Life (cf.
biota) is
a characteristic that distinguishes
object that have self-sustaining
biological processes from those
that do not—either because such functions have ceased (
death), or else because they lack such functions and
are classified as "
inanimate."
In
biology, the science of living organisms,
"life" is the condition which distinguishes active
organisms from
inorganic matter, including the capacity
for growth,
functional activity and the
continual change preceding death. A diverse array of living
organisms (life forms) can be found in the
biosphere on
Earth, and
properties common to these organisms—
plants,
animals,
fungi,
protists,
archaea,
and
bacteria — are a
carbon- and
water-based
cellular form with complex
organization and heritable
genetic information. Living organisms undergo
metabolism, maintain
homeostasis, possess a capacity to
grow, respond to
stimuli,
reproduce and, through
natural selection, adapt to their
environment in successive generations. More complex living
organisms can communicate through various means.
In
philosophy and
religion, the
conception of
life and its nature varies. Both offer interpretations as to how
life relates to
existence and
consciousness, and both touch on many related
issues, including
life stance,
purpose,
conceptions
of God, the
soul and the
afterlife.
Early theories about life
Materialism
[[Image:Grand prismatic spring.jpg|thumb|An
aerial photo of microbial mats around the Grand Prismatic
Spring
ofYellowstone National Park
.]]
Some of the earliest theories of life were
materialist, holding that all that exists is
matter, and that all life is merely a complex form or arrangement
of matter.
Empedocles (430 B.C.) argued
that every thing in the universe is made up of a combination of
four eternal 'elements' or 'roots
of all': earth, water, air, and fire. All change is explained by
the arrangement and rearrangement of these four elements. The
various forms of life are caused by an appropriate mixture of
elements. For example, growth in plants is explained by the natural
downward movement of earth and the natural upward movement of
fire.
Democritus (460 B.C.), the disciple of
Leucippus, thought that the essential
characteristic of life is having a soul (
psychê). In
common with other ancient writers, he used the term to mean the
principle of living things that causes them to function as a living
thing. He thought the soul was composed of fire atoms, because of
the apparent connection between life and heat, and because fire
moves. He also suggested that humans originally lived like animals,
gradually developing communities to help one another, originating
language, and developing crafts and agriculture.
In the
scientific revolution of
the seventeenth century, mechanistic ideas were revived by
philosophers like
Descartes.
Hylomorphism
Hylomorphism is the theory (originating
with
Aristotle (322 BC)) that all things
are a combination of matter and form. Aristotle was one of the
first ancient writers to approach the subject of life in a
scientific way. Biology was one of his main interests, and there is
extensive biological material in his extant writings. According to
him, all things in the material universe have both matter and form.
The form of a living thing is its
soul (Greek
'psyche', Latin 'anima'). There are three kinds of souls: the
'vegetative soul' of plants, which causes them to grow and decay
and nourish themselves, but does not cause motion and sensation;
the 'animal soul' which causes animals to move and feel; and the
rational soul which is the source of consciousness and reasoning
which (Aristotle believed) is found only in man. Each higher soul
has all the attributes of the lower one. Aristotle believed that
while matter can exist without form, form cannot exist without
matter, and therefore the soul cannot exist without the body.
Consistent with this account is a
teleological explanation of life. A teleological
explanation accounts for phenomena in terms of their purpose or
goal-directedness. Thus, the whiteness of the polar bear's coat is
explained by its
purpose of camouflage. The direction of
causality is the other way round from materialistic science, which
explains the consequence in terms of a prior cause. Most modern
biologists now reject this functional view in terms of a material
and causal one: biological features are to be explained not by
looking
forward to future optimal results, but by looking
backwards to the past evolutionary history of a species,
which led to the
natural selection
of the features in question.
Vitalism
Vitalism is the belief that the
life-principle is essentially immaterial. This originated with
Stahl, and held sway until the
middle of the nineteenth century. It appealed to philosophers such
as
Henri Bergson,
Nietzsche,
Wilhelm
Dilthey, anatomists like
Bichat, and chemists like
Liebig.
