The Definition of Life: Part 1
"Swamp Girl of Venus" was an "exiting adventure on another planet" when it appeared in the September 1949 issue of Amazing Stories. We no longer see stories in that setting because we have learned that those swamps can't exist. On the other hand, stories set onboard starships traveling at warp speed are everyday fare because we can still speculate on how cosmological inflationary theory could be applied to interstellar craft.
A story in which we communicate with crystalline beings who live on Jupiter's core seems to be perfectly legitimate. After all, even though we are familiar with life-as-we-know-it, we have no way to predict the properties of life-as-we-don't-know-it. We might not even recognize it as life. However, I would dispute those statements. We do know what life is. Unfortunately, attempts to define life are often muddied by attempts to combine the definition of life with its technology. We know what life is, but we do not know what technologies exist other than those found here on Earth. Creation of fictional life forms is a proper thing for writers to do, but we should remain within the boundaries that the definition of life sets for us.
Over the years, I have seen many descriptions (I hesitate to call them definitions) of life. None has succeeded at being a true definition. Many of them failed because they included the "technology" of life, i.e., composition and behavior--things like growth, reproduction, organic chemicals. If life must reproduce, what about mules? If it must grow, am I still alive, since I've stopped growing? If life must contain organic compounds in water, could there be silicon-based fish swimming in liquid ammonia on some other planet? Not only might we ask if androids dream of electric sheep, we could ask, "Are androids alive?"
Life can be defined: Life is a persistent, homeostatic, nonspontaneous system.
A system is an ordered set of interacting or interdependent processes forming an integrated whole. We use the adjectives alive or living to describe a system which fits the above definition of life. A living system is an organism.
For terrestrial organisms, the processes are chemical reactions and systems of chemical reactions. Photosynthesis is an ordered group of chemical reactions which interact to convert carbon dioxide and water into oxygen and glucose. Photosynthesis is a component system found in some organisms. The conversion of hydrogen and oxygen to water is not a system since it involves a single chemical change.
A persistent system tends to continue once it has come into existence. Specific living systems may persist over widely varying time spans, depending on the organism and its environment. Organisms like bacteria have no known life span.
A homeostatic system resists changes which would prevent the system from persisting. If you lose water, which could disrupt your living processes (you could die), then you get a drink. If you get too warm, you cool yourself by perspiring or moving into a cooler environment.
According to the Second Law of Thermodynamics, a nonspontaneous process occurs because it absorbs energy. The photosynthetic conversion of carbon dioxide and water to oxygen and glucose will not occur unless energy is supplied to the system. The decomposition of water to hydrogen and oxygen will not occur unless energy is added, as in the electrolysis of water. Photosynthesis and electrolysis are nonspontaneous processes. The conversion of hydrogen and oxygen to water releases energy and is referred to as a spontaneous change.
Note that all changes are reversible: hydrogen and oxygen can react to form water (spontaneous direction), and water can decompose to form hydrogen and oxygen (nonspontaneous direction.) Whether the direction of change is spontaneous or nonspontaneous depends on whether energy is released or absorbed. The reversibility of changes has important consequences for living systems because of the Second Law of Thermodynamics.
Living systems, consistent with the Second Law of Thermodynamics, use energy to cause nonspontaneous changes. A consequence of nonspontaneous changes is an increase in order in their part of the universe: The atoms which make up a tree are much more ordered than when they were scattered randomly throughout their environment as the carbon dioxide, water, and inorganic ions from which the plant was made. However, should the living system be disrupted or the input of energy discontinued, then the reversible, spontaneous processes would prevail and the living system would decay back into disorder.
If "Life" can be defined in just seven words, why hasn't an accepted definition been available before? I'll look at this question in Part 2 of this series.
The Definition of Life: Part 2
Life is a persistent, homeostatic, nonspontaneous system.
If "Life" can be defined in just seven words, why hasn't an accepted definition been available before?
A definition is a statement of the limits which allow us to tell one thing from another. Some concepts/things have rigorous definitions (a circle is a line for which all its points are the same distance from its center), some things have arbitrary definitions (day and night can only be defined arbitrarily), some things have no definition (like biological species or pornography--as Supreme Court Justice Potter Stewart said, we know them when we see them.)
Because definitions are made up of words which themselves have definitions made up of words, we run into the danger of potential infinite regress or self-reference (circular definitions.) Not everyone will agree with some specific definition because different people use the same words differently--they don't use the same definitions. For example, if a tree falls in the forest and no one hears it, is there a sound? Your answer depends on your definition of "sound."
