Life/Addendum

Selected definitions of life

Marcello Bárbieri, 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". — Herbert 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." — L. 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." — E. 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)

Published collections of definitions of 'Life'

• 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 (Quotes and source-citations from 1885 to 2002)
• 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)

The exobiologists' view

Exobiologists (also known as astrobiologists) consider issues relating to the possible existence of extraterrestrial living systems. Dirk Schulze-Makuch and Louis Irwin attempted to distill the essential characteristics of a living system in their book Life in the Universe. 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 ability of that microenvironment to transform energy and matter from the environment to maintain a highly ordered, 'organizational' state (a low entropy state),

• therefore, the ability of that microenvironment to remain in thermodynamic disequilibrium with its environment,

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

The gray zone

Not all entities that otherwise qualify as living reproduce themselves, although 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 capacity to reproduce may die before reaching the reproductive stage in their life-cycle. Conversely, 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 would not qualify strictly as living things, but manage to 'reproduce' in living systems.

One might ask whether a spermatozoon qualifies as a living entity. From the thermodynamic perspective, one might answer affirmatively, as it keeps itself ‘living’ by doing cellular work. It has a compartmentalized internal organization functioning to keep it far-from-equilibrium. In that respect it resembles a motile bacterium. A spermatozoon reproduces, but in a different way than a motile bacterium: it does it through its parent’s progeny, which the spermatozoon plays an essential role in generating. It doesn’t have to hijack a cell’s machinery to reproduce; it cooperates with another cell (an ovum) to generate cells with machinery to reproduce it. Moreover, in reproducing that way, it subjects itself to meiotic crossover variation, just as its parent’s progeny does, contributing to the variation needed by natural selection to perpetuate the process of living on an earth with ever-changing environments.