Comments on the NAS report “The Limits of Organic Life in Planetary Systems”

Recently, I mentioned a newly released National Academy of Sciences report about the search for extraterrestrial life. After reading the report, I have a few comments.

The Executive Summary begins, appropriatetely, with a discussion of life itself and its characteristics:

Reflecting the near inevitability of human missions to Mars and other locales in the solar system where life might exist, and given the interest of the public in the question, Are we alone?, the National Aeronautics and Space Administration (NASA) commissioned the National Research Council, which formed the Committee on the Limits of Organic Life in Planetary Systems, to address the following questions:

  • What can be authoritatively said today about limits of life in the cosmos?
  • What Earth-based research must be done to explore those limits so that NASA missions would be able to recognize, conserve, and study alien life that is encountered?

Theory, data, and experiments suggest that life requires (in decreasing order of certainty):

  • A thermodynamic disequilibrium;
  • An environment capable of maintaining covalent bonds, especially between carbon, hydrogen, and other atoms;
  • A liquid environment; and
  • A molecular system that can support Darwinian evolution.

These represent rather broad criteria. Indeed, the report soon states:

     The committee found that using thermal and chemical energy to maintain thermodynamic disequilibria, covalent bonds between carbon atoms, water as the liquid, and DNA as a molecular system to support Darwinian evolution is not the only way to create phenomena that would be recognized as life. Indeed, the emerging field of synthetic biology has already provided laboratory examples of alternative chemical structures that support genetics, catalysis, and Darwinian evolution. Organic chemistry offers many examples of useful chemical reactivity in nonwater liquids. Macromolecular structures reminiscent of those found in terran biology can be formed with silicon and other elements. [bold added]

As a result, the report attempts to explore some possible alternative chemistry and chemical structures as a means by which to hope to broaden the applicability of these criteria so as to have a better grasp on where life might be found elsewhere and how we might recognize it.  

However, none of these criteria, nor any collection of them, specifically define life, even approximately. The report notes that “most locales in the solar system are at thermodynamic disequilibrium”, and as already quoted above, there is nothing necessarily unique about any particular choice of chemistry or solvents. In short, reliance upon chemical composition  as a means of determining whether a natural system is a living organism or not is suggestive at best, and misleading at worst. This leaves the last criteria: a molecular system that can support Darwinian evolution.

There is something that seems to me rather obtuse and almost circular about this criteria, since individual organisms do not evolve; instead, evolution is a property of species. Thus, to use this criteria in the search for extraterrestrial life would require identifying populations of certain natural systems found in an extraterrestrial environment, tracking any reproduction of those natural systems over an adequately large number of generations, and then determining whether these natural systems are indeed living or not. Such a determination would rest upon identifying the genomes of the systems under study, and the relationships of those genomes across generations. As a practical matter, since this kind of multi-generational investigation in an extraterrestrial environment would be quite resource-intensive, it would of course be extremely preferable to make – with very high probability – the initial determination that the natural system being considered is in fact a living organism. If so, then the matter is already answered at the level of the individual organism to a high degree of probability, which would render somewhat moot the further testing of generations of a population of these organisms.

Therefore, I suggest that the criteria as specified in the report are not terribly helpful as starting points in an investigation for extraterrestrial life. But if not these criteria, then what? One likely answer is suggested in the paragraphs on Darwinian evolution:

     Many of the definitions of life include the phrase undergoes Darwinian evolution. The implication is that phenotypic changes and adaptation are necessary to exploit unstable environmental conditions, to function optimally in the environment, and to provide a mechanism to increase biological complexity. The canonical characteristics of life are inherent capacities to adapt to changing environmental conditions and to increase in complexity by multiple mechanisms, particularly by interactions with other living organisms.

     One of the apparent generalizations that can be drawn from knowledge of Earth life is that lateral gene transfer is an ancient and efficient mechanism for rapidly creating diversity and complexity. The unity of biochemistry among all Earth’s organisms emphasizes the ability of organisms to interact with other organisms to form coevolving communities, to acquire and transmit new genes, to use old genes in new ways, to exploit new habitats, and, most important, to evolve mechanisms to help to control their own evolution. Those characteristics are likely to be present in extraterrestrial life even if it has had a separate origin and a very different unified biochemistry from that of Earth life.

