Life on Earth:
This article is one of a series on:
LIFE IN THE UNIVERSE
LIFE IN THE SOLAR SYSTEM
Life on Venus
Life on Earth
Life on Mars
Life on Europa
Life on Titan
Life on Titan
LIFE OUTSIDE THE SOLAR SYSTEM
Circumstellar habitable zone
LIFE is a characteristic distinguishing physical entities having
biological processes , such as signaling and self-sustaining
processes, from those that do not, either because such functions have
ceased , or because they never had such functions and are classified
as inanimate. Various forms of life exist, such as plants , animals ,
fungi , protists , archaea , and bacteria . The criteria can at times
be ambiguous and may or may not define viruses , viroids , or
potential artificial life as "living".
Biology is the primary science
concerned with the study of life, although many other sciences are
The definition of life is controversial. The current definition is
that organisms maintain homeostasis , are composed of cells , undergo
metabolism , can grow , adapt to their environment, respond to stimuli
, and reproduce . However, many other biological definitions have been
proposed, and there are some borderline cases of life, such as
viruses. Throughout history, there have been many attempts to define
what is meant by "life" and many theories on the properties and
emergence of living things, such as materialism , the belief that
everything is made out of matter and that life is merely a complex
form of it; hylomorphism , the belief that all things are a
combination of matter and form, and the form of a living thing is its
soul; spontaneous generation , the belief that life repeatedly emerges
from non-life; and vitalism , a now largely discredited hypothesis
that living organisms possess a "life force" or "vital spark". Modern
definitions are more complex, with input from a diversity of
Biophysicists have proposed many definitions
based on chemical systems ; there are also some living systems
theories , such as the
Gaia hypothesis , the idea that the Earth
itself is alive. Another theory is that life is the property of
ecological systems , and yet another is elaborated in complex systems
biology , a branch or subfield of mathematical biology . Abiogenesis
describes the natural process of life arising from non-living matter,
such as simple organic compounds . Properties common to all organisms
include the need for certain core chemical elements to sustain
Earth first appeared as early as 4.28 billion years ago, soon
after ocean formation 4.41 billion years ago , and not long after the
formation of the
Earth 4.54 billion years ago. Earth's current
life may have descended from an
RNA world , although
RNA -based life
may not have been the first. The mechanism by which life began on
Earth is unknown, though many hypotheses have been formulated and are
often based on the
Miller–Urey experiment . The earliest known life
forms are microfossils of bacteria. In July 2016, scientists reported
identifying a set of 355 genes believed to be present in the last
universal common ancestor (LUCA) of all living organisms.
Since its primordial beginnings, life on
Earth has changed its
environment on a geologic time scale . To survive in most ecosystems ,
life must often adapt to a wide range of conditions. Some
microorganisms , called extremophiles , thrive in physically or
geochemically extreme environments that are detrimental to most other
life on Earth.
Aristotle was the first person to classify organisms.
Carl Linnaeus introduced his system of binomial nomenclature
for the classification of species . Eventually new groups and
categories of life were discovered, such as cells and microorganisms ,
forcing dramatic revisions of the structure of relationships between
Cells are sometimes considered the smallest units
and "building blocks" of life. There are two kinds of cells,
prokaryotic and eukaryotic , both of which consist of cytoplasm
enclosed within a membrane and contain many biomolecules such as
proteins and nucleic acids .
Cells reproduce through a process of cell
division , in which the parent cell divides into two or more daughter
Though currently only known on
Earth , life need not be restricted to
it, and many scientists believe in the existence of extraterrestrial
Artificial life is a computer simulation or man-made
reconstruction of any aspect of life, which is often used to examine
systems related to natural life.
Death is the permanent termination of
all biological functions which sustain an organism, and as such, is
the end of its life.
Extinction is the process by which an entire
group or taxon , normally a species , dies out.
Fossils are the
preserved remains or traces of organisms.
* 1 Definitions
* 1.1.1 Alternative definitions
* 1.1.2 Viruses
Living systems theories
* 1.3.2 Nonfractionability
Life as a property of ecosystems
Complex systems biology
* 1.3.5 Darwinian dynamic
* 1.3.6 Operator theory
* 2 History of study
* 3 Origin
* 4 Environmental conditions
* 4.2 Range of tolerance
* 4.4 Chemical elements
* 5 Classification
* 5.1 Biota (taxonomy)
* 7 Extraterrestrial
* 8 Artificial
* 10 See also
* 11 Notes
* 12 References
* 13 Further reading
* 14 External links
It is a challenge for scientists and philosophers to define life.
This is partially because life is a process, not a substance. Any
definition must be general enough to both encompass all known life and
any unknown life that may be different from life on Earth.
