What constitutes life?
From my 10th grade Biology class I remember that Life has three criteria.
1) Ability to reproduce.
2) Reacts to Stimuli
3) Ingest Nutrients for sustenance
If it reproduces, reacts to its environment and eats to sustain itself and grow then it is life. This can be very, very minimal. In the case of a virus, the definition of life may be stretched a little beyond its limit. A virus can reproduce, but it is non-reactive and does not have a metabolism. It is more like a computer program in that it is a piece of genetic code capable of invading a host cell and taking over the mechanics for its own reproduction.
A computer program that replicates itself (as does a computer virus) might have to be considered life if you consider a virus life.
The next rule – “reacts to stimuli”; is a little trickier. There are chemicals that change when stimulated which are obviously not life. This reaction is more a mechanical thing that is a result of the actual structure of the living thing. A plant moving towards sunlight (phototropism) or a microbe moving away from an area of low pH, is what the definition is referring to. In the case of a plant, it is the result of a chain of chemical reactions which cause the plant to grow more on the dark side and thereby bending the stem. In the case of a microbe it is a chain of chemical reactions which cause the cilia to move faster on the low pH side than the high pH side. These are simple mechanistic reactions which can easily be simulated with a computer. Inert matter sometimes has similar reactions to stimuli.
In the case of higher animals the reaction to stimuli is chemical, but it is filtered through a neural net that can result in a variety of reactions depending on the combinations of stimuli factors. In the extreme case many higher animals seem to make decisions – conscious decisions we might even say.
My personal opinion is that this rule is a “brain-centric”; chauvinistic interpretation of the behavior of living things. The plant and the microbe are certainly reaction to stimulation, but not in the sense that a more complicated creature is. We are attributing behavior to things that have evolved a simple chemical reaction. The reactions may benefit the organism (detrimental ones would have caused extinction), but they are not as reactive as something as simple as a flatworm. We have to be careful not to attribute brain-like reactions to reactions, although having the appearance of being complicated, are not the result of real choices. Our own viewpoint tends to explain simple reactions as being more complex than they actually are.
I think that that life might have to be sub-classed using this second criterion into a non-reactive, simple reactive, compound reactive and complex reactive. The non-reactive would have to be viral, (biological and digital). The simple reactions would be non-neural chemical reactions. Compound reactive would be the reflexive behavior of simple neural nets, and would include only simple learning and no real memory of events. Complex reactance would be the instances where there appears to be an element of choice as would appear in vertebrates. Complex behavior would include memory of events and the recognition of situations and scenarios.
The next criterion is the one about eating and growing. Are we saying that for something to be alive that it must be self sustaining and self repairing? Perhaps we have too much of a previous conception of earth life. The cell based life that we know is a little chemical factory using sugar and oxygen to produce co2, water and energy. Plants have a neat trick of turning sunlight, co2 and water into sugar which it uses to get energy.
The energy from this process is used to power muscles, the creation of complex sugars, starches and proteins as well as hormones. All biological life (except virus which steel their food from the host) utilize this process to fuel for themselves. Since we observe this in all life, we make it a requirement of life.
In addition to sustaining life by burning sugar, life is required to sustain itself. A life form must be grow and be self repairing. Most life forms progress through a process from a birth of some kind to eventual death. In the meantime, minor (and sometimes major) repairs are made to damage done by the environment.
What is life in a Science Fiction Story?
Now there is an interesting question. Let’s examine the three criteria.
Reproduction: Does a life form have to reproduce? There are a couple of instances when this is not a requirement for life. These are 1) Artificial life and 2) Singletons.
A fabricated life form may not have the ability to reproduce itself. If a life form is designed and built, it may not have included in its design any reproductive capability. A sterile or celibate creature may not reproduce and yet it is surely alive. If we extend that to a species or class of life then rule one is not a true criterion.
A singleton is a life form that comes about spontaneously. This is probably very rare. There is probably a built in bias toward a molecule that can consume something and reproduce itself. The first pre-life on earth was probably a reproducing molecule. The nature of the molecule and the chemical reaction changed as time went on, producing more and more complex results. As the raw material that fueled the reactions began to run out, more complicated reactions took over that used other chemicals for fuel or consumed the end products of other chemical reactions. Eventually a sack was formed around these little chemical reaction chains and one of them came up with RNA and the rest is pre-history.
But consider a cosmic accident that makes a chemical reaction that can consume and grow and even evolve without reproducing.
