Editor's Note: This story was originally published in the December 2004 issue of Scientific American.
In an episode of the classic 1950s television comedy The Honeymooners, Brooklyn bus driver Ralph Kramden loudly explains to his wife, Alice, “You know that I know how easy you get the virus.” Half a century ago even regular folks like the Kramdens had some knowledge of viruses—as microscopic bringers of disease. Yet it is almost certain that they did not know exactly what a virus was. They were, and are, not alone.
For about 100 years, the scientifi c community has repeatedly changed its collective mind over what viruses are. First seen as poisons, then as life-forms, then biological chemicals, viruses today are thought of as being in a gray area between living and nonliving: they cannot replicate on their own but can do so in truly living cells and can also affect the behavior of their hosts profoundly. The categorization of viruses as nonliving during much of the modern era of biological science has had an unintended consequence: it has led most researchers to ignore viruses in the study of evolution. Finally, however, scientists are beginning to appreciate viruses as fundamental players in the history of life.
Coming to Terms
It is easy to see why viruses have been diffi cult to pigeonhole. They seem to vary with each lens applied to examine them. The initial interest in viruses stemmed from their association with diseases—the word “virus” has its roots in the Latin term for “poison.” In the late 19th century researchers realized that certain diseases, including rabies and foot-and-mouth, were caused by particles that seemed to behave like bacteria but were much smaller. Because they were clearly biological themselves and could be spread from one victim to another with obvious biological effects, viruses were then thought to be the simplest of all living, gene-bearing life-forms.
Their demotion to inert chemicals came after 1935, when Wendell M. Stanley and his colleagues, at what is now the Rockefeller University in New York City, crystallized a virus— tobacco mosaic virus—for the fi rst time. They saw that it consisted of a package of complex biochemicals. But it lacked essential systems necessary for metabolic functions, the biochemical activity of life. Stanley shared the 1946 Nobel Prize— in chemistry, not in physiology or medicine—for this work.
Further research by Stanley and others established that a virus consists of nucleic acids (DNA or RNA) enclosed in a protein coat that may also shelter viral proteins involved in infection. By that description, a virus seems more like a chemistry set than an organism. But when a virus enters a cell (called a host after infection), it is far from inactive. It sheds its coat, bares its genes and induces the cell’s own replication machinery to reproduce the intruder’s DNA or RNA and manufacture more viral protein based on the instructions in the viral nucleic acid. The newly created viral bits assemble and, voilà, more virus arises, which also may infect other cells.
These behaviors are what led many to think of viruses as existing at the border between chemistry and life. More poetically, virologists Marc H. V. van Regenmortel of the University of Strasbourg in France and Brian W. J. Mahy of the Centers for Disease Control and Prevention have recently said that with their dependence on host cells, viruses lead “a kind of borrowed life.” Interestingly, even though biologists long favored the view that viruses were mere boxes of chemicals, they took advantage of viral activity in host cells to determine how nucleic acids code for proteins: indeed, modern molecular biology rests on a foundation of information gained through viruses.
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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.
The 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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