- It has not been decided whether religious links are appropriate for this article or not. Please see the talk page before adding further links. -->
]
* Muslim attitudes towards death
* (Vedic/Hindu view)
* By Maurice Lamm
* Christian beliefs on death and the afterlife.

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Revision as of 09:51, 5 June 2007

Template:Two other uses

Life
Life colonizing a rocky peak
Scientific classification
(unranked): Life (Biota)
Domains and Kingdoms

Life is a condition that distinguishes organisms from inorganic objects, i.e. non-life, and dead organisms, being manifested by growth through metabolism, reproduction, and the power of adaptation to environment through changes originating internally. In physical terms, life is an organism that feeds on negative entropy. In more detail, according to physicists such as John Bernal, Erwin Schrodinger, 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).

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 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.

An entity with the above properties is considered to be a living organism, 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 viruses, which do not qualify under narrower definitions as they are acellular and do not metabolise. Broader definitions of life may also include theoretical non-carbon-based life and other alternative biology. Some forms of artificial life, however, especially wet alife, 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.

Conventional definition: Often scientists say that life is a characteristic of organisms that exhibit the following phenomena:

  1. Homeostasis: Regulation of the internal environment to maintain a constant state; for example, sweating to reduce temperature.
  2. Organization: Being composed of one or more cells, which are the basic units of life.
  3. Metabolism: Consumption of energy by converting nonliving material 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 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.
  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 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.
  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.
Plant life

However, others cite several limitations of this definition. 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 mule, liger or eunuchs are alive although they are not capable of self reproduction. However, non-reproducing organisms may still propagate through mechanisms such as kin selection.

Viruses and aberrant prion 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 Rickettsia and Chlamydia are examples of bacteria that cannot independently fulfill many vital biochemical processes, and depend on entry, growth, and replication within the cytoplasm of eukaryotic host cells. However, most forms of life rely on foods produced by other species, or at least the specific chemistry of Earth's environment.

File:Rhesus Macaques 4528.jpg
Rhesus Macaques 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. 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 systemic definition of life is that living things are self-organizing and autopoietic (self-producing). These objects are not to be confused with dissipative structures (e.g. fire).

Variations of this definition include Stuart Kauffman's definition of life as an autonomous agent or a multi-agent system capable of reproducing itself or themselves, and of completing at least one thermodynamic work cycle.

File:Nwhi - French Frigate Shoals reef - many fish.jpg
Marine life around a coral reef

Yet other definitions of life are:

  1. 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.
  2. Life is a characteristic of self-organizing, cannibalistic systems consisting of a population of replicators that are capable of mutation, around most of which homeostatic, metabolizing organisms evolve. This definition does not include flames, but does include worker ants, viruses and mules. Self reproduction and energy consumption is only one means for a system to promote its own continuation. This explains why bees can be alive and yet commit suicide in defending their hive. In this case the whole colony works as such a living system.
  3. 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 adaptative benefits.
  4. Life is a potentially self-perpetuating open system of linked organic reactions, catalyzed simultaneously and almost isothermally by complex chemicals (enzymes) that are themselved produced by the open system.

Origin of life

Main article: Origin of life
Microbial mats around the Grand Prismatic Spring of Yellowstone National Park

Although it cannot be pinpointed exactly, evidence suggests that life on Earth has existed for about 3.7 billion years .

There is no truly "standard" model for the origin of life, but most currently accepted scientific models build in one way or another on the following discoveries, which are listed roughly in order of postulated emergence:

  1. Plausible pre-biotic conditions result in the creation of the basic small molecules of life. This was demonstrated in the Miller-Urey experiment, and in the work of Sidney Fox.
  2. Phospholipids spontaneously form lipid bilayers, the basic structure of a cell membrane.
  3. Procedures for producing random RNA molecules can produce ribozymes, 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 organic molecules 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 do not fit into either of these categories.

Extraterrestrial life

Main articles: Extraterrestrial life, Astrobiology

Earth is the only planet in the universe known to harbour life. The Drake equation 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, panspermia and exogenesis are theories proposing that life originated elsewhere in the universe and was subsequently transferred to Earth perhaps via meteorites, comets or cosmic dust. However those theories do not help explain the origin of this extraterrestrial life.

Classification of life

Main article: Scientific classification

Traditionally people divided living things into plants and animals, 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 Aristotle, 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 New World 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 morphology 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 scientific name 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 Species Plantarum (1st edn. 1753) and to the species of animals in the 10th edition of Systema Naturae (1758). By consistently using these specific epithets, Linnaeus separated nomenclature from taxonomy. 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 Species Plantarum and the 10th edn. of Systema Naturae were chosen as starting points for the Botanical and Zoological Nomenclature respectively. This convention for naming species is referred to as binomial nomenclature. Today, nomenclature is regulated by Nomenclature Codes, which allows names divided into ranks; separately for botany and for zoology. Whereas Linnaeus classified for ease of identification, it is now generally accepted that classification should reflect the Darwinian principle of common descent.