I point to two concepts therein: biological complexity, and biological function. The report does not specify what is meant here by ‘biological complexity’, but by common usage it refers not to any measure of number of atoms which comprise the system, a system’s spatial extent, nor other such measure; but that it instead refers to the complexity of the organization within the system. However, it is possible that non-living systems could possess very complicated internal structural organizations. So as the quote implies, the specific relevant organization within the system is not particularly the system’s structural organization; but rather, that organization which  serves the biological functions described above – its functional organization.

It is hardly arguable that such organization does not exist within every known living organism, and it is unthinkable that anything we could consider as life anywhere, regardless of its chemical composition, would not possess such internal functional organization, in order to carry out its biological activities, even if such organizations differ from those of organisms with which we are familiar.

By using functional organization as the criteria in a search for extraterrestrial life, a number of things change. First, we are no longer looking for suggestive properties or symptoms of life which provide inconclusive results; instead, we are looking for certain characteristic causal patterns with the organism itself. Further, since functional organization models (known as relational models) of these causal patterns possess no information about any specific chemistry, they are inherently amenable to the search for life based upon alien chemistries. Finally, functional organization is independent of scale: the criteria applies equally to microscopic and macroscopic organisms.  

Much of the initial theoretical work on modelling of functional organization of organisms has already been completed, as the field called Relational Biology. This work was initiated by biophysicist Nicolas Rashevsky (e.g., [2-4]), and brought to fruition by theoretical biologist Robert Rosen (e.g., [5-9], see the bibliography for a complete listing). What remains is primarily technological — how to design and construct the necessary sensory devices and analytic abilities in order for a space probe to carry out the task of detecting functional organizations with a reasonable degree of competence.

This would be a nontrivial engineering task, to say the least. On the other hand, it is of little use to design and construct expensive space probes if the onboard detectors for extraterrestrial life are based upon criteria which provide inconclusive results. It would, I think, be more effective to study the nature of functional organization of terrestrial organisms, and learn how to build detectors of them. Once the technology of detection of functional organization is accomplished, it can be applied to probes destined for any kind of environment, with hopefully only a change of its sensory apparatus to accomodate that particular environment. This flexibility also indicates the possibility that the sensory and analytic aspects of such a detector could reasonably be separated. This might allow for the space probe to carry primarily only sensory apparatus, leaving the analytic processing to be done remotely, here on Earth.

One of the sections in the report is titled “Strategies to Mitigate Anthropocentricity”. The strategies essentially boil down to: given what we know of the chemical and energetic properties of terrestrial life, how do we generalize those properties in a reasonable and logical manner, such that these generalized properties will likely encompass those properties possessed by various extraterrestrial life? An alternate question I pose is this: is it better to mitigate anthropocentricity by attempting  to generalize from the modes of analysis which preoccupy our biological sciences, or do we better mitigate anthropocentricity by not assuming that such modes of analysis – which have failed to even provide us with a solid definition of terrestrial life – are the appropriate modes of analysis for finding extraterrestrial life?



[1] 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. NAS National Academies Press. ISBN 0-309-10484-X. 116 pgs.

[2] Rashevsky, N. 1954. Topology and Life: In search of General Mathematical Principles in Biology and Sociology. Bull. of Math. Bio. 16:317-348.

[3] Rashevsky, N. 1960. Mathematical Biophysics: Physico-Mathematical Foundations of Biology, Vol. Three. Dover. (3rd. Ed.)

[4] Rashevsky, N. 1961. Mathematical Principles in Biology and Their Applications. Charles C. Thomas, Springfield, Il.

[5] Rosen, R. 1958. A Relational Theory of Biological Systems. Bull. of Math. Bio. 20:245-260.

[6] Rosen, R. 1958. The Representation of Biological Systems from the Standpoint of the Theory of Categories. Bull. of Math. Bio. 20:317-341.

[7] Rosen, R. 1959. A Relational Theory of Biological Systems II. Bull. of Math. Bio. 21:109-128.

[8] Rosen, R. 1972. Some Relational Cell Models: The Metabolism-Repair System. Foundations of Mathematical Biology Vol. 2, Ch. 4. Academic Press.

[9] Rosen, R. 1991. Life Itself. Columbia Univ. Press.



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