Organism The characteristics of life
Since there is no unequivocal definition of life, most current
definitions in biology are descriptive.
Life is considered a
characteristic of something that exhibits all or most of the following
* HOMEOSTASIS : regulation of the internal environment to maintain a
constant state; for example, sweating to reduce temperature
* ORGANIZATION : being structurally composed of one or more cells
– 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 time in response to the
environment. This ability is fundamental to the process of evolution
and is determined by the organism's heredity , diet, and external
* 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 chemotaxis .
* REPRODUCTION : the ability to produce new individual organisms,
either asexually from a single parent organism or sexually from two
These complex processes, called physiological functions , have
underlying physical and chemical bases, as well as signaling and
control mechanisms that are essential to maintaining life.
Entropy and life
From a physics perspective, living beings are thermodynamic systems
with an organized molecular structure that can reproduce itself and
evolve as survival dictates. Thermodynamically, life has been
described as an open system which makes use of gradients in its
surroundings to create imperfect copies of itself. Hence, life is a
self-sustained chemical system capable of undergoing Darwinian
evolution . A major strength of this definition is that it
distinguishes life by the evolutionary process rather than its
Others take a systemic viewpoint that does not necessarily depend on
molecular chemistry. One 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 . This definition is extended by the apparition of novel
functions over time.
Adenovirus as seen under an electron
Whether or not viruses should be considered as alive is
controversial. They are most often considered as just replicators
rather than forms of life. They have been described as "organisms at
the edge of life" because they possess genes , evolve by natural
selection, and replicate by creating multiple copies of themselves
through self-assembly. However, viruses do not metabolize and they
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 .
To reflect the minimum phenomena required, other biological
definitions of life have been proposed, with many of these being
based upon chemical systems .
Biophysicists have commented that living
things function on negative entropy . In other words, living
processes can be viewed as a delay of the spontaneous diffusion or
dispersion of the internal energy of biological molecules towards more
potential microstates . 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 that 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.
LIVING SYSTEMS THEORIES
Living systems are open self-organizing living things that interact
with their environment . These systems are maintained by flows of
information, energy , and matter.
Some scientists have proposed in the last few decades that a general
living systems theory is required to explain the nature of life. Such
a general theory would arise out of the ecological and biological
sciences and attempt to map general principles for how all living
systems work. Instead of examining phenomena by attempting to break
things down into components, a general living systems theory explores
phenomena in terms of dynamic patterns of the relationships of
organisms with their environment.
The idea that the
Earth is alive is found in philosophy and religion,
but the first scientific discussion of it was by the Scottish
James Hutton . In 1785, he stated that the
Earth was a
superorganism and that its proper study should be physiology . Hutton
is considered the father of geology , but his idea of a living Earth
was forgotten in the intense reductionism of the 19th century. :10 The
Gaia hypothesis, proposed in the 1960s by scientist
James Lovelock ,
suggests that life on
Earth functions as a single organism that
defines and maintains environmental conditions necessary for its
survival. This hypothesis served as one of the foundations of the
Earth system science .
The first attempt at a general living systems theory for explaining
the nature of life was in 1978, by American biologist James Grier
Miller . Robert Rosen (1991) built on this by defining 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
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
Robert Ulanowicz (2009) highlights mutualism as the key to
understand the systemic, order-generating behavior of life and
Complex systems biology See also: Mathematical biology
Complex systems biology (CSB) is a field of science that studies the
emergence of complexity in functional organisms from the viewpoint of
dynamic systems theory. The latter is also often called systems
biology and aims to understand the most fundamental aspects of life. A
closely related approach to CSB and systems biology called relational
biology is concerned mainly with understanding life processes in terms
of the most important relations, and categories of such relations
among the essential functional components of organisms; for
multicellular organisms, this has been defined as "categorical
biology", or a model representation of organisms as a category theory
of biological relations, as well as an algebraic topology of the
functional organization of living organisms in terms of their dynamic,
complex networks of metabolic, genetic, and epigenetic processes and
signaling pathways . Alternative but closely related approaches
focus on the interdependance of constraints, where constraints can be
either molecular, such as enzymes, or macroscopic, such as the
geometry of a bone or of the vascular system.
It has also been argued that the evolution of order in living systems
and certain physical systems obeys a common fundamental principle
termed the Darwinian dynamic. The Darwinian dynamic was formulated
by first considering how macroscopic order is generated in a simple
non-biological system far from thermodynamic equilibrium, and then
extending consideration to short, replicating
RNA molecules. The
underlying order-generating process was concluded to be basically
similar for both types of systems.
Another systemic definition called the operator theory proposes that
"life is a general term for the presence of the typical closures found
in organisms; the typical closures are a membrane and an autocatalytic
set in the cell" and that an organism is any system with an
organisation that complies with an operator type that is at least as
complex as the cell.