Vitalism underpinned the idea of a fundamental separation of
'organic' and inorganic material, and the belief that organic
material can only be derived from living things. This was disproved
in 1828 when
Wöhler prepared urea from
inorganic materials. This so-called
Wöhler synthesis is considered the
starting point of modern
organic
chemistry. It is of great historical significance because for
the first time an
organic compound
was produced from
inorganic
reactants.
Later,
Helmholtz, anticipated
by
Mayer, demonstrated that
no energy is lost in muscle movement, suggesting that there were no
vital forces necessary to move a muscle. These empirical
results led to the abandonment of scientific interest in vitalistic
theories, although the belief lingered on in non-scientific
theories such as
homeopathy, which
interprets diseases and sickness as caused by disturbances in a
hypothetical vital force or life force.
Definitions
It is still a challenge for scientists and philosophers to define
life in unequivocal terms. Any definition must be sufficiently
broad to encompass all life with which we are familiar, and it
should be sufficiently general that, with it, scientists would not
miss life that may be fundamentally different from earthly
life.
Biology
Since there is no unequivocal definition of life, the current
understanding is descriptive, where life is a 'characteristic' of
organisms that exhibit all or most of the following
phenomena:
- Homeostasis:
Regulation of the internal environment to maintain a constant
state; for example, electrolyte concentration or sweating to reduce
temperature.
- Organization: Being
structurally composed of one or more cell, which are the basic units of life.
- Metabolism:
Transformation of energy by converting chemicals and energy into
cellular components (anabolism) and
decomposing organic matter (catabolism).
Living things require energy to
maintain internal organization (homeostasis) and to produce the other phenomena
associated with life.
- Growth: Maintenance
of a higher rate of anabolism than catabolism. A growing organism
increases in size in all of its parts, rather than simply
accumulating matter.
- 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 to
external chemicals, to complex reactions involving all the senses
of multicellular organisms. A response is often expressed by
motion, for example, the leaves of a plant turning toward the sun
(phototropism) and by chemotaxis.
- Reproduction: The
ability to produce new individual organisms, either asexually from a single parent
organism, or sexually from two
parent organisms.
- Proposed
To reflect the minimum phenomena required, some have proposed other
biological definitions of life:
- 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.
- A network of inferior negative feedbacks (regulatory
mechanisms) subordinated to a superior positive feedback (potential
of expansion, reproduction).
- A systemic definition of
life is that living things are self-organizing and autopoietic (self-producing). Variations of this
definition include Stuart Kauffman's
definition as an autonomous agent
or a multi-agent system capable
of reproducing itself or themselves, and of completing at least one
thermodynamic work cycle.
- Life is a self-sustained chemical system capable of undergoing
Darwinian evolution.
- Viruses
Viruses are most often considered
replicators rather than forms of life. They have
been described as "organisms at the edge of life", since they
possess
genes,
evolve
by
natural selection, and
replicate by creating multiple copies of themselves through
self-assembly. However, viruses do not
metabolise and require a host cell to make new
products. Virus self-assembly within host cells has implications
for the study of the
origin of life,
as it may support the hypothesis that life could have started as
self-assembling organic molecules.
Biophysics
Biophysicists have also commented on the
nature and qualities of life forms—notably that they function on
negative entropy. In more detail,
according to physicists such as
John
Bernal,
Erwin
Schrödinger,
Eugene Wigner, and
John Avery, life is a member of
the class of
phenomena which are open or
continuous systems able to decrease their internal
entropy at the expense of substances or
free energy taken in from the
environment and
subsequently rejected in a degraded form (see:
entropy and life).
Living systems theories
In order to answer the question ‘What is life?’, some scientists
have recently proposed that a general
Living systems theory is required. Such
general theory, arising out of the
ecological and
biological
sciences, attempts to map general principles for how all living
systems work. Instead of examining phenomena by attempting to break
things down into component parts, a general living systems theory
explores phenomena in terms of dynamic patterns of the
relationships of organisms with their environment.
