Life/Citable Version

For other uses, see Life (disambiguation), Lives (disambiguation) or Living (disambiguation)

Biologists use the term life to refer to the process(es) comprising the activity of living, and to the entities that embody that/those process(es)—complex adaptive systems. The question turns on what precisely characterizes the "process(es) of living". In answering that question, biologists hope to find answers to many other questions in biology, perhaps even some not yet asked (see Biology and Systems biology). For example, do viruses count as living things? Can we construct, as artefacts, living things not based on carbon and water chemistry (see Artificial life)? Could living things exist that consisted of purely computational processes [bits] or purely immaterial processes [quantum fields]?

This article will elaborate on the above issues and other considerations related to defining life.

Shared Characteristics of Living Things: Systems and Thermodynamic Perspectives

Biologists traditionally approach explanations of living entities by observing their shared characteristics. In addition to sharing a common carbon- and water-based chemistry, those entities generally acknowledged as living systems——namely organisms (e.g., trees, fish, chimpanzees)——share a common basic building block, the cell, the smallest system capable of independent living. Some organisms live freely as single cells, others as multicellular systems, the latter with various cell types specialized for different functions. Nature has produced an enormous array of different cell types.

Biologists view the commonalities and uniquenesses of cell types from several perspectives, including those recalling Aristotle's perspective:

• from the list of organic and inorganic parts (carbon-containing molecules and inorganic ions (Aristotle’s 'material' explanation);

• from the way the parts organize themselves in relation to each other to form substructures (patterns of form) (Aristotle’s 'formal' [form-like] explanation);

• from the way the components of the substructures interact with each in a coordinated dynamic manner and the way the substructures interact among themselves in a coordinated dynamic, and hierarchical manner (Aristotle’s 'efficient' [effective] explanation); and,

• from the way the cell as-a-whole functions and behaves and the properties that characterize it (Aristotle’s 'final' explanation).

Those different perspectives together now have formalized into Systems biology, and apply not just to cells but to all living systems.

Biologists also view the commonalities among living things from the perspective of thermodynamics (thermo-, heat; -dynamics, movement)---the physics of the relationships among energy (capacity to do work), heat (thermal energy), work (movement through force), and entropy (degree of disorder or of missing information), which define what the thermodynamic system can and cannot do during the process of converting energy to work or other forms of energy.

Unlike classical thermodynamics, in which the states of the system studied remain in equilibrium (i.e., without tendency for a spontaneous change to occur, because opposing forces remain in balance), non-equilibrium thermodynamics can describe the behavior of systems that remain for a time (=lifespan) in a near steady-state far-from-equilibrium while transfering energy from one place to another, or converting energy from one form to another, in processes that ultimately move the system towards an irreversible state of stability (equilibrium) characterized by randomness or disorder---whereupon the near steady-state far-from-equilibrium system ceases.

Living systems qualify as far-from-equilibrium systems, moving toward the equilibrium-randomness of a non-living state. During their lifespans, living systems produce organization and maintain a state of non-randomness for prolonged periods because they carry out processes that keep them in a condition far from the stability (equilibrium) of randomness. They achieve that by importing and transforming energy and matter from their external environment into the work and structures required to sustain their organization as a functioning system. In addition, they produce waste (entropy=disorder) and export it into the external environment. The biological system, having open access to its external environment, maintains its internal state of organization and process at the expense of the external environment, leaving the environment more disorganized than the gain in organization of the living system.

Thus, the following could serves as one fundamental characterization of life, or of living things:

• The ability to remain for a time as an organized, coordinated functioning system in which spontaneous and externally forced tendencies to change meet offsetting built-in self-correcting mechanisms fueled by external resources (energy, matter) and facilitated by production and exportation of disorder, all the while operating far-from-equilibrium.

But that characteristic, while applicable to cells and organisms, might also apply to systems generally recognized as non-living: a tornado, a candle flame. Tornados and candle flames cannot reproduce themselves, however, whereas cells and organisms can. One might then characterize living systems as having:

• The ability to remain as an organized, coordinated functioning system, in which spontaneous and externally forced tendencies to change meet offsetting built-in self-correcting mechanisms fueled by external resources (energy, matter) and facilitated by production and exportation of disorder, allthe while operating far-from an ever-approaching equilibrium (aka death), and reproducing before equilibrium arrives.

In a living system's activity of reproducing, however, random events introduce variations in the reproduced system's properties, functions and behavior. Some variations offer some progeny, or the progeny of some conspecific living systems, less opportnity to reproduce than others, and other progeny better opportunity to reproduce, sometimes better even than their forebears, given either changes in environmental conditions or limitations of environmental resources. Accordingly, new conspecific groups with different system properties arise, and older groups may discontinue reproducing. Biologists call that process evolution by means of natural selection.

