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.