For some people, a certain definition might threaten a person's economic or mental well being. Some people have a vested interest in one definition over another--or in saying that there is no definition.
With the definition of life, for instance, some people will want to define life in a way that will allow them to declare that they are the first to create it in a lab or are the first to find it on another world. Alternately, suppose Researcher A is someone who is competing in research leading to the creation of life in the laboratory. He may insist that there is no definition of life so that, if Researcher B claims to have created life, Researcher A can say that B's work was interesting or useful but does not mean that the Researcher A has lost the race.
Unlike biologists, exobiologists, biochemists, etc., I have no vested interest in a definition of life. I'm not working in the field. I expect no social immortality from creating life in the laboratory or from finding it on some other world. I do, however, have a PhD in Biochemistry. My research was done at the University of Miami's Institute for Molecular Evolution under Dr. Sidney W. Fox, a man very much interested in the origin (or origins) of life. Because of this background and because of my interest in science fiction, I am interested in a definition of life. I think that anyone working in exobiology should also be interested. If you don't know what you're looking for, how will you know if you've found it or, more importantly, how to look for it?
Any definition of life is going to be problematic. Over the years, I've seen a lot of attempts to define life, as well as attempts to ignore the need for a definition.
Sometimes the problem is because of the scientific competition mentioned above.
Part of the problem is a family of words which impinges on the concept of life. These are words like alive/dead, living/dying, or brain-dead. In some contexts, these words become very emotionally charged. Some have legal (or need legal) definitions quite apart from the definition of "life." I am not even going to try to get into this mess.
Often the problem is because of confusion between the technology of life and the definition of life. You wouldn't include in the definition of a hammer the requirement that it have an iron or steel head and a wooden handle (the technology of some hammers). A hammer is a lever with some firm material attached at a right angle to one end and made for the purpose of pounding. Including iron or steel (the technology) in the definition would eliminate some tools which are hammers, but which, for example, have plastic or hard rubber heads. Attempts have been made to include such things as carbon compounds, nucleic acids, phosphate, or proteins (the technology) into the definition of life, but this limits us to an attempt to define life-as-we-know-it.
The discovery that NASA scientists could select for strains of bacteria which could incorporate some arsenic into their DNA in place of some of its phosphorus led someone to conclude that we had to change the definition of life. That made about as much sense as saying that we'd have to change the definition of hammer if we found a tool for pounding that had a plastic or hard rubber head.
Aside from establishing bragging rights when someone finally creates it in the laboratory or finds it on another world, what can we do with a definition of life? I'll look at that question in Part 3 of this series.
The Definition of Life - Part 3
Life is a persistent, homeostatic, nonspontaneous system.
Aside from establishing bragging rights when someone finally creates it in the laboratory or finds it on another world, what can we do with a definition of life? Well, it lets us answer questions like "Are blank alive?" You just fill in the blank. For instance,...
Are viruses alive? The answer: No. Viruses are not systems. They are no more alive than hemoglobin or ribosomes. They may become part of a living system, like hemoglobin and ribosomes, but they are not alive. They may have some components which can damage or modify living systems, but they are not alive.
Are androids, such as Star Trek's Commander Data or Star Wars' C3PO, alive?
Right now, Commander Data, C3PO and similar computer devices are in the same category as warp drives and subspace travel--they are pure science fiction. We can't have the faintest inkling as to whether they are alive or not because not enough information exists about the mechanism responsible for their life-like behaviors to allow us to draw any conclusions. However, I would like to make a prediction: If we "pull the plug" on Commander Data, the same thing will happen to him as will happen to your computer or refrigerator. Nothing. All the processes going on in Commander Data will just cease until you plug him in again. As the Second Law of Thermodynamics takes over, you will see no spontaneous reversal of the processes which have made him life-like. Even without the actions of bacteria and molds, you and I will end up as a smelly, disgusting pile of chemicals soon after our plugs are pulled. But Data's positronic components will just wait to power up when someone switches him back on. Androids are not alive.
"Wait," someone objects. "Exemptions to the Second Law of Thermodynamics do not exist. An android will be subject to the same Second Law decay as you and I. Metals will oxidize. Sensitive components will be damaged by internal decay of radioisotopes or from cosmic rays. It may take a while, but an android will eventually be reduced to a pile of junk." True, but the android is not experiencing "the same Second Law decay as you and I." Living systems will decay because, without an input of energy, their nonspontaneous changes will cease and spontaneous changes--going in the reverse direction--will take over. The changes described in the objection have nothing to do with spontaneous reversal of the changes that made the android seem to be alive. The nonspontaneous changes found in an android would be completely different than those found in a living organism. No, Commander Data will probably be waiting for us to switch him back on at The Restaurant at the End of the Universe.
