Molecular biologists went on to crystallize most of the essential components of cells and are today accustomed to thinking about cellular constituents—for example, ribosomes, mitochondria, membranes, DNA and proteins—as either chemical machinery or the stuff that the machinery uses or produces. This exposure to multiple complex chemical structures that carry out the processes of life is probably a reason that most molecular biologists do not spend a lot of time puzzling over whether viruses are alive. For them, that exercise might seem equivalent to pondering whether those individual subcellular constituents are alive on their own. This myopic view allows them to see only how viruses co-opt cells or cause disease. The more sweeping question of viral contributions to the history of life on earth, which I will address shortly, remains for the most part unanswered and even unasked.
To Be or Not to Be
The seemingly simple question of whether or not viruses are alive, which my students often ask, has probably defi ed a simple answer all these years because it raises a fundamental issue: What exactly defi nes “life?” A precise scientifi c defi nition of life is an elusive thing, but most observers would agree that life includes certain qualities in addition to an ability to replicate. For example, a living entity is in a state bounded by birth and death. Living organisms also are thought to require a degree of biochemical autonomy, carrying on the metabolic activities that produce the molecules and energy needed to sustain the organism. This level of autonomy is essential to most definitions.
Viruses, however, parasitize essentially all biomolecular aspects of life. That is, they depend on the host cell for the raw materials and energy necessary for nucleic acid synthesis, protein synthesis, processing and transport, and all other biochemical activities that allow the virus to multiply and spread. One might then conclude that even though these processes come under viral direction, viruses are simply nonliving parasites of living metabolic systems. But a spectrum may exist between what is certainly alive and what is not.
A rock is not alive. A metabolically active sack, devoid of genetic material and the potential for propagation, is also not alive. A bacterium, though, is alive. Although it is a single cell, it can generate energy and the molecules needed to sustain itself, and it can reproduce. But what about a seed? A seed might not be considered alive. Yet it has a potential for life, and it may be destroyed. In this regard, viruses resemble seeds more than they do live cells. They have a certain potential, which can be snuffed out, but they do not attain the more autonomous state of life.
Another way to think about life is as an emergent property of a collection of certain nonliving things. Both life and consciousness are examples of emergent complex systems. They each require a critical level of complexity or interaction to achieve their respective states. A neuron by itself, or even in a network of nerves, is not conscious—whole brain complexity is needed. Yet even an intact human brain can be biologically alive but incapable of consciousness, or “brain-dead.” Similarly, neither cellular nor viral individual genes or proteins are by themselves alive. The enucleated cell is akin to the state of being braindead, in that it lacks a full critical complexity. A virus, too, fails to reach a critical complexity. So life itself is an emergent, complex state, but it is made from the same fundamental, physical building blocks that constitute a virus. Approached from this perspective, viruses, though not fully alive, may be thought of as being more than inert matter: they verge on life.
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12 Comments
Add CommentI think that whether or not viruses are "alive" (whateve THAT means) is immaterial to whether they impact evolution. There are plenty of non-living things that impact evolution (like plate tectonics and other non-biological environmental factors). Evolution, by it's very nature, does not exist in a vacuum. Even if viruses are "non-living" the fact that they affect "living" things (particularly the fact that tey do this by phyiscally invading and biochemically changing "living" cella) means their impact on evolution cannot be ignored. Their impact on evolution may be different from, say, bacteria's effect, but that doesn't mean the effect doesn't exist.
Reply | Report Abuse | Link to thisJust another example of how narrow-minded people can be when the become highly specialized. Specialists are essential, but they need generalists to keep them grounded in something akin to reality.
My, my, it sounds to me that viruses may be God's little screwdriver used to tweak his creation. Just another mild blurring of the spirit/science boundary.
Reply | Report Abuse | Link to thisThe issue of whether or not a virus is alive is simple to resolve, in my opinion. So long as we recognize that life is just functioning molecular machinery, mesomachinery, then grasping the place of the virus becomes trivial.
Reply | Report Abuse | Link to thisA virus is a static bit of mesomachinery, of molecular machinery, on its own, which only functions when inserted into an active machine, such as a cell.
There is no need to specify 'life' as some special or magical thing. Life is molecular machines in action, death is cessation of that function without hope of restart, and a virus is just a snippet of organic computer code that requires a running program in order to be acted upon. The code is mechanical, biology as a molecular Babbage engine, virus as... a virus, a computer virus, in physical, molecular machine form. Nothing more, nothing less.
And as for you and I? We are electrochemical programs running on walking, talking mesomachine systems. Nothing more, and nothing less.
'Life' is a process, the program running, not a thing.
If life is defined by the ability to respire then a virus is not living!
Reply | Report Abuse | Link to thisMissing from this article is the idea that the mitochondria in eucaryotic cells may well be viral in origin. The DNA is unique, passed down by the mother only, and could not have evolved by any accepted evolutionary standards.
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Reply | Report Abuse | Link to thisPer the virus question, here's my 2 cents worth: The virus has an existence dependent on DNA/RNA sequences (whatever that may amount to), plus it can be killed off... Genetics, plus it can die, means to me that it must be alive..
Reply | Report Abuse | Link to thisViruses are both, dead AND alive. Outside a living cell they are nothing but a large complex of organic chemicals. Inside the cell they assume some properties of life. I believe it's impossible to truly create life from inert matter, without using another life form to assist in the process. Or at least we are a very long way from it.
Reply | Report Abuse | Link to thisSince it is now fairly easy to create certain viruses from scratch, as done by Eckard Wimmer of Stony Brook University, I think it's not really life that's being created, but rather at best "re-created" from known information. This life can be "molded" within certain parameters to be put to good use, for instance in the making of new vaccines, as the groups around Eckard Wimmer and Steffen Mueller subsequently showed (some explanation found here: http://ms.cc.sunysb.edu/~smueller/index.html )
The closest thing to creating some type of new life form is the group around Craig Venter. They use a completely synthetic bacterial genome, and try to transplant it into an empty bacterium, which had its genome completely taken out. However, as I said this process depends on an empty surrogate bacterium, which contains all the "stuff of life" in form of thousands of different proteins, EXCEPT the information (i.e genome) to replicate itself. So, even if they succeed in doing that, it may new life form, and after "booting" it from the synthetic genome, it will assume the properties encoded by the new genome, but it first needed that "empty" bacterial shell, and THAT perhaps can never be made from scratch, because it is way too complex.
I like the idea of looking at viruses as seeds. Viruses are seeds that do not give rise to an organism, but only produce more seeds. Sometimes these seeds implant themselves into the human genome and grow into part of the human organism.
Reply | Report Abuse | Link to this1+1=2; 1x1=1; abc; point,line,plane; triangle, square, circle. what is the minimum definition of life?
Reply | Report Abuse | Link to thisvirus does NOT qualify. there is NO gray area. unless you want to change the definition soley to include the lowly virus.
go back to the basics. biology 101.
maybe a more appropriate term: sub-life?
Well, anything that isn't dead...is alive...yes?
Reply | Report Abuse | Link to thisObligate parasites that depend on their host as replication machines - I hardly call that alive, since they are not independent, not even at a cellular level. This is no different than computer viruses that depend on executable programs to replicate itself: Not life, but replication machines only. Technically that's what all life does - EXCEPT - living things are of their own organism even if it's an individual cell. (Even though all life feeds off of life but that's a moot point - living things are their own entity).
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