Life can also be modeled as a network of
inferior negative feedbacks of regulatory mechanisms subordinated to a
superior positive feedback formed by the potential of expansion and
HISTORY OF STUDY
Plant growth in the
Herds of zebra and impala gathering on the
Maasai Mara plain An
aerial photo of microbial mats around the
Grand Prismatic Spring
Grand Prismatic Spring of
Yellowstone National Park
Yellowstone National Park
Some of the earliest theories of life were materialist, holding that
all that exists is matter, and that life is merely a complex form or
arrangement of matter.
Empedocles (430 BC) argued that everything 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
Democritus (460 BC) thought that the essential characteristic of life
is having a soul (psyche). Like other ancient writers, he was
attempting to explain what makes something a living thing. His
explanation was that fiery atoms make a soul in exactly the same way
atoms and void account for any other thing. He elaborates on fire
because of the apparent connection between life and heat, and because
Plato's world of eternal and unchanging Forms , imperfectly
represented in matter by a divine Artisan , contrasts sharply with the
Weltanschauungen , of which atomism was, by the
fourth century at least, the most prominent ... This debate persisted
throughout the ancient world. Atomistic mechanism got a shot in the
Epicurus ... while the
Stoics adopted a divine teleology ...
The choice seems simple: either show how a structured, regular world
could arise out of undirected processes, or inject intelligence into
the system. — R. J. Hankinson, Cause and Explanation in Ancient
The mechanistic materialism that originated in ancient Greece was
revived and revised by the French philosopher
René Descartes , who
held that animals and humans were assemblages of parts that together
functioned as a machine. In the 19th century, the advances in cell
theory in biological science encouraged this view. The evolutionary
Charles Darwin (1859) is a mechanistic explanation for the
origin of species by means of natural selection .
Hylomorphism The structure of the souls of
plants, animals, and humans, according to
Hylomorphism is a theory first expressed by the Greek philosopher
Aristotle (322 BC). The application of hylomorphism to biology was
important to Aristotle, and biology is extensively covered in his
extant writings . In this view, everything in the material universe
has both matter and form, and 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
of the attributes of the lower ones.
Aristotle believed that while
matter can exist without form, form cannot exist without matter, and
that therefore the soul cannot exist without the body.
This account is consistent with teleological explanations of life,
which account for phenomena in terms of purpose or goal-directedness.
Thus, the whiteness of the polar bear's coat is explained by its
purpose of camouflage. The direction of causality (from the future to
the past) is in contradiction with the scientific evidence for natural
selection, which explains the consequence in terms of a prior cause.
Biological features are explained not by looking at future optimal
results, but by looking at the past evolutionary history of a species,
which led to the natural selection of the features in question.
Spontaneous generation was the belief on the ordinary formation of
living organisms without descent from similar organisms. Typically,
the idea was that certain forms such as fleas could arise from
inanimate matter such as dust or the supposed seasonal generation of
mice and insects from mud or garbage.
The theory of spontaneous generation was proposed by
Aristotle , who
compiled and expanded the work of prior natural philosophers and the
various ancient explanations of the appearance of organisms; it held
sway for two millennia. It was decisively dispelled by the experiments
Louis Pasteur in 1859, who expanded upon the investigations of
predecessors such as
Francesco Redi . Disproof of the traditional
ideas of spontaneous generation is no longer controversial among
Vitalism is the belief that the life-principle is non-material. This
Georg Ernst Stahl
Georg Ernst Stahl (17th century), and remained popular
until the middle of the 19th century. It appealed to philosophers such
Henri Bergson ,
Friedrich Nietzsche , and
Wilhelm Dilthey ,
Marie François Xavier Bichat , and chemists like
Justus von Liebig
Justus von Liebig .
Vitalism included the idea that there was a
fundamental difference between organic and inorganic material, and the
belief that organic material can only be derived from living things.
This was disproved in 1828, when
Friedrich Wöhler prepared urea from
inorganic materials. This
Wöhler synthesis is considered the
starting point of modern organic chemistry . It is of historical
significance because for the first time an organic compound was
produced in inorganic reactions.
During the 1850s,
Hermann von Helmholtz
Hermann von Helmholtz , anticipated by Julius
Robert von Mayer , demonstrated that no energy is lost in muscle
movement, suggesting that there were no "vital forces" necessary to
move a muscle. These results led to the abandonment of scientific
interest in vitalistic theories, although the belief lingered on in
pseudoscientific theories such as homeopathy , which interprets
diseases and sickness as caused by disturbances in a hypothetical
vital force or life force.
