Life on Earth:
Non-cellular life[note 1] [note 2]
This article is one of a series on:
Life in the Universe
Habitability in the Solar System
Habitability of Venus
Life on Earth
Habitability of Mars
Habitability of Enceladus
Habitability of Europa
Habitability of Titan
Life outside the Solar System
Circumstellar habitable zone
Life is a characteristic that distinguishes physical entities that do
have 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 involved.
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
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 biochemical functions.
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. 3.45 billion year old Australian
rocks are reported to have contained microorganisms. In 2016,
scientists reported identifying a set of 355 genes thought to be
present in the last universal common ancestor (LUCA) of all living
organisms, already a complex organism and not the first living
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
living organisms. 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 speculate 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.
1.1.1 Alternative definitions
Living systems theories
1.3.1 Gaia hypothesis
Life as a property of ecosystems
1.3.4 Complex systems biology
1.3.5 Darwinian dynamic
1.3.6 Operator theory
2 History of study
2.3 Spontaneous generation
4 Environmental conditions
4.2 Range of tolerance
4.4 Chemical elements
5.1 Biota (taxonomy)
10 See also
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.
See also: 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 preserves, furthers or reinforces its
existence in the given environment. This characteristic exhibits all
or most of the following traits:
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 parent
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.
See also: 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.
Main article: Virus
Adenovirus as seen under an electron microscope
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.
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.
Main article: Gaia hypothesis
The idea that the
Earth is alive is found in philosophy and religion,
but the first scientific discussion of it was by the Scottish
scientist 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 modern
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
Main article: Complex systems biology
See also: Mathematical biology
Complex systems 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 reproduction.
History of study
Main article: Materialism
Plant growth in the Hoh Rainforest
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
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 various
mechanistic 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
arm from 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 Greek Thought
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.
Main article: Hylomorphism
The structure of the souls of plants, animals, and humans, according
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.
Main article: Spontaneous generation
Spontaneous generation was the belief that living organisms can form
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
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
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 biologists.
Main article: Vitalism
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
as Henri Bergson, Friedrich Nietzsche, and Wilhelm Dilthey,
anatomists like Marie François Xavier Bichat, and chemists like
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, 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.
view • discuss • edit
Earliest sexual reproduction
Axis scale: million years
Orange labels: ice ages.
Human timeline and
Main article: Abiogenesis
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
Late Heavy Bombardment theory, suggest that
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
More than 99% of all species of life forms, amounting to over five
billion species, that ever lived on
Earth are estimated to be
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
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, Acetyl-CoA, NADH, etc.) are
either nucleotides or substances clearly related to them. The
catalytic properties of
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
RNA molecules under the conditions that existed prior to life
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
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 on
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
Earth has 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.
Main article: Biosphere
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
near-weightlessness of space and to survive in the vacuum of
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. According to one researcher, "You can find
microbes everywhere — they're extremely adaptable to conditions, and
survive wherever they are."
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
Earth 4.54 billion years ago. According to biologist
Stephen Blair Hedges, "If life arose relatively quickly on
Earth ... then it could be common in the universe."
In a general sense, biospheres are any closed, self-regulating systems
containing ecosystems. This includes artificial biospheres such as
Biosphere 2 and BIOS-3, and potentially ones on other planets or
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, heat, 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
Further information: Extremophile
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.
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 in the Earth's oceans. Microbes also thrive inside
rocks up to 1,900 feet (580 m) below the sea floor under 8,500
feet (2,600 m) 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 beyond 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
Main article: DNA
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 acids; 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
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
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
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
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 acquired by Rosalind Franklin.
Main article: Biological classification
The hierarchy of biological classification's eight major taxonomic
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
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
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 kingdom Protista; later,
the prokaryotes were split off in the kingdom Monera, which would
eventually be divided into two separate groups, the
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.
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.
Main article: Cell (biology)
Cells are the basic unit of structure in every living thing, and all
cells arise from pre-existing cells by division.
Cell theory was
formulated by 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
them. 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
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 of identical 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
Main articles: Extraterrestrial life, Astrobiology, and Astroecology
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
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 Synthetic biology
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.
Main article: Death
Animal corpses, like this African buffalo, are recycled by the
ecosystem, providing energy and nutrients for living creatures
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 is difficult, as cessation of life functions
is often not simultaneous across organ systems. Such
determination therefore requires drawing conceptual lines 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.
Main article: Extinction
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 diversify.
Main article: 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. 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
^ The "evolution" of viruses and other similar forms is still
uncertain. Therefore, this classification may be paraphyletic because
cellular life might have evolved from non-cellular life, or
polyphyletic because the most recent common ancestor might not be
^ Infectious protein molecules prions are not considered living
organisms, but can be described as "organism-comparable organic
^ Certain specific organism-comparable organic structures may be
considered subviral agents, including virus-dependent entities:
satellites and defective interfering particles, both of which require
another virus for their replication.
^ a b Dodd, Matthew S.; Papineau, Dominic; Grenne, Tor; Slack, John
F.; Rittner, Martin; Pirajno, Franco; O'Neil, Jonathan; Little,
Crispin T. S. (1 March 2017). "Evidence for early life in Earth's
oldest hydrothermal vent precipitates". Nature. 543 (7643): 60–64.
Bibcode:2017Natur.543...60D. doi:10.1038/nature21377. Archived from
the original on 8 September 2017. Retrieved 2 March 2017.
^ a b Zimmer, Carl (1 March 2017). "Scientists Say Canadian Bacteria
Fossils May Be Earth's Oldest". New York Times. Archived from the
original on 2 March 2017. Retrieved 2 March 2017.
^ a b Ghosh, Pallab (1 March 2017). "Earliest evidence of life on
Earth 'found". BBC News. Archived from the original on 2 March 2017.
Retrieved 2 March 2017.
^ a b Dunham, Will (1 March 2017). "Canadian bacteria-like fossils
called oldest evidence of life". Reuters. Archived from the original
on 2 March 2017. Retrieved 1 March 2017.
^ Tyrell, Kelly April (18 December 2017). "Oldest fossils ever found
show life on
Earth began before 3.5 billion years ago". University of
Wisconsin-Madison. Retrieved 18 December 2017.
^ Schopf, J. William; Kitajima, Kouki; Spicuzza, Michael J.;
Kudryavtsev, Anatolly B.; Valley, John W. (2017). "SIMS analyses of
the oldest known assemblage of microfossils document their
taxon-correlated carbon isotope compositions". PNAS. 115: 53.
doi:10.1073/pnas.1718063115. Retrieved 19 December 2017.
^ a b Wade, Nicholas (25 July 2016). "Meet Luca, the Ancestor of All
Living Things". New York Times. Archived from the original on 28 July
2016. Retrieved 25 July 2016.
^ a b A. Tsokolov, Serhiy A. (May 2009). "Why Is the Definition of
Life So Elusive? Epistemological Considerations" (PDF). Astrobiology.
