NASA Astrobiology Assessment 2008

From the National Academies Press comes the 2008 “Assessment of the NASA Astrobiology Institute” [1]. The Executive Summary section begins:

Astrobiology is a scientific discipline devoted to the study of life in the universe—its origins, evolution, distribution, and future. It brings together the physical and biological sciences to address some of the most fundamental questions of the natural world: How do living systems emerge? How do habitable worlds form and how do they evolve? Does life exist on worlds other than Earth? As an endeavor of tremendous breadth and depth, astrobiology requires interdisciplinary investigation in order to be fully appreciated and examined.

The fundamental question missing from this paragraph? “What is life?” Indeed, this question is not present anywhere within the entire document.

Indeed, of all the goals and objectives of astrobiology, it would seem paramount that one of the first, if not the first, goal would be to answer “what is life?” However, this task is notably absent from The Goals and Objectives list in the Assessment. In turn, this list is excerpted from a more complete list of goals and objectives in the NASA Astrobiology Roadmap 2008 [2].

The closest the Roadmap comes to acknowledging a need for a definition of life is a paragraph in which, ironically, a tacit definition of life is given, in the form of several conditions which must be met for a system to be alive:

A bounded system of replicating and catalytic molecules capable of undergoing Darwinian evolution is by definition a cell. At some point life on Earth became cellular, either from its inception or soon thereafter. Boundary membranes divide complex molecular mixtures into large numbers of individual structures that can undergo selective processes required for biological evolution. They also have the capacity to develop substantial ion gradients that represent a central energy source for virtually all life today. A primary objective of research is to understand self-organizing and evolutionary processes that lead to the emergence of cellular structures and test this understanding by creating laboratory models of primitive cells. These are systems of interacting molecules within bounded environments capable of working in concert to capture energy and nutrients from the surroundings, transduce environmental signals, form metabolic networks that allow for growth through polymerization, and reproduce some of their polymeric components. Approaching this challenging problem will lead to a more refined definition of the living state, and will clarify the hurdles faced by self-assembled systems of organic molecules as they evolved toward life.

From the perspective of the Roadmap, then, to answer “what is life?” is essentially a matter of refining those conditions which they have just tacitly specified. However, I argue that none of the conditions alone, nor any combination thereof, comprise a set of sufficient conditions for life. That is, there is no set of those conditions such that if system X satisfies conditions {F,G,H} then X must be alive.

Indeed, the references to evolution are irrelevant, since it is neither necessary nor sufficient for a system to be capable of evolution in order that it be alive. Individual organisms do not evolve, species evolve. Moreover, if it were seriously the case that being capable of evolution was a condition of life, then it would not be possible to say with any certainty that any given natural system is alive until one has empirically observed it (or more correctly, observed it’s species) evolving, which is a preposterous situation.

The Roadmap also discusses how to recognize signatures of life, on Earth and elsewhere:

Astrobiological exploration is founded upon the premise that signatures of life (biosignatures) will be recognizable in the context of their environments. A biosignature is an object, substance and/or pattern whose origin specifically requires a biological agent. The usefulness of a biosignature is determined, not only by the probability of life creating it, but also by the improbability of nonbiological processes producing it. An example of such a biosignature might be complex organic molecules and/or structures whose formation is virtually unachievable in the absence of life. A potential biosignature is a feature that is consistent with biological processes and that, when it is encountered, challenges the researcher to attribute it either to inanimate or to biological processes. Such detection might compel investigators to gather more data before reaching a conclusion as to the presence or absence of life.

I contend that while the search for these chemical artifacts of life may resonate well with the practical limitations on the complexity and scope of the technology which we are able to launch to other worlds, that detecting these chemical artifacts are at best only suggestive in discerning life. As such, the justification for the cost of these programs must be weighed against the inherently inconclusive nature of the results they provide.

Ultimately, whatever it’s chemical makeup, physical size or local environs, living organisms are organized systems, not aggregations of molecules. Organisms possess an internal organization of biological functions which is not found in non-living natural systems. All the discussions about polymerization, energy transduction, and chemical artifacts of life arise only as a consequence of this internal organization of biological functions. What distinguishes a chemical reaction or an energy transduction as being biological or not rests entirely upon whether those processes have a functional role in an organism or not. This is why an inability to determine whether a functional role is played by such processes leads to inherently inconclusive results from probes and experiments which detect only the material aspects – the molecular artifacts – of organisms. 

As such, defining life, and the search for life elsewhere, is best addressed by directly understanding and modelling the aforementioned internal functional organization, abstracted away from the particular chemical and material composition of any given organism. These kinds of formal models of internal biological functional organization are called relational models [3]. By focusing specifically on this internal organization, from which all other recognizable characteristics of organisms arise, relational models provide us with the most direct way of addressing the question “what is life?”.



[1] Committee on the Review of the NASA Astrobiology Institute, National Research Council. 2008. Assessment of the NASA Astrobiology Institute. National Academies Press. ISBN-10: 0-309-11497-7 / ISBN-13: 978-0-309-11497-4. (Available as PDF)

[2] NASA ASTROBIOLOGY ROADMAP 2008 . Draft of 28 December 2007.

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

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