Life timeline view • discuss • edit -4500 — – -4000 —
– -3500 — – -3000 — – -2500 — – -2000 — – -1500 —
– -1000 — – -500 — – 0 — WATER Single-celled
life PHOTOSYNTHESIS EUKARYOTES Multicellular
life LAND LIFE DINOSAURS MAMMALS FLOWERS ←
Earth (−4540 ) ← Earliest water ← Earliest
life ← LHB meteorites ← Earliest oxygen ←
Atmospheric oxygen ←
Oxygen crisis ← Earliest sexual
reproduction ← Ediacara biota ←
← Earliest humans P
n Pongola Huronian
Cryogenian Andean Karoo Quaternary
Axis scale : millions of years .
Orange labels: known ICE AGES.
Human timeline and
Nature timeline Main article:
The age of the
Earth is about 4.54 billion years. Evidence
suggests that life on
Earth has existed for at least 3.5 billion years
, with the oldest physical traces of life dating back 3.7
billion years; however, some theories, such as the Late Heavy
Bombardment theory , suggest that life on
Earth may have started even
earlier, as early as 4.1–4.4 billion years ago, and the
chemistry leading to life may have begun shortly after the
Big Bang ,
13.8 billion years ago , during an epoch when the universe was only
10–17 million years old.
More than 99% of all species of life forms, amounting to over five
billion species, that ever lived on
Earth are estimated to be extinct
Although the number of Earth's catalogued species of lifeforms is
between 1.2 million and 2 million, the total number of species in
the planet is uncertain. Estimates range from 8 million to 100
million, with a more narrow range between 10 and 14 million, but it
may be as high as 1 trillion (with only one-thousandth of one percent
of the species described) according to studies realized in May 2016.
The total amount of related
DNA base pairs on
Earth is estimated at
5.0 x 1037 and weighs 50 billion tonnes. In comparison, the total
mass of the biosphere has been estimated to be as much as 4 TtC
(trillion tons of carbon ). In July 2016, scientists reported
identifying a set of 355 genes from the Last Universal Common Ancestor
(LUCA) of all organisms living on Earth.
All known life forms share fundamental molecular mechanisms,
reflecting their common descent ; based on these observations,
hypotheses on the origin of life attempt to find a mechanism
explaining the formation of a universal common ancestor , from simple
organic molecules via pre-cellular life to protocells and metabolism.
Models have been divided into "genes-first" and "metabolism-first"
categories, but a recent trend is the emergence of hybrid models that
combine both categories.
There is no current scientific consensus as to how life originated.
However, most accepted scientific models build on the Miller–Urey
experiment and the work of Sidney Fox , which show that conditions on
Earth favored chemical reactions that synthesize amino
acids and other organic compounds from inorganic precursors, and
phospholipids spontaneously form lipid bilayers , the basic structure
of a cell membrane .
Living organisms synthesize proteins , which are polymers of amino
acids using instructions encoded by deoxyribonucleic acid (DNA).
Protein synthesis entails intermediary ribonucleic acid (RNA)
polymers. One possibility for how life began is that genes originated
first, followed by proteins; the alternative being that proteins came
first and then genes.
However, because genes and proteins are both required to produce the
other, the problem of considering which came first is like that of the
chicken or the egg . Most scientists have adopted the hypothesis that
because of this, it is unlikely that genes and proteins arose
Therefore, a possibility, first suggested by
Francis Crick , is that
the first life was based on
RNA , which has the DNA-like properties
of information storage and the catalytic properties of some proteins.
This is called the
RNA world hypothesis , and it is supported by the
observation that many of the most critical components of cells (those
that evolve the slowest) are composed mostly or entirely of RNA. Also,
many critical cofactors (ATP ,
NADH , etc.) are either
nucleotides or substances clearly related to them. The catalytic
RNA had not yet been demonstrated when the hypothesis
was first proposed, but they were confirmed by
Thomas Cech in 1986.
One issue with the
RNA world hypothesis is that synthesis of
simple inorganic precursors is more difficult than for other organic
molecules. One reason for this is that
RNA precursors are very stable
and react with each other very slowly under ambient conditions, and it
has also been proposed that living organisms consisted of other
molecules before RNA. However, the successful synthesis of certain
RNA molecules under the conditions that existed 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 world hypothesis more plausible.
Geological findings in 2013 showed that reactive phosphorus species
(like phosphite ) were in abundance in the ocean before 3.5 Ga, and
Schreibersite easily reacts with aqueous glycerol to generate
phosphite and glycerol 3-phosphate . It is hypothesized that
Schreibersite -containing meteorites from the Late Heavy Bombardment
could have provided early reduced phosphorus, which could react with
prebiotic organic molecules to form phosphorylated biomolecules, like
In 2009, experiments demonstrated
Darwinian evolution of a
two-component system of
RNA enzymes (ribozymes ) in vitro. The work
was performed in the laboratory of
Gerald Joyce , who stated "This is
the first example, outside of biology, of evolutionary adaptation in a
molecular genetic system."
