What is a living thing?

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I know that the scientific community has guidelines to determine whether or not something is a living thing, but even their standards are inconsistent. For instance, the scientific community has been arguing back and forth for well over a hundred years about whether or not the virus is a living thing. A virus is just DNA or RNA surrounded by a protein shell. They do reproduce, but only with the help of the cell they attack. So is a virus a living thing or not? That is an example of what I mean. Where is the cutoff line that determines whether or not something is a living thing? I'm sure it is a very fine line. I believe in animism and believe that there is no such thing as a nonliving thing. The computer I am typing on at this moment is made of molecules and atoms just like I am. Hell, doesn't an atom even seem like it's alive? The electrons are constantly moving around the nucleus in a rapid order. So the question is: What is living and what is not? How do we determine what is living and what is not living?

I consider a virus to be a living thing because it uses nucleic acid to encode genes that are translated into proteins that give the organism specific traits and functions. A viroid is not a living thing because it does not code for proteins; it is just replicating nucleic acid that gets in the way of plant cell function. However, it could be argued (based on ribozymes and the function of cellular RNAs) that life should even include viroids as well. Prions are not alive because they do not have nucleic acid. Anything that does not reproduce and pass on heritable traits to its offspring is definitely not alive.

Atoms do have electrons moving around in constant motion, although random. They don't really create other atoms (unless you count fission and fusion..., reproduce, seek nourishment, etc.

But we've only seen a small part of the Universe....

there is no adequate definition for life.

Though I did spend a semester with a Biochemist trying to mathematically define life. Which you also cannot do. ahhhhh chaos theory, makes me warm and happy inside.

A relevant article came out on physorg.com just today:

Link: Why Life Originated (And Why it Continues)

"Life" is just a word, an abstraction. To apply the word "life" or "alive" to something depends entirely on the definition that you have given to the word.

that which is was before i made it unalive

This is a neat question but it's also impossible to answer based on our current understanding. Most often it's shelved as an "I know it when I see it" issue. Every Intro to Bio textbook tries to tackle this in the first chapter.

Extant living things do indeed form a natural category and, from my current understanding, do form a monophyletic group, i.e. all current living things seem to have enough things in common that they are more like other living things than non-living things and therefore probably derived from a single common ancestor.

One of the main criteria is the ability to self-replicate, i.e. reproduce. However inorganic crystals also self-replicate. One of the current ideas regarding the origin of life on Earth involves the possibility that nucleic acids first "learned" to self-organize and replicate on a scaffold of clay crystals. Clay would form these self-replicating structures and nucleic acids would ionically bind to the clay in a complimentary pattern. Over several "generations" one particularly stable arrangement was achieved and that nucleic acid arrangement was able to float off the scaffold and maintain integrity... and the ability to self replicate through association with complimentary sequences.

Most of the other criteria for defining "life" are just characteristics shared by all the things that are unquestionably accepted as "alive." So far there doesn't seem to be much of a gradient between life and non-life and, under our current understanding, the category ca be regarded as discrete.

For the record, I don't consider viruses as living since they lack the cellular mechanisms for self-propagation. They don't count as life any more than a piece of computer code can count as a full-blown computer.

i always thought rocks were alive; they just live much more slowly than what we're used to. they grow, reproduce, consume nutrients to do all of that.. they were born and eventually die into dust... just very very slowly.

And what about all the obligate parasites that only reproduce inside a host organism? There are eukaryotes that fall into that category, are they non-life?

Parasitic organisms still produce their own replication "machinery." They depend on their hosts for nutrients and habitat but, as far as I am aware, the cellular and molecular structures are endogenous tot he organism and not hijacked from the host. Again, the analogy of program on a disk (or flash drive to be up-to-date) versus full-blown computer complete with drive, CPU and OS is still pretty applicable.

And again, I'm prefacing this all with a big, "As far as we can tell..." and "Based on current findings..." Hell, we're (the field of biology in general, not me specifically) still trying to come to grips with the massive overhaul of Pauling's "Central dogma of molecular biology," i.e. DNA -> RNA -> Protein. The past decade (the past couple years, even) have shown us that RNA is a hell of a lot more catalytically active than ever imagined and, well, there's still a lot of people trying to accept the whole "prion" thing (self-replicating proteins).

We just have to remember Box's maxim: "All models are false. Some models are useful."

viruses fail 3 of the 4 criteria for life.

otherwise if it is a eukaryotic cell then it can reproduce, metabolize and grow on its own (though it might require energy or some resource from another cell or its "host")

viruses are a strand of RNA or DNA and a protein shell, they cannot reproduce metabolize or grow.

Though amusingly enough... fire meets all 4 criteria for life.

and that is irony.

The endosymbiont theory for the origin of eukaryotes relies on the possibility of intracellular parasites. I don't see much fundamental difference anyways- there are a wide variety of parasites that are incapable of reproducing outside a host organism and take advantage of the host's biological processes. I'm not quite following the analogy.


The "central dogma" was Crick actually. And yeah, we still have some major gaps in our knowledge of molecular biology. I don't think there was a single chapter in my genetics book that didn't include, at some point, a disclaimer that they weren't completely sure about something they were discussing. I mean, we still have no real idea how viral self-assembly works, we've practically been ignoring an entire domain of life in the Archaea, we're still pretty fuzzy on the protists, and proteomics is still in its infancy at best. It's an exciting field to be in. Lots and lots of room for new developments.


What are these "4 criteria for life?" Google didn't give me as solid results for that as I'd have liked.

I would consider a virus to be a living thing on these criteria:

They seem to 'know' which cells they have to target in order to sustain themselves and make copies of themselves.
They die in the incorrect environment e.g. the AIDS virus cannot survive if out of a human (or otherwise) host for more than 20 minutes (or so it is believed, you may have to check up on that time wise, besides I'm just using it as an example to illustrate my point). How can a virus be deemed 'dead' if in the incorrect conditions if it was never a living thing in the first place?
How something that isn't living to a certain degree cause the damage that it can potentially do to it's hosts e.g. the Ebola virus: if this was not alive, then how come it has the power to kill? Another way that this can be explained is to say that a virus is a special kind of poison - a toxin. But then if this be the case, viruses should not be described as being 'dead' in the incorrect environments etc.

well fluidly you can combine them as 4 or expand them into more. Science is not accurate. Basically,

Metabolism-Obtain and use energy
Growth-Grow, develop, and die
Reproduction
Adaption/Homeostasis/Response to Stimuli-Respond to the environment

Hm. Viruses reproduce, and they to respond to stimuli. Metabolism is harder to justify, since viruses mostly hijack a host's metabolism, but that is still a way of obtaining and using energy. Growth is not something I feel like arguing.