[编辑首段]维基百科，自由的百科全书
自然界的一切系統可分為生命（例如：动物、植物）與無生命（例如：礦物）。
而生命泛指由有机物，水构成的细胞组成，一类具有稳定的物质和能量代谢，能够稳定地从外界获取物质和能量，能回应刺激，复制自我繁殖的半开放物质系统，经过自然选择，渐渐进化过程中，一代代适应其环境。生命通常都要经历出生、成长和死亡。
具有以上特征的个体均被视为生物。无论如何，并非所有对生命的定义都以上述条件为标准。例如「在繁衍过程中改变的能力」有时被视判断生命的根本条件，我们称之为生命现象。病毒在有寄主可寄生的時候，會表現出生命現象；但在沒有寄主可寄生的時候，不會表現生命現象，所以病毒是介於生命與無生命之間的一種奇妙的生物。
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定義
生命沒有公認的定義；不同科學家也提出過種種的定義，但如何以明確的詞彙定義生命對科學家仍然是一個難題[1][2]。
傳統定義：科學家經常認為只有生物體會展現以下全部現象：
- 體內平衡：調節體內環境以維持身體處於一個恆定狀態，例如發汗以降低體溫。
- 組織性：由一個或以上的生物基本單位──細胞所組成。
- 新陳代謝：透過轉換非生物為細胞成分（組成代謝）以及分解有機物（分解代謝）以消耗能量。生物體需要能量以維持體內平衡以及製造其他生命跡象。
- 生長：維持組合的速率高於催化的速率。一個生長中的有機體增加所有其部分的大小，多於純粹積聚物質。特別的物種開始倍大和膨大體積用以進化、繼續維持物種旺盛。
- 適應：在一段時間後對環境作出反應的能力。這種能力對進化是很重要的，並由生物的遺傳和代謝物的構造，以及外界的因素決定。
- 對刺激作出反應：反應可以以很多方式進行，由觸碰後單細胞的收縮到高等生物在不同感覺下的複雜反應。最常反應的方法是運動，例如植物的葉片轉向太陽以及動物追捕其獵物。
- 繁殖：是產生新個體的能力。包括只需一個親本的無性生殖，或需要至少兩個親本的有性生殖）。
大部分科學家稱這樣的現象為生命的表現方式。通常必須具備全部七個特徵才能當作為生命。
但是，其他引用限制了這個定義[3]。因此，有些物種的某些成員不會生育，可能因為它們屬於一些特別的不生育等級（比如工蟻），也算有生命的。有些人說生命的特性是可遺傳的；因此，儘管不孕的以及雜種的：例如宦官、騾、獅虎獸等不能繁衍的有機體，它們也是有生命的。但是，不能繁殖的生物仍可以親屬選擇等機理散佈。
病毒和變體朊毒體蛋白通常考慮為複製品多於一種生物，因為它們不能在缺乏特別基質如宿主細胞或蛋白質下生殖。同樣，立克次體和衣原體都是細菌中不能獨立地執行很多重要生化過程的例子，並依賴進入真核生物宿主細胞的細胞質內進行生長和自我複製。但是幾乎所有的生命都倚賴其他物種製造的食物，或至少也需要地球上某些特殊化學作用，如光合作用。
具系統性的生命定義是生物是自我組織並自我製造的。這些物質不與耗散結構混淆（如：火）。
這個定義變種包括了Stuart
Kauffman定義生命為能夠複製自己或他人的一種autonomous
agent或一種multi-agent
system，並最少完成一次熱力學循環。
其他定義包括：
- 生命是具自我組織，自相殘殺的系統的特徵，而其中包含了可以突變的族群。這定義不包括哲學上所定義為有生命的火焰，但包括了工蟻、病毒和騾。自我複製以及能量消耗只是系統要保持延續的方法之一。這解釋了為何蜂有生命但又會為了保護蜂巢而自殺。在這個個案中整個群體運作的方式與生物無異。[來源請求]
- 一種製造不同且可變複雜性的互動物質組織，透過利用物質和能量複製「接近完美」的個體。這個定義中的「接近完美」便是複製中有利於使生物適應環境的突變。[來源請求]
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生命的起源
-
儘管不能準確地找到確實時間，但有證據表現地球上的生命已存在了大約37億年[4]。
雖然沒有標準表示生命起源的模型，但現時最為公認的科學模型[5]建立於一個或以上下面的發現之上，可以粗略地列出有以下假設：
- 模擬真實的史前生物環境以製造形成生命的基本細小分子。這已由米勒-尤里實驗以及Sidney
W. Fox的工作所證明。
- 磷脂自發地形成脂雙層，而脂雙層是細胞膜的基本結構。
- 製造隨機核糖核酸分子的過程可能製造出核酶，而可以在非常特殊的環境下製造更多核糖核酸。
很多不同的假說可能使用了取自簡單有機分子轉為原始細胞以及新陳代謝的過程。很多模型都可分為「先有基因」或「先有新陳代謝」兩類，但最近趨勢表現了混合模型並不符合任何一類[6]。不管現時所推測的生命歷史，生命的起源對科學家仍然是一個很大的謎團。
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外星生命
- 主條目：外星生命，天體生物學
地球是宇宙中唯一已知有生命存在的星球。德雷克公式可以估算其他地方出現生命的機率，但科學家不同意很多公式中變量的值（可以嚴謹地說德雷克公式計算於銀河系中且我們可能接觸的外星生物的數量，而不是有生命的機率）。取決於不同的值，方程式可以暗示生命的形成是頻繁或稀少的。德雷克計算我們在任何時間可能接觸的外星生命只有1個。
有關地球生命的起源，胚種論和exogenesis都是提出生命來自宇宙其他地方而其後透過隕石，彗星或宇宙塵等天體到達地球的理論。但是這些理論對解釋外星生命的起源卻沒有幫助。
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生命的終結
即死亡。
以人類為例，呼吸及心臟停止跳動為判定死亡的標準。詳細見（死亡）
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參見
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参考文献
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外部链接
From Wikipedia, the free encyclopedia
