Life/Citable Version

Biologists use the term life to refer to the process(es) comprising the activity of living, to the entities that embody that/those process(es), and to the interrelations and interactions among those entities---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 artifacts, living entities not based on carbon and water chemistry (e.g., androids)?
  • Can we produce living entities that consisted of purely computational processes, or 'bits' (e.g., 'artificial life')?
• Could living entities exist that consisted of purely matter-free processes (e.g., '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 in context 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 children learn heuristically from their usage in the socio-cultural matrix in which they live. One can define any other word, i.e., 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'.

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.

To explain 'life', then, one must first explain what characterizes the processes that enable an entity to live, or what characterizes the proceses that enable the activity of living. 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 biology professor Antonio Lazcano's remark: "Life is like music; you can describe it but not define it". (Lazcano 1994, cited in ).

Shared Characteristics of Living Things: Perspectives from Systems Biology, Thermodynamics, Evolutionary Biology, Exobiology, and the Concept of 'Autonomous Agents'

As well as sharing a common carbon- and water-based chemistry, entities that biologists generally acknowledge as living ——(bacteria, trees, fish, chimpanzees etc.)——share a common basic building block, the cell. The cell is the smallest system thought capable of independent living. Many organisms live as single cells, some are cooperative colonies of single cells, others are complex multicellular systems, with many different cell types specialized for different functions. Nature has produced an enormous variety of cell types in three vast ‘domains’ of living systems: Archaea, Bacteria, and Eukarya. (See article, Biology) Yet all three domains 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 are ‘manufactured’ by pre-existing cells. All extract the chemical energy in simple sugar molecules’ chemical bounds, converting it by chemical reactions into energy forms for a variety of purposes, ultimately enabling them to respond to their evolutionary imperative of self-reproduction. They all share the possession of a molecular-(i.e., DNA)-embodied code, using the same code (the ‘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 faithfully or nearly so, and thereby have the capacity to replicate themselves.

The Systems Perspective

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

• from how 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' [effect-producing] explanation); and,

• from how 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 an academic discipline, Systems biology, and they apply not just to cells but to all living systems.

The Thermodynamic Perspective

Biologists also view the commonalities among living things from the perspective of thermodynamics (thermo-, heat; -dynamics, movement)---the science of interactions among energy (capacity to do work, a driving force), heat (thermal energy), work (movement through force), and entropy (degree of disorder or of missing information). The nature of those interactions define what the thermodynamic system can and cannot do in the process of interconverting energy and work. For example, experiments in thermodynamics find that in converting one form of energy to one or more other forms of energy, no net loss of energy occurs, and no net gain (First Law of thermodynamics: energy conservation).

The special field of 'non-equilibrium' thermodynamics, involving the Second Law of Thermodynamics, can describe narratively and mathematically many of the characteristics of systems that remain, for a more or less long time (=lifespan) in a steady-state of organized functional activity, indicating that they perform their organized functional activities far from the 'equilibrium' state of activity of the system if it did not have access to and ability to store and utilize available energy from outside the system. Such systems can store energy and perform work on themselves and outside. The available outside energy ultimately supplies the driving force that keeps the system functioning far-from-equilibrium and in disequilibrium with its environment.

Biological cells, and other living systems, fall in the category of non-equilibrium thermodynamic systems, since they must consume energy to live, and since they reach an equilibrium state only in death---whereupon all parts relate to each other according to spontaneous physicochemical processes. Viewing living systems from the perspective of non-equilibrium thermodynamics give biologists mathematical tools to work with to learn how the system organizes itself. Those tools may help find ways to eliminate accumulated dysfunctions in the organizational activity of a living system that eventually causes it fail to maintain its optimal organization sufficently within bounds to keep the system organized and therefore far-from-equilibrium. Slowing or eliminating dysfunctions of organizational activities might increase lifespan.

