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 entities?
• Can we construct, as artefacts, living entities not based on carbon and water chemistry (see Artificial life)?
  • Could living entities exist that consisted of purely computational processes [bits]?
• Could living entities exist that consisted of purely immaterial processes [quantum fields]?

This article will elaborate on the above issues and other considerations related to defining life as biologists use the term. It will also provide a heuristic enabling the reader to understand what constitutes 'living'. Until that understanding emerges full-blown, the reader must, so to speak, live with the word as context suggests during the unfolding of this article.

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 declares that understanding 'life' is one of the major objectives of biology; he states:

"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 [emphasis added]. 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 (aka reification], '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 other word, 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:

In semantic primes, 'Life' defines as 'that which lives' or 'something lives', where 'lives' speakers and listeners understand primitively, through experience with the language speakers in their environment. They also know primitively that things which live 'die' and they generate the word 'death' to refer to 'that which died'. '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 to live, or 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)

Carol Cleland, philosopher and member of NASA’s Astrobiology Institute, adds:

"Definitions tell us about the meanings of words in our language, as opposed to telling us about the nature of the world. In the case of life, scientists are interested in the nature of life; they are not interested in what the word "life" happens to mean in our language. What we really need to focus on is coming up with an adequately general theory of living systems, as opposed to a definition of "life.""

That resonates with Lazcano's remark: "Life is like music; you can describe it but not define it".(Lazcano 1994).

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'. This article will not take that approach, as indicated in the Introduction.

Shared Characteristics of Living Things: Systems and Thermodynamic Perspectives

As well as sharing a common carbon- and water-based chemistry, entities that biologists generally acknowledge as living ——namely organisms (e.g., bacteria, trees, fish, chimpanzees)——share a common basic building block, the cell, the smallest system thought capable of independent living. Many organisms live as single cells, other organisms as cooperative colonies of single cells, others as complex multicellular systems, with many different cell types specialized for different functions. Nature has produced an enormous array of different cell types in three distinguishable ‘domains’ of living systems: Archaea, Bacteria, and Eukarya. See article, Biology.

Nevertheless, all three major ‘domains’ of living entities share the characteristic cellular feature of compartmentalization from their environment by a surrounding lipid-protein membrane, the plasma membrane. Moreover the cells in all three domains share their existence as ‘manufactured’ products of pre-existing cells. All share the ability to extract the chemical energy in simple sugar molecules’ chemical bounds, converting it by chemical reactions into energy forms usable by the cell for a variety of purposes, ultimately for the purpose of responding to their evolutionary imperative of self-reproduction. They all share the possession of a molecular-(i.e., DNA)-embodied code, using the same code (aka ‘genetic code’) that by similar processes guides the production of the variety of proteins (polypeptide chains of amino acids) that give structure and function to the cells. All replicate their DNA semi-faithfully.

From those basic shared characteristics, biologists view the commonalities and uniquenesses of cell types from several perspectives, including those recalling Aristotle's four perspectives:

• 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). Those relationships 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 to operate.

Living systems are far-from-equilibrium systems that maintain a state of organization (non-randomness) for prolonged periods 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 order to live, or remain alive. In addition, living systems 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--in keeping with the reality described by the second law of thermodynamics.

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

• The ability to remain for a time (=lifspan) 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 waste (disorder)---thus all the while operating far-from an ever-approaching equilibrium (aka death).

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

• The ability to remain for a time (=lifspan) 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 waste (disorder)---thus all the while operating far-from an ever-approaching equilibrium (aka death)---and reproducing themselves before equilibrium arrives.

In a living system's activity of reproducing, however, random events (e.g., mutations) introduce variations in the reproduced system's properties, functions and behavior. Some system variations offer some progeny, or the progeny of some conspecific living systems, less opportunity 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. Living systems evolve by other mechanisms as well..

Thus, living systems extract environmental resources and export waste (disorder) to produce and temporarily maintain functional organization, reproduce with variation, and evolve by natural selection of variations favorable to enabling another reproductive cycle. Biologists refer to the process of extraction of environmental resources (energy, matter) and production of waste (heat, disorder) by living systems as metabolism.

Other Shared Characteristics of Living Things

Living things share some very specific features; for example:

• all living entities descended from a common ancestor;
• only pre-existing cells can "manufacture" new cells;
• only pre-existing multicellular organisms can 'manufacture" new muticellular organisms;

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

• a membrane encloses every cell, protecting them from dissolution into their external environment;

• the cell membrane contains molecular systems that enable matter and energy to be imported into and exported out of the cell, and to send and receive signals to and from other cells;

• all cells have an inherited "blueprint" for constructing its components, and mechanisms for carrying out the construction;
• all cells have the capability to assemble and organize themselves from more rudimentary states (e.g., to grow, adapt, complexify);
• all cells and multicellular systems eventually die.

Emergence

Perhaps one can reduce all other common characteristic of cells or multicellular systems to those delineated in the previous section.

With perhaps one important exception: All cell and cell systems exhibit properties, functions and behaviors that in principle arise from the properties of the system's components—more specifically from the organization of those components (see below)—but that one could not have predicted from those component properties. Two interrelated reasons:

• the properties of the system's components do not of themselves determine those of the whole system, but rather their organization within the system does, where 'organization' includes the interrelations of the components and their dynamic, coordinated, hierarchical interactions;
• the system as a whole operates in its own context, or external environment, which impacts on the properties, functions and behaviors of the system-as-a-whole. The results of that contextual impact on the whole system's properties and behaviors in turn influence the organization of the system's components—-a 'downward causation'.