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. A physical
characteristic of life is that it feeds on negative
entropy.[1][2]
In more detail, according to physicists such as John
Bernal, Erwin
Schrödinger, 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).[3][4]
A diverse array of living organisms can be found in the biosphere
on Earth. Properties common to these organisms—plants,
animals, fungi,
protists, archaea
and bacteria—are
a carbon-
and water-based
cellular
form with complex organization
and heritable genetic
information. They undergo metabolism,
possess a capacity to grow, respond to stimuli,
reproduce
and, through natural
selection, adapt to their environment in successive generations.
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
artificial life, 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[5][6].
Conventional definition: Often scientists say that life is a
characteristic of organisms that exhibit the following phenomena:
- Homeostasis:
Regulation of the internal environment to maintain a constant state; for
example, sweating to reduce temperature.
- Organization: Being composed of one or more cells,
which are the basic units of life.
- 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.
- 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.
- 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.
- 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 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.
.jpg/250px-Herds_Maasi_Mara_(cropped_and_straightened).jpg)
Herds of zebra and impala gathering on the
Masai
Mara plain
However, others cite several limitations of this definition[7].
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 mules,
ligers, and eunuchs
are alive although they are not capable of self-reproduction. However, (a) The
species as a whole does reproduce, (b) There are no cases of species where
100% of the individuals reproduce, and (c) specialized non-reproducing
individuals of the species may still partially propagate their DNA or other
master pattern 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.
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.[citation
needed] 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.
Proposed definitions of life include:
- 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.[citation
needed]
- Life is a characteristic
of self-organizing,
self-recycling systems
consisting of populations
of replicators
that are capable of mutation,
around most of which homeostatic,
metabolizing
organisms evolve.
The above definition includes worker
caste ants, viruses
and mules while
precluding flames.