Thus, living systems extract environmental resources and export waste (disorder) to produce and temporarily maintain a functional organization, reproduce with variation, and evolve by natural selection of variations favorable to enabling another reproductive cycle.

Some Definitions of Life Resonating with the Preceding Exposition

Marcello Barbieri, Professor of Morphology and Embryology, University of Ferrara, Italy, in his Book, The Organic Codes, collected an extensive list of definitions of “Life” from scientists and philosophers of the 19th and 20th centuries. Many resonate with the above exposition:

• The broadest and most complete definition of life will be "the continuous adjustment of internal to external relations". —Hebert Spencer (1884)

• It is the particular manner of composition of the materials and processes, their spatial and temporal organisation which constitute what we call life. — A. Putter (1923)

• A living organism is a system organised in a hierarchic order of many different parts, in which a great number of processes are so disposed that by means of their mutual relations, within wide limits with constant change of the materials and energies constituting the system, and also in spite of disturbances conditioned by external influences, the system ts generated or remains in the state characteristic of it, or these processes lead to the production of similar systems. — Ludwig von Bertalanffy (1933)

• Life seems to be an orderly and lawful behaviour of matter, not based exclusively on its tendency to go from order to disorder, but based partly on existing order that is kept up. —Erwin Schrodinger (1944)

• Life is made of three basic elements: matter, energy and information. ..Any element in life that is not matter and energy can be reduced to information. — P.Fong (1973)

• A living system is an open system that is self-replicating, self-regulating, and feeds on energy from the environment. —R. Sattler (1986)

The persistence of a living system's status as a far-from-equilibrium system requires a set of mechanisms that enable it to import matter and energy from the environment and convert those to the work of self-maintenance. Those mechanisms--transcription and translation, cell-signalling, metabolism, transmembrane transport, and many others---generically common but differing in details among cell types---allow further consideration of the commonalities and differences among living things.

Other Shared Characteristics of Living Things

At a somewhat less general level of commonality than that of the systems and themodynamic perspective, living things share the characteristics that:

• all biological cells come into existence from pre-existing cells, by one or another 'manufacturing' process;
• all muticellular organisms come into existence from pre-existing organisms, by one or another 'manufacturing' process.

Other articles detail the various 'manufacturing' processes (see Biology).

• all cells possess an enclosing membrane that gives them more or less protection against dissoution into their external environment;

• the enclosing membrane contains molecular systems that enable importation of matter and energy for use in maintaining its system properties and functions, in the face of changing external conditions, through energy conversions and work, and that involve production of and enable exportation of unusable matter and energy;

• all cells possess an inherited blueprint for constructing its components, and mechanisms for carrying out the construction process;
• all cells have the capability to assemble and organize themselves from more rudimentary states;
• all cells and multicellular systems exist interdependently with other cells and multicellular systems;
• all cells and multicellular systems eventually die.

Perhaps one can reduce all other common characteristic of cells or multicellular systems to those delineated above.

With perhaps one important exception: All cell and cell systems exhibit properties, functions and behaviors that in principle arise from the organizational processes of the system, but that one cannot have predicted from those processes---because the system as a whole operates in its own environment that impacts on the system-as-a-whole, the results of which in turn influence the properties and behaviors of the system's subsystems--a kind of downward causation. In other words, the activities of cellular systems give rise to novel behaviors not predictable from knowledge of the systems' components or subsystems without knowledge of how the behavior of the whole system impacts on its subsystems. Those novel behaviors "emerge" from the activities of the system as a whole.

Thus emergent properties, functions and behaviors qualify as another general common characteristic of living things.

[Not best placed here.] The entire Earth contains about 75 billion tons of living matter (needs source-citation) (biomass), living in nearly every conceivable niche of the earth---the interior of "solid" rocks, ocean-bottom volcanic vents at boiling-water temperatures---all comprising a so-called biosphere.