- Gaia hypothesis
The idea that the Earth is alive is probably as old as human kind,
but the first public expression of it as a fact of science was by a
Scottish scientist,
James Hutton. In
1785 he stated that the Earth was a superorganism and that its
proper study should be physiology. Hutton is rightly remembered as
the father of geology, but his idea of a living Earth was forgotten
in the intense reductionism of the nineteenth century. The
Gaia hypothesis, originally proposed in the
1960s by scientist
James Lovelock,
explores the idea that the life on Earth functions as a single
organism which actually defines and maintains environmental
conditions necesary for its survival.
- Nonfractionability
Robert Rosen (1991) built on the
assumption that the explanatory powers of the mechanistic worldview
cannot help understand the realm of living systems. One of several
important clarifications he made was to define a system component
as "a unit of organization; a part with a function, i.e., a
definite relation between part and whole." From this and other
starting concepts, he developed a "relational theory of systems"
that attempts to explain the special properties of life.
Specifically, he identified the "nonfractionability of components
in an organism" as the fundamental difference between living
systems and 'biological machines.'
- Life as a property of ecosystems
A systems view of life treats environmental
fluxes and biological fluxes together as a "reciprocity
of influence", and a reciprocal relation with environment is
arguably as important for understanding life as it is for
understanding ecosystems. As Harold J. Morowitz (1992) explains it,
life is a property of an
ecological system
rather than a single organism or species. He argues that an
ecosystemic definition of life is preferable to a strictly
biochemical or physical one.
Robert
Ulanowicz (2009) also highlights mutualism as the key to
understand the systemic, order-generating behavior of life and
ecosystems.
Origin of life
- For religious beliefs about the creation of life, see
creation myth.
Evidence suggests that
life on Earth
has existed for about 3.7
billion years. All known life forms
share fundamental molecular mechanisms, and based on these
observations, theories on the origin of life attempt to find a
mechanism explaining the formation of a primordial single cell
organism from which all life originates. There are many different
hypotheses regarding the path that might have been taken from
simple
organic molecules via
pre-cellular life to protocells and metabolism. Many models fall
into the "
genes-first" category or the
"
metabolism-first" category, but a recent
trend is the emergence of hybrid models that combine both
categories.
There is no scientific consensus as to how life originated and all
proposed theories are highly speculative. However, most currently
accepted scientific models build in one way or another on the
following hypotheses:
Life as we know it today synthesizes proteins, which are
polymers of amino acids using instructions encoded
by cellular
genes — which are polymers of
deoxyribonucleic acid (DNA).
Protein synthesis also entails
intermediary
ribonucleic acid (RNA)
polymers. One possibility is that genes came first and then
proteins. Another possibility is that proteins came first and then
genes. However, because genes are required to make proteins, and
proteins are required to make genes, the problem of considering
which came first is like that of the
chicken or the egg. Most scientists have
adopted the hypothesis that because DNA and proteins function
together so intimately, it's unlikely that they arose
independently. Therefore, many scientists consider the possibility,
apparently first suggested by
Francis
Crick, that the first life was based on the DNA-protein
intermediary:
RNA. In fact, RNA has the DNA-like
properties of information storage and replication and the
catalytic properties of some proteins. Crick and
others actually favored the
RNA-first hypothesis even before the
catalytic properties of RNA had been demonstrated by
Thomas Cech.
A significant issue with the RNA-first hypothesis is that
experiments designed to synthesize RNA from simple precursors have
not been nearly as successful as the Miller-Urey experiments that
synthesized other organic molecules from inorganic precursors. One
reason for the failure to create RNA in the laboratory is that RNA
precursors are very stable and don't react with each other under
ambient conditions. However, the successful synthesis of certain
RNA molecules under conditions hypothesized to exist prior to life
on Earth has been achieved by adding alternative precursors in a
specified order with the precursor
phosphate present throughout the reaction. This
study makes the RNA-first hypothesis more plausible to many
scientists.
Recent experiments have demonstrated true
Darwinian evolution of unique RNA
enzymes (
ribozymes) made up of two
separate catalytic components that replicate each other
in
vitro. In describing this remarkable work from his laboratory,
Gerald Joyce stated: "This is the first
example, outside of biology, of evolutionary adaptation in a
molecular genetic system." Such experiments make the possibility of
a primordial
RNA World even more attractive to many
scientists.
