This is well-known if only implicitly stated to imply life as we live it and understand it in our labs. There are many good reasons to think this way, not least being that one doesn't know how else to think, and we can't sell grants or stories to the media without conjuring up at least recognizable images. That may not be the best kind of science, but it is certainly understandable.
Going further, the real hunger is to find intelligent life out there somewhere. SETI, the Search for Extra-Terrestrial Intelligence, has been scanning the skies for decades with this in mind. The underlying idea is that such life is like our (that is, human) intelligence. That means, we assume, that LifeOutThere communicates long distance by electromagnetic emanations, since that is what we currently know could best accomplish this. But that also means controlling the emanations in a language-like way. For us to detect it, the emanations must be non-random relative to space's electromagnetic background, which is very complex but presumably all due to natural physical processes; and unlike intelligent life whose signals would be 'unnatural' in being highly patterned, just as this message is: part of the physical world, but composed by directed processes and designed to contain abstract or symbol-based information.
There's nothing wrong with searching for a kind of life with which we could get in touch. After all, if it exists here on Earth (i.e., in the form of you and us), it must be physically possible elsewhere. However, since we've been on Earth as 'intelligent' beasts for only about 100,000 years (depending on what fossils you count) but only using such communication for 100 of those years, perhaps you can see a problem.
The known (and only currently knowable) universe is about 13.7 billion years old. Our existence comprises something like only a ten-millionth of the age of the universe, and our use of telecom only about a billionth. So, if we are typical and reflect how long it takes a planet to evolve to our state of smarts, then we need to ask how plausible it is that other civilizations exist in a similar state, at this very time, so we can communicate with it. Is it too much coincidence to expect? There are about a hundred billion galaxies each comprising a hundred or more billions of stars, and many of those stars would have 'habitable' planets. So the idea that They may be out there somewhere is not entirely fanciful, even taking the coincidence factor into account.
But if you look at our posts about Gliese 667C (here and here), the reported star with some juicy-looking 'habitable' planets, and our earlier set (here's the first of those posts) on life in space and the 'infinity' game, you'll see that the idea that we'll find such companions in brainpower becomes pretty far-fetched (assuming brains is what They have). Indeed, even if SETI does some day detect signals, even then it will not be the kind of coincidence that press releases will probably suggest: the signal will likely be from a source millions or billions of light-years away, meaning that the message was sent that long ago. We're only seeing intelligence as it was way back then!
Given how ephemeral the lifespans of we earthly species' are, one wouldn't hold out much hope that those They whom we detect still even exist! Perhaps even stranger, any They who do exist probably haven't been detected because their emanations just haven't reached us yet. By the time we get those signals, the senders' will again likely be gone to evolutionary oblivion. Or, by the time messages from sender cultures that exist today reach the earth, it will find only bleached skeletons on windswept plains.
But there's another aspect of the search for 'intelligent' life that we want to discuss. What, in the first place, is 'intelligent' life? It seems like a meaningful term, but is it?
Just like us?
The usual vision invoked as we noted above, is that intelligent life is like us, communicating by something we'd recognize as language, and in that sense 'thinking' in terms that are commensurate with our thinking. If we're on totally different conceptual wavelengths, so to speak, entirely different ways of 'thinking', then there won't be any recognition much less communication.
But the very definition of 'intelligent' is usually left out of the discussion, since the idea as implicitly taken doesn't really bear a close look--unless it simply means Just Like Us. We can't dictate a proper definition of 'intelligent' but we want to cite an example that shows the confused nature of the notion.
An insect robot
There were recent stories of robotic flying devices that could maneuver more or less like small flies or midges. They can flit around, change directions, explore their environment. As with other robots, there is feedback, such as from light, sound, heat, or chemical sensors, even video camera transmissions to some computational device that directed the 'flynsect' to go this way or that, and so on. They could, for example, be programmed to pollinate crops so we no longer rely on bees.
Currently, as shown in the figures (from the website of the Harvard research group that made these), the flynsect is tethered with a power wire, but surely that is only a temporary restriction. But compared to real midges (can you guess which are which?) the work is remarkably real and life-like.
This achievement is part of the general development of robotic drones these days, and clearly works quite well for such an early model. All sorts of plans are afoot for such objects that can do all sorts of checking things out (and you can be sure you'll be observed by them, whether you want to be or not!). Indeed, plans are for military robots that can even make kill decisions without a monitor back at home base pushing a 'shoot' button, as is the case with current military drones.
Now, this is hailed (properly) as quite an achievement of engineering. In that sense it is computers developing 'artificial intelligence.' Indeed, in the figures above, the damn thing, preliminary as it is, looks and moves remarkably like a real midge! And it is clearly the product of a whole range of intelligent designers and builders. It may or not may not be 'self aware', but it certainly is an extension of self-aware consciousness--that of the makers without whom it would not exist or do anything. So in every useful sense it is an 'intelligent' device.
But, so is a real fly! Why don't we consider a real fly to be intelligent life? Trees are very different from inanimate rocks, and they act as whole organisms in many ways. But they haven't much central control, nothing corresponding to neural wiring, so perhaps they don't qualify. But insects, especially ones like the above electronic midges, that do just what we think is intelligent when we do it, perhaps should qualify. If we want to insist that consiousness is a requisite for 'intelligence', we might say that the flynsects aren't conscious so what they do doesn't count, but how can we know whether they are conscious if we don't even know what consciousness is, or how to detect it? If it requires heavy-duty consciousness (humans') to design a very imperfect fly-bot, why do we deny that the real thing, tiny and made all on its own without any external engineers, has some such cognitive-aware powers, whether they resemble our experience or not?
We could put our point in another way. Were we to discover 'midges' on some planet elsewhere, of a sort we couldn't relate to our particular earth's evolutionary history, but stumbled upon them busy in their highly orderly activity, would we not have every reason to consider them 'intelligent', even if they didn't have radios or speak the way we do? Their nature and existence, could we find them, would be wondrous comparisons for what has happened here on Earth.
Of course, since they don't send out intentional emanations, don't code language into radio waves, or don't build spaceships, our radio telescopes wouldn't find them, so that, really, we are just searching the cosmos for mirror images of our noble selves. Or could they emanate? Since electromagnetic waves do (eventually, in theory) traverse all the universe, and since the motion of real midges involve energy release, and hence photon emanations, they probably do emit signals. They'd be highly organized, not just random electromagnetic noise like deep-space waves. Would we recognize them as purposive, problem-solving phenomena worth sending a spaceship to visit? Would they seem like some sort of language?
Just as interesting to contemplate is that if the highly patterned and nonrandom emanations from such beasts were detected by finely honed radiotelescopy, could SETI infer that they were the ethereal musings of minds? Would we mount expensive efforts to communicate with them?
A dreamy world
When we think about the nature of space, we sometimes wish we had become cosmologists. But reality beckons, and while we find this to be as fascinating to think about as anything can be, we find the promotion of the fanciful aspects of space to be at least as big a boondoggle as GWAS. We only wonder whether the people pushing it aren't much more aware of its fanciful nature than most human geneticists are of the GWAS bubble they're blowing. Physics has better theories than biology does in this respect, and hence perhaps fewer excuses.