Debating Life’s Boundaries

Astrobiology Magazine

Summary (Dec 21, 2006): Astrobiology Magazine organized a debate about alien life at the Astrobiology Science Conference last March. In part five of this seven-part series, the panelists toss around the question of how to define “life,” and wonder if we should even bother with definitions at all.

Lynn Rothschild: I’m going to try to get a discussion going on what is “life as we know it.” Can we put any boundaries around this? We’ve had suggestions all the way from lunar rovers and the sun to different solvents, to even the idea that we don’t know enough biology to even pose the question, “What is life?” So who would like to take the first stab at that?

Neville Woolf: I’m going to go back to Schrödinger and say that life has to involve at least one process whereby the free energy of the environment is used to harvest more free energy and that automatically creates certain things that we have associated with life. And those things are that life reaches an ecological limit, and that because of that ecological limit, anything else that comes along is liable to latch onto life to try and find sustenance from it either as a symbiotic form or as a predator.

These kinds of things therefore are absolutely fundamental to the basic thing that Schrödinger brought up, the thermodynamics, and it’s after that that we have our disagreements as to which ways that thermodynamics can be accepted as part of life.

Steve Benner: I’m going to join Carol Cleland and talk about the futility of making definitions. But I don’t quite agree with her in one respect. Because I think that definitions and the attempt to make them are useful because they tell you something about the people who are attempting to make the definitions.

When you say you believe that carbon is essential for life, and when you believe that this rules out silicon as being essential to life, then you tend to come up with the argument that, because silicon is an order of magnitude lower in concentration in the galaxy than carbon, silicon life is not possible. In point of fact, I happen to believe that molybdenum is essential for life, and molybdenum is much more scarce than either silicon or carbon. Boron might also be essential to life, and that is still less abundant. So I happen to think that that’s not a good argument against silicon life.

But because we’re familiar with carbon life, we would tend to define life as requiring carbon. So definitions are useful, not because they understand or deliver information about the underlying concept, but because they help us understand ourselves.

Carol Cleland: We just increased our degree of disagreement, particularly about the scientific method. I’m going to try not to launch into discussion of the scientific method, which is extremely problematic. There’s a mythology about the scientific method having to do with falsificationism and inductivism, and all of it is logically deeply flawed and historically inaccurate. But I won’t go there.

I do want to say something with regard to the idea that you must define ‘life’ in order to run an experiment. I just think that’s not true. People like Steve Benner have been running experiments and engaging in a lot of interesting theoretical thought about the possibility of designing nucleic acids with different nucleotide bases, so I don’t think we need a definition to run experiments. In fact, I think a definition is going to act as a blinker. It could actually guide you along lines that make it impossible to recognize strange forms of life if in fact you encounter them. So I think that is self-defeating.

Everybody has his own basic principles for life. But I think there are some certain rules of chemistry which one should obey. Carbon is just an incredible element which can form three dimensional structures, and there is no other element which can do this. Actually, that’s not completely true -- I read in a paper which Dirk Schulz-Makuch gave me yesterday that silanes can form some kind of large aggregates. But still, that happens only under certain conditions. I think that our carbon-based life is very powerful. The carbon abundance in space argues that this is probably also possible in other environments. I think we should rely on that argument.

Pam Conrad: I think there are two ways you could look at what life is in terms of whether we can agree or not agree. One way is to look at life by what it is, what it’s made out of, what we think its properties are. Another way we can look at life is by what it does, by its function. And the problem with looking at life as a function or as some processes, is that once you define life as some stuff that happens through time, you’ve got to come up with a sampling rate and all sorts of definitions of the length of experiment, and then we’re back to defining things again.

Steve Benner: I agree, but the question that I think we, as well as the public, finds most interesting is whether we have a chance of encountering, in a trek to the stars, life forms that behave like we do, where the fundamental chemistry is based on a polymer of silicon, for example, or living in a solvent other than water.

If you go to the astrobiology web page on silicon life, you’ll find it to be a fountain of misinformation. They tell you, for example, that there are no chiral silicon compounds. Of course there are chiral silicon compounds. There are chains of silicon up to 30 or 35 –- they are very interesting; they are studied by chemists. These compounds carry nitrogen, they carry sulfur. In fact, carbon-based life is not really carbon-based life either, it’s carbon-scaffolded life, with the oxygen, nitrogen, and phosphorus playing absolutely critical roles.

Now, the argument is going to be more subtle. Maybe silicon is not as easy to generate in prebiotic conditions as carbon-based life. I think that’s probably a true statement. Obviously the affinity of silicon for oxygen is going to be a problem when you try to assemble silicon-based life.

Neville Woolf: (holds up his laptop computer) It depends on what kind of life you’re asking for if you want silicon life. The question is whether you insist that life has to be a chemical phenomenon, or whether you’re willing to allow that other physical processes could indeed form life.

I think we can see from the vacuum cleaner that goes around on its own and sweeps where it needs to, that indeed it is possible to have many of these behaviors that we consider to be associated with chemical life also in other principles. And in fact, following the Linnaean system, I’d like to suggest that there’s a higher level than Peter Ward suggested, and that it depends on the principles on which the life is built, therefore they should be called “Principalities.”