Conditions for life
The diversity of life on Earth today is a result of the dynamic
interplay between
genetic
opportunity, metabolic capability and
environmental challenges. For most
of its existence, Earth's habitable environment has been dominated
by
microorganisms and subjected to
their
metabolism and
evolution. As a consequence of such microbial
activities on a
geologic time
scale, the physical-chemical environment on Earth has been
changing, thereby determining the path of evolution of subsequent
life. For example, the release of molecular
oxygen by
cyanobacteria
as a by-product of
photosynthesis
induced fundamental, global changes in the Earth's environment. The
altered environment, in turn, posed novel evolutionary challenges
to the organisms present, which ultimately resulted in the
formation of our planet's major animal and plant species. Therefore
this 'co-evolution' between organisms and their environment is
apparently an inherent feature of living systems.
Range of tolerance
The inert components of an
ecosystem are
the physical and chemical factors necessary for life – energy
(
sunlight or
chemical energy),
water,
temperature,
atmosphere,
gravity,
nutrients, and
ultraviolet solar
radiation protection. In most ecosystems the conditions vary
during the day and often shift from one
season to the next. To live in most ecosystems, then,
organisms must be able to survive a range of conditions, called
'range of tolerance'. Outside of that are the 'zones of
physiological stress', where the survival and reproduction are
possible but not optimal. Outside of these zones are the 'zones of
intolerance', where life for that organism is implausible. It has
been determined that organisms that have a wide range of tolerance
are more widely distributed than organisms with a narrow range of
tolerance.
Extremophiles
Life has evolved strategies that allow it to survive even beyond
the physical and chemical limits to which it has adapted to grow.
To survive, some microorganisms can assume forms that enable them
to withstand
freezing,
complete desiccation,
starvation, high-levels of
radiation exposure, and other physical or
chemical challenges. Furthermore, some microorganisms can survive
exposure to such conditions for weeks, months, years, or even
centuries.
Extremophiles are microbial
life forms that thrive outside the ranges life is commonly found
in. They also excel at exploiting uncommon sources of energy. While
all organisms are composed of nearly identical
molecules, evolution has enabled such microbes to
cope with this wide range of physical and chemical conditions.
Characterization of the
structure and metabolic diversity of
microbial communities in such
extreme environments is ongoing. An
understanding of the tenacity and versatility of life on Earth, as
well as an understanding of the molecular systems that some
organisms utilize to survive such extremes, will provide a critical
foundation for the search for
life
beyond Earth.
Classification of life
Traditionally, people have divided organisms into the classes of
plants and
animals,
based mainly on their ability of movement. The first known attempt
to classify organisms was conducted by the Greek philosopher
Aristotle (384-322 BC). He classified all
living organisms known at that time as either a plant or an animal.
Aristotle distinguished animals with blood from animals without
blood (or at least without red blood), which can be compared with
the concepts of
vertebrates and
invertebrates respectively. He divided the
blooded animals into five groups: viviparous quadrupeds (
mammals),
birds, oviparous
quadrupeds (
reptiles and
amphibians),
fishes and
whales. The bloodless animals were also
divided into five groups:
cephalopods,
crustaceans, insects (which also included
the
spiders,
scorpions, and
centipedes,
in addition to what we now define as
insects), shelled animals (such as most
molluscs and
echinoderms)
and "
zoophytes". Though Aristotle's work in
zoology was not without errors, it was the grandest biological
synthesis of the time and remained the ultimate authority for many
centuries after his death.
The exploration of the
American
continent revealed large numbers of new plants and animals that
needed descriptions and classification. In the latter part of the
16th century and the beginning of the 17th, careful study of
animals commenced and was gradually extended until it formed a
sufficient body of knowledge to serve as an anatomical basis for
classification.
In the late 1740s,
Carolus Linnaeus
introduced his method, still used, to formulate the
scientific name of every species. Linnaeus
took every effort to improve the composition and reduce the length
of the many-worded names by abolishing unnecessary rhetoric,
introducing new descriptive terms and defining their meaning with
an unprecedented precision. By consistently using his system,
Linnaeus separated
nomenclature from
taxonomy. This convention for naming
species is referred to as
binomial
nomenclature.
