Monday, December 16, 2013

Still Can't Define Life

After years of trying to find a working definition of life, Ferris Jabr, an associate editor at Scientific American, has given up:
No one has ever managed to compile a set of physical properties that unites all living things and excludes everything we label inanimate. There are always exceptions. Most people do not consider crystals to be alive, for example, yet they are highly organized and they grow. Fire, too, consumes energy and gets bigger. In contrast, bacteria, tardigrades and even some crustaceans can enter long periods of dormancy during which they are not growing, metabolizing or changing at all, yet are not technically dead. How do we categorize a single leaf that has fallen from a tree? Most people would agree that, when attached to a tree, a leaf is alive: its many cells work tirelessly to turn sunlight, carbon dioxide and water into food, among other duties. When a leaf detaches from a tree, its cells do not instantly cease their activities. Does it die on the way to the ground; or when it hits the ground; or when all its individual cells finally expire? If you pluck a leaf from a plant and keep its cells nourished and happy inside a lab, is that life?
Among other things, Jabr points out that the definition of life NASA dreamed up for its Mars missions -- a self-sustaining system capable of Darwinian evolution -- excludes viruses and parasites, which are not self-sustaining. For that matter, are we self-sustaining, or do we depend on millions of other living things from potato plants to the bacteriophages that live in our nasal mucous? Every definition ever proposed can be picked apart in this same way. Jabr has decided, he says, that life does not exist:
Truthfully, that which we call life is impossible without and inseparable from what we regard as inanimate. If we could somehow see the underlying reality of our planet—to comprehend its structure on every scale simultaneously, from the microscopic to the macroscopic—we would see the world in innumerable grains of sand, a giant quivering sphere of atoms. Just as one can mold thousands of practically identical grains of sand on a beach into castles, mermaids or whatever one can imagine, the innumerable atoms that make up everything on the planet continually congregate and disassemble themselves, creating a ceaselessly shifting kaleidoscope of matter. Some of those flocks of particles would be what we have named mountains, oceans and clouds; others trees, fish and birds. Some would be relatively inert; others would be changing at inconceivable speed in bafflingly complex ways.
I think this is getting carried away. Things can exist without our being able to define them in a sentence. It may be true that life is a continuum of organizational complexity rising from RNA molecules to human cities, rather than a yes-no question. But when a human-sized, self-mobile mass of water and protein suddenly falls several big steps down the ladder and can no longer move, eat, speak, breathe, or philosophize, something dramatic and important has still happened.

No comments: