I remember when I took tenth grade biology, our first discussion about what is alive. By that time I’d already had intense interest in biology. I had an internal premonition that I was destined to be a doctor since elementary school and considered medicine merely a part of biology devoted to humans. I was naive of the Venn diagram representation I have today, medicine as intersection of areas of study, social, psychological, chemistry, physics. At any rate, deciding what was alive and what wasn’t occupied me then.
Living things were separate from non-living things by a series of verbs. These catalogued functions of body organs. Living things, so my teacher said, eat, excrete, reproduce, react to their environment, move, breathe (or respire), form cells, strive to remain living, perceive and act. Did something need to be doing all these things to be considered alive? Of course not. Then how many of these criteria are needed to separate the living from the dead? Some us around the classroom thought one or more of these characteristics to be most important. Reproduction was the favored in the post pubertal mind, but if a machine could make copies of itself would it then be alive? What about a something that took things in and then passed out waste? We all had a pretty good idea about what was living and in the end decided that while these verbs were useful, none of themselves defined life. Life was something like pornography for the Supreme court, you know it when you see it.
Our teacher solicited our opinion as to whether viruses were alive. It all went haywire with viruses. We were reduced to seeking the wisdom of crowds. We took it to a vote. To start with, viruses, everyone agreed, did not do anything but make copies of themselves, so if we admit viruses we might as well say that reproduction defines life. Most persons thought of viruses as non-living chemical particles, nothing more. So what if those chemicals were the same nucleic acids common to all living things. The majority of my classmates did not feel that viruses were organisms, although they felt good about admitting bacteria. Some felt so strongly they wished to alter our definition of life to exclude viruses as living. I was and am firmly in the opposite camp. To me viruses are definitely living, otherwise why study them in a biology class? Well that was not my reason. It seemed to me there was much more to viruses than meets the eye. I was just as sure there was something we weren’t considering that would place viruses among the living.
Viruses intimately interact with, cause disease and bring down living organisms and thoroughly use genetic machinery, DNA or RNA. In the days when I was in school, viruses were presented as mere snippets of DNA or RNA informational chemical material plus some protein, so how could mere chemicals be alive? But today,it seems to me, we are asking the wrong question. A virus may not simply be that Nucleic acid/protein unit but a bit more. There are some naked viruses composed of Nucleic acid material without even an envelope or a membrane and others at the opposite extreme that are encapsulated in complex dodecahedron envelopes housing actively reproducing units.
Many biological organisms exist in two or more forms. Lots of parasites like malaria plasmodia, assume many forms over their life cycle, coursing through blood and organs, as schizonts, merozoids and the like, some in diploid some in haploid form. Humans exist in haploid and diploid form as well, as germ cells, ova and sperm, and in our full bodied human form. The same goes for viruses. What we consider viruses cannot reproduce without commandeering the machinery of intact cells. Viruses are not considered alive by some people because they are only considering one form as infectious agents and don’t see them in their full splendor having taken over certain cells in order to live and reproduce. Perhaps it should be the infected cell that is considered the fully developed form of the virus and the viral particle only the germ cell, that is the reproductive unit, of the species.
Ordinary viruses attach to proteins on the membranes of cells and bacteria then their nucleic acid is admitted into the cell which then takes over cellular machinery to make their numerous copies of their own DNA or RNA, the RNA requiring reverse transcriptase to make DNA copies first then reproduce RNA. They also use the cell’s machinery to make the viral proteins necessary for further transmission. This protein synthesis, done by the enslaved infected cell, may reform the viral envelope and other proteins necessary for the next viral transmission. The infected cell, reproduces the viral DNA or RNA many times before often lysing and liberating new viral particles. Thus the infected cell may be considered the more macroscopic form of a viral organism containing many viral units almost like a syncytium or multinucleate cell. It is in this latter form, the infected cell, containing its multiple viral units that the virus may be considered truly alive as this cell performs many of the functions of full fledged living organisms. Moreover it should be emphasized, viruses use the same nucleic acid code that is the hallmark of all known living things.