9 (4): 401–12. Bibcode:2009AsBio...9..401T.
doi:10.1089/ast.2007.0201. PMID 19519215. Retrieved 11 April
^ Mullen, Leslie (19 June 2002). "Defining Life". Astrobiology
Magazine. NASA. Archived from the original on 21 April 2012. Retrieved
12 November 2016.
^ Emmeche, Claus (1997). "Defining Life, Explaining Emergence". Niels
Bohr Institute. Archived from the original on 14 March 2012. Retrieved
25 May 2012.
^ "Can We Define Life". Colorado Arts & Sciences. Archived from
the original on 10 June 2010. Retrieved 22 June 2009.
^ Strother, Paul K. (22 January 2010). "What is life?". Origin and
Life on Earth. Boston College. Archived from the original
on 20 December 2016. Retrieved 12 November 2016.
^ Mautner, Michael N. (1997). "Directed panspermia. 3. Strategies and
motivation for seeding star-forming clouds" (PDF). Journal of the
British Interplanetary Society. 50: 93–102.
Bibcode:1997JBIS...50...93M. Archived (PDF) from the original on 2
^ Mautner, Michael N. (2000). Seeding the
Universe with Life: Securing
Our Cosmological Future (PDF). Washington D. C.: Legacy Books
(www.amazon.com). ISBN 978-0-476-00330-9. Archived (PDF) from the
original on 2 November 2012.
^ McKay, Chris (18 September 2014). "What is life? It's a Tricky,
Often Confusing Question".
^ Nealson, K. H.; Conrad, P. G. (December 1999). "Life: past, present
and future" (PDF). Philosophical Transactions of the Royal Society of
London B. 354 (1392): 1923–39. doi:10.1098/rstb.1999.0532.
PMC 1692713 . PMID 10670014.
^ a b McKay, Chris P. (14 September 2004). "What Is Life—and How Do
We Search for It in Other Worlds?". PLoS Biology. 2 (2(9)): 302.
doi:10.1371/journal.pbio.0020302. PMC 516796 .
^ Mautner, Michael N. (2009). "Life-centered ethics, and the human
future in space" (PDF). Bioethics. 23 (8): 433–40.
doi:10.1111/j.1467-8519.2008.00688.x. PMID 19077128. Archived
(PDF) from the original on 2 November 2012.
^ Koshland, Jr., Daniel E. (22 March 2002). "The Seven Pillars of
Life". Science. 295 (5563): 2215–16. doi:10.1126/science.1068489.
PMID 11910092. Archived from the original on 28 February 2009.
Retrieved 25 May 2009.
^ "life". The American Heritage Dictionary of the English Language
(4th ed.). Houghton Mifflin. 2006. ISBN 978-0-618-70173-5.
^ "Life". Merriam-Webster Dictionary. Archived from the original on 10
November 2016. Retrieved 12 November 2016.
^ "Habitability and Biology: What are the Properties of Life?".
Phoenix Mars Mission. The University of Arizona. Archived from the
original on 24 April 2014. Retrieved 6 June 2013.
^ Trifonov, Edward N. (2012). "Definition of Life: Navigation through
Uncertainties" (PDF). Journal of Biomolecular Structure &
Adenine Press. 29 (4): 647–50.
doi:10.1080/073911012010525017. ISSN 0739-1102. Archived (PDF)
from the original on 27 January 2012. Retrieved 12 January 2012.
^ Zimmer, Carl (11 January 2012). "Can scientists define 'life' ...
using just three words?". NBC News. Archived from the original on 14
April 2016. Retrieved 12 November 2016.
^ Luttermoser, Donald G. "ASTR-1020: Astronomy II Course Lecture Notes
Section XII" (PDF). East Tennessee State University. Archived from the
original (PDF) on 22 March 2012. Retrieved 28 August 2011.
^ Luttermoser, Donald G. (Spring 2008). "
Physics 2028: Great Ideas in
Science: The Exobiology Module" (PDF). East Tennessee State
University. Archived from the original (PDF) on 22 March 2012.
Retrieved 28 August 2011.
^ Lammer, H.; Bredehöft, J. H.; Coustenis, A.; Khodachenko, M. L.; et
al. (2009). "What makes a planet habitable?" (PDF). The Astronomy and
Astrophysics Review. 17 (2): 181–249.
Archived from the original (PDF) on 2 June 2016. Retrieved 2016-05-03.
Life as we know it has been described as a (thermodynamically) open
system (Prigogine et al. 1972), which makes use of gradients in its
surroundings to create imperfect copies of itself.
^ Joyce, Gerald F. (1995). The
RNA world: life before
DNA and protein.
Cambridge University Press. pp. 139–51.
doi:10.1017/CBO9780511564970.017. Retrieved 27 May 2012.
^ Overbye, Dennis (28 October 2015). "Cassini Seeks Insights to Life
in Plumes of Enceladus, Saturn's Icy Moon". New York Times. Archived
from the original on 28 October 2015. Retrieved 28 October 2015.
^ Domagal-Goldman, Shawn D.; Wright, Katherine E. (2016). "The
Astrobiology Primer v2.0" (PDF). Astrobiology. 16 (8): 561–53.
PMC 5008114 . PMID 27532777. Retrieved 2016-08-29.
^ Kaufmann, Stuart (2004). Barrow, John D.; Davies, P. C. W.; Harper,
Jr., C. L., eds. "Autonomous agents". Science and Ultimate Reality:
Quantum Theory, Cosmology, and Complexity. Cambridge University Press:
654–66. ISBN 978-0-521-83113-0. Archived from the original on 3
^ Longo, Giuseppe; Montévil, Maël; Kauffman, Stuart (1 January
2012). "No Entailing Laws, but Enablement in the
Evolution of the
Biosphere". Proceedings of the 14th Annual Conference Companion on
Genetic and Evolutionary Computation. GECCO '12. New York, NY, USA:
ACM: 1379–92. doi:10.1145/2330784.2330946.
ISBN 978-1-4503-1178-6. Archived from the original on 11 May
^ Koonin, E. V.; Starokadomskyy, P. (7 March 2016). "Are viruses
alive? The replicator paradigm sheds decisive light on an old but
misguided question". Stud Hist Philos Biol Biomed Sci. 59: 125–34.
doi:10.1016/j.shpsc.2016.02.016. PMC 5406846 .
^ Rybicki, EP (1990). "The classification of organisms at the edge of
life, or problems with virus systematics". S Aft J Sci. 86:
^ Holmes, E. C. (October 2007). "
Viral evolution in the genomic age".
PLoS Biol. 5 (10): e278. doi:10.1371/journal.pbio.0050278.
PMC 1994994 . PMID 17914905. Retrieved 13 September
^ Forterre, Patrick (3 March 2010). "Defining Life: The Virus
Life Evol Biosph. 40 (2): 151–60.
PMC 2837877 . PMID 20198436.
^ Koonin, E. V.; Senkevich, T. G.; Dolja, V. V. (2006). "The ancient
Virus World and evolution of cells".
Biology Direct. 1: 29.
doi:10.1186/1745-6150-1-29. PMC 1594570 . PMID 16984643.
Retrieved 14 September 2008.
