What does “life” mean? Hold on, now, I’m not talking about the “meaning of life.” We all know that’s forty-two, anyway. No, the question that I’m getting at is how do we know that something is alive? And no, at no point in this discussion beyond this sentence will I raise the topic of abortion.
I ask the question because I watched an episode of The Universe on Netflix last night. For those of you who aren’t familiar with that program, it’s the History Channel’s attempt to present space sciences to the masses. Occasionally, it does let slip some deep thoughts, and the pictures and animations are well done. The episode that I watched was on the subject of astrobiology. As a writer of science fiction, among other kinds of writing, this is important to me.
So here again is the question: What is life? In what follows, I’ll propose a possible definition. See what you think.
1. A living organism must reduce local entropy.
Entropy, or the second law of thermodynamics, is the observation that in a closed system, energy states must run to the lowest possible level over time. Put another way, the flow always runs from organized to disorganized. (See my desk for an example.) In an internal combustion engine, for example, the fuel, which exists at a relatively high state of potential energy, is burned. That does work, but the products of combustion now are at a lower level of potential energy than before. The universe is likely a closed system, but since it started out at a high state of energy, it has a long time to run down. Earth is not a closed system, speaking in the local sense, because the Sun provides lots of new energy. Some life instead uses geothermal energy of volcanic vents on the sea floor.
Now that you see what entropy is, consider my requirement for life. A living organism must use energy from the environment to create something more complex than its surroundings. Yes, cells eat each other, but what they produce–more of themselves–is still at a higher level of organization than minerals, atmospheric gases, and the like.
I raise this qualification to answer whether fire is alive. Yes, fire consumes food and reproduces, but what it leaves behind is less organized–at a higher state of entropy–than what was there before.
2. A living organism must create discrete units.
Is a rock crystal alive? It reproduces, and its structure is often more complex than its surroundings, but a crystal is a repetition of a relatively simple pattern. Cells, by contrast, have discrete boundaries and internal parts. Some parts absorb nutrients; some contain the genetic instructions for building and reproducing the cell, and so forth. It’s interesting to speculate as to whether a crystal could develop in that manner, but for now, what we know of them says that they are mere repetitions of an organized, but simple pattern.
3. A living organism must have the capacity to reproduce.
Is our Sun alive? In some ways, it acts like a living cell, but it can’t make more of itself. By reproduction, I mean the ability to pass on the form of the living organism through a set of instructions that will assemble a new organism that is separate from the parent. Life on Earth does this through DNA. Making more of something through accumulation–such as what happened in the formation of the solar system when small rocky bodies collided and joined to form planets–isn’t reproduction in this sense.
4. A living organism must be self-contained in its functions.
Yes, living things have to have external sources of energy to live. We all must eat. But the functions of building, repairing, and reproducing the organism have to be contained within the organism.
Is an assembly line alive, for example? It takes in parts–call them food sources–and assembles them into something more complex. But an assembly line doesn’t contain all of its functions within itself. It’s made up of the cooperative efforts of smaller units, the workers. There certainly is a level of analogy here, and the definition gets strained when we consider that multicellular organisms are something like assembly lines. I’m not happy with this part of the definition, even though I have the feeling that I know what it means. This item remains to be made rigorous.
I do want this item, though, because we have to consider whether viruses are alive. A virus is a string of RNA that takes over the machinery of a cell to make more viruses. If the virus is alive, it is only alive in a derivative sense.
What this all means:
You may be wondering why I don’t simply say that a living organism has DNA. The answer is that we may come on something that looks alive on another planet that does not use our method of encoding instructions for life functions. Consider again a crystal that is complex enough to have differentiation of parts and that does more than just copy its structure ad infinitum. Can it be alive?
Or what about computer viruses, worms, and the like? When do they reach a level that qualifies them as being alive? There’s a lot of discussion about what is required for machine intelligence, but if computers gain the ability to assemble themselves, won’t they be a kind of life?
I hot that this isn’t going to be like pornography: We may not be able to define life in a rigorous sense, but we’ll know it when we see it. But even loose definitions can work as guides.
Feel free to offer suggestions for improvement in the definition or alternative ways to define the idea.