Turned up to eleven: Fair and Balanced

Thursday, May 22, 2003

What does it mean to be alive?

So, here are some thoughts that I find interesting, but I am not sure they fit in with my book. Oh, well, here goes...How do we decide what is alive and what isn't? Of course, at the margins, it is very easy. I am alive, a rock is not. A tree is alive, a glass of water is not. But it can't be that we just look at things, and on a case by case basis, call them "alive" and "not alive", we need some rules (or at least, heuristics). I know that many rules have been proffered, and I will recapitulate some of the best ones. A living thing must eat, excrete, and reproduce. That is a pretty good one, but there are some problems. For example, it is not clear that all living things excrete. Some bacteria that live on minerals as their supply of energy (lithotrophs) are not clearly "excreting" anything, although minerals pass through them, and come out in altered forms. Eating is pretty good, too, but problems arise when you try and define it. For example, when water crystallizes to form ice, is the gowing ice crystal "eating" the liquid water? If you say no, and I expect you will, why not? If you accept this as eating, then you can accept melting as "excretion" it seems to me. So all that keeps ice from being alive is the inability to reproduce; but of course, if you put an ice cube in warm water and it cracks, you have two (at least) ice cubes, so it seems that we have discovered a new form of life!! Incidentally, lots of polymers can form spontaneously, with potentially substantial heterogeneity, and display all of these characteristics of life. Crystals, in particular, occupy an important nether region between life and non-life, as do, in a more complicated way, viruses.

My point here, and I could go on with more and more examples, is that categorizing living and non-living things is very difficult in some cases, and there seems, rather than very distinct categories, to be a sort of broad continuum from rocks, elements, and simple chemicals on one end, to people and trees (and reptiles, and dogs, etc.) on the other, with everything else sort spread out in the middle. Most people draw the line, more or less, just to one side or the other of viruses. What is interesting, however, is that for the most part, we don't have terrifically good reasons for this line. We have pretty good reasons that mark, for example, bacteria as alive, but we don't have very good reasons why a micelle, a liposome, a prion, or a virion isn't. We can't use genetic material (viruses have them) or reproduction (prions reproduce) or excretion and eating (materials can pass through lipid layers, and be incorporated into the lipids themselves, or undergo chemical reaction inside).

Interestingly, this parallels the best theories for the origins of life, and suggests that we may never pinpoint a "most distant ancestor" because we won't be able to agree on the precise change that made it alive. The best guess is that a lipid bilayer encompassed some elements of nucleic acid (RNA, probably with enzymatic activity), and this assembly was somehow able to incorporate the necessary chemical reactions to allow it to reproduce. It is not clear, however, when in this path it goes from being "not alive" to being "alive". Of course, once we did it, there was no turning back.