The
fungi were originally treated as plants.
For a short period Linnaeus had placed them in the taxon
Vermes in Animalia. He later placed them back in
Plantae.
Copeland classified the
Fungi in his Protoctista, thus partially avoiding the problem but
acknowledged their special status. The problem was eventually
solved by
Whittaker, when he gave
them their own kingdom in his
five-kingdom system. As it
turned out, the fungi are more closely related to animals than to
plants.
As new discoveries enabled us to study
cells and
microorganisms, new groups of life were
revealed, and the fields of
cell
biology and
microbiology were
created. These new organisms were originally described separately
in
protozoa as animals and
protophyta/thallophyta as plants, but were
united by
Haeckel in his kingdom
protista, later the group of
prokaryotes were split off in the kingdom
Monera, eventually this kingdom would be
divided in two separate groups, the
Bacteria and the
Archaea,
leading to the
six-kingdom system and
eventually to the current
three-domain system. The classification
of eukaryotes is still controversial, with protist taxonomy
especially problematic.
As
microbiology,
molecular biology and
virology developed, non-cellular reproducing agents
were discovered, such as
viruses and
viroids. Sometimes these entities are considered to
be alive but others argue that viruses are not living organisms
since they lack characteristics such as
cell membrane,
metabolism and do not grow or respond to their
environments. Viruses can however be classed into "species" based
on their biology and genetics but many aspects of such a
classification remain controversial.
Since the 1960s a trend called
cladistics
has emerged, arranging taxa in an
evolutionary or phylogenetic tree. It is
unclear, should this be implemented, how the different codes will
coexist.
Extraterrestrial life
Earth is the only planet in the
universe known to harbour life. The
Drake equation, which relates the number of
extraterrestrial civilizations in our galaxy with which we might
come in contact, has been used to discuss the probability of life
elsewhere, but scientists disagree on many of the values of
variables in this equation. Depending on those values, the equation
may either suggest that life arises frequently or
infrequently.
Panspermia, also called exogenesis, is a
hypothesis proposing that life originated
elsewhere in the universe and was subsequently transferred to Earth
in the form of
spores perhaps via
meteorites,
comets or
cosmic dust. However, this hypothesis
does not help explain the ultimate origin of life.
Death
Death is the permanent termination of all
vital functions or life processes in an organism or cell. After
death, the remains of an organism become part of the
biogeochemical cycle. Organisms may be
consumed by a
predator or a
scavenger
and leftover
organic material may
then be further decomposed by
detritivores, organisms which recycle
detritus, returning it to the environment for reuse
in the
food chain.
One of the challenges in defining death is in distinguishing it
from life. Death would seem to refer to either the moment at which
life ends, or when the state that follows life begins. However,
determining when death has occurred requires drawing precise
conceptual boundaries between life and death. This is problematic
however because there is little consensus over how to define life.
The nature of death has for millennia been a central concern of the
world's religious traditions and of philosophical enquiry. Many
religions maintain faith in either some kind of
afterlife,
reincarnation, or
resurrection.
Extinction
Extinction is the gradual process by
which a group of
taxa or
species dies out, reducing
biodiversity. The moment of extinction is
generally considered to be the death of the last individual of that
species. Because a species' potential
range may be very large, determining this
moment is difficult, and is usually done retrospectively after a
period of apparent absence. Species become extinct when they are no
longer able to survive in changing
habitat
or against superior competition. Over the history of the Earth,
over 99% of all the species that have ever lived have gone
extinct.
Fossils
Fossils are the preserved remains or
traces of animals, plants, and other organisms
from the remote past. The totality of fossils, both discovered and
undiscovered, and their placement in fossil-containing
rock formations and
sedimentary layers (
strata) is known as the
fossil record. Such
a preserved specimen is called a "fossil" if it is older than the
arbitrary date of 10,000 years ago. Hence, fossils range in age
from the youngest at the start of the
Holocene Epoch to the oldest from the
Archaean Eon, a few
billion years old.
See also
References
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Further reading
External links