^ Rybicki, Ed (November 1997). "Origins of Viruses". Archived from the
original on 9 May 2009. Retrieved 12 April 2009.
^ "Giant Viruses Shake Up Tree of Life".
Astrobiology Magazine. 15
September 2012. Archived from the original on 17 September 2012.
Retrieved 13 November 2016.
^ Popa, Radu (March 2004). Between Necessity and Probability:
Searching for the Definition and Origin of
Life (Advances in
Astrobiology and Biogeophysics). Springer.
^ Schrödinger, Erwin (1944). What is Life?. Cambridge University
Press. ISBN 978-0-521-42708-1.
^ Margulis, Lynn; Sagan, Dorion (1995). What is Life?. University of
California Press. ISBN 978-0-520-22021-8.
^ Lovelock, James (2000). Gaia – a New Look at
Life on Earth.
Oxford University Press. ISBN 978-0-19-286218-1.
^ Avery, John (2003). Information Theory and Evolution. World
Scientific. ISBN 978-981-238-399-0.
^ Woodruff, T. Sullivan; John Baross (8 October 2007). Planets and
Life: The Emerging Science of Astrobiology. Cambridge University
Press. Cleland and Chyba wrote a chapter in Planets and Life:
"In the absence of such a theory, we are in a position analogous to
that of a 16th-century investigator trying to define 'water' in the
absence of molecular theory." [...] "Without access to living things
having a different historical origin, it is difficult and perhaps
ultimately impossible to formulate an adequately general theory of the
nature of living systems".
^ Brown, Molly Young (2002). "Patterns, Flows, and Interrelationship".
Archived from the original on 8 January 2009. Retrieved
^ a b Lovelock, James (1979). Gaia: A New Look at
Life on Earth.
Oxford University Press. ISBN 978-0-19-286030-9.
^ Lovelock, J. E. (1965). "A physical basis for life detection
experiments". Nature. 207 (7): 568–70. Bibcode:1965Natur.207..568L.
doi:10.1038/207568a0. PMID 5883628.
^ Lovelock, James. "Geophysiology". Papers by James Lovelock. Archived
from the original on 6 May 2007.
^ Woodruff, T. Sullivan; John Baross (8 October 2007). Planets and
Life: The Emerging Science of Astrobiology. Cambridge University
Press. ISBN 978-0-521-82421-7. Cleland and Chyba wrote a
chapter in Planets and Life: "In the absence of such a theory, we are
in a position analogous to that of a 16th-century investigator trying
to define 'water' in the absence of molecular
theory."... "Without access to living things having a different
historical origin, it is difficult and perhaps ultimately impossible
to formulate an adequately general theory of the nature of living
^ Robert, Rosen (November 1991).
Life Itself: A Comprehensive Inquiry
into the Nature, Origin, and Fabrication of Life.
^ Fiscus, Daniel A. (April 2002). "The Ecosystemic
Bulletin of the Ecological Society of America. Archived from the
original on 6 August 2009. Retrieved 28 August 2009.
^ Morowitz, Harold J. (1992). Beginnings of cellular life: metabolism
recapitulates biogenesis. Yale University Press.
ISBN 978-0-300-05483-5. Archived from the original on 5 September
^ Ulanowicz, Robert W.; Ulanowicz, Robert E. (2009). A third window:
natural life beyond Newton and Darwin. Templeton Foundation Press.
ISBN 978-1-59947-154-9. Archived from the original on 3 September
^ Baianu, I. C. (2006). "Robert Rosen's Work and Complex Systems
Biology". Axiomathes. 16 (1–2): 25–34.
^ * Rosen, R. (1958a). "A Relational Theory of Biological Systems".
Bulletin of Mathematical Biophysics. 20 (3): 245–60.
^ * Rosen, R. (1958b). "The Representation of Biological
the Standpoint of the Theory of Categories". Bulletin of Mathematical
Biophysics. 20 (4): 317–41. doi:10.1007/bf02477890.
^ Montévil, Maël; Mossio, Matteo (7 May 2015). "Biological
organisation as closure of constraints". Journal of Theoretical
Biology. 372: 179–91. doi:10.1016/j.jtbi.2015.02.029.
PMID 25752259. Archived from the original on 17 November
^ a b Harris Bernstein; Henry C. Byerly; Frederick A. Hopf; Richard A.
Michod; G. Krishna Vemulapalli (June 1983). "The Darwinian Dynamic".
The Quarterly Review of Biology. The University of Chicago Press. 58
(2): 185. doi:10.1086/413216. JSTOR 2828805.
^ Michod, Richard E. (2000). Darwinian Dynamics: Evolutionary
Transitions in Fitness and Individuality. Princeton: Princeton
University Press. ISBN 978-0-691-05011-9.
^ Jagers, Gerard (2012). The Pursuit of Complexity: The Utility of
Biodiversity from an Evolutionary Perspective. KNNV Publishing.
^ "Towards a Hierarchical Definition of Life, the Organism, and
Death". Foundations of Science. 15.
^ "Explaining the Origin of
Life is not Enough for a Definition of
Life". Foundations of Science. 16.
^ "The role of logic and insight in the search for a definition of
life". J. Biomol. Struct. Dyn. 29.
^ Jagers, Gerald (2012). "Contributions of the Operator Hierarchy to
the Field of Biologically Driven Mathematics and Computation". In
Ehresmann, Andree C.; Simeonov, Plamen L.; Smith, Leslie S. Integral
Biomathics. Springer. ISBN 978-3-642-28110-5.
^ Korzeniewski, Bernard (7 April 2001). "Cybernetic formulation of the
definition of life". Journal of Theoretical Biology. 209 (3):
275–86. doi:10.1006/jtbi.2001.2262. PMID 11312589.
^ Parry, Richard (4 March 2005). "Empedocles". Stanford Encyclopedia
of Philosophy. Retrieved 25 May 2012.
^ Parry, Richard (25 August 2010). "Democritus". Stanford Encyclopedia
of Philosophy. Retrieved 25 May 2012.
^ Hankinson, R. J. (1997). Cause and Explanation in Ancient Greek
Thought. Oxford University Press. p. 125.
ISBN 978-0-19-924656-4. Archived from the original on 4 September
^ Thagard, Paul (2012). The Cognitive Science of Science: Explanation,
Discovery, and Conceptual Change. MIT Press. pp. 204–05.
ISBN 978-0-262-01728-2. Archived from the original on 3 September
^ Aristotle. On the Soul.
^ Marietta, Don (1998). Introduction to ancient philosophy. M. E.
Sharpe. p. 104. ISBN 978-0-7656-0216-9.
^ Stewart-Williams, Steve (2010). Darwin, God and the meaning of life:
how evolutionary theory undermines everything you thought you knew of
life. Cambridge University Press. pp. 193–94.
ISBN 978-0-521-76278-6. Archived from the original on 3 September
^ Stillingfleet, Edward (1697). Origines Sacrae. Cambridge University
Press – via Internet Archive.
^ André Brack (1998). "Introduction" (PDF). In André Brack. The
Molecular Origins of Life. Cambridge University Press. p. 1.
