The Definition of Life
|October 18, 2006||Posted by Patrick under Biology, Comp. Sci. / Eng., Darwinism, Evolution, Science, Self-Org. Theory|
The opening discussion:
To decide whether life has a common chemical plan, we must decide what life is. A panel assembled by NASA in 1994 was one of many groups to ponder this question. The panel defined life as a Ã¢â‚¬Ëœchemical system capable of Darwinian evolutionÃ¢â‚¬â„¢ . This definition, which follows an earlier definition by Sagan , will be used here. This definition contrasts with many others that have been proposed, and avoids many of their pitfalls. For example, some definitions of life confuse Ã¢â‚¬ËœlifeÃ¢â‚¬â„¢ with the concept of being Ã¢â‚¬ËœaliveÃ¢â‚¬â„¢. Thus, asking if an entity can move, eat, metabolize or reproduce might ask whether it is alive. But an individual male rabbit (for example) that is alive cannot (alone) support Darwinian evolution, and therefore is not Ã¢â‚¬ËœlifeÃ¢â‚¬â„¢ . Further, many efforts to define Ã¢â‚¬ËœlifeÃ¢â‚¬â„¢ fall afoul of the fact that no non-trivial term can be defined to philosophical completeness . The general difficulty of defining terms, theoretically or operationally, was one of the discoveries of 20th century philosophy, and is not unique to the definition of Ã¢â‚¬ËœlifeÃ¢â‚¬â„¢. It is impossible, for example, to define Ã¢â‚¬ËœwaterÃ¢â‚¬â„¢ in a complete way. We can say that water is Ã¢â‚¬Ëœdihydrogen oxideÃ¢â‚¬â„¢, but are we speaking of a water molecule, water as a substance, or water operationally? And what is Ã¢â‚¬ËœhydrogenÃ¢â‚¬â„¢? Any effort to deal with the definition of life at this level encounters analogous questions, which can easily be paralyzing. The hrase Ã¢â‚¬ËœDarwinian evolutionÃ¢â‚¬â„¢ carries baggage from 150 years of discussion and elaboration. It makes specific reference to a process that involves a molecular system (DNA on Earth) that is replicated imperfectly, where the imperfections are themselves heritable. Therefore, Darwinian evolution implies more than reproduction, a trait that ranks high in many definitions of life. The panelÃ¢â‚¬â„¢s definition also avoids confusion from many non-living systems that reproduce themselves. For example, a crystal of sodium chlorate can be powdered and used to seed the growth of other crystals ; the crystal thereby reproduces. Features of the crystal, such as its chirality, can be passed to descendants . The replication is imperfect; a real crystal of sodium chlorate contains defects. To specify all of the defects would require enormous information, easily the amount of information in the human genome. But the information in these defects is not itself heritable. Therefore, the crystal of sodium chlorate cannot support Darwinian evolution. Therefore, a sodium chlorate system is not life .]. The NASA panelÃ¢â‚¬â„¢s definition of life is interesting for other reasons. First, it provides information about what forms of life were believed to be possible, not just conceivable. As fans of Star Trek know, forms of life that are not chemical systems capable of Darwinian evolution are easily conceivable. Besides aliens resembling Hollywood actors with prostheses, the Enterprise has encountered conceptual aliens that do not fit the panelÃ¢â‚¬â„¢s definition. The nanites that infected the Enterprise computer in Episode 50 of Star Trek: The Next Generation (Ã¢â‚¬ËœEvolutionÃ¢â‚¬â„¢) are informational in essence; their Darwinian evolution is not tied to an informational molecule, like DNA (although they require a chemical matrix to survive). The Crystalline Entity of Episodes 18 (Ã¢â‚¬ËœHome SoilÃ¢â‚¬â„¢) and 104 (Ã¢â‚¬ËœSilicon AvatarÃ¢â‚¬â„¢) appears to be chemical, but not obviously Darwinian. The Calamarain (Episode 51: Ã¢â‚¬ËœDeÃ‚Â´ ja` QÃ¢â‚¬â„¢) are a conceptual life form that is surely energy, not evidently requiring matter. And Q (Episode 1: Ã¢â‚¬ËœEncounter at FarpointÃ¢â‚¬â„¢) appears to be neither matter nor energy, flitting instead in and out of the Continuum without the apparent need of either. If we were to encounter any of these other conjectural entities during a real, not conceptual, trek through the stars, we would be forced to concede that they represent living systems. We would be obligated to change our definition of life. We do not change it now simply because we do not believe that the weirder life conceived in the Star Trek scripting room is possible outside of that room. Nanites and the fictional android Data are examples of artificial life. We do not doubt that androids can be created, including androids who (note the pronoun) desire to be human. We do not, however, believe that Data could have arisen spontaneously, without a creator that had already emerged by Darwinian process (as is indeed the case with Data). Hence, we might regard Data as a biosignature, or even agree that he is alive, even if we do not regard him as a form of life. For the same reason, a computer holding nanites would be evidence that a life form existed to create it; the computer is a biosignature, and the nanites are an artificial life form, something requiring natural life to emerge.
Dr. Benner then continues with a discussion on the requirements for life.
Maybe I’m just a trekker but I have to grin like a cheschire cat whenever I see scientists reference Star Trek.