Let's just assume, for the sake of argument, that something like Commander Data will exist some day. If he's not alive, what is he? After all, he can do many of the things that some living organisms do. Let me ask a parallel question: Is a brick a hammer? After all, it can be used to do some of the things that hammers can do. But a brick does not fit the definition of a hammer. Regardless of how hammer-like we might use it, a brick is not a hammer. So what is it? Because it might be considered hammer-like, why not just say that it's a hammeroid? (No snickering, please.) Just like calling human-like aliens humanoid, calling a brick a hammeroid simply recognizes that there can be tools with functions similar to hammers, but which are still not hammers.
The same could be said for androids, if we should ever find that such things can exist. Androids could certainly be life-like. We might say they are--oh, I hesitate to coin the word--lifeoid. I'm sure that someone can come up with a better word, or, perhaps, we should stick to "android."
How about those silicon-based fish swimming in liquid ammonia on some other planet? Or maybe we'll find them schooling in the methane lakes of Titan. Are they alive? Yes. They are fighting against the Second Law's spontaneous decay in much the same way as a goldfish. But if NASA scientists plan on finding them, like they tried on Mars, by watching for the release of carbon dioxide, they'll never succeed. They're confusing life with its technology.
Personally, I have a hunch that we will never find life which is not a persistent, homeostatic, nonspontaneous system of chemical reactions. "System of chemical reactions" has been a part of my definition of life since I first came up with it over forty years ago. This may be a provincial attitude based on lack of experience with anything beyond Earth. It wasn't until I started writing and rewriting this article that I concluded that perhaps chemical reactions were part of the technology available to living things, not part of the nature of life itself. Although I doubt the validity of this conclusion, I'm willing to give life the benefit of the doubt.
Since living systems must be built from some sort of "technology," we should realize that there may be a limited set, perhaps a very limited set, of technologies which are available. A hammerhead may be made of plastic, hard rubber, wood, iron, brass, or even some yet-to-be-discovered substance, but we will never use a hammer with a head made of Jell-O.
However, the simplicity suggested by a seven-word definition probably makes it difficult for some people to separate life's technologies from its definition. Let's look at what I've been calling the "technology of life" in the final part of this series.
The Definition of Life: Part 4
Life is a persistent, homeostatic, nonspontaneous system.
The technology of terrestrial life includes the atoms, molecules, and structures that function in its systems of chemical reactions. These include a vast array of carbon (organic) compounds and an assortment of inorganic ions. Also a part of the technology of life are structures like cell walls, cell membranes, ribosomes, etc.
The carbon compounds found in terrestrial life include such things as carbohydrates, lipids, proteins, nucleic acids and a vast number of small molecules involved in the synthesis and use of these compounds. The inorganic components include ions such as hydrogen, hydroxide, ammonium, bicarbonate, iron, and magnesium, to name just a few.
It is certainly fair to ask, "Why chemical reactions? Why these types of chemical reactions? Why these types of chemicals? Why these particular chemicals? Why not silicon-based chemistry in ammonia or methane? Why not crystalline life forms based on something other than chemistry?" I think I can at least make a stab at answering these questions.
Spontaneous changes go on with the release of energy, often as heat. According to the Second Law of Thermodynamics, one consequence of a spontaneous change is an increase in disorder.
Nonspontaneous changes will not go on without the absorption of energy, and one of the consequences is an increase in order. Nonspontaneous changes must be coupled to a spontaneous change which supplies the energy and which guarantees that the total change in order is in the direction of more disorder. In other words, the presence of an energy source can cause nonspontaneous changes and a local increase in order.
The spontaneous fusion of hydrogen to helium in a star is accompanied by the release of energy in a variety of forms, including radiant energy. The radiant energy from a star can be coupled with nonspontaneous changes on a planet circling the star. These nonspontaneous changes can store energy in their new state which can be used as energy sources for other nonspontaneous changes. For instance, the ultraviolet radiation from the sun can cause a mixture of methane, ammonia, and water to form a variety of more complicated organic compounds, such as amino acids. Such changes can occur in space as well as on a planet--meteorites have been found to contain amino acids. On the surface of a planet where these are formed, other products (such as polyphosphates) and amino acids can participate in a series of reactions to form peptides. Large peptides are called proteins.