ISBN 978-0-521-56475-5. Archived (PDF) from the original on 26
March 2009. Retrieved 7 January 2009.
^ Levine, Russell; Evers, Chris. "The Slow
Death of Spontaneous
Generation (1668–1859)". North Carolina State University. National
Health Museum. Archived from the original on 9 October 2015.
^ Tyndall, John (1905). Fragments of Science. 2. New York: P. F.
Collier. Chapters IV, XII, and XIII – via Internet Archive.
^ Bernal, J. D. (1967) [Reprinted work by A. I. Oparin originally
published 1924; Moscow: The Moscow Worker]. The Origin of Life. The
Weidenfeld and Nicolson Natural History. Translation of Oparin by Ann
Synge. London: Weidenfeld & Nicolson. LCCN 67098482.
^ Zubay, Geoffrey (2000). Origins of Life: On
Earth and in the Cosmos
(2nd ed.). Academic Press. ISBN 978-0-12-781910-5.
^ Smith, John Maynard; Szathmary, Eors (1997). The Major Transitions
in Evolution. Oxford Oxfordshire: Oxford University Press.
^ Schwartz, Sanford (2009). C. S. Lewis on the Final Frontier: Science
and the Supernatural in the
Space Trilogy. Oxford University Press.
p. 56. ISBN 978-0-19-988839-9. Archived from the original on
4 September 2016.
^ a b Wilkinson, Ian (1998). "History of Clinical Chemistry –
Wöhler & the Birth of Clinical Chemistry" (PDF). The Journal of
the International Federation of Clinical Chemistry and Laboratory
Medicine. 13 (4). Archived (PDF) from the original on 5 January 2016.
Retrieved 27 December 2015.
Friedrich Wöhler (1828). "Ueber künstliche Bildung des
Harnstoffs". Annalen der Physik und Chemie. 88 (2): 253–56.
Bibcode:1828AnP....88..253W. doi:10.1002/andp.18280880206. Archived
from the original on 10 January 2012.
^ Rabinbach, Anson (1992). The
Human Motor: Energy, Fatigue, and the
Origins of Modernity. University of California Press.
pp. 124–25. ISBN 978-0-520-07827-7. Archived from the
original on 4 September 2016.
^ "NCAHF Position Paper on Homeopathy". National Council Against
Health Fraud. February 1994. Retrieved 12 June 2012.
^ "Age of the Earth". U.S. Geological Survey. 1997. Archived from the
original on 23 December 2005. Retrieved 10 January 2006.
^ Dalrymple, G. Brent (2001). "The age of the
Earth in the twentieth
century: a problem (mostly) solved".
Special Publications, Geological
Society of London. 190 (1): 205–21. Bibcode:2001GSLSP.190..205D.
^ Manhesa, Gérard; Allègre, Claude J.; Dupréa, Bernard &
Hamelin, Bruno (1980). "Lead isotope study of basic-ultrabasic layered
complexes: Speculations about the age of the earth and primitive
Earth and Planetary Science Letters. 47 (3):
^ a b Tenenbaum, David (14 October 2002). "When Did
Life on Earth
Begin? Ask a Rock".
Astrobiology Magazine. Archived from the original
on 20 May 2013. Retrieved 13 April 2014.
^ a b c d Borenstein, Seth (19 October 2015). "Hints of life on what
was thought to be desolate early Earth". Excite. Yonkers, NY:
Mindspark Interactive Network. Associated Press. Archived from the
original on 23 October 2015. Retrieved 20 October 2015.
^ a b c Bell, Elizabeth A.; Boehnike, Patrick; Harrison, T. Mark; et
al. (19 October 2015). "Potentially biogenic carbon preserved in a 4.1
billion-year-old zircon" (PDF). Proc. Natl. Acad. Sci. U.S.A.
Washington, D.C.: National Academy of Sciences. 112 (47): 14518–21.
ISSN 1091-6490. PMC 4664351 . PMID 26483481. Archived
(PDF) from the original on 6 November 2015. Retrieved 20 October
2015. Early edition, published online before print.
^ a b Courtland, Rachel (2 July 2008). "Did newborn
life?". New Scientist. Archived from the original on 14 November 2016.
Retrieved 14 November 2016.
^ a b Steenhuysen, Julie (20 May 2009). "Study turns back clock on
origins of life on Earth". Reuters. Archived from the original on 14
November 2016. Retrieved 14 November 2016.
^ Schopf, J. William; Kudryavtsev, Anatoliy B; Czaja, Andrew D;
Tripathi, Abhishek B (2007). "Evidence of
Archean life: Stromatolites
and microfossils". Precambrian Research. 158 (3–4): 141.
Fossil evidence of Archaean life". Philos. Trans. R. Soc. Lond. B
Biol. Sci. 29.
^ Hamilton Raven, Peter; Brooks Johnson, George (2002). Biology.
McGraw-Hill Education. p. 68. ISBN 978-0-07-112261-0.
Archived from the original on 1 January 2014. Retrieved 7 July
^ Milsom, Clare; Rigby, Sue (2009).
Fossils at a Glance (2nd ed.).
John Wiley & Sons. p. 134. ISBN 1-4051-9336-0. Archived
from the original on 4 September 2016.
^ a b Ohtomo, Yoko; Kakegawa, Takeshi; Ishida, Akizumi; Nagase,
Toshiro; Rosing, Minik T. (8 December 2013). "Evidence for biogenic
graphite in early Archaean Isua metasedimentary rocks". Nature
Geoscience. 7: 25–28. Bibcode:2014NatGe...7...25O.
^ a b Borenstein, Seth (13 November 2013). "Oldest fossil found: Meet
your microbial mom". Associated Press. Archived from the original on
29 June 2015.
^ a b Noffke, Nora; Christian, Daniel; Wacey, David; Hazen, Robert M.
(8 November 2013). "Microbially Induced Sedimentary Structures
Recording an Ancient
Ecosystem in the ca. 3.48 Billion-Year-Old
Dresser Formation, Pilbara, Western Australia". Astrobiology. 13 (12):
1103–24. Bibcode:2013AsBio..13.1103N. doi:10.1089/ast.2013.1030.
PMC 3870916 . PMID 24205812.
^ Loeb, Abraham (October 2014). "The Habitable Epoch of the Early
Universe". International Journal of Astrobiology. 13 (4): 337–39.
arXiv:1312.0613 . Bibcode:2014IJAsB..13..337L.
CiteSeerX 10.1.1.680.4009 . doi:10.1017/S1473550414000196.
Archived from the original on 6 July 2015. Retrieved 15 December
^ Loeb, Abraham (2 December 2013). "The Habitable Epoch of the Early
Universe". International Journal of Astrobiology. 13 (4): 337–39.
arXiv:1312.0613v3 . Bibcode:2014IJAsB..13..337L.
^ Dreifus, Claudia (2 December 2014). "Much-Discussed Views That Go
Way Back – Avi Loeb Ponders the Early Universe,
Life". New York Times. Archived from the original on 3 December 2014.
