1. Homeostasis: your insides stay pretty constant all the time. You've got a constant temperature, for example.
2. Organization: you've got cells. You're not just made of free flowing energy or some kind of gelatinous mush.
3. Metabolism: Your cells can convert energy (food). You need to do this to maintain homeostasis and to grow and so forth.
4. Growth: yup, you started off as a single cell and now look at you!
5. Response to stimuli: Things that are alive react to the environment. Fire bad!
6. Adaptation: No one organism can do this but populations of organisms, over evolutionary time, change by means of natural selection to become better suited to their environment.
7. Reproduction: things that are alive can make more of themselves.
So that seems pretty exhaustive and it does a great job of explaining the unique characteristics of, say, an animal. Animals all eat and grow and evolve and all that good stuff. But there are some things in the natural world that look like they really should be alive but aren't and some things that seem like they really shouldn't be alive but act like they are.
Viruses are science's most notorious culprits for looking a lot like living things but not fulfilling all seven criteria. Viruses reproduce (that's what's going on when you have a cold, they're invading your cells and making more of themselves) but they can't do it by themselves; they've got to have a "host." Viruses definitely adapt, that's why everyone's worried about the potential of "super viruses," because they do evolve and (for example) become resistant to antiviral agents. They have organization even though they don't have cells but they don't grow and they don't really respond to stimuli, or at least not in the sense that we normally think of stimulus and response (even single celled amoebas and things like that look more like they have "behavior" in the way we tend to think of that phenomenon).
So scientists have been stymied over whether or not viruses are a life form or not. Perhaps they are some thing that's not life but not not alive either...maybe we just need some new language.
I've been thinking about this recently for two reasons: a friend of mine told me that scientists have observed metabolic reactions (chains of molecules processing "food" into other stuff) happening outside of "living" cells. I don't think anyone wants to claim that these metabolic reactions are alive just as is but it raises some interesting questions about how life began and how life works and what exactly the differences are between "living" and "not living."
And finally I watched this TED talk the other day. You should just go ahead and watch it. I'll wait....
I know, crazy, right?! That bit where the "cell" splits in two, essentially showing reproduction happening in a system that for all intents and purposes is "not alive" is just so cool and bizarre and throws a lot of other assumptions about cells and life into question.
Or maybe it doesn't. Maybe it just teaches us about what things on primordial Earth were like. Maybe viruses sort of do the same. Maybe 3.5 billion years ago there were lots of things that were sort of alive but by means of natural selection and competition the truly alive mostly won out. One thing is for sure: that's some pretty neat science.