Prebiotic compounds may have originated extraterrestrially. NASA
findings in 2011, based on studies with meteorites found on Earth,
RNA components (adenine , guanine and related organic
molecules) may be formed in outer space .
In March 2015,
NASA scientists reported that, for the first time,
RNA organic compounds of life, including uracil ,
cytosine and thymine , have been formed in the laboratory under outer
space conditions, using starting chemicals, such as pyrimidine , found
in meteorites . Pyrimidine, like polycyclic aromatic hydrocarbons
(PAHs), the most carbon -rich chemical found in the universe , may
have been formed in red giants or in interstellar dust and gas clouds,
according to the scientists.
According to the panspermia hypothesis, microscopic life
—distributed by meteoroids , asteroids and other small Solar System
bodies —may exist throughout the universe.
Cyanobacteria dramatically changed the composition of life forms
Earth by leading to the near-extinction of oxygen-intolerant
The diversity of life on
Earth is a result of the dynamic interplay
between genetic opportunity , metabolic capability, environmental
challenges, and symbiosis . For most of its existence, Earth's
habitable environment has been dominated by microorganisms and
subjected to their metabolism and evolution. As a consequence of these
microbial activities, the physical-chemical environment on
been changing on a geologic time scale , thereby affecting the path of
evolution of subsequent life. For example, the release of molecular
oxygen by cyanobacteria as a by-product of photosynthesis induced
global changes in the Earth's environment. Because oxygen was toxic to
most life on
Earth at the time, this posed novel evolutionary
challenges, and ultimately resulted in the formation of Earth's major
animal and plant species. This interplay between organisms and their
environment is an inherent feature of living systems.
The biosphere is the global sum of all ecosystems. It can also be
termed as the zone of life on
Earth , a closed system (apart from
solar and cosmic radiation and heat from the interior of the Earth),
and largely self-regulating. By the most general biophysiological
definition, the biosphere is the global ecological system integrating
all living beings and their relationships, including their interaction
with the elements of the lithosphere , geosphere , hydrosphere , and
Life forms live in every part of the Earth's biosphere , including
soil , hot springs , inside rocks at least 19 km (12 mi) deep
underground, the deepest parts of the ocean, and at least 64 km (40
mi) high in the atmosphere. Under certain test conditions, life
forms have been observed to thrive in the vacuum of outer space .
Life forms appear to thrive in the
Mariana Trench , the deepest spot
in the Earth's oceans. Other researchers reported related studies
that life forms thrive inside rocks up to 580 m (1,900 ft; 0.36 mi)
below the sea floor under 2,590 m (8,500 ft; 1.61 mi) of ocean off the
coast of the northwestern United States, as well as 2,400 m (7,900
ft; 1.5 mi) beneath the seabed off Japan. In August 2014, scientists
confirmed the existence of life forms living 800 m (2,600 ft; 0.50 mi)
below the ice of Antarctica.
The biosphere is postulated to have evolved , beginning with a
process of biopoesis (life created naturally from non-living matter,
such as simple organic compounds) or biogenesis (life created from
living matter), at least some 3.5 billion years ago. The earliest
evidence for life on
Earth includes biogenic graphite found in 3.7
billion-year-old metasedimentary rocks from
Western Greenland and
microbial mat fossils found in 3.48 billion-year-old sandstone from
Western Australia . More recently, in 2015, "remains of biotic life
" were found in 4.1 billion-year-old rocks in Western Australia. In
2017, putative fossilized microorganisms (or microfossils ) were
announced to have been discovered in hydrothermal vent precipitates in
the Nuvvuagittuq Belt of Quebec, Canada that were as old as 4.28
billion years, the oldest record of life on earth, suggesting "an
almost instantaneous emergence of life" after ocean formation 4.4
billion years ago , and not long after the formation of the
billion years ago. According to one of the researchers, "If life
arose relatively quickly on
Earth ... then it could be common in the
In a general sense, biospheres are any closed, self-regulating
systems containing ecosystems. This includes artificial biospheres
Biosphere 2 and
BIOS-3 , and potentially ones on other planets
RANGE OF TOLERANCE
Deinococcus radiodurans is an extremophile that can resist
extremes of cold, dehydration, vacuum, acid, and radiation exposure.
