It may not be the oldest question in science fiction (that distinction may go to Mary Shelley's Frankenstein, and "Can Man create life?"), but it is certainly the most intriguing and defining question for science fiction: "Is there life elsewhere in the cosmos?"

And like the best of questions... it's complicated.  Even moreso than Shelley's question; after all, men and women create life all the time (it's called sex... ask your parents, and watch them squirm).  But the intention of the question was to ask whether there are ways of creating life beyond traditional techniques of procreation.  When it comes to life outside of Earth, there are rules, unknowns and (cough) math involved, and the resultant paperwork is pretty messy.

Not that that hasn't stopped science fiction writers from dreaming up all manner of forms of life off-Earth, in every form from Wells' tentacled Martian invaders, to Burroughs' four-armed warriors, to Lem's sentient ocean, and every variation in-between.  All of that is fiction, of course, and fiction writers can handwave and write whatever they want; the question is moot.  

But for those of us who really want to know, the hands have to stop waving, and some things need to be addressed. For instance, Life itself: It exists on this planet, in myriad forms.  Science and history teach us that a pretty incredible set of nearly-miraculous physical and chemical interactions had to happen over billions of years to create life on Earth... but as the evidence shows, it did happen.  And scientists have done numerous experiments to see if those steps could conceivably happen again, and have discovered that most of them seem to happen pretty spontaneously, given enough time.  So, given the right circumstances, which may not be so hard to replicate, life arises.  Life is something we consider miraculous... and somehow, almost inevitable.

And of the circumstances that created life here, are any or all of them repeated anywhere else in the cosmos?  Observation of our galaxy suggests the answer is Yes; that there are planets out there, capable of being able to generate the proper circumstances—including some circumstances we may not have considered yet—that can create life of some sort.  We suspect we share a Solar System with a few of them.  Not to mention the incredibly unlikely event that, among all the stars in the galaxy, absolutely none of them have planets that are even slightly like Earth, or the other planets and moons in the Solar System, that can potentially generate life.  Given that, it's more than likely that Yes, Virginia, there is life among the stars.

But ah, that's only part of the real question, isn't it?  Has any of that life out there risen to the level of "higher life forms;" that is, more advanced than paramecium, possibly even self-aware?  Well, if the numbers make it statistically likely that life exists, the numbers are conversely not so positive beyond that.  Although the right ecological balance must be struck to support life, the history of life on Earth has shown that life had to aggressively overcome serious hurdles to evolve into a higher form... beginning with the ability to evolve, itself, and continuing with the opportunity to suffer the trial-and-error, hit-and-miss, lucky-breaks of survival-long-enough-to-procreate cycle of life over the millennia.  And for every successful strain of life, there were uncountable failures and dead-ends, and some that simply hit an unscalable wall and lost the evolutionary battle.

But if there were enough instances of life to take on all of those incredible odds, at least a small subset of those instances would be likely to evolve and survive.  And if their ecological situation provided enough resources, that life might have enough to allow it to thrive.  

So far, the possibilities don't seem too bad.  But higher life forms are one thing... what about intelligence?  What about sentience?  When we get to this level of life, I consider intelligence and sentience as survival traits; something life forms develop in order to preserve themselves.  An organism figures out what sensory information equates to safe food to eat: That is intelligence.  An organism reasons that it needs to eat to survive: That is sentience. Do these things develop autonomously?  Only if some foods are inedible... or if the life form is driven to search for the food in the first place.  Think of a tree: It does not need to seek food, food is in the soil where it grows, and it ingests that food autonomously.  If food is everywhere, and eating is safe and effortless, there is no drive for intelligence or sentience for survival.

But often, there is more to life than just safe food.  Competition for food (or to avoid becoming food) can mean developing new skills or strategies to figure out what is food, how to get to the food, and when to eat it (usually before it maybe eats you).  Where there is a lot of life, there is likely to be competition, and possibly survival at stake, the primary drivers of intelligence and sentience.

