This is a part of an ongoing discussion about usefulness of the term ‘life’. See also:
Null hypothesis: the term ‘life’ is not needed in biology – by Spacek
Cleland & Grinspoon response to Benner: Do synthetic microbes provide another example of life?
Why astrobiologists should NOT get rid of the term ‘life’ – by Cleland
Why the term ‘life’ is useless in astrobiology – by Spacek
Cite as: Cleland, C. (2022) “Why the term ‘life’ is important to research in astrobiology”. Primordial Scoop, e20220601. https://doi.org/10.52400/TIZX8856
Suppose someone asks: What natural phenomena can the common noun ‘water’ be used to explain that cannot be explained by reference to theoretical concepts from modern chemistry? The answer is that modern chemistry explains the observable stuff called ‘water’. The alchemists explained this stuff differently: Water is an indivisible element. The alchemists were wrong. The modern chemical concept of water, as a substance primarily (but even in the purest samples, not exclusively) composed of molecules of H2O, now dominates our scientific understanding of the stuff in my water bottle. We can’t, however, rule out discovering that current molecular theory is defective in an important way and hence will someday be substantially revised. Thus, it would be a mistake to jettison the observation-based concept of water in favor of concepts from current chemistry. Doing so is to ignore the direction of explanation, which is from chemical theory to observable phenomena. It is (to exploit an old idiom) to put the cart before the horse.
Scientific theories explain phenomena in the observable world (not vice versa), and sometimes well entrenched theories run into serious empirical challenges—inconsistencies between observation and theory—and are revised or even replaced as a result. As a case in point, consider the replacement of Newton’s theory of gravity by Einstein’s theory of general relativity, which solved the observed deviations of Mercury’s orbit from that predicted by Newton’s theory. Something similar almost happened in chemistry with the discovery of isotopes. Isotopic differences don’t affect the chemical reactivity of most elements in the periodic table. If they did, the periodic table wouldn’t provide a very good systematization of chemical elements for purposes of prediction and explanation and undoubtedly would have been replaced by a different taxonomy of chemical elements, one in which isotopes of the same element (in the old periodic table) were classified as distinct chemical elements. An important exception to the principle that isotopes of an element don’t play a significant role in its chemical reactivity is water. Isotopes of hydrogen do make a difference to the chemistry of water. A good illustration is “heavy water”, D2O, which exhibits different chemical and physical properties from “normal” (protium) water; among other things, drinking it in large quantities is potentially harmful. Fortunately, for the periodic table, however, water is chemically unusual in this respect.
The point is to replace an observation-based concept, such as water or life, with concepts from a current scientific theory about the phenomenon concerned is to assume (in the absence of evidence) that the theory is correct—that it will never be revised in relevant ways or (like Newton’s theory) be replaced. But as the history of science reveals this is unlikely to be the case for any of our current theories and it is impossible to say in advance of acquiring the pertinent evidence which concepts of a scientific theory will be retained and which will be rejected.
In any case, the water example isn’t a good analogy for life because our scientific understanding of life is much more limited than our scientific understanding of water. We don’t yet know whether familiar Earth life is representative or unrepresentative of life considered generally (wherever and whenever it may be found in the universe). There is reason (from synthetic biology) to believe that life-as-we-know-it may be unrepresentative of life considered generally. Biochemists have synthetized nonstandard genomes (using alternative genetic codes as well as nucleobases) and nonstandard proteins (using alternative suites of amino acids). And we don’t know how different genetic, structural, and enzymatic molecules could be from familiar life in environments that are chemically and physically quite different from that of Earth’s, such as perhaps Saturn’s moon Titan or the clouds of Venus. The point is our understanding of life is based on a single example that may well be unrepresentative of life. To eliminate the general concept of life from the scientific vocabulary in favor of using theoretical concepts based on the biochemistry of familiar Earth life would blind us to alternative possibilities for life.
Keeping the term ‘life’ and its associated concept as part of the scientific lexicon allows astrobiologists to ask whether a chemico-physical phenomenon that they observe in an extraterrestrial environment might be a novel form of life. In short, the term ‘life’ is far from useless in astrobiology.