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 from one season to the next. To live in most
ecosystems, then, organisms must be able to survive a range of
conditions, called the "range of tolerance." Outside that are the
"zones of physiological stress," where the survival and reproduction
are possible but not optimal. Beyond these zones are the "zones of
intolerance," where survival and reproduction of that organism is
unlikely or impossible. Organisms that have a wide range of tolerance
are more widely distributed than organisms with a narrow range of
To survive, selected microorganisms can assume forms that enable them
to withstand freezing , complete desiccation , starvation , high
levels of radiation exposure , and other physical or chemical
challenges. These microorganisms may survive exposure to such
conditions for weeks, months, years, or even centuries. Extremophiles
are microbial life forms that thrive outside the ranges where life is
commonly found. They 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.
Microbial life forms thrive even in the
Mariana Trench , the deepest
spot on the Earth. Microbes also thrive inside rocks up to 1900 feet
below the sea floor under 8500 feet of ocean.
Investigation 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, is important for the search for life
Earth . For example, lichen could survive for a month in a
simulated Martian environment .
All life forms require certain core chemical elements needed for
biochemical functioning. These include carbon , hydrogen , nitrogen ,
oxygen, phosphorus , and sulfur —the elemental macronutrients for
all organisms —often represented by the acronym
CHNOPS . Together
these make up nucleic acids , proteins and lipids , the bulk of living
matter. Five of these six elements comprise the chemical components of
DNA, the exception being sulfur. The latter is a component of the
amino acids cysteine and methionine . The most biologically abundant
of these elements is carbon, which has the desirable attribute of
forming multiple, stable covalent bonds . This allows carbon-based
(organic) molecules to form an immense variety of chemical
arrangements. Alternative hypothetical types of biochemistry have
been proposed that eliminate one or more of these elements, swap out
an element for one not on the list, or change required chiralities or
other chemical properties.
Deoxyribonucleic acid is a molecule that carries most of the genetic
instructions used in the growth, development, functioning and
reproduction of all known living organisms and many viruses .
RNA are nucleic acid s; alongside proteins and complex carbohydrates ,
they are one of the three major types of macromolecule that are
essential for all known forms of life. Most
DNA molecules consist of
two biopolymer strands coiled around each other to form a double helix
. The two
DNA strands are known as polynucleotides since they are
composed of simpler units called nucleotides . Each nucleotide is
composed of a nitrogen-containing nucleobase —either cytosine (C),
guanine (G), adenine (A), or thymine (T)—as well as a sugar called
deoxyribose and a phosphate group . The nucleotides are joined to one
another in a chain by covalent bonds between the sugar of one
nucleotide and the phosphate of the next, resulting in an alternating
sugar-phosphate backbone . According to base pairing rules (A with T,
and C with G), hydrogen bonds bind the nitrogenous bases of the two
separate polynucleotide strands to make double-stranded DNA. The total
amount of related
DNA base pairs on
Earth is estimated at 5.0 x 1037,
and weighs 50 billion tonnes . In comparison, the total mass of the
biosphere has been estimated to be as much as 4 TtC (trillion tons of
DNA stores biological information . The
DNA backbone is resistant to
cleavage, and both strands of the double-stranded structure store the
same biological information. Biological information is replicated as
the two strands are separated. A significant portion of
DNA (more than
98% for humans) is non-coding , meaning that these sections do not
serve as patterns for protein sequences.
The two strands of
DNA run in opposite directions to each other and
are therefore anti-parallel . Attached to each sugar is one of four
types of nucleobases (informally, bases). It is the sequence of these
four nucleobases along the backbone that encodes biological
information. Under the genetic code ,
RNA strands are translated to
specify the sequence of amino acids within proteins. These
are initially created using
DNA strands as a template in a process
called transcription .
DNA is organized into long structures called
chromosomes . During cell division these chromosomes are duplicated in
the process of
DNA replication , providing each cell its own complete
set of chromosomes. Eukaryotic organisms (animals , plants , fungi ,
and protists ) store most of their
DNA inside the cell nucleus and
some of their
DNA in organelles , such as mitochondria or chloroplasts
. In contrast, prokaryotes (bacteria and archaea ) store their DNA
only in the cytoplasm . Within the chromosomes, chromatin proteins
such as histones compact and organize DNA. These compact structures
guide the interactions between
DNA and other proteins, helping control
which parts of the
DNA are transcribed.
DNA was first isolated by
Friedrich Miescher in 1869. Its molecular
structure was identified by
James Watson and
Francis Crick in 1953,
whose model-building efforts were guided by
X-ray diffraction data
Rosalind Franklin .
Biological classification The
hierarchy of biological classification 's eight major taxonomic ranks
Life is divided into domains, which are subdivided into further
groups. Intermediate minor rankings are not shown.
Life is usually classified by eight levels of taxa—domains,
kingdoms, phyla, class, order, family, genus, and species. In May
2016, scientists reported that 1 trillion species are estimated to be
Earth currently with only one-thousandth of one percent described.