Retrieved 3 December 2014.
^ a b Kunin, W.E.; Gaston, Kevin, eds. (31 December 1996). The Biology
of Rarity: Causes and consequences of rare—common differences.
ISBN 978-0-412-63380-5. Archived from the original on 5 September
2015. Retrieved 26 May 2015.
^ a b Stearns, Beverly Peterson; Stearns, S. C.; Stearns, Stephen C.
(2000). Watching, from the Edge of Extinction. Yale University Press.
p. preface x. ISBN 978-0-300-08469-6. Archived from the
original on 17 July 2017. Retrieved 30 May 2017.
^ a b Novacek, Michael J. (8 November 2014). "Prehistory's Brilliant
Future". New York Times. Archived from the original on 29 December
2014. Retrieved 25 December 2014.
^ a b c G. Miller; Scott Spoolman (2012). Environmental Science -
Biodiversity Is a Crucial Part of the Earth's Natural Capital. Cengage
Learning. p. 62. ISBN 1-133-70787-4. Archived from the
original on 18 March 2015. Retrieved 27 December 2014. We do not know
how many species there are on the earth. Estimates range from 8
million to 100 million. The best guess is that there are 10–14
million species. So far, biologists have identified almost 2 million
^ a b Mora, C.; Tittensor, D.P.; Adl, S.; Simpson, A.G.; Worm, B. (23
August 2011). "How many species are there on
Earth and in the ocean?".
PLOS Biology. 9 (8): e1001127. doi:10.1371/journal.pbio.1001127.
PMC 3160336 . PMID 21886479. In spite of 250 years of
taxonomic classification and over 1.2 million species already
catalogued in a central database, our results suggest that some 86% of
existing species on
Earth and 91% of species in the ocean still await
^ a b Staff (2 May 2016). "Researchers find that
Earth may be home to
1 trillion species". National Science Foundation. Archived from the
original on 4 May 2016. Retrieved 6 May 2016.
^ Pappas, Stephanie (5 May 2016). "There Might Be 1 Trillion Species
on Earth". LiveScience. Archived from the original on 7 June 2017.
Retrieved 7 June 2017.
^ a b Nuwer, Rachel (18 July 2015). "Counting All the
DNA on Earth".
The New York Times. New York: The
New York Times
New York Times Company.
ISSN 0362-4331. Archived from the original on 18 July 2015.
Retrieved 18 July 2015.
^ a b "The Biosphere: Diversity of Life". Aspen Global Change
Institute. Basalt, CO. Retrieved 2015-07-19.
^ Coveney, Peter V.; Fowler, Philip W. (2005). "Modelling biological
complexity: a physical scientist's perspective". Journal of the Royal
Society Interface. 2 (4): 267–80. doi:10.1098/rsif.2005.0045.
PMC 1578273 . PMID 16849185.
^ "Habitability and Biology: What are the Properties of Life?".
Phoenix Mars Mission. The University of Arizona. Archived from the
original on 17 April 2014. Retrieved 6 June 2013.
^ Senapathy, Periannan (1994). Independent birth of organisms.
Madison, Wisconsin: Genome Press. ISBN 0-9641304-0-8. Archived
from the original on 5 September 2016.
^ Eigen, Manfred; Winkler, Ruthild (1992). Steps towards life: a
perspective on evolution (German edition, 1987). Oxford University
Press. p. 31. ISBN 0-19-854751-X.
^ a b Barazesh, Solmaz (13 May 2009). "How
RNA Got Started: Scientists
Look for the Origins of Life". U. S. News & World Report. Archived
from the original on 23 August 2016. Retrieved 14 November 2016.
^ Watson, James D. (1993). Gesteland, R. F.; Atkins, J. F., eds.
Prologue: early speculations and facts about
RNA templates. The RNA
World. Cold Spring Harbor, New York: Cold Spring Harbor Laboratory
Press. pp. xv–xxiii.
^ Gilbert, Walter (20 February 1986). "Origin of life: The
Nature. 319 (618): 618. Bibcode:1986Natur.319..618G.
^ Cech, Thomas R. (1986). "A model for the RNA-catalyzed replication
of RNA". Proceedings of the National Academy of Sciences USA. 83 (12):
4360–63. Bibcode:1986PNAS...83.4360C. doi:10.1073/pnas.83.12.4360.
Archived from the original on 4 September 2015. Retrieved 25 May
^ Cech, T.R. (2011). "The
RNA Worlds in Context". Cold Spring Harb
Perspect Biol. 4 (7): a006742. doi:10.1101/cshperspect.a006742.
PMC 3385955 . PMID 21441585.
^ Powner, Matthew W.; Gerland, Béatrice; Sutherland, John D. (14 May
2009). "Synthesis of activated pyrimidine ribonucleotides in
prebiotically plausible conditions". Nature. 459 (7244): 239–42.
^ Szostak, Jack W. (14 May 2009). "Origins of life:
on early Earth". Nature. 459 (7244): 171–72.
^ a b Pasek, Matthew A.; et at.; Buick, R.; Gull, M.; Atlas, Z. (18
June 2013). "Evidence for reactive reduced phosphorus species in the
Archean ocean". PNAS. 110 (25): 10089–94.
PMC 3690879 . PMID 23733935. Archived from the original on
4 September 2015. Retrieved 16 July 2013.
^ Lincoln, Tracey A.; Joyce, Gerald F. (27 February 2009).
"Self-Sustained Replication of an
RNA Enzyme". Science. 323 (5918):
1229–32. Bibcode:2009Sci...323.1229L. doi:10.1126/science.1167856.
PMC 2652413 . PMID 19131595.
^ Joyce, Gerald F. (2009). "
Evolution in an
RNA world". Cold Spring
Harbor Symposium on Quantitative Biology. 74: 17–23.
doi:10.1101/sqb.2009.74.004. PMC 2891321 .
^ Callahan; Smith, K.E.; Cleaves, H.J.; Ruzica, J.; Stern, J.C.;
Glavin, D.P.; House, C.H.; Dworkin, J.P. (11 August 2011).
"Carbonaceous meteorites contain a wide range of extraterrestrial
nucleobases". PNAS. 108 (34): 13995–98. Bibcode:2011PNAS..10813995C.
doi:10.1073/pnas.1106493108. PMC 3161613 . PMID 21836052.
Archived from the original on 18 September 2011. Retrieved 15 August
^ Steigerwald, John (8 August 2011). "
Blocks Can Be Made in Space". NASA. Archived from the original on 23
June 2015. Retrieved 10 August 2011.
DNA Building Blocks Can Be Made in Space,
NASA Evidence Suggests".
ScienceDaily. 9 August 2011. Archived from the original on 5 September
2011. Retrieved 9 August 2011.
^ Gallori, Enzo (November 2010). "
Astrochemistry and the origin of
genetic material". Rendiconti Lincei. 22 (2): 113–18.
doi:10.1007/s12210-011-0118-4. Retrieved 11 August 2011.
