Every so often someone will come up with a new theory to explain the Fermi Paradox, a contradiction between the number of alien civilizations we might expect to see in a universe so vast and the actual number of alien civilizations we have been able to detect with 150 years of science. The latest entry (click here) provides a perfectly cromulent theory based on the probable difficulty of life to sustain itself even on planets that are generally habitable. Life is more than liquid water and temperate climates, but a complex system of feedbacks dependent on stable geological processes and sufficient biomass to perform the critical regulatory role life has in sustaining its own habitat. Perhaps thousands of galactic ecologies were strangled in the cradle by their own fragility, perhaps even the majority of them. That would remove a lot of potential aliens from the equation.
But even for those ecologies able to get past this Gaian Bottleneck and maintain habitability there are plenty of other barriers between life and civilization. Many point out the rapidity with which life arose on Earth as a sure sign that the galaxy must be lousy with civilizations, but as rapid as that may have occurred, the critical jump to multi-cellular complexity took almost three billion years. Any ETs that may have emerged and taken a look at Earth during those three billion years would have noticed next to nothing in the way of life, unless they were able to look very closely. So maybe the Golden Age of Galactic Civilization peaked two billion years ago, and those guys are long gone. Or maybe complexity is so rare that few life forms have ever gotten there at all. Based on the history of life on our own planet (the only case study available), we can expect most alien life is too small to see and probably, as microbes, have nothing much to say.
Still, the odds are fairly good that life evolved more complexity at least somewhere else in the universe. Maybe even within the confines of our own galaxy. But that level of complexity is no guarantee of intelligence either, and again using own world as model, it was only within the last 200,000 years that civilization building intelligence emerged (this out of a 700 million year history of evolving complex organisms). In all that time, from well before the age of dinosaurs to deep into the age of mammals, millions of species adapted to survive on a constantly changing planet, but none of them ever found a high level of what we call intelligence to be a useful adaptation. Self-reflective hubris may have convinced us that our intelligence is the bestest and the brightest old adaptation that nature ever selected, but given how rare it is, and how late in the game it has come along, it would seem that nature has nothing but contempt for the trait.
But even intelligence only gets you so far. Our own, emerging within the last 200,000 years, was stuck in the Stone Age for a long time. The stone hand ax was the pinnacle of human technology for over 150,000 years, much of that time well past our transition into Homo sapiens. Then the agricultural revolution allowed us to settle down and gather into city-states and build empires, but even as our technology and culture grew we were blissfully unaware of, and incapable of making contact with, whatever other life forms may exist in the universe. We excelled in the following millennia in city building and war, but did not reach the technological sophistication necessary to communicate with extraterrestrials until quite recently. There is a common conceit that had Christianity not strangled European thought during the Dark Ages we would be a thousand years ahead in our scientific and technological progress. This notion is not only racist, but assumes progress is inevitable. That may not be any more true for the evolution of civilizations than it is for the evolution of life.
Life – Complexity – Intelligence – Civilization – Technology. Because all of these things have happened on our own world we tend to see them as being inevitable, but it is probably closer to the truth that each one is significantly less likely to occur than the one that proceeds it. Which does not mean that there aren't any aliens out there, only that they are probably so very far away that it would be impossible to have an intelligent conversation with them.