The first known attempt to classify organisms was conducted by the
Aristotle (384–322 BC), who classified all living
organisms known at that time as either a plant or an animal , based
mainly on their ability to move. He also 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, and divided the blooded animals into five
groups: viviparous quadrupeds (mammals ), oviparous quadrupeds
(reptiles and amphibians ), birds , fishes and whales . The bloodless
animals were also divided into five groups: cephalopods , crustaceans
, insects (which included the spiders , scorpions , and centipedes ,
in addition to what we define as insects today), shelled animals (such
as most molluscs and echinoderms ), and "zoophytes " (animals that
resemble plants). 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
Americas 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,
Carl Linnaeus introduced his system
of binomial nomenclature for the classification of species. Linnaeus
attempted to improve the composition and reduce the length of the
previously used many-worded names by abolishing unnecessary rhetoric,
introducing new descriptive terms and precisely defining their
The fungi were originally treated as plants. For a short period
Linnaeus had classified them in the taxon
Vermes in Animalia, but
later placed them back in Plantae. Copeland classified the
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 .
Evolutionary history shows that the fungi are more closely related to
animals than to plants.
As new discoveries enabled detailed study of 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 the
Protista ; later, the prokaryotes were split off in the
Monera , which would eventually be divided into two separate
Bacteria and the
Archaea . This led to the six-kingdom
system and eventually to the current three-domain system , which is
based on evolutionary relationships. However, the classification of
eukaryotes, especially of protists, is still controversial.
As microbiology, molecular biology and virology developed,
non-cellular reproducing agents were discovered, such as viruses and
viroids . Whether these are considered alive has been a matter of
debate; viruses lack characteristics of life such as cell membranes,
metabolism and the ability to grow or respond to their environments.
Viruses can still be classed into "species" based on their biology and
genetics , but many aspects of such a classification remain
In the 1960s a trend called cladistics emerged, arranging taxa based
on clades in an evolutionary or phylogenetic tree .
1969 Woese et al.
Kingdom (biology) § Summary
THIS SECTION NEEDS EXPANSION. You can help by adding to it . (March
In systems of scientific classification , BIOTA is the superdomain
that classifies all life.
Cells are the basic unit of structure in every living thing, and all
cells arise from pre-existing cells by division .
Cell theory was
Henri Dutrochet ,
Theodor Schwann ,
Rudolf Virchow and
others during the early nineteenth century, and subsequently became
widely accepted. The activity of an organism depends on the total
activity of its cells, with energy flow occurring within and between
Cells contain hereditary information that is carried forward as
a genetic code during cell division.
There are two primary types of cells. Prokaryotes lack a nucleus and
other membrane-bound organelles , although they have circular
Archaea are two domains of prokaryotes. The
other primary type of cells are the eukaryotes , which have distinct
nuclei bound by a nuclear membrane and membrane-bound organelles,
including mitochondria , chloroplasts , lysosomes , rough and smooth
endoplasmic reticulum , and vacuoles . In addition, they possess
organized chromosomes that store genetic material. All species of
large complex organisms are eukaryotes, including animals, plants and
fungi, though most species of eukaryote are protist microorganisms .
The conventional model is that eukaryotes evolved from prokaryotes,
with the main organelles of the eukaryotes forming through
endosymbiosis between bacteria and the progenitor eukaryotic cell.
The molecular mechanisms of cell biology are based on proteins . Most
of these are synthesized by the ribosomes through an enzyme-catalyzed
process called protein biosynthesis . A sequence of amino acids is
assembled and joined together based upon gene expression of the cell's
nucleic acid. In eukaryotic cells, these proteins may then be
transported and processed through the
Golgi apparatus in preparation
for dispatch to their destination.
Cells reproduce through a process of cell division in which the
parent cell divides into two or more daughter cells. For prokaryotes,
cell division occurs through a process of fission in which the
replicated, then the two copies are attached to parts of the cell
membrane. In eukaryotes , a more complex process of mitosis is
followed. However, the end result is the same; the resulting cell
copies are identical to each other and to the original cell (except
for mutations ), and both are capable of further division following an
Multicellular organisms may have first evolved through the formation
of colonies like cells. These cells can form group organisms through
cell adhesion . The individual members of a colony are capable of
surviving on their own, whereas the members of a true multi-cellular
organism have developed specializations, making them dependent on the
remainder of the organism for survival. Such organisms are formed
clonally or from a single germ cell that is capable of forming the
various specialized cells that form the adult organism. This
specialization allows multicellular organisms to exploit resources
more efficiently than single cells. In January 2016, scientists
reported that, about 800 million years ago , a minor genetic change in
a single molecule , called GK-PID, may have allowed organisms to go
from a single cell organism to one of many cells .