It also explains why bees
can be alive and yet commit suicide in defending their hive.
They are only individual instances of the living system that comprises all
life forms on planet Earth
(which is the only living system known to mankind).
- 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 article: Origin
of life
Although it cannot be pinpointed exactly, evidence suggests that life
on Earth has existed for about 3.7 billion
years [8].
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:
- 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.
- Phospholipids
spontaneously form lipid
bilayers, the basic structure of a cell
membrane.
- 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.[9]
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 relate 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
-
The hierarchy of
biological
classification's major eight levels.
Life
is divided into domains, which are subdivided into further groups.
Intermediate minor rankings are not shown.
Traditionally, people have divided organisms into the classes of plants
and animals,
based mainly on their ability of movement. The first known attempt to classify
organisms, as per personal observations, was conducted by the Greek
philosopher Aristotle.
He classified all bliving organisms known at that time as either a plant or
an animal. Aristotle distinguished animals with blood from animals without
blood (or at least without red blood), which can be compared with the concepts
of vertebrates
and invertebrates
respectively. He divided the blooded animals into five groups: viviparous
quadrupeds (mammals),
birds, oviparous
quadrupeds (reptiles
and amphibians),
fishes and whales.
The bloodless animals were also divided into five groups: cephalopods,
crustaceans,
insects (which also included the spiders,
scorpions,
and centipedes,
in addition to what we now define as insects),
shelled animals (such as most molluscs
and echinoderms)
and "zoophytes".
Though Aristotle's work in zoology was not without errors, it was the grandest
biological synthesis of the time, and remained the ultimate authority for many
centuries after his death. His observations on the anatomy of octopus,
cuttlefish, crustaceans, and many other marine invertebrates are remarkably
accurate, and could only have been made from first-hand experience with
dissection. [10]
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.
Carolus
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,
leading to the six-kingdom
system and eventually to the three-domain
system. The 'remaining' protists would later be divided into smaller
groups in clades in relation to more complex organisms. Thomas
Cavalier-Smith, who has published extensively on the classification of
protists, has recently proposed that the Neomura,
the clade which groups together the Archaea
and Eukarya,
would have evolved from Bacteria,
more precisely from Actinobacteria.
As microbiology,
molecular
biology and virology
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 all 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,
groups based on traits which have evolved separately and where the most
recent common ancestor is not included are called polyphyletic.
A new formal code of nomenclature, the PhyloCode,
to be renamed "International Code of Phylogenetic
Nomenclature" (ICPN), is currently under development, intended to
deal with clades, which do not have set ranks, unlike conventional Linnaean
taxonomy. It is unclear, should this be implemented, how the different
codes will coexist.
See also
- Biology,
the scientific study of life
- Entropy
and life
- Artificial
life
- Synthetic
life
- Extraterrestrial
life
- Cellular
life
- Non-cellular
life
- Organic
life
- Carbon-based
life
- Cellular
automaton, a discrete model of an infinite, regular grid of cells
- Organism
- Extremophile,
organisms that live in so called 'extreme' conditions e.g. hydrothermal
vents
- Biological
kingdom
- Origin
of life
- Prehistoric
life, life from before the human history started on Earth
- Death,
the termination of life
- Non-life
- Gaia
hypothesis
- Taxonomy,
the science of describing, categorising and naming organisms
- Phylogenetics,
is the study of evolutionary relatedness among species
- Conway's
Game of Life, simple mathematical 'cellular automaton' that mimicks
the dynamics of an ecosystem.
- Nature,
in the original meaning, it is strongly associated with life.
- Personal
life
- Quality
of life
- Meaning
of life
References
Further reading
- Kauffman, Stuart. The Adjacent Possible: A Talk with Stuart Kauffman.
Retrieved Nov. 30, 2003 from [1]
- Walker, Martin G. LIFE! Why We Exist...And What We Must Do to Survive
(Book
Page) (Web
Site), Dog Ear Publishing, 2006, ISBN
1-59858-243-7
External links
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