^ Marlaire, Ruth (3 March 2015). "
NASA Ames Reproduces the Building
Life in Laboratory". NASA. Archived from the original on 5
March 2015. Retrieved 5 March 2015.
^ Rampelotto, P.H. (2010). "Panspermia: A Promising Field Of Research"
(PDF). Archived (PDF) from the original on 27 March 2016. Retrieved 3
^ a b c d e Rothschild, Lynn (September 2003). "Understand the
evolutionary mechanisms and environmental limits of life". NASA.
Archived from the original on 11 March 2012. Retrieved 13 July
^ King, G.A.M. (April 1977). "
Symbiosis and the origin of life".
Evolution of Biospheres. 8 (1): 39–53.
Bibcode:1977OrLi....8...39K. doi:10.1007/BF00930938. Retrieved 22
^ Margulis, Lynn (2001). The Symbiotic Planet: A New Look at
Evolution. London, England: Orion Books Ltd.
^ Douglas J. Futuyma; Janis Antonovics (1992). Oxford surveys in
Symbiosis in evolution. 8. London, England:
Oxford University Press. pp. 347–74.
^ The Columbia Encyclopedia, Sixth Edition. Columbia University Press.
2004. Archived from the original on 27 October 2011. Retrieved 12
^ University of Georgia (25 August 1998). "First-Ever Scientific
Estimate Of Total
Earth Shows Far Greater Numbers Than
Ever Known Before". Science Daily. Archived from the original on 10
November 2014. Retrieved 10 November 2014.
^ Hadhazy, Adam (12 January 2015). "
Life Might Thrive a Dozen Miles
Beneath Earth's Surface".
Astrobiology Magazine. Archived from the
original on 12 March 2017. Retrieved 11 March 2017.
^ Fox-Skelly, Jasmin (24 November 2015). "The Strange Beasts That Live
In Solid Rock Deep Underground". BBC online. Archived from the
original on 25 November 2016. Retrieved 11 March 2017.
^ Dvorsky, George (13 September 2017). "Alarming Study Indicates Why
Bacteria Are More Resistant to Drugs in Space". Gizmodo.
Archived from the original on 14 September 2017. Retrieved 14
^ Caspermeyer, Joe (23 September 2007). "
Space flight shown to alter
ability of bacteria to cause disease". Arizona State University.
Archived from the original on 14 September 2017. Retrieved 14
^ Dose, K.; Bieger-Dose, A.; Dillmann, R.; Gill, M.; Kerz, O.; Klein,
A.; Meinert, H.; Nawroth, T.; Risi, S.; Stridde, C. (1995).
"ERA-experiment "space biochemistry"". Advances in
Space Research. 16
(8): 119–29. Bibcode:1995AdSpR..16..119D.
doi:10.1016/0273-1177(95)00280-R. PMID 11542696.
^ Vaisberg, Horneck G.; Eschweiler, U.; Reitz, G.; Wehner, J.;
Willimek, R.; Strauch, K. (1995). "Biological responses to space:
results of the experiment "Exobiological Unit" of ERA on EURECA I".
Space Res. 16 (8): 105–18. Bibcode:1995AdSpR..16..105V.
doi:10.1016/0273-1177(95)00279-N. PMID 11542695.
^ a b c d e Choi, Charles Q. (17 March 2013). "Microbes Thrive in
Deepest Spot on Earth". LiveScience. Archived from the original on 2
April 2013. Retrieved 17 March 2013.
^ a b Glud, Ronnie; Wenzhöfer, Frank; Middelboe, Mathias; Oguri,
Kazumasa; Turnewitsch, Robert; Canfield, Donald E.; Kitazato, Hiroshi
(17 March 2013). "High rates of microbial carbon turnover in sediments
in the deepest oceanic trench on Earth".
Nature Geoscience. 6 (4):
284–88. Bibcode:2013NatGe...6..284G. doi:10.1038/ngeo1773. Retrieved
17 March 2013.
^ a b Oskin, Becky (14 March 2013). "Intraterrestrials:
Ocean Floor". LiveScience. Archived from the original on 2 April
2013. Retrieved 17 March 2013.
^ Morelle, Rebecca (15 December 2014). "Microbes discovered by deepest
marine drill analysed". BBC News. Archived from the original on 16
December 2014. Retrieved 15 December 2014.
^ Fox, Douglas (20 August 2014). "Lakes under the ice: Antarctica's
secret garden". Nature. 512 (7514): 244–46.
Bibcode:2014Natur.512..244F. doi:10.1038/512244a. PMID 25143097.
Archived from the original on 21 August 2014. Retrieved 21 August
^ Mack, Eric (20 August 2014). "
Life Confirmed Under Antarctic Ice; Is
Space Next?". Forbes. Archived from the original on 22 August 2014.
Retrieved 21 August 2014.
^ Campbell, Neil A.; Brad Williamson; Robin J. Heyden (2006). Biology:
Exploring Life. Boston, Massachusetts: Pearson Prentice Hall.
ISBN 0-13-250882-6. Archived from the original on 2 November
^ Zimmer, Carl (3 October 2013). "Earth's Oxygen: A Mystery Easy to
Take for Granted". New York Times. Archived from the original on 3
October 2013. Retrieved 3 October 2013.
^ "Meaning of biosphere". WebDictionary.co.uk. WebDictionary.co.uk.
Archived from the original on 2 October 2011. Retrieved 12 November
^ "Essential requirements for life". CMEX-NASA. Archived from the
original on 17 August 2009. Retrieved 14 July 2009.
^ a b Chiras, Daniel C. (2001). Environmental Science –
Creating a Sustainable Future (6th ed.).
^ a b Chang, Kenneth (12 September 2016). "Visions of
Life on Mars
Life on Mars in
Earth's Depths". New York Times. Archived from the original on 12
September 2016. Retrieved 12 September 2016.
^ Rampelotto, Pabulo Henrique (2010). "Resistance of microorganisms to
extreme environmental conditions and its contribution to
astrobiology". Sustainability. 2 (6): 1602–23.
^ Baldwin, Emily (26 April 2012). "
Lichen survives harsh Mars
environment". Skymania News. Archived from the original on 28 May
2012. Retrieved 27 April 2012.
^ de Vera, J.-P.; Kohler, Ulrich (26 April 2012). "The adaptation
potential of extremophiles to Martian surface conditions and its
implication for the habitability of Mars" (PDF). European Geosciences
Union. Archived from the original (PDF) on 8 June 2012. Retrieved 27
^ Hotz, Robert Lee (3 December 2010). "New link in chain of life".
Wall Street Journal. Dow Jones & Company, Inc. Archived from the
original on 17 August 2017. Until now, however, they were all thought
to share the same biochemistry, based on the Big Six, to build
proteins, fats and DNA.
^ Neuhaus, Scott (2005). Handbook for the Deep Ecologist: What
Everyone Should Know About Self, the Environment, And the Planet.
iUniverse. pp. 23–50. ISBN 978-0-521-83113-0. Archived
from the original on 4 September 2016.
^ Committee on the Limits of Organic
Life in Planetary Systems;
Committee on the Origins and
Evolution of Life; National Research
Council (2007). The Limits of Organic
Life in Planetary Systems.