A variety of nonspontaneous changes can be coupled to a star's spontaneous output of energy. Weather systems, oceanic circulation, and erosion are three examples which come to mind. How many of these changes could be coupled with other changes in such a way as to result in a persistent, homeostatic, nonspontaneous system? I do not know. Perhaps, if we ever find life on another planet, we will find out. However, if atoms and molecules are free to move about and interact with each other, then chemical reactions seem to be likely candidates for participation in such systems. In fact, since we know that life exists on Earth, then it is obvious that chemical reactions can and do participate in such systems. Normally, I say that lack of evidence is not evidence. However, I think that the lack of other spontaneous changes which do the same on Earth might be taken at least as an indication that life is most likely to be a persistent, homeostatic, nonspontaneous system of chemical reactions wherever we may find it. Which chemical reactions?
What particular chemicals we find in life on Earth, and will find in alien life, depends on the chemicals and conditions that we find on the planet (or satellite) that harbors it. I mentioned above that, for chemical reactions to be the technology on which life is based, atoms and molecules need to be free to move about and interact. Chemical reactions, spontaneous or not, occur very slowly, if at all, in the solid state. Only in the gaseous or liquid state can chemicals react easily.
On Earth, we have no life forms in which the chemical reaction systems occur in the gaseous state. Perhaps this is because, at the temperature and pressure on Earth, atoms and molecules in the gaseous state are too far apart to interact well. But under pressure, such as we find someplace like Jupiter, this might not be a problem. I've thought for long time that Jupiter was probably another place in our solar system where we might expect to find life.
Lakes of methane, ethane, and dissolved nitrogen exist on Saturn's moon, Titan. Some other satellites of the outer planets are thought to have subsurface oceans of water. Jupiter, as atmospheric pressure increases toward the center, probably has a layer of liquid hydrogen. The temperature and pressure on Earth allow some substances to be liquid--medium-sized hydrocarbons, a variety of alcohols, mercury, a few other compounds, and water.
Molecules of water, an abundant compound on the surface of Earth, consist of an oxygen atom bonded strongly to two hydrogen atoms. Water molecules behave as though they have a partial negative charge on the oxygen and a partial positive charge on the hydrogens, so water molecules are attracted electrostatically to each other. Ions are also attracted to these partial charges, forming temporary bonds with water molecules. They can move out of the solid state into solution. Some organic compounds which contain oxygen or nitrogen also behave as if they contain partial charges, and, like true ions, can go into solution with water. Once in solution, these ions and molecules are free to react with each other. So, on any body where we find liquid water, we should not be surprised to find life using systems of chemical reactions.
Except for water and some inorganic ions or molecules, the chemicals found in terrestrial life are nearly all carbon compounds. This is probably because the stability of carbon-carbon bonds allows molecules with long chains or large rings of carbon atoms to exist. A vast number of different compounds of greatly differing size and shapes can be formed and are available for the chemical reactions found in terrestrial life. Other elements rarely, if ever, form similar chains and rings.
Sometimes the suggestion is made that silicon chemistry might replace carbon chemistry. After all, silicon is in the same family as carbon. I know practically nothing about the chemistry of silicon, but I doubt that, under terrestrial conditions, silicon can form the vast number and vast size range of compounds that carbon does. If it did, our industries would probably have sweet, but indigestible, "sugars" in our foods and a host of other useful silicon compounds available. They don't. The instability of silicon-silicon bonds is probably why life on Earth uses carbon compounds, not silicon compounds. Perhaps in a colder environment on some other world, silicon compounds might be stable enough to form organic analogs.
Silicon-oxygen compounds (silicones) are fairly stable, so these might also be used somewhere. I still ask, if they are versatile enough that life might be based on them, why haven't our industries been churning them out?
For an interesting article considering alternative biochemistries, you might like to read Wikipedia's Hypothetical Types of Biochemistry.
Life on Earth is based on some of the most common elements in the universe, so is it surprising that life is based on their chemical reactions? I think not. We could still ask, will life elsewhere be based on the same chemicals as on Earth? Is it an accident of history that glucose is the end product of photosynthesis but ribose (and deoxyribose) are the sugars in our nucleic acids? Why do our proteins contain the specific set of twenty amino acids that they do? Why do we use nucleic acids to store our genetic information? Do the aliens? Perhaps if we knew how life originated on Earth or could find some aliens, we could answer these questions. Or vise versa.
Keep reading/keep writing - Jack
Page updated: 2021 04 05
Page created: 2021 04 05