Life Further Characterized

Without universal agreement on the definition of life, scientists generally accept that the biological manifestation of living systems include the following:

1. Organization: A temporary (=lifespan) organization of interrelated, coordinated, dynamically interacting hierarchy of molecular components within [cells]], cellular components within [organs] and [organisms], organisms within species, and species within ecosystems.
2. Metabolism: Production of any or all of various forms of energy (e.g., chemical, electrical, mechanical, thermal) by importing and converting available external energy (exergy) and nonliving material, including organic and inorganic, into cellular components (synthesis).
3. Growth: At certain stages of its life-cycle, cells, organs, and organisms maintain a higher rate of synthesis (anabolism) than breakdown (catabolism) of structure. Growth occurs largely according to plan for survival and reproduction. Species populations tend to grow as resources and other factors permit.
4. Reproduction: The ability to reproduce itself, for example, the division of one cell to form two new cells. Usually the term is applied to the production of a new individual (either asexually, from a single parent organism, or sexually, from at least two differing parent organisms), although strictly speaking it also describes the production of new cells in the process of growth.
5. Gain of New Inheritable Traits.: Inheritable diversity among progeny organisms, whether adaptive, neutral or disadvantageous, is a common feature of living things, and the starting point for natural selection. (See also:)
6. Adaptation: At the species level, the ability to gain traits through evolutionary processes that improve the members of the species chance for reproductive success; at the individual organism level, the ability to change (e.g., through learning) in ways that improve the individual's chances for reproductive success.
7. Response to stimuli: A response can take many forms, from the contraction of a unicellular organism when touched, to complex reactions involving all the senses of higher animals. A response is often expressed by motion, for example, the leaves of a plant turning toward the sun or an animal chasing its prey.
8. Homeostasis: Regulation of the internal environment to maintain a near-constant state in response to perturbations; for example, sweating to cool off.

Exceptions to the foregoing exposition of the characteristics of living things

It is important to note that life is a definition that applies primarily at the level of species, so even though many individuals of any given species do not reproduce, possibly because they belong to specialized sterile castes (such as ant workers), these are still considered forms of life. One could say that the property of life is inherited; hence, sterile hybrid species such as the mule are considered life although not themselves capable of reproduction. It is also worth noting that non-reproducing individuals may still help the spread of their genes through such mechanisms as kin selection.

For similar reasons, viruses and aberrant prion proteins are often considered replicators rather than forms of life, a distinction warranted because they cannot reproduce without very specialized substrates such as host cells or proteins, respectively. However, most forms of life rely on foods produced by other species, or at least the specific chemistry of Earth's environment.

Some individuals contest such definitions of life on philosophical grounds, and offer the following as examples of life: viruses which reproduce; flames which "grow"; certain computer software programs which are programmed to mutate and evolve; future software programs which may evince (even high-order) behavior; machines which can move; and some forms of proto-life consisting of metabolizing cells without the ability to reproduce. Template:Citation needed

Still, most scientists would not call such phenomena expressive of life. Generally all six characteristics are required for a population to be considered a life form.

Descent with modification

A useful characteristic upon which to base a definition of life is that of descent with modification: the ability of a life form to produce offspring that are like its parent or parents, but with the possibility of some variation due to chance. Descent with modification is sufficient by itself to allow evolution, assuming that the variations in the offspring allow for differential survival. The study of this form of heritability is called genetics. In all known life forms (assuming prions are not counted as such), the genetic material is primarily DNA or the related molecule, RNA.

Unlike other definitions, this definition of life includes viruses, as they are replicators with a genotype and phenotype, making them capable of natural selection and evolution. The definition may also include other replicating elements, including plasmids, which are otherwise considered part of a larger organism.

Also difficult for this definition is organisms which cannot reproduce directly, such as worker bees—which may also continue their gene-line by helping to produce siblings, and sterilised organisms, such as spayed or neutered pets, which are no longer capable of descent.

More abstract concepts may also be considered alive by this definition, including memes and the artificial life of computer software, such as self-modifying computer viruses and programs created through genetic programming.

The chemoton model

The chemoton is an abstract model for life introduced by Tibor Gánti in 1971. Its aim was to define the minimal model of a living organism.

A living system:

1. Has to be separated from its environment.
2. Has to perform metabolism with its environment.
3. It must replicate itself.
4. It has to have a polimer type subsystem carrying information.
5. It must have an autocatalytic system, which is connected to the metabolism and creates the stuff needed to grow its boundary and to replicate its information system.

Such a system may be called alive, since it can live, replicate in its proper environment and it can evolve, since there is an information system.

Other definitions

The systemic definition is that living things are self-organizing and autopoietic (self-producing). These objects are not to be confused with dissipative structures (e.g. fire).

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

Another definition is : "Living things are systems that tend to respond to changes in their environment, and inside themselves, in such a way as to promote their own continuation."

Yet another definition: "Life is a self-organizing, cannibalistic system consisting of a population of replicators that are capable of mutation, around most of which homeostatic, metabolizing organisms evolve." This definition does not include flames, but does include worker ants, viruses and mules. Without 'most of', it does not include viruses.

Self reproduction and energy consumption is only one means for a system to promote its own continuation. This explains why bees can be alive and yet commit suicide in defending their hive. In this case the whole colony works as such a living system.