National Academy of Sciences. ISBN 0-309-66906-5. Archived from
the original on 10 May 2012. Retrieved 3 June 2012.
^ Benner, Steven A.; Ricardo, Alonso; Carrigan, Matthew A. (December
2004). "Is there a common chemical model for life in the universe?"
(PDF). Current Opinion in Chemical Biology. 8 (6): 672–89.
doi:10.1016/j.cbpa.2004.10.003. PMID 15556414. Archived from the
original (PDF) on 8 June 2012. Retrieved 3 June 2012.
^ Purcell, Adam (5 February 2016). "DNA". Basic Biology. Archived from
the original on 5 January 2017. Retrieved 15 November 2016.
^ Russell, Peter (2001). iGenetics. New York: Benjamin Cummings.
^ Dahm R (2008). "Discovering DNA:
Friedrich Miescher and the early
years of nucleic acid research". Hum. Genet. 122 (6): 565–81.
doi:10.1007/s00439-007-0433-0. PMID 17901982.
^ Portin P (2014). "The birth and development of the
DNA theory of
inheritance: sixty years since the discovery of the structure of DNA".
Journal of Genetics. 93 (1): 293–302. doi:10.1007/s12041-014-0337-4.
^ "Aristotle". University of California Museum of Paleontology.
Archived from the original on 20 November 2016. Retrieved 15 November
^ Knapp S, Lamas G, Lughadha EN, Novarino G (April 2004). "Stability
or stasis in the names of organisms: the evolving codes of
nomenclature". Philosophical Transactions of the Royal Society of
London B. 359 (1444): 611–22. doi:10.1098/rstb.2003.1445.
PMC 1693349 . PMID 15253348.
^ Copeland, Herbert F. (1938). "The Kingdoms of Organisms". Quarterly
Review of Biology. 13 (4): 383. doi:10.1086/394568.
^ Whittaker, R. H. (January 1969). "New concepts of kingdoms or
organisms. Evolutionary relations are better represented by new
classifications than by the traditional two kingdoms". Science. 163
(3863): 150–60. Bibcode:1969Sci...163..150W.
CiteSeerX 10.1.1.403.5430 . doi:10.1126/science.163.3863.150.
^ a b Woese, C.; Kandler, O.; Wheelis, M. (1990). "Towards a natural
system of organisms: proposal for the domains Archaea, Bacteria, and
Eucarya". Proceedings of the National Academy of Sciences of the
United States of America. 87 (12): 4576–9.
PMC 54159 . PMID 2112744.
^ Adl SM, Simpson AG, Farmer MA, et al. (2005). "The new higher level
classification of eukaryotes with emphasis on the taxonomy of
protists". J. Eukaryot. Microbiol. 52 (5): 399–451.
doi:10.1111/j.1550-7408.2005.00053.x. PMID 16248873.
^ Van Regenmortel MH (January 2007). "
Virus species and virus
identification: past and current controversies". Infection, Genetics
and Evolution. 7 (1): 133–44. doi:10.1016/j.meegid.2006.04.002.
^ Pennisi E (March 2001). "Taxonomy. Linnaeus's last stand?". Science.
New York, N.Y. 291 (5512): 2304–07.
doi:10.1126/science.291.5512.2304. PMID 11269295.
^ Linnaeus, C. (1735). Systemae Naturae, sive regna tria naturae,
systematics proposita per classes, ordines, genera &
^ Haeckel, E. (1866). Generelle Morphologie der Organismen. Reimer,
^ Chatton, É. (1925). "Pansporella perplexa. Réflexions sur la
biologie et la phylogénie des protozoaires". Annales des Sciences
Naturelles - Zoologie et Biologie Animale. 10-VII: 1–84.
^ Copeland, H. (1938). "The kingdoms of organisms". Quarterly Review
of Biology. 13: 383–420. doi:10.1086/394568.
^ Whittaker, R. H. (January 1969). "New concepts of kingdoms of
organisms". Science. 163 (3863): 150–60.
^ Cavalier-Smith, T. (1998). "A revised six-kingdom system of life".
Biological Reviews. 73 (03): 203–66.
doi:10.1111/j.1469-185X.1998.tb00030.x. PMID 9809012.
^ Systema Naturae 2000 "Biota" Archived 14 June 2010 at the Wayback
^ Taxonomicon "Biota" Archived 15 January 2014 at the Wayback Machine.
^ Sapp, Jan (2003). Genesis: The
Evolution of Biology. Oxford
University Press. pp. 75–78. ISBN 0-19-515619-6. Archived
from the original on 3 September 2016.
^ a b Wolfram, Stephen (2002). A New Kind of Science. Wolfram Media.
pp. 170–83, 297–362. ISBN 1-57955-008-8.
^ Lintilhac, P. M. (Jan 1999). "Thinking of biology: toward a theory
of cellularity—speculations on the nature of the living cell" (PDF).
BioScience. 49 (1): 59–68. doi:10.2307/1313494. JSTOR 1313494.
PMID 11543344. Archived from the original (PDF) on 6 April 2013.
Retrieved 2 June 2012.
^ Whitman, W.; Coleman, D.; Wiebe, W. (1998). "Prokaryotes: The unseen
majority". Proceedings of the National Academy of Sciences of the
United States of America. 95 (12): 6578–83.
PMC 33863 . PMID 9618454.
^ Pace, Norman R. (18 May 2006). "Concept
Time for a change" (PDF).
Nature. 441 (7091): 289. Bibcode:2006Natur.441..289P.
doi:10.1038/441289a. PMID 16710401. Archived from the original
(PDF) on 8 June 2012. Retrieved 2 June 2012.
^ "Scientific background". The Nobel Prize in Chemistry 2009. Royal
Swedish Academy of Sciences. Archived from the original on 2 April
2012. Retrieved 10 June 2012.
^ Nakano A, Luini A (2010). "Passage through the Golgi". Curr Opin
Cell Biol. 22 (4): 471–78. doi:10.1016/j.ceb.2010.05.003.
^ Panno, Joseph (2004). The Cell. Facts on
File science library.
Infobase Publishing. pp. 60–70. ISBN 0-8160-6736-8.
Archived from the original on 4 September 2016.
^ Alberts, Bruce; et al. (1994). "From Single Cells to Multicellular
Biology of the Cell (3rd ed.). New York: Garland
Science. ISBN 0-8153-1620-8. Archived from the original on 6
March 2016. Retrieved 12 June 2012.
^ Zimmer, Carl (7 January 2016). "Genetic Flip Helped Organisms Go
From One Cell to Many". New York Times. Archived from the original on
7 January 2016. Retrieved 7 January 2016.
^ Alberts, Bruce; et al. (2002). "General Principles of Cell
Biology of the Cell. New York: Garland
Science. ISBN 0-8153-3218-1. Archived from the original on 4
September 2015. Retrieved 12 June 2012.
