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This paper reflects the research and thoughts of a student at the time the paper was written for a course at Bryn Mawr College. Like other materials on Serendip, it is not intended to be "authoritative" but rather to help others further develop their own explorations. Web links were active as of the time the paper was posted but are not updated.
The definition of "life," it seems, must remain ambiguous. For the most part, life on earth is easily distinguished from non-life. A horse, for example, is alive, while a rock is not. As computer technology becomes more advanced and software more complex, the possibility of artificial intelligence grows—thus begging new questions concerning the nature of life. However, there is one kind of thing on the planet that straddles the boundaries of life already: the virus. A.I., as we can imagine, could be considered alive because of its staggering complexity and vast array of life like features. Viruses, while made up of organic material, are simple enough that they seem closer to the more complex organic molecules than complex forms of life. Allowing viruses the label, "life", changes many conceptions of "life." Examining closely the reasons why viruses are straddling the boundaries of many definitions of life can help us to understand exactly why life is so difficult to contain in one definition.
Viruses are essentially strands of DNA or RNA protected by a protein shell. They cannot reproduce on their own. Instead, viruses require cells of living beings to reproduce. This process involves the virus invading the host cell through a variety of means. The HIV virus, for example, attaches to receptors on the membrane of the cell and wait to be mistakenly absorbed. (1) Once inside the cell, the genetic material is released, which then makes the cell into a virus producing machine, abandoning its original function and using its resources to create as many viruses as possible before the cell dies. The new viruses then move on to repeat the process on other nearby cells. (2)
From this description of the viral reproduction process, we see that viruses have something that would strongly move its status towards "alive": genes. The RNA or DNA inside of a virus functions identically as in other life forms, controlling the production of proteins in cells. Thus, a virus recreates itself, though it requires another cell to do so. By many definitions, nothing alive requires complicated systems outside itself to reproduce, at least on a cellular level. Thinking on larger scales however, shows that most, if not all known forms of life, require other forms of life in order to survive. Many use other life forms in their reproductive process—many species of plants require that animals carry their seed to new places in order to best spread out. Flowers often use insects to pollinate—without the insects, pollination and therefore reproduction would be essentially impossible for them. Autonomous self-reproduction is a tricky term, seeing as reproduction is an enormously complicated process for so many forms of life, and using it as a characteristic of life is problematic in that it is so ambiguous.
Viruses are simple in construction—however, their interaction with more structurally complex organisms is anything but. While their process of reproduction is simple, viruses cause vastly different reactions in all the different forms of life they infect. The common cold, herpes, and HIV all are viruses that effect humans, but each works on different cells in different parts in the body to create vastly different symptoms in the host. Many viruses do not kill their host. Some, like herpes, can live in a host for an entire lifetime, feeding off of the cells but not killing the system. Some, like the common cold, are easily overtaken by a healthy immune system's antibodies. Others, like HIV, inevitably destroy their hosts, self-destructing along with them. These examples show the complicated cycles of viruses. It would be unconventional to say that a kind of rhythm is a characteristic of life, but surely many would agree that life does move in a rhythmic way.
Surely rocks do not behave similarly. Viruses have a relationship with life, for sure, in both their similarities in genetic/protein structures and in the fact that viruses require life. Giving up the requirement that things must autonomously self-reproduce in order to be alive eliminates most of the debate about calling viruses alive, and, so, for the purposes of this paper, we will do just that. So, now that we assume something like a virus is alive, we have a problem in front of us. What else, then can be considered alive in this framework? Surely a self-replicating nanotechnology is possible. Scientists already use viruses as tools in genetic study, carrying specific genes to cells. (2) Should we then consider these tools alive? The difference between them and the naturally occurring virus is insignificant.
Why is it that we feel uncomfortable calling our own creations "living"? That the virus arose from the natural world creates in us a sense that it should at least be considered half-living. However, very small self-replicating machines do not seem to fit comfortably in any of the standard definitions of life. Viruses are made up of organic material similar to what exists in our cells—if we build machines out of this material, could we then call it "alive"? We must, if we consider viruses to be alive. Still—as of now, all life has arisen out of the natural world, not out of human construction.
The debate concerning the status of viruses continues—and we have not answered any questions, only explored reasons why there is this problem of defining "life." This discussion does hint at an interesting trend—life seems to be composed of complicated interactions based in some kind of self-reproduction. The more we allow for simpler things to be alive and the less terms we ask of things in order to consider them alive, the more we have to consider wilder conceptions of what it means to be alive. Weather systems, planetary systems, even molecules all might some day fall under such a definition. Surely our own creations will someday be causing fierce debates. Already people fight over the imagined nature of A.I.—which, if "alive," will not necessarily reproduce nor will have an earthly body of the sort every other kind of life does.
Life must not be containable in any kind of principle or rule. Instead, it seems to be only containable in an idea of a community; in an idea of a specific kind of system and the parts of that system. With that kind of definition, we may be able to approach the future with a new and open mind for what we might find, alive, half-alive, machine, or otherwise.(1) CellsAlive HomepageSerendip