Okay—competition and survival generate intelligence and sentience.  Under the right circumstances, we might find a lot of that in the cosmos: Creatures that exist because of their ability to compete and survive.  But we haven't reached higher intelligence yet; the ability to think abstractly, to use tools, to develop language... which supposes a level of group cooperation.  Some of this can be driven by a heightened sense of competition or survival; but it could also be driven by an abundance of free time (time not devoted to just surviving).  Higher intelligence might require a sense of curiosity, the ability to recognize that everything is not known, and to be able to ask, "What if?"

This might be what sets human being apart from, say, dolphins: Dolphins may exhibit group cooperation, and have a form of language... they have even demonstrated playfulness... but they may not be particularly curious about things.  If a dolphin breaches the surface and looks into the night sky, it may see the stars above; but it may not be particularly curious about what those tiny lights are, nor do they consider how to reach them.

But maybe dolphins are more curious about other things... like the human beings that build such amazing things and seem to be so interested in them... or why their oceans are getting harder to live in.  They may not be interested in the stars... but they may be interested in the state of their environment, how to fix it... and if humans can help.  So we may not be so different from dolphins, after all.

If there are higher lifeforms out there, they could be more like us... or more like dolphins.  Or they may look like nothing we've seen, or can barely even imagine, but live in the right circumstances to make them wonder, "What if?"

But all of these variables... planets in the right condition, reasonably supportive chemistry, a reasonably supportive environment, the pressures of survival, the time to develop, and the luxury of considering life beyond that survival... add up to an incredible string of odds.  And the fact is, although the chance of all these variables being positive is not zero... the chance may be pretty small.  And the other fact is... there are no facts.  Without evidence—which we largely don't have—we can't support many of these variables.

All of which adds up to the ultimate fact that, although life off Earth is theoretically possible, at the time of this writing... no one knows.

But as I said, that never stopped science fiction writers from imagining alien life, just as it never kept people from reading such stories.  Science fiction is about conjecture... and imagination.  It's allowed to ask the big questions—including the BIG question—and answer it any way it wants.

My thoughts: Yes, there's life out there.  Probably some that are even intelligent.  Most likely, it lived long before we crawled out of the slime, or will live far beyond the point at which we are dust.  There may be life out there similar to us, but so far away that we'll never have the chance to meet.  But it's not likely that we'll ever know for sure.  But here's the kicker: Those questions pale in comparison to the real BIG QUESTION, which—if you haven't guessed—is: 

Why is there life?  Is there a reason behind organized collections of organic and inorganic molecules, that consume resources, reproduce themselves, and incidentally support other organisms, only to die and return to its component molecules?  Is there a point, a value, a purpose behind the impacts life makes to the landscape around it, however large or small, when inevitably the universe will erode those impacts in its inexorable voyage to entropy, leaving no trace of itself behind?  Does life mean anything to everything?

Speaking of everything: Carl Sagan suggested, in his book Cosmos, that life is the universe's way of knowing itself; that life creates self-awareness.  Is self-awareness the whole point to there being life, then?  Just an unfortunate by-product of overly-complex collections of molecules?  Or does some inevitable quantum mechanism force awareness on organisms once they become complex enough?  These could be questions to keep physicists and philosophers at each other's throats for centuries.

I suppose we can also debate about how self-aware the tiniest paramecium can be, but taken as a whole, we can say that life is aware that it exists as part of a larger world.  Maybe those individual life forms aren't aware of any purpose to their life—they may not be finely aware that they are even living—but maybe awareness itself really is the only thing that matters.  We may not know where we came from... we may not know what we're doing... we may not know if there are others like us... and we may not know where or how we will end up.  But we know we were here. 

Sounds very egotistical, I know.  As if knowing that fact elevates our importance to the world.  As if the cosmos cares that we know of it, will do anything differently in deference to that fact, or will end up any differently at the end.  It won't.  But, through part of its journey at least, existence will have a witness, a cosmological Kilroy out there, doing what it does in a world that does what it does... and continually reminding itself that it is here.