How will we know if we have discovered extraterrestrial life on Mars, Venus, or the Moon? It might not use DNA or proteins or lipids; it might not be cellular; it might not even be carbon-based (although I think that’s unlikely given the uniqueness of carbon). If we discover some interesting chemistry, how will we know if it is biotic or abiotic? Can we recognize a metabolism that looks very different from the present core used by all living things on our planet?
One approach to check if what is observed corresponds to an agreed-upon definition of life. NASA has a pithy definition that’s both easy to remember and useful: “Life is a self-sustaining chemical system capable of Darwinian evolution.” It is also pregnant with meaning. Its heart is chemical (which I favor as a chemist!), but also systemic (with all that entails). It has the contrast of being self-sustaining and yet open to change via evolution. Must that evolution be Darwinian? What would it mean, and how is that distinguished from other evolutionary mechanisms? Is this definition too limiting? Is it too broad?
In their paper “Towards a General Definition of Life” (Origins Life Evol. Biosphere 2019, 49, 77-88), Vitas and Dobovisek propose an updating of NASA’s definition. Here’s the abstract.
Piece-by-piece, the authors analyze and critique the NASA definition that was formulated twenty-five years prior. Notably: (1) Thermodynamics is incorporated through the idea that life seems to stay away from falling into the equilibrium state; (2) Information becomes a central piece of their thesis; (3) Dynamic interactions between the living system and its environment are emphasized. Here’s their updated definition; it’s not as pithy, but perhaps a little improved over the original by being both more precise and more encompassing.
Life is a far from equilibrium self-maintaining chemical system capable of processing, transforming and accumulating information acquired from the environment.
The same journal issue contains a workshop report discussing “Hidden Concepts in the History and Philosophy of Origins-of-Life Studies” (Origins Life Evol. Biosphere 2019, 49, 111-145). There are twenty-five co-authors, and the discussion is indeed far and wide. One section considers the roles of necessity and contingency in origin-of-life explanations: Some explanations have a universal aspect; others have a historical aspect; and yet others seek to test the model by synthesizing (artificial) life. These three aspects were subsequently discussed in a definition of Lyfe (with a diagram illustrating these aspects).
The group also discussed two main strands in origin-of-life research. The first connects biology, chemistry and geology. The second connects physics, information theory and computation. For a while there was little cross-fertilization between the two groups, but that has changed in recent years. My own journey started with a little-known book by Jeffrey Wicken: “Evolution, Thermodynamics and Evolution: Extending the Darwinian Program” (published in 1987). I read it a decade ago but didn’t quite grasp its significance. More recently, I read “The Origin of Nature and Life on Earth” by Morowitz and Smith, combining the two strands. This was influential in pushing me towards tackling the question of proto-metabolism as my current main research thrust. You also see this in Vitas and Dobovisek’s updated NASA definition, where they weave the second strand into the first.
However, the 25-person panel also wonders if trying to find an agreed-upon definition of life might compound the problem, and this leads to what I think is the most interesting paragraph in their paper.
This brings us to a central problem with tailoring a model of the origins of life closely on a definition of life: there may be little reason to suppose that one could extract a causal “recipe” for life (in general, or specifically with regard to life on Earth) from a description of the fundamental properties of life (even assuming that we know what they are). It is not true in general that knowledge of the identifying properties of a material thing will reveal how it was produced. As an analogy, descriptions of quartz at the macro-mineralogical level (hardness, crystal habit, etc.) or the molecular level (SiO2) both explain how to identify quartz. Neither, however, explains how quartz is produced under natural conditions. Geochemists have discovered that quartz crystallizes in magma and precipitates in hot springs, and there are possibly other ways in which it forms under conditions very different from those found on Earth. Moreover, there is a fear in any subject that our observations are ‘theory-laden’ and so what appears essential to us is merely essential to the implicit theoretical commitments with which we approach life. The point is a lack of clarity about one’s commitments to the nature of life can lead to theoretical confusions and ambiguities over what needs to be included in a model of the origins of life, and in fact, a commitment to a theory or definition of life may not even be entirely necessary for the sake of making progress in understanding steps in the emergence of life.
Can a well-meaning and well-crafted definition of life cause us to look at life with narrowly blinkered vision? I don’t know the answer, but I should keep the question constantly in mind.
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