Linguistic Considerations Relating to the Definition of Life

Ernst Mayr, a 20th century giant among evolutionary biologists, in his last decade as a centenarian, wrote a book called This is Biology: The Science of the Living World (Mayr 1997). In his opening chapter, What Is the Meaning of “Life” [his quotation marks], he states:

"To elucidate the nature of this entity called "life" has been one of the major objectives of biology. The problem here is that "life" suggests some "thing" -- a substance or force -- and for centuries philosophers and biologists have tried to identify this life substance or vital force, to no avail. In reality, the noun "life" is merely a reification of the process of living. It does not exist as an independent entity. One can deal with the process of living scientifically, something one cannot do with the abstraction "life". One can describe, even attempt to define, what living is; one can define what a living organism is; and one can attempt to make a demarcation between living and nonliving. Indeed, one can even attempt to explain how living, as a process can be the product of molecules that themselves are not living." (Mayr 1997, page 2).

Professor Mayr’s thoughts suggest we should fuss less about defining the nominalization, “life”, and concentrate more on defining the process, “living”.

Scientist Eric Schneider and science writer Dorian Sagan echo Mayr:

"Indeed, the word is a grammatical misnomer: life is a noun, but the phenomenon to which it refers is a process. And it is vitalistic: when we say life, we think we know what we are talking about when often we have simply applied a label that allows us to categorize, rather than examine closely, the phenomenon about which we are speaking."

Ultimately, all definitions of words in terms of other words converge on a set of some 70 so-called semantic primes, viz., primitive words, each undefinable in terms of other words, universal among languages, whose meaning one learns heuristically from their usage in the socio-cultural matrix in which one lives. One can define any non-semantic-prime word with some combination of semantic primes. The linguists who pioneered the theory of semantic primes do not list “life” as a primitive word, though they do list the verb “live” as such. See list of semantic primes at this site:

“Life” defines as “this lives” or “something lives”, where “lives” both speakers and listeners understand primitively, through experience. They also know primitively that things which live “die” and they generate the word “death” to refer to “dying”. “Plants” define as “things that live”, machines as “things that do not live” or “things that people make”.

To explain life, then, one must first explain living.

Interestingly, in English, according to the earliest references the Oxford English Dictionary finds, the verb “to live” preceded usage of the noun “life” by some 300 years.

The question, then, not “what is life?”, but “what characterizes things that live?” In fact, biologists act on the latter question, even as they ask it in terms of the former.

Quoting biologist, logician and historian J.H. Woodger (1929):

• ”It does not seem necessary to stop at the word "life" because this term can be eliminated from the scientific vocabulary since it is an indefinable abstraction and we can get along perfectly well with "living organism" which is an entity which can be speculatively demonstrated.” — J.H.Woodger (1929)

One way to compose an encyclopedia entry on “Life”: define it as a nominalization of the process of living, and redirect to the entry “Living”.

Origin of life

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

1. Plausible pre-biotic conditions result in the creation of the basic small molecules of life. This was demonstrated in the Miller-Urey experiment.
2. Phospholipids spontaneously form lipid bilayers, the basic structure of a cell membrane.
3. Procedures for producing random RNA molecules can produce ribozymes, which are able to produce more of themselves under very specific conditions.

There are many different hypotheses regarding the path that might have been taken from simple organic molecules to protocells and metabolism. Many models fall into the "genes-first" category or the "metabolism-first" category, but a recent trend is the emergence of hybrid models that do not fit into either of these categories.

The possibility of extraterrestrial life

Main articles: Extraterrestrial life, Astrobiology

Earth is the only planet in the universe known to harbor life. The Drake equation has been used to estimate the probability of life elsewhere, but scientists disagree on many of the values of variables in this equation. Depending on those values, the equation may either suggest that life arises frequently or infrequently.

See also

• Artificial life
• Extraterrestrial life
• Biological kingdom
• Origin of life

References

Further reading

• Kauffman, Stuart. The Adjacent Possible: A Talk with Stuart Kauffman. Retrieved Nov. 30, 2003 from [1]
• Lynn Margulis and Dorion Sagan - What is Life? (1995). Simon & Schuster. ISBN 0-684-81087-5
• Erwin Schrodinger - What is Life? (1944 to 2000). Cambridge University Press (Canto). ISBN 0-521-42708-8

See also

• Origin of life
• Evolution of cells

External links

• "In the Beginning..." (The Economist)
• "The Adjacent Possible: A Talk with Stuart Kauffman"
• Stanford Encyclopedia of Philosophy entry
• The Biologist
• Understanding Life
• Life under extreme conditions An in depth look at how life can form under the most extreme conditions.
• The universal nature of biochemistry