^ Race, Margaret S.; Randolph, Richard O. (2002). "The need for
operating guidelines and a decision making framework applicable to the
discovery of non-intelligent extraterrestrial life". Advances in Space
Research. 30 (6): 1583–91. Bibcode:2002AdSpR..30.1583R.
doi:10.1016/S0273-1177(02)00478-7. ISSN 0273-1177. There is
growing scientific confidence that the discovery of extraterrestrial
life in some form is nearly inevitable
^ Cantor, Matt (15 February 2009). "Alien
Astronomer". Newser. Archived from the original on 3 May 2013.
Retrieved 3 May 2013. Scientists now believe there could be as many
habitable planets in the cosmos as there are stars, and that makes
life's existence elsewhere "inevitable" over billions of years, says
^ Schulze-Makuch, Dirk; Dohm, James M.; Fairén, Alberto G.; Baker,
Victor R.; Fink, Wolfgang; Strom, Robert G. (December 2005). Venus,
Mars, and the Ices on Mercury and the Moon: Astrobiological
Implications and Proposed Mission Designs. Astrobiology. 5.
pp. 778–95. Bibcode:2005AsBio...5..778S.
^ Woo, Marcus (27 January 2015). "Why We're Looking for Alien
Moons, Not Just Planets". Wired. Archived from the original on 27
January 2015. Retrieved 27 January 2015.
^ Strain, Daniel (14 December 2009). "Icy moons of Saturn and Jupiter
may have conditions needed for life". The University of Santa Cruz.
Archived from the original on 31 December 2012. Retrieved 4 July
^ Selis, Frank (2006). "Habitability: the point of view of an
astronomer". In Gargaud, Muriel; Martin, Hervé; Claeys, Philippe.
Lectures in Astrobiology. 2. Springer. pp. 210–14.
ISBN 3-540-33692-3. Archived from the original on 3 September
^ Lineweaver, Charles H.; Fenner, Yeshe; Gibson, Brad K. (January
2004). "The Galactic Habitable Zone and the age distribution of
complex life in the Milky Way". Science. 303 (5654): 59–62.
arXiv:astro-ph/0401024 . Bibcode:2004Sci...303...59L.
doi:10.1126/science.1092322. PMID 14704421.
^ Vakoch, Douglas A.; Harrison, Albert A. (2011). Civilizations beyond
Earth: extraterrestrial life and society. Berghahn Series. Berghahn
Books. pp. 37–41. ISBN 0-85745-211-8.
^ "Artificial life". Dictionary.com. Archived from the original on 16
November 2016. Retrieved 15 November 2016.
^ Chopra, Paras; Akhil Kamma. "Engineering life through Synthetic
Biology". In Silico Biology. 6. Archived from the original on 5 August
2008. Retrieved 9 June 2008.
^ Definition of death. Archived from the original on 1 November
^ a b "Definition of death". Encyclopedia of
Death and Dying. Advameg,
Inc. Archived from the original on 3 February 2007. Retrieved 25 May
^ Henig, Robin Marantz (April 2016). "Crossing Over: How Science Is
Life and Death". National Geographic. Archived from the
original on 1 November 2017. Retrieved 23 October 2017.
^ Extinction – definition. Archived from the original on 1
^ "What is an extinction?". Late Triassic. Bristol University.
Archived from the original on 1 September 2012. Retrieved 27 June
^ Van Valkenburgh, B. (1999). "Major patterns in the history of
carnivorous mammals". Annual Review of
Earth and Planetary Sciences.
27: 463–93. Bibcode:1999AREPS..27..463V.
^ "Frequently Asked Questions". San Diego Natural History Museum.
Archived from the original on 10 May 2012. Retrieved 25 May
^ Vastag, Brian (21 August 2011). "Oldest 'microfossils' raise hopes
for life on Mars". The Washington Post. Archived from the original on
19 October 2011. Retrieved 21 August 2011.
^ Wade, Nicholas (21 August 2011). "Geological Team Lays Claim to
Oldest Known Fossils". The New York Times. Archived from the original
on 1 May 2013. Retrieved 21 August 2011.
Kauffman, Stuart. The Adjacent Possible: A
Talk with Stuart Kauffman
Life Legacy Books, Washington D. C., 2000,
Walker, Martin G. [permanent dead link] LIFE! Why We Exist ...
And What We Must Do to Survive Dog Ear Publishing, 2006,
Find more aboutLifeat's sister projects
Media from Wikimedia Commons
Quotations from Wikiquote
Texts from Wikisource
Look up life or living in Wiktionary, the free dictionary.
Life (Systema Naturae 2000)
Wikispecies – a free directory of life
Resources for life in the
Solar System and in galaxy, and the
potential scope of life in the cosmological future
"The Adjacent Possible: A
Talk with Stuart Kauffman"
Stanford Encyclopedia of Philosophy entry
The Kingdoms of Life
Life – related articles
Elements of nature
Hierarchy of life
Biosphere > Ecosystem > Biocoenosis >
Population > Organism > Organ system >
Organ > Tissue > Cell > Organelle >
Biomolecular complex > Macromolecule > Biomolecule
Themes and subjects
Chronology of the universe
1: Creation -
Big Bang and cosmogony
2: Stars - creation of stars
3: Elements - creation of chemical elements inside dying stars
4: Planets - formation of planets
Life - abiogenesis and evolution of life
6: Humans - development of Homo sapiens
7: Agriculture - Agricultural Revolution
Modernity - modern era
Big History Project
Crash Course Big History
Cynthia Stokes Brown
Evolutionary history of life
Index of evolutionary biology articles
Outline of evolution
Timeline of evolution
Earliest known life forms
Evidence of common descent
Last universal common ancestor
Origin of life
Evolutionary developmental biology
dolphins and whales
Programmed cell death
Life cycles/nuclear phases
Tempo and modes
Renaissance and Enlightenment
Transmutation of species
On the Origin of Species
History of paleontology
The eclipse of Darwinism
History of molecular evolution
Extended evolutionary synthesis
Teleology in biology
Pollution / quality
Ambient standards (USA)
Clean Air Act (USA)
Fossil fuels (peak oil)
Non-timber forest products
Types / location
storage and recovery
Earth Overshoot Day
Renewable / Non-renewable
Agriculture and agronomy
Molecules detected in outer space
Magnesium monohydride cation
Hydrogen cyanide (HCN)
Hydrogen isocyanide (HNC)
Protonated molecular hydrogen
Protonated carbon dioxide
Protonated hydrogen cyanide
Buckminsterfullerene (C60 fullerene, buckyball)
Ethyl methyl ether
Atomic and molecular astrophysics
Diffuse interstellar band
Earliest known life forms
Extraterrestrial liquid water
Helium hydride ion
Iron–sulfur world theory
Molecules in stars
Nexus for Exoplanet
PAH world hypothesis
Polycyclic aromatic hydrocarbon
Polycyclic aromatic hydrocarbon (PAH)
RNA world hypothesis
Organisms and comparable organic structures
RNA satellite virus
DNA satellite virus (Virophage)
RNA satellite (Virusoid)
Defective Interfering RNA
Defective interfering DNA
BNF: cb11933780m (d