What if the primeval soup had created a long polymer that could burn sugar along its side and add complex byproducts to the ends of its branching structure? What if lives on some planets were not cell based, but fiber based? A fiber might break in half from time to time, but that might not be real reproduction.
Consider a planet where there is a silicon crystal that is able to grow as a crystal grows, but is so complex that it can direct its own growth.
Consider a sun where the magnetic eddies become complex enough to sustain themselves and expand in complex and yet self sustaining ways.
Does life have to react to stimuli?
If life is non-reactive is it even interesting? A life form which is totally oblivious to its environment is not quite possible. If it can’t squeeze energy from its environment it will stop and maybe die. All life forms must be able to sustain environmental damage of some sort. I think you can call damage to the life form a stimuli and death might be a reaction.
A creature that has no built in reflexive action is quite possible. How could you influence or communicate with a creature that has no interaction with its environment except to consume energy?
As far as Science Fiction story goes, it would hard to have such a creature be a character, but they could easily be a force. In “Nitrogen Fix”; by Hal Clement, an artificial life form starts a chemical reaction that fixes all of the oxygen in the air to the nitrogen, making the air into nitrites, nitrates and co2 and all oxygen breathers die. The artificial microbe never plays any other role in the book other than to set up the situation that men must deal with. The life form is a simple reproducing chemical reaction. It is so simple that all it does is reproduce and fix nitrogen.
Is ingestion and growth a true requirement of life?
I think of life as a counter to entropy. Entropy is the tendency of all things to slide down a hill from high energy to low energy. Life is the localized reversal of entropy. Life organizes itself and controls the environment. It directs entropy through itself and takes advantage of energy potentials to improve itself. Any life form which does not ingest some form of energy will cool off and die.
Life must involve some sort of energy consumption to continue for more than a very limited time. It may be possible for a life form to be “charged”; once at its creation and spend the rest of its life running down and using itself up. The EverReady bunny consumes batteries, but what if the battery was built into the structure of the bunny?
What about growth and self repair? I doubt if either of these is a requirement of life. We observe on earth that life tends to grow and mostly can heal itself in the case of limited damage. These are neat tricks that make life successful, but are by no means a requirement of life.
So what could alien life be like?
Life that we encounter will probably be chemical based and might even be carbon based with a Krebs Cycle to turn sugar into energy. The most common elements in the universe are the light elements starting with Hydrogen, Oxygen and Carbon. (Helium is not chemically reactive). It is likely that these will be the stuff of life with a few spicy elements like Sulfur, Nitrogen, Sodium and Potassium thrown in. The problem with silicon life is that Carbon seems to be so much better for life. Carbon and Silicon are chemically similar in the way that they join with oxygen and hydrogen to form complex molecules. Wherever you have Silicon, you will probably have Carbon and Carbon is much more reactive than Silicon. Carbon compounds dissolve in water. (Compare sand to sugar). I doubt if there are any Silicon based life forms out there.
Chemical life will probably be Carbon based because carbon has the ability to join to 4 other atoms to form very complex molecules. This chemical complexity is probably a requirement of life. Carbon compounds are often soluble in liquid water which makes planets with liquid water a good place to look for life.
What about cold or hot planets? Cold planets such as gas giants could use liquid methane or ammonia as a medium for life. The energy cycles could be built up around energy transfers back and forth between states of ammonia. I think that Jupiter’s red spot is an area where a Jovian bacterium is fixing nitrogen to make crystals of Nitric Acid which would appear red. Entropy would have it that the atmosphere of Jupiter would be mixed up by winds to the point where it would be very homogenous, but then why is there a big red spot on the planet?
Hot planets would have trouble with many chemical reactions. The heat would make most molecules unstable or cause the lighter ones to boil off. Maybe liquid tin or lead could provide a solvent to a silicon or a carbon based chemical reaction. Highly unlikely, but there are a lot of planets out there, enough to say the impossible doesn’t mean anything.
So here are some thoughts science fiction and life.
I like the idea of singletons. These could be produced by artificial means or freaks of accident. I think that the tar creature in the STNG episode that kills Ensign Yar is a singleton, possibly of the fiber life variety.
I don’t think that there is an environment that could not give rise to life. Of course many environments are so unlikely to create a life form that it would be virtually impossible. On the other hand, fiction is the art of making impossible coincidences appear possible.
Artificial life tends to break all the rules. If you can conceive of it, it is likely that someone will build it someday.
Life on earth is strange. We live among monsters. Life from other planets will be much stranger. If we ever get to the stars, we will meet monsters that make spiders and snakes, lobsters and slime molds appear nice and familiar friends.