Life is amazing. It is all around us in a diversity of forms,ranging from microscopic bacteria to ancient towering trees, from almost inert lichen to transient insect blooms, from birds flocking in the sky to thriving colonies of tube worms at inky deep-sea vents. The first forms of life on earth spontaneously arose out of a preexisting prebiotic chemical soup. From those simple origins has evolved a diverse hierarchy of forms of life, which includes the most complex objects in the known universe. Individual living entities (organisms) maintain their self-identity and their self- organization while continually exchanging materials and energy and information with their local environment. Different species of life flexibly and tenaciously exploit various niches in the environment. When viewed on a long enough time scale, life forms are always changing, adjusting, producing novel responses to unpredictable contingencies, adapting and evolving through blindly opportunistic natural selection. Not all the diversity and complexity and change in life is adaptive, of course. Random drift, architectural constraints and other non-adaptive factors have their influence. But what is especially distinctive and striking about life in the long run is the supple, open-ended evolutionary process that perpetually produces novel adaptations. In fact, I will contend in this paper that supple adaptation defines life at its most general. There are plenty of puzzles about the concept of life. The concrete objects ready to hand are usually easily classified as living or non-living. Fish and ants are alive while candles, crystals and clouds are not. Yet many things are genuinely puzzling to classify as living or not. Viruses are one borderline case, biochemical soups of evolving RNA strings in molecular genetics laboratories are another. The Gaia hypothesis (Lovelock 1988), according to which the entire chemical and biological environment around the surface of the earth (including things like the oceans and the atmosphere) constitute one living organism, also strains the ordinary concept of life. So does the search for extraterrestrial life. Extraterrestrial life forms, if any exist, might well not depend on DNA-encoded information or, indeed, any familiar carbon chemistry processes. How would we recognize extraterrestrial life if we found it? We have no reason to suppose it will have any of the accidental characteristics found in familiar forms of life. What, then, are the essential properties possessed by all possible forms of life? The search for extraterrestrial life needs some answer to this question, for we can search for life only if we have a prior conception of what life is. The phenomena of life raise a variety of subtle and controversial questions. Borderline cases like viruses raise the general issue of whether life is a black-or-white property, as it seems at first blush, or whether it comes in shades of gray. Early life forms somehow originated from pre-biotic chemical soup. Does this imply that there is an ineliminable continuum of things being more or less alive, as many suppose (e.g., Cairns-Smith 1985, Küppers 1985, Bagley and Farmer 1992, Emmeche 1994, Dennett 1995)? Another subtle question concerns the different levels of living phenomena&endash;such as cells, organs, organisms, ecosystems&endash;and asks in what senses (if any) the concept of life applies at these various levels. Mayr (1982) seems to be especially sensitive to this question, although he has no ready answer. Recently a third question has been receiving lots of attention (e.g., Langton 1989a , Emmeche 1992): Does the essence of life concern matter or form? On the one hand, certain distinctive carbon-based macromolecules play a crucial role in the vital processes of all known living entities; on the other hand, life seems to be more in the nature of a process than a kind of substance. The relationship between life and mind raises a fourth question. When we consider plants, bacteria, insects, and mammals, for example, we apparently find different kinds of mental activity, and it seems that different degrees of behavioral sophistication correspond to different levels of intelligence. Might the various forms of life and mind be somehow connected? To answer questions like these and make sense of the puzzling phenomena of life, we need a sound and compelling grasp of the nature of life. Can any property embrace and unify not only life's existing diversity but also all its possible forms? What is the philosophically and scientifically most plausible way to account for the characteristic life-like features of this striking diversity of phenomena? How can we resolve the controversies about life? The concept of life as supple adaptation, explained below, is my attempt to address these issues. Notice that our ordinary, everyday concept of life does not settle what the true nature of life is. Thus, we are not concerned here with careful delineation of the paradigms and stereotypes that we commonly associate with life. We want to know what life is , not what people think life is. Glass does not fall under the everyday concept of a liquid, even though chemists tell us that glass really is a liquid. Likewise, we should not object if the true nature of life happens to have some initially counterintuitive consequences.
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