We can, then, view a living system as a state of organizational activity (non-randomness) maintained in that far-from-equilibrium state for prolonged periods by importing, storing and transforming energy and matter from its external environment into the work and structures required to sustain its organizational activity as a system functioning in its own behalf. In doing so, living systems produce waste (pollution) and export it to the external environment, lowering the organizational state of the enviroinment. The biological system maintains its internal organizational activity at the expense of that the external environment, leaving the environment more disorganized than the gain in organization of the living system--in keeping with the second law of thermodynamics that the total level of disorder (system plus environment) always increases in this universe.

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

• The ability to remain for a time (=lifespan) as an organized, coordinated functioning system, in which spontaneous and externally forced tendencies to disturb the organizational activity are opposed by 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).

The Evolutionary Perspective

But that characteristization might also apply to some non-living systems such as a tornado or the flame of a candle. However, tornados and candle flames cannot 'reproduce' themselves, as cells and organisms do. One might then characterize living systems as having:

• The ability to remain for a time (=lifespan) as a compartmentalized, organized, coordinated functioning system, in which spontaneous and externally forced tendencies to disorganization 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 capable in principle of reproducing themselves before equilibrium arrives.

In a living system's activity of reproducing, however, random events (e.g., 'mutations') introduce variations in the system's properties, functions and behavior. Some 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 progenitors, 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", and regard it as the most important way, though not the only one, whereby living systems evolve over historical time..

Therefore, 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. Evolution by means of natural selection enables descent with modification, 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 have very few of the characteristics of living systems described earlier in this article, but they do 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.

Enlarging the description, we can say then that living systems have:

• The ability to remain for a time (=lifespan) as a compartmentalized, organized, coordinated functioning system, in which spontaneous and externally forced tendencies to disorganization 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 capable in principle of reproducing themselves before equilibrium arrives, and capable of evolving ove historical time to adapt to environmental conditions.

Thus, living systems extract environmental resources and export waste (disorder) to produce (for a while) functional organization with the ability to do work, reproduce with variation, and evolve transgenerationally by natural selection of variations favorable to enabling another reproductive cycle.

An Exobiological Perspective

Exobiologists (aka astrobiologists) concern themselves with issues relating to the possible existence of extraterrestrial living systems. Dirk Schulze-Makuch and Louis N. Irwin, interested in guiding the recognition of extraterrestrial living systems, attempt to distill the essential characteristics of a living system. They stress these characteristics, which resonate with the systems, thermodynamic and evolutionary perspectives discussed above:

• a microenvironment with a boundary between it and its external environment,

• the capability of that microenvironment to transform energy and matter from the environment to maintain itself in a low entropy state (i.e., highly ordered or organizational state),

• therefore, the capability of that microenvironment to sustain itself in thermodynamic disequilibrium with its environment,

• the capability of that microenvironment to encode and transmit information.

'Autonomous Agents' Perspective

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

He gives the hypothetical example of an enzyme that catalyzes the binding of two smaller sub-component molecules into a copy of itself---self-replication by 'auto-catalysis'. It requires energy to produce an excess of the enzyme over the concentrations of its sub-components prevailing when all are at equilibrium. That energy arrives through release of the energy in an energy-rich bond of another molecule, which results from breaking apart the bond connecting that enegy-rich molcule's sub-components. The self-replication stops when that energy-rich molecule is depleted, so to sustain self-replication, external energy---perhaps coming from light-energy (photons) impinging on the system from outside---must drive the repair of the broken chemical bond, re-establishing an excess of that energy-supplying energy-rich molecule. A new cycle of 'auto-catalytic self-replication' can then begin, provided an ample supply of the sub-compnents of the 'auto-catalytic' enzyme.

Kauffman conceives, then, of a self-replicating autocatalytic molecule in a network of molecules that has repeat cycles of self-replication sourced by external 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 that 'drives' the self-replication.