The Fungi 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 Vermes in Animalia because he was misinformed: the hyphae were said to have been worms. He later placed them back in Plantae. Copeland classified the Fungi in his Protoctista, thus partially avoiding the problem but acknowledging 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 where 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 of in the kingdom Monera, eventually this kingdom would be divided in two separate groups, the Bacteria and the Archaea. The 'remaining' protists would later be divided into smaller groups in clades in relation to more complex organisms. As the molecular biology developed, non-cellular reproducing agents were discovered, sometimes these are considered to be alive and are treated in the domain of non-cellular life named Acytota or Aphanobionta.

And thus the primary taxonomical ranks were established: Domain, Kingdom, Phylum, Class, Order, Family, Genus, Species

Since the 1960s a trend called cladistics has emerged, arranging taxa in an evolutionary or phylogenetic tree. If a taxon includes all the descendants of some ancestral form, it is called monophyletic, as opposed to paraphyletic. Other groups, where neither the most recent common ancestor nor all the descendants are included, are called polyphyletic.

A new formal code of nomenclature, the PhyloCode, 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.

Linnaeus
1735 		Haeckel
1866 		Chatton
1925 		Copeland
1938 		Whittaker
1969 		Woese et al.
1990 		Cavalier-Smith
1998, 2015
2 kingdoms 3 kingdoms 2 empires 4 kingdoms 5 kingdoms 3 domains 2 empires,
6/7 kingdoms
(not treated) Protista Prokaryota Monera Monera Bacteria Bacteria
Archaea Archaea (2015)
Eukaryota Protoctista Protista Eucarya "Protozoa"
"Chromista"
Vegetabilia Plantae Plantae Plantae Plantae
Fungi Fungi
Animalia Animalia Animalia Animalia Animalia

See also

References

  1. Schrödinger, Erwin (1944). What is Life?. Cambridge University Press. ISBN 0-521-42708-8.
  2. Margulis, Lynn (1995). What is Life?. University of California Press. ISBN 0-520-22021-8. {{cite book}}: Unknown parameter |coauthors= ignored (|author= suggested) (help)
  3. Lovelock, James (2000). Gaia – a New Look at Life on Earth. Oxford University Press. ISBN 0-19-286218-9.
  4. Avery, John (2003). Information Theory and Evolution. World Scientific. ISBN 9812383999.
  5. http://www.astrobio.net/news/article226
  6. http://www.nbi.dk/~emmeche/cePubl/97e.defLife.v3f.html
  7. http://forums.hypography.com/biology/6702-what-exactly-constitutes-life.html
  8. http://www.ucmp.berkeley.edu/exhibits/historyoflife.php
  9. http://www.scribd.com/doc/1569/Origin-of-Life-in-Universe
  10. http://www.journals.royalsoc.ac.uk/openurl.asp?genre=article&id=doi:10.1098/rsif.2005.0045
  11. Linnaeus, C. (1735). Systemae Naturae, sive regna tria naturae, systematics proposita per classes, ordines, genera & species.
  12. Haeckel, E. (1866). Generelle Morphologie der Organismen. Reimer, Berlin.
  13. Chatton, É. (1925). "Pansporella perplexa. Réflexions sur la biologie et la phylogénie des protozoaires". Annales des Sciences Naturelles - Zoologie et Biologie Animale. 10-VII: 1–84.
  14. Copeland, H. (1938). "The kingdoms of organisms". Quarterly Review of Biology. 13 (4): 383–420. doi:10.1086/394568. S2CID 84634277.
  15. Whittaker, R. H. (January 1969). "New concepts of kingdoms of organisms". Science. 163 (3863): 150–60. Bibcode:1969Sci...163..150W. doi:10.1126/science.163.3863.150. PMID 5762760.
  16. Woese, C.; Kandler, O.; Wheelis, M. (1990). "Towards a natural system of organisms:proposal for the domains Archaea, Bacteria, and Eucarya". Proceedings of the National Academy of Sciences of the United States of America. 87 (12): 4576–9. Bibcode:1990PNAS...87.4576W. doi:10.1073/pnas.87.12.4576. PMC 54159. PMID 2112744.
  17. Cavalier-Smith, T. (1998). "A revised six-kingdom system of life". Biological Reviews. 73 (3): 203–66. doi:10.1111/j.1469-185X.1998.tb00030.x. PMID 9809012. S2CID 6557779.
  18. Ruggiero, Michael A.; Gordon, Dennis P.; Orrell, Thomas M.; Bailly, Nicolas; Bourgoin, Thierry; Brusca, Richard C.; Cavalier-Smith, Thomas; Guiry, Michael D.; Kirk, Paul M.; Thuesen, Erik V. (2015). "A higher level classification of all living organisms". PLOS ONE. 10 (4): e0119248. Bibcode:2015PLoSO..1019248R. doi:10.1371/journal.pone.0119248. PMC 4418965. PMID 25923521.

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

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