life, living matter and, as such, matter that shows certain attributes that include responsiveness, growth, metabolism, energy transformation, and reproduction. – Encyclopedia Brittanica
Close enough, and encompassing the generally accepted criteria: responsiveness, reproduction, metabolism and adaptation. My older son asked the question about viruses the other day. I have been looking forward to this question. It means that he is prepared to understand some things about life which are important. It is a tricky question if considered from the wrong viewpoint. A virus displays some of the characteristics which define a living organism. It can respond to stimuli, attaching to the proper cells and injecting its genetic material through the cell membrane when it makes contact. It can replicate. It can adapt to avoid a host immune response. But it does not have the capacity to metabolize. It cannot, in other words, run its own show. It is entirely dependent on its host organism in that respect. Nor is the virus alone on the gray borders of life. Certain families of bacteria lack some essential metabolic processes which would make them autonomous. They must live inside another cell, and depend on their host’s metabolism to survive. Yet, they too can reproduce, adapt, and respond to stimuli in their environment. Because they have a membrane which is active, biologists are prone to give obligate intracellular bacteria, like mycoplasma and Rickettsia, a break. Most biologists are less charitable when it comes to prions. Prions are mis-folded proteins which replicate by somehow inducing their own conformal change in normally folded proteins with which they come in contact. Prions can reproduce, but they cannot metabolize. They cannot adapt much (although they have managed to pass from cows to humans), but they can respond to their environment, albeit in a very limited way. Still, the difference between the prion and the obligate intracellular bacterium would seem to be one of magnitude rather than quality. Differences in their classification reflect a little bit of membrane chauvinism on the part of biologists. The same prejudice is evident in the gray zone at the other end of the complexity scale. By our criteria for life, is a male angler fish alive? The fish can survive for a short period of time independently, but it cannot carry on its own metabolic processes independently for the long-term. It must rely on a female angler fish. It must quickly sniff out a female and attach itself to her, permanently. The male fish spends most of its existence as a tissue of the female angler fish’s body; its brief, free swimming existence is a transitional aberration. Its ability to adapt is extremely limited. Its existence can be mapped on an algorithm only barely more complex than the one which describes a prion’s lifestyle. So what does differentiate the male angler fish from a mycoplasma bacterium, a virus, or even a prion? A few extra membranes make the only difference. Even our own status as living things is at risk if we apply our criteria strictly. We can certainly reproduce, just like the viruses, obligate intracellular bacteria, prions, and angler fish. But it is questionable whether or not we can independently metabolize. We actually rely on hereditary intracellular symbionts for our primary metabolic process. Without these symbionts, our mitochondria, we could live only minutes on the metabolic processes encoded by our own genetic material. So, we can hardly be blamed for fudging our criteria. We certainly want to call ourselves alive. Since it looks and acts alive, we want to call the male angler fish alive. For practical purposes, we also want to call Rickettsia and mycoplasma alive, as well as viruses from time to time. As for the prions, it is often more convenient to view them as sophisticated toxins rather than living things. And that’s the upshot of my son’s question. The issue of whether or not a virus is alive is only confusing if we consider “life” an actual, efficacious thing. But life is just a category. When we look out across the terrible landscape of things, we see phenomena which cluster about each other by dint of their shared heritage. Our account of our cluster is biology, and our criteria for life provide the outline for our biological stories. This is correct viewpoint on the question of life, and what is alive. But this is not the popular viewpoint. The popular viewpoint attempts to preserve life as a thing, as vital essence or emergent property. Unfortunately, the popular viewpoint is not feasible. It leads inexorably back to the original question rephrased, “where is the life in a thing to be found?” In the end, we find that the essence or the emergent property is explained by the operational mechanisms and properties of the thing in question, but it in turn, explains nothing about the thing; it just notes where that particular thing lies on the vast, terrible landscape of things. Despite its glaring inadequacy, the popular viewpoint remains popular because it seems to save us from losing an idea that we don’t feel comfortable losing. But we don’t need to worry, becoming a category doesn’t vitiate life. We have the things which the category marks clustered around us after all, even if it’s only according to our viewpoint. We can’t escape life anymore than we can climb out of our skins. So, the answer to the question? Sure, a virus is alive – as long as you can explain why.