3/7/2008 15:51

Life, Actually

We tend to take far too mechanistic a view of life. It must be something about our human nature, or else it is deeply embedded in our cultural history.

Back in the 1600s and 1700s, more or less the dawn of both physics and mechanical industry, the whole universe as well as life was frequently spoken of in terms of machines: Life as a clock, for example, was a metaphor used by John Amos Comenius and by Deists in America.

One of the flaws of the mechanical view of life is that it leaves little room to recognize change and growth, which are central to our actual experience of being alive.

Today, at the dawn of molecular biology, the dominant metaphor of life seems to be the computer program. One might have expected me to name our time "the dawn of the information age", especially in view of the programming metaphor, but the metaphor is drawn out from an analogy between computer programs and the genetic information encoded in deoxyribonucleic acid, or DNA. It is not uncommon to speak of DNA as a "program" for a human being.

The program metaphor is useful in some contexts but, like the clock metaphor, should not be pressed too far. Even an actual computer program is not self-defining. The computer program is nothing but an ordered series of numbers; you need to have a receptive environment to transform those numbers into any practical actuality. For computer programs, that environment includes the processor chip, operating system, power supply, and all the electronic interconnections among them. For a living cell, there must be functional proteins, mitochondria, and the cellular organization provided by lipid membranes and a molecular cytoskeleton. Otherwise, nothing happens; the program doesn't execute.

Following the metaphor of the program of life can lead to defining a life, in particular a human life, by the unique program code - the inherited DNA. This extension of the idea is clearly false, of course; one need only to think of identical triplets who have one genetic program but three lives. (There are many reasons why the DNA does not define a unique life,even without leaving the mechanistic view. We know that there effects of environment, which must be at least slightly different even among identical triplets. Molecular silencing of certain genes can, cell by cell and with approximate randomness, turn off either a father's or a mother's gene.)

Many people accept a more subtle extension of the programming metaphor for life. This is the idea that once the program is "written" a life is immediately begun. For diploid sexual organisms like us, that means the moment when an egg is fertilized. This has a very reasonable appeal, as metaphor. As a literal statement of actual reality it is just as flawed as limiting human life to the content of the DNA program.

For example, not every combination of DNA strands is a valid program for a human life. Some possible sequences of nucleic acids encode a "program" which is inconsistent with a living being. This doesn't even take us outside the programming metaphor, since it is equally true that not every sequence of numbers forms an executable computer program. But if the DNA of the fertilized egg is not a "program" for a human life, could life exist when that sequence was finalized at fertilization? One could arrive at a tortuous definition of life that would make this true, but the more parsimonious approach is to simply answer "No".

I don't want to overemphasize the example the special case of a completely nonfunctional genome. What I want to point out is that a DNA "program" by itself is not a sufficient definition of a life. There is something else in addition to the DNA which is necessary for a life to exist. At the least, there must be some external definition of what constitutes a valid program. Conceptually, this is the key point: For the genetic data to define a human life, there must also be something beyond the DNA.

My phrase, something else, is deliberately vague. What could this something else be? Must it be only one other thing? Of course not. This something else certainly includes the realities of molecular themodynamics; the DNA molecule must be conformable to transcription into RNA and (subsequently) into proteins; the chemistry of the cell must be able to turn genes on and off appropriately to sustain life. This something else must include some sort of higher semantic structure, though the possible nature of that higher structure could range from a rational design to a purely emergent property of the physical and chemical nature of the cell.

And, of course, there could be a soul. Surely there remains opportunity to write another essay.