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Viruses at this period of time do not fit the current definition of life.

Much of the reasoning behind this is that we currently believe that all life must be made up of cells.

Also, many biologist/scientists say that viruses are not living because all they are is just chemicals carrying out their predefined chemistry.

Has anyone every considered the fact that cells, the supposed building blocks of life, themselves simply only carry out predefined chemical actions? All actions cells carry out are chemically driven, not consciously or spiritually driven. Cells do not make choices, they follow chemical directions present in their DNA and RNA. Viruses act very similarly, following the action transcribed in their DNA or RNA.

How can we really even draw a defined line of what life is?

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migrated from skeptics.stackexchange.com Sep 18 '13 at 0:18

This question came from our site for scientific skepticism.

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Drawing a clear line is difficult as you note. But science typically excludes viruses because they don't have the proteins and other parts necessary to reproduce or survive on their own. They contain the genetic code necessary to hijack the functional parts of cells in other organisms, but nothing else. Viruses don't follow chemical directions - they rely on the host cell to do that. –  Mark Sep 17 '13 at 23:24
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Who believes life must consist of cells? I've never seen that included in the definition of life. Who believes free will is required for life? No one I have ever read. –  Oddthinking Sep 17 '13 at 23:32
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Relevant: biology.stackexchange.com/a/9455/4101 –  Amory Sep 18 '13 at 1:33
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@MiguelÁngelNaranjoOrtiz: I disagree. The "classic" point against viruses being alive is that they are obligate intracellular parasites. That said, even if cellularity (whatever that means) was the "rule" for being alive that surely would not be because "we want to exclude viruses". There are definitions of life and virus may or may not fit into them, but to say (without bringing any proof whatsoever) that the definition is made specifically to exclude viruses sounds very unscientific to me... –  nico Sep 18 '13 at 11:44
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But there are a lot of protozoa and bacteri athat are obligate intracellular parasites. In fact, many viruses have more genomic complexity than Micoplasma and many other insect associated intracellular bacteria, for example. "Life" as a term is very unclear. It is assumed that implies the ability to transform energy in order to reduce its own entropy, the ability to reproduce and the ability to evolve by natural selection. All of this is very hard to specify to many parasitic microbes, and thus the ultimate limit is celularity. Megaviruses are in a limbo, thought. –  Miguel Ángel Naranjo Ortiz Sep 18 '13 at 13:45

2 Answers 2

How can really even draw a defined line of what life is?

It's easy, really: just make one. In the answer I linked to I go into the reasons why viruses aren't alive (spoiler alert: no ribosomes) and what our current definition is, but the bigger question is why does it matter? We can draw the line wherever we want to, but it's still an arbitrary construct placed onto the universe by humans so we can sleep at night. It's similar to intelligence. We draw arbitrary lines around primates and dolphins and elephants and pigs and what have you, because they're the smartest and most thinking, but that's only for our own (legal) convenience. Intelligence and awareness is a spectrum, there needn't be a point beyond which you "count." There's no reason life can't be the same way.

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...many biologist/scientists say that viruses are not living because everything they do is just chemicals carrying out their predefined chemistry.

I don't think that's correct. Many definitions of life exclude viruses because they lack the apparatus to perform the life functions themselves, especially reproduction. They don't seem to qualify as parasites because, when they've infected a cell, they no longer occupy a bounding structure (other then the cell itself.)

When viruses are acting like a living thing, they look like misbehaving cell. Here's a similar puzzle; what organism is cancer? Viruses are basically rogue genes, just like cancer cells are basically rogue cells.

"Organisms carrying out their predefined chemistry," is basically saying that living systems are deterministic. The interesting question is, are very complex systems that emerge from stochastically-driven events themselves deterministic? Are organisms like a clockwork?

Probably you should check out Schrödinger's What is Life? to get started.

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"They don't seem to qualify as parasites": they do, they are obligate intracellular parasites. –  nico Sep 18 '13 at 21:26
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I appreciate your point, @nico, but I wonder, if they were clearly qualified as parasites there would no debate over whether they are alive. Parasitism is an ecological niche, not a definition of life. So (to split the hair) they are clearly parasitic when they've infected a cell, but I question whether the annotation as parasite is accurate. –  Ryan Sep 23 '13 at 14:21
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it's not really the "parasite" part which is important, but rather the fact that they are obligate, i.e. they cannot reproduce by themselves, they need a host to do that. –  nico Sep 23 '13 at 15:17
    
Every gene needs a host to replicate! Every gene is obligate; the question of parasitism is key to defining a virus as something other than a gene. –  Ryan Sep 25 '13 at 19:48
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well, I do not know many people who think genes are alive... –  nico Sep 27 '13 at 16:33

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