Facts and puzzles about the phenomena of life.

Life is wonderful. It is all around us in a variety of forms, ranging from microscopic bacteria to very old towering trees, from almost inert lichen to transient insect blooms, from birds flocking in the sky to thriving colonies of tube worms at inky ocean 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. person living entities (organisms) maintain their self-identity and their self-organization while continually exchanging materials and energy and information with their restricted 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. hit and miss drift, architectural constraints and other non-adaptive factors have their influence. But what is particularly distinctive and striking about life in the long run is the supple, open-ended evolutionary method that continually 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 simply 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 average case, biochemical soups of evolving RNA strings in molecular genetics laboratories are another. 

The phenomena of life raise a variety of subtle and divisive 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 range 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 delicate 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 material.