nature of life
Inquiries into the origin of life and the possible existence of extraterrestrial life build upon assumptions made about life's very nature. It was once commonly believed that life was fundamentally different from non-life: that living things possessed a "vital principle" which was absent from non-living matter. This notion of "vitalism" began to seem less sustainable following the laboratory synthesis, in 1828, by Wöhler, of an organic substance (urea). Although Henri Bergson and others continued to champion vitalism into the 20th century, the overwhelming consensus emerged that life could ultimately be reduced to a complex series of physical and chemical processes. The debate focused onto what the fundamental unit of life might be. In the mid-19th century, Thomas Huxley had maintained that it was "protoplasm". In the early decades of the 20th century, smaller living entities were proposed, including enzymes, viruses, and genes, though some researchers, including the American biochemist Edmund B. Wilson, argued that only a complete, functioning cell could properly be considered alive.
Today, there is still no agreement among biologists as to a clear definition of life. Several factors are generally reckoned to be involved, including the ability to self-replicate, actively to maintain a boundary between the inner environment of the organism and the outside world (see cell membrane), and to carry out metabolism. To these, some might add the ability to mutate and evolve, or to be part of a community capable of Darwinian evolution. If all of these factors are required to be satisfied for something to be considered alive, then it is questionable, for example, whether viruses are alive. Viruses replicate not by themselves but by hijacking the cellular machinery of some host. Moreover, when dormant, they enter an inert, crystalline, non-metabolizing state.
In addition to the general features we might expect of all life in the cosmos (the ability to metabolize, reproduce, differentiate "self" from the surroundings, and evolve), it may be that life, wherever it occurs, displays more specific features. For instance, it may be that life always utilizes cells; liquid water; the principal elements carbon, hydrogen, oxygen, nitrogen, phosphorus, and sulfur; a common set of organic molecules; a common set of metabolic reactants; and a common cell membrane structure. Some researchers posit that extraterrestrial life, in its biochemical, metabolic, and cellular make-up, will closely resemble the terrestrial variety, even though the gross appearance of life elsewhere might be different from anything we know. On the other hand, there are those who argue that extraterrestrial life might have a fundamentally different basis and therefore be alien, not only in its external aspects but also in its basic constitution.
A further problem in establishing the limits of what may be considered alive is that "life" is a label of our own invention. Try as we might to impose a definition of life on nature, there are bound to be instances where the distinction between living and non-living is blurred or indeterminate. Given that this is true even of terrestrial organisms, we can only guess what difficulties will arise in assessing the credentials of possible life elsewhere. In exploring other worlds, we may encounter life-forms far more different from any terrestrial species than, say, a sponge is from a condor. Some of these alien forms may, in spite of their unfamiliarity, be obviously alive. Yet others, like viruses, will challenge our definitions. Our real problem, however, may not be with simple biological systems on the lower margins of life but with entities that are vastly more evolved than ourselves (see extraterrestrial intelligence, more advanced than us), entirely different in appearance or composition (see extraterrestrial life, variety), or perhaps even of a partially or wholly artificial nature. Regarding this last possibility, there have already been claims that some computer programs are, in a sense, alive (see artificial life). Others researchers have challenged this view, arguing that life must have a corporeal or material basis. In any event, we can envisage a time when genetic engineering, artificial body parts, and direct brain-computer links may lead to an exponential growth of human physical and intellectual capabilities. It is impossible to predict where such developments might lead if they were to continue for thousands or millions of years. Yet such technology-driven evolution may already have happened with other races in the Galaxy. The question then is whether the state to which such races may have evolved could be contained within any conventional notion of life or whether we would be forced to admit that there was another category of existence – one that is as far elevated above human life as we are, for example, above algae.1, 2, 3, 4
1. Margulis, Lynn. What is Life? New York: Simon & Schuster (1995).
2. Miller, J. G. Living Systems. New York: McGraw-Hill (1978).
3. Sagan, Carl. "Life." In Encyclopedia Britannica, 15th ed., Macropedia, vol. 10 (1974), p. 908.
4.Schrödinger, Erwin. What is Life? New York: Anchor Books (1956).