Читать книгу The Explosion of Life Forms - Группа авторов - Страница 23

What is life? Do we really know? The way to define life as long as we did not know how it functions was to describe the conglomerate of attributes which characterize it. A similar approach led to describing water according to its properties – like “being wet, transparent, odorless, tasteless, thirst quenching, and a good solvent”, which was a helpful approximation until we found out its H₂O structure, which is the scientifically most informative answer to define water today (Cleland and Chyba 2010, p. 326).

Similar, a (1) physiological approach to define life, long-time popular, describing life according to all of its functions – like eating, metabolizing, excreting, breathing, moving, growing, reproducing, and being responsive to external stimuli – was a first approximation, but is less in use today, as “such properties are either present in machines that nobody is willing to call alive, or absent from organisms that everybody is willing to call alive” (Sagan 2010, p. 303). Automobiles share too many of these properties of the “living” and are certainly not alive. Also, some obviously living bacteria do not breathe at all.

More promising appears to be an approach focused on (2) metabolism, describing “a living system as an object with a definite boundary, continually exchanging some of its materials with its surroundings, but without altering its general properties, at least over some period of time” (Sagan 2010, p. 303). Here as well, however, some known examples of life do not fit into the picture. What about seeds, which can lay dormant quite a while before awakening to revive? A very popular counterexample is also fire, which is a metabolism that is, in addition, even showing growth. Clearly, metabolism alone – while a necessary property of all actual life – cannot be sufficient to describe its nature.

Maybe a (3) biochemical description does better. Physicist Erwin Schrödinger had speculated about an “aperiodic crystal” being an essential ingredient to chromosomes and hence to life (Schrödinger 1992, p. 5). Even more accurate stipulations regarding molecular information storage have been made earlier by Nikolai Koltsov (Soyfer 2001, p. 726). As Watson and Crick did not notice Koltsov, it was Schrödinger’s speculations which made Crick think “that great things were just around the corner” (Crick 1990, p. 18), and as is well-known, Crick’s efforts have been crowned by the discovery of the double-helix structure of the DNA. So, something like Schrödinger’s aperiodic crystal did exist. Today, “reproducible hereditary information coded in nucleic acid molecules” (Sagan 2010, p. 304) is seen as a key ingredient of life, but still, there are exceptions even to this characterization. What about viruses without a DNA of their own? (On this question see Morange 2017, 103f).

A (4) genetic definition of life would be “a system capable of Darwinian evolution by natural selection”; it takes into account the progressive development of life forms. Even without the assumption of an increasing complexity throughout evolutionary history, the evolutionary scenario is appealing. How so? It simply seems to make sense: in the moment self-replicating units emerged on Earth, those units filled their respective environment. Any replication carries with it the possibility for duplication errors, which leads (besides a lot of “waste”) to mutant versions of the original unit. Those mutants replicating more efficiently replicated more quickly. Limited resources provided, this leads to the dominance of particular replicators over others in a given environment, while some mutants were able to make use of new environments. As a result, Earth is soaked through with life, even in extreme environments totally hostile to human life, microbes (called extremophiles) exist.

While the genetic definition seems to stay valid and is in use today, there is a further way of defining life via (5) thermodynamics. Also, Schrödinger had hinted towards something like negative entropy as life’s property (Schrödinger 1992, p. 70). What does this mean? Normally, according to the laws of thermodynamics, no processes can occur that increase the net order of a given system. Structures tend to dissolve over time, entropy is increasing, and our universe develops into a state of disorder, called the heat death of the universe (which is an extremely cold state because matter is so spread out). How, then, can the scale of order show us which life forms it is? The reason for this is that what has been said is only valid for closed systems. Living systems are, in contrast, open systems and create localized regions with an increasing order. The difference between open and closed systems is the open system’s exchange with their environment. As we looked at metabolism as a characteristic even to define it, life certainly has this property and is an open system. Think of sunlight nourishing the plants, for instance. Now, this local increase in order comes at the cost of an overall increase in disorder (entropy). The sun loses energy steadily by fueling the thermonuclear processes necessary to radiate and shine on Earth. This way, the laws of thermodynamics are still valid, while local order is allowed to grow. Life could be described as an open system. There is a counterexample to this thermodynamic definition of life, however, crystals, which are open systems as well, are not considered living entities (Cleland and Chyba 2010, p. 327).

So, what is life? Popular today is a version of the genetic or Darwinian definition, characterizing life as a “self-sustained chemical system capable of undergoing Darwinian evolution“ (Joyce et al. 1994, xi), including the processes of “self-reproduction, material continuity over a historical lineage, genetic variation, and natural selection” (Cleland and Chyba 2010, p. 328). This definition excludes any potential life based on other components than chemistry-based Earth life. This is why the quest to define life has been revived not only in the context of synthetic biology but also in that of astrobiology. The definition also excludes viruses, as they are not self-sustained but need a host to replicate.