Cells have evolved methods to perceive and respond to their
microenvironment, thereby enhancing their adaptability. Cell signaling
coordinates cellular activities, and hence governs the basic functions
of multicellular organisms. Signaling between cells can occur through
direct cell contact using juxtacrine signalling , or indirectly
through the exchange of agents as in the endocrine system . In more
complex organisms, coordination of activities can occur through a
dedicated nervous system .
Extraterrestrial life ,
Astrobiology , and
Though life is confirmed only on Earth, many think that
extraterrestrial life is not only plausible, but probable or
inevitable. Other planets and moons in the
Solar System and other
planetary systems are being examined for evidence of having once
supported simple life, and projects such as
SETI are trying to detect
radio transmissions from possible alien civilizations. Other locations
Solar System that may host microbial life include the
subsurface of Mars , the upper atmosphere of Venus , and subsurface
oceans on some of the moons of the giant planets . Beyond the Solar
System, the region around another main-sequence star that could
support Earth-like life on an Earth-like planet is known as the
habitable zone . The inner and outer radii of this zone vary with the
luminosity of the star, as does the time interval during which the
zone survives. Stars more massive than the Sun have a larger habitable
zone, but remain on the main sequence for a shorter time interval.
Small red dwarfs have the opposite problem, with a smaller habitable
zone that is subject to higher levels of magnetic activity and the
effects of tidal locking from close orbits. Hence, stars in the
intermediate mass range such as the Sun may have a greater likelihood
for Earth-like life to develop. The location of the star within a
galaxy may also affect the likelihood of life forming. Stars in
regions with a greater abundance of heavier elements that can form
planets, in combination with a low rate of potentially habitat
-damaging supernova events, are predicted to have a higher probability
of hosting planets with complex life. The variables of the Drake
equation are used to discuss the conditions in planetary systems where
civilization is most likely to exist. Use of the equation to predict
the amount of extraterrestrial life, however, is difficult; because
many of the variables are unknown, the equation functions as more of a
mirror to what its user already thinks. As a result, the number of
civilizations in the galaxy can be estimated as low as 9.1 x 10^-11 or
as high as 156 million; for the calculations, see
Drake equation .
Artificial life and
Artificial life is the simulation of any aspect of life, as through
computers , robotics , or biochemistry . The study of artificial life
imitates traditional biology by recreating some aspects of biological
phenomena. Scientists study the logic of living systems by creating
artificial environments—seeking to understand the complex
information processing that defines such systems. While life is, by
definition, alive, artificial life is generally referred to as data
confined to a digital environment and existence.
Synthetic biology is a new area of biotechnology that combines
science and biological engineering . The common goal is the design and
construction of new biological functions and systems not found in
Synthetic biology includes the broad redefinition and
expansion of biotechnology , with the ultimate goals of being able to
design and build engineered biological systems that process
information, manipulate chemicals, fabricate materials and structures,
produce energy, provide food, and maintain and enhance human health
and the environment.
Animal corpses, like this
African buffalo ,
are recycled by the ecosystem , providing energy and nutrients for
Death is the permanent termination of all vital functions or life
processes in an organism or cell. It can occur as a result of an
accident, medical conditions , biological interaction , malnutrition ,
poisoning , senescence , or suicide. After death, the remains of an
organism re-enter 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 that 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
Death would seem to refer to either the moment life ends, or
when the state that follows life begins. However, determining when
death has occurred requires drawing precise conceptual boundaries
between life and death. This is problematic, however, because there is
little consensus over how to define life. The nature of death has for
millennia been a central concern of the world's religious traditions
and of philosophical inquiry. Many religions maintain faith in either
a kind of afterlife or reincarnation for the soul , or resurrection of
the body at a later date.
Extinction is the process by which a group of taxa or species dies
out, reducing biodiversity. The moment of extinction is generally
considered 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
Species become extinct when they are no longer able
to survive in changing habitat or against superior competition. In
Earth\'s history , over 99% of all the species that have ever lived
are extinct; however, mass extinctions may have accelerated
evolution by providing opportunities for new groups of organisms to
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. 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, up to 3.4 billion years old.
Biology , the study of life
Evolutionary history of life
Lists of organisms by population
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hopes for life on Mars". The Washington Post. Retrieved 21 August
* ^ Wade, Nicholas (21 August 2011). "Geological Team Lays Claim to
Oldest Known Fossils". The New York Times. Retrieved 21 August 2011.
* Kauffman, Stuart. The Adjacent Possible: A
Talk with Stuart
* Seeding the
Life Legacy Books, Washington D. C.,
2000, ISBN 0-476-00330-X
* Walker, Martin G. LIFE! Why We Exist ... And What We Must Do to
Survive Dog Ear Publishing, 2006, ISBN 1-59858-243-7
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Life (Systema Naturae 2000)
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