> * (discounting viruses, which are oddities that probably arose from cellular life)

This is a general post, not intended at you specifically, but you imply that viruses are living, so I feel the need to throw this out there: I think the key reason the exception has to be made for viruses here is that viruses simply aren't alive. They have no biological processes, they don't reproduce, don't eat, don't breathe, don't create or consume energy, etc. A virus is really nothing more than a piece of data in a container. One of the traditional arguments is that they do reproduce, they just do it via a living cell, but to that I say: does a CD reproduce when a human copies it?

I think that in a couple decades, we'll look back on viruses being defined as living organisms and wonder what we were thinking.

You seem to think the definition of life is going to get sharper in the future. I'd say all signs point towards it getting a lot fuzzier instead; what will die is the idea that it is a binary yes/no instead of yet another continuum. Is a sufficiently advanced AI alive? Well, some yes, some no.

Actually, I don't think it'll get much sharper, if at all, largely for the reason you mentioned. I think that what we consider to be "alive" will largely grow more fuzzy, but what isn't alive will be sharper. I don't find it hard to believe that a computer running advanced software (e.g. an advanced AI) is alive, but I find the idea that a piece of inert data (a virus) is alive to be counterintuitive and wrong.

  > I say: does a CD reproduce when a human copies it?

If CDs somehow found their way to my burner and caused themselves to be copied, I'd be inclined to say yes.

Viruses don't "find their way" into cells, though. Cells stumble upon them, take them in, and are then infected. In this way, it's like you plugging a USB thumbdrive into your computer, and becoming infected.

It's really too bad that brownian motion doesn't work at the macroscale, then. :)

The music "ecosystem" undergoes artificial selection by humans. By definition, there will exist more copies (legitimately manufactured or burned at home) of more popular albums. This is similar to the concept of a meme. (By the way, it would make more sense to ponder whether the information reproduces itself, rather than the physical disc.)

No one would say the albums reproduce themselves, but then again no one is saying the biological reproduction so crucial to natural selection requires the organism to deliberately reproduce or even be aware of the reproduction.

All living organisms require something external to survive and reproduce—in the case of viruses, the requirement may be another living cell. The reproduction argument is insufficient to declare viruses non-living. I don't think biologists are trying to pass judgement on viruses, rather I believe the definition of life is less important to them than, say, the cell. Viruses aren't cells, and I think that's why textbooks waste time on the "are viruses living" debate.

I mis-spoke; it wasn't my intention to call viruses alive. My intention was actually the opposite: I was trying to anticipate a possible objection to what I said about all known life having the same genome type (double stranded DNA). That's true as long as one discounts viruses as a sort of life.

Really though, whether one wants to call viruses alive or not is a philosophical point, detached from any real significance.

Who has defined viruses as living organisms? Biologists don't.

Ah, then what about prions?

I don't know much at all about prions, so take this with a large grain of salt, but it seems to me that most -- if not all -- of the arguments against viruses being living organisms apply to prions. In the case of prions, the data is just in the form of a malformed protein, rather than a snippet of RNA or DNA. There may be something about them that I don't know that negates one or more of those arguments, though.