Kauffman calls that open thermodynamic system a ‘molecular autonomous agent’. The reason for that appellation: Given a source of external energy (e.g., photons) and the excess materials (the sub-component molecules needed to assemble the autocatalytic enzyme), the system perpetuates itself autonomously, i.e., not controlled by outside forces even though dependent on outside energy. The ‘agent’is the the system doing work, in this case the work of auto-catalytic self-replication. That's what 'agents' do---they do work. That work gets done because the system starts far-from-equilibrium, i.e., excess amounts of the sub-components of the autocatalyst and plentiful outside energy. The agent continues 'to live' so long as that far-from-equilibrium state exists.

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

Kauffman then argues that, from his hypothetical example, cells, and indeed all living systems, qualify as autonomous agents, constructed from such molecular autonomous agents.

Emergence

All cell and cellular 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 no matter how well one knows those properties, functions and behaviors independently of the those they might have when embedded in the functioning system to which they belong.

Two interrelated reasons for that constraint on prediction:

• the intrinsic properties, functions and behaviors 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 separate context, its 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 affect 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 intrinsic properties, functions and behaviors of the components. The system's behavior itself influences the organization of its components. Novel behaviors of the system 'emerge' that are not predictable from knowledge of the intrinsic properties, functions and behaviors of the components alone. For example, the behavior of a kidney cell depends not only on the intrinsic characteristics of its components, but also on the organ (kidney) in which it resides, because that environment influences the cell’s structure and behavior, which in turn influence the organization of the cell’s components.

Systems biologists refer to those two determinants of a system's behavior as "bottom-up" and "top-down" effects. The novel, emergent properties, functions and behaviors that result from a combination of bottom-up and top-down effects constiture another general characteristic of living systems.

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.

Mentionables

Living systems generally have the following features:

1. Organization: A temporary (=lifespan) organization of interrelated, coordinated, dynamically interacting hierarchy of molecular components within [cells]], of cellular components within [organs] and [organisms], of organisms within species, and of species within ecosystems---each importing energy and matter, and using it in its own behalf by performing work to build, grow and sustain its structural organization for performing the functional activities needed to maintain that organization for reproducing itself.
2. Metabolism: Conversion of imported energy into any or all of the various forms of energy (e.g., chemical, electrical, mechanical, thermal), needed to utilize imported matter for maintaining functional organization.
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 and increase in organizational complexity. 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, an animal chasing its prey, or neuronal membrane potentials traveling during thought..
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.

See also

• Artificial life
• Extraterrestrial life
• Biological kingdom
• Origin of life
• Biology
• Systems biology

Appendix A

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

Marcello Barbieri, Professor of Morphology and Embryology at the University of Ferrara, Italy, 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

Published Colections of Definitions of 'Life' or 'Living Systems'

• Popa R. (2004) Chronology of Definitions and Interpretations of Life. In: Popa R, ed. Between Necessity and Probability: Searching for the Definition and Origin of Life. Berlin: Springer-Verlag 2004: pp. 197-205 Abstract & Link to Full-Text

• Quotes and source-citations from 1885 to 2002 CE

• Barbieri M. (2003) Appendix: Definitions of Life. In: The Organic Codes: An Introduction to Semantic Biology. Cambridge, UK: Cambridge University Press ISBN 0521824141

• Quotes from 1802 to 2002

Further reading not cited in text

Books and Book Chapters

• Margulis L, Sagan D. (1995) What is Life? Simon & Schuster. ISBN 0-684-81087-5
• Schrodinger E. (1944-2000) What is Life? Cambridge University Press (Canto). ISBN 0-521-42708-8 Full-Text Copy Found on Internet

Articles

• Hazen R. (2006) The Big Questions: What is Life? New Scientist Issue 2578, 17 November 2006, page 46-51 Full-Text

Interviews and Commentaries

• Kauffman S. The Adjacent Possible: A Talk with Stuart Kauffman. Interview

See also

• Origin of life
• Evolution of cells
• Systems biology
• Biology

External links

• "In the Beginning..." (The Economist)

• From the preface: "How life on Earth got going is still mysterious, but not for want of ideas."

• "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