In other words, the organization of the system's components determine the system's behavior, but that organization does not arise solely because of the properties of the system's components. The system's behavior itself influences the organization of its components. Novel behaviors of the system 'emerge' not predictable from knowledge of the properties of the systems' components without knowledge of how the behavior of the whole system impacts back on the organization of those components. Those novel behaviors 'emerge' from the system's components' organization as it modulates in response to the system's behavior, in turn as the latter modulates in response to the context (environment) in which it interacts.

For example, a kidney cell's behavior depends not only on the properties of its components—even as they would inherently organize themselves—but also on the effects of the organ (kidney) in which it resides, as those contextual effects influence the cell’s structure and behavior, which in turn influence the organization of the cell’s components.

Systems biologists refer to that dual determination of a system's 'emergent' behavior as a combination of bottom-up and top-down effects.

The novel, emergent properties, functions and behaviors that result from such a combination of bottom-up and top-down effects qualify as another general common characteristic of living systems.

Life Further Characterized

Although there is no universally agreed definition of life, living systems generally have the following features:

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

Not all entities that otherwise qualify as living ordinarily reproduce themselves, though they exist as reproduced living things. Biologists call such living things 'sterile'. Examples include programmed sterility (e.g., worker ants, mules); acquired sterility (due to acquired injury (disease) to the reproductive process; access sterility (lack of reproductive fitness); voluntary sterility (e.g., human couples). Obviously living things with the inherent capacity to reproduce may die before reaching the reproductive stage in their life-cycle.

Non-reproducing individuals may still effect reproduction of copies of their genes by facilitating the reproduction of kin, who share many genes (see kin selection.

Viruses and aberrant prions would not qualify strictly as living things, but manage to 'reproduce' in living systems.

Living Systems from an Evolutionary Perspective

An important, but not unique, characteristic of living systems is descent with modification: the ability of a living system to produce offspring that inherit its features, but with some variation among offspring due to chance. Descent with modification can by itself allow evolution by means of natural selection, assuming that the variations in the offspring allow for differential reproductive fitness. The variations occur due to chance variations in the inherited genetic recipe (genotype) for constructing the offspring's phenotype. In all biological systems, DNA or the related molecule, RNA, primarily provides the genetic recipe.

Viruses, not sharing most of the characteristics of living systems described earlier in this article, nevertheless have a genotype and phenotype, making them capable of natural selection and evolution.

Accordingly, descent with modification is not uniquely a characteristic of living systems. Beyond the scope of this article, we find descent with modification in memes and the artificial life of computer software, such as self-modifying computer viruses and programs created through genetic programming. Descent with modification has also been proposed to account for the evolution of the universe.

Living Systems From the Perspective of Autonomous Agents

Stuart Kauffman emphasizes the concept of ‘autonomous agents’ in explaining living systems.

He gives the hypothetical example of a molecule, an enzyme, that catalyzes the binding of two smaller ones into an identical copy of itself—self-replication by auto-catalysis. It requires energy to produce an excess of the enzyme over its equilibrium concentration of components, energy which comes from the breaking an energy-rich bond in another molecule, a process coupled to and transferring the released energy to the binding process . The non-spontaneous process of such enriched self-replication stops on depletion of that energy-rich molecule. For the work of enriched self-replication to repeat, available external energy—photons impinging on the system, say—must spontaneously drive the repairing of the broken chemical bond and re-establishing excess of that replication-driving energy-rich molecule. A new work cycle, comprising the coupling of a spontaneous and non-spontaneous process, then can commence.

Kauffman conceives, then, of a self-replicating autocatalytic molecule in a network of molecules that has repeat cycles of the energy-driven work of self-replication ultimately sourced by energy external to the network. In effect, the cycles consist in repeat work cycles performed in the network from outside energy. The network, or system, has a self-replication process as a subsystem, and a ‘motor’, namely, the recycling breakup and repair of an energy-rich molecule’s chemical bond supplying energy for the self-replication process.

Kauffman calls that open thermodynamic system a ‘molecular autonomous agent’. Given the excess outside-the-system available energy (photons) and the excess materials (the molecules need to assemble the autocatalyst) it needs, the system functions autonomously in it own behalf of self-perpetuation. The ‘agent’—the system doing work, what ‘agents’ do—arises because the system starts far-from-equilibrium, i.e., excess components of the autocatalyst and excess photons, and it remains ‘alive’ so long as the far-from-equilibrium state exists.

Left out of that account: Within the system interactions (couplings) among its components have effects—technically referred to as ‘allosteric’ effects—that help ‘organize’ the system’s processes , allowing the system to flow coordinately.

Kauffman then argues that from his hypothetical example, one can extrapolate that cells, and indeed all living systems, qualify as autonomous agents, building up from molecular autonomous agents.

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. Must be separated from its environment.
2. Must perform metabolism with its environment.
3. It must replicate itself.
4. Must have a polymer-type subsystem carrying information.
5. 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.

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
• Biology
• Systems biology

Appendix A

Some Definitions of Life Offered by 19th and 20th Century Thinkers

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. Those selected below resonate with the systems and thermodynamic perspectives of living systems:

• 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)

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