On the basis of the DNA or RNA nature of viruses, their appearance including absence or presence of a viral envelope, the types of cells they infect, whether bacteria, plants and animals, regular logical Linnaean classification schemes have been devised for viruses just as they have for all other living things. By the way there are many non-bacterial small parasitic non bacterial forms that are not viruses which we know best by the diseases they cause, mycoplasma, Rickettsiae, spirochetes, among them.
You may think, viruses are simple and primitive. They can’t exist apart from other complete cells on which they are dependent. They are composed mostly of nucleic acid which is the chemical basis of life and therefore viruses must have been the first organisms on earth. Admitting these theories to be somewhat conjectural, that is not fully known, to classify viruses as the earliest form of life would be an error. One strong argument is that all viruses cannot reproduce on their own and cannot have evolved before intact cells.
Since viruses cannot make others of their own kind on their own, they had evolve from out of cellular organisms at least at the level of complexity as bacteria which can replicate. Richard Dawkins talks about the possibility that the goal of life is not the furtherance and reproduction of living things, but how an individual organism is merely a container which allows for replication of selfish genes. If that is so then viruses are the most elegant and simple example of selfish genes reproducing without their own an infrastructure. In other pages here I wrote about the El Capitan Rock climbers of Yellowstone, how the first pioneers developed ever more ingenious ropes, hooks, hammers, screws and all manner of equipment allowing the intrepid to climb the rock which in those days could take well over a month. But later climbers developed ingenious ways of “free climbing” the rock using less and less equipment, until finally the best of them ascended with nothing but a bit of talc, their own fingers and special shoes. making it to the top in as little as 2.5 hours! There is no better illustration of the elegance of simplicity. The complex does not always evolve from the simple, the definition of elegance is simplicity derived from complexity. There are many examples of this type of evolution. In the twentieth century we had large electronic devices, big wooden cabinets housing tiny black and white TV screens. Now our screens may be big or small but the electronics have shrunken to minuscule proportions and great sound is produced by tiny speakers not dreamed of a generation ago. So it is with viruses which had to have evolved from primitive larger organisms serving the same purpose, furtherance of certain genes. It may be one day be true that the largest musical ensembles, chorus and orchestra of a thousand performers, as powerful and beautiful a the sounds are, may give way to a few devices that fit on a table top and to the same emotional effect of tickling the same intellectual and emotional parts of our brain making us one with the universe.
Nor is it true that parasites as pejorative a term as it still is, who can only live and reproduce by depending on the largess of other organisms on which they depend, are necessarily primitive organisms. Parasites are no lower than we who self-sufficiently replicate. In fact none of us reproduce without the help of others. On careful consideration the complex interdependence of all living things is revealed. In recent years we humans have come to appreciate just how dependent we are on microscopic parasites around us, especially bacteria, but very likely viruses as well. Our own guts contain many more bacteria than we have cells of our own, and our skin is also covered with bacteria and viruses many of which undoubtedly serve functions unknown to us. Our cells, contain organelles such as mitochondria, remnants of invaders from the very early in the evolution of cells, that handle most of our energy needs, former parasitic invaders. The dividing line between parasitism and symbiosis is so blurred as to make these words virtually synonymous.
How then is life to be defined? The first sign of life was the separation of inside and the outside by a self organizing bi-lipid cell membrane, the formation of a soap-bubble in water in other words. Fatty acid carbon chains organized themselves automatically in two layers, such that the polar parts attracted to water, faced the outside world and the non-polar inner and outer layers faced each other with the inner layer of water soluble polar portions facing cytosol composed also of water (aqueous) layer. You have water inside and outside a cell. When the inner milieu is separated from the outer milieu you have the beginnings of what is a cell, the cradle of life, separation of the inner living individual organism from the outside world. The second requisite for life is a process of self-organization that leads to the furtherance of the living cell’s continuance of existence, in its own self-interest. Life is in simplest terms separation of self from other. The virus is living as it exists in a second form which is cellular and separate even though it must take over the reproductive machinery of another cell. The second viral form is as cellular syncytium.