Much has been given to Fermi's Paradox – the lack of evidence (radio signals, spaceships) of intelligent life from the assuredly millions of habitable planets in our universe. Given that intelligent life (humans) evolved on planet earth in just under 4 billion years, and would have millions and millions of opportunities to evolve on other planets during the universe's 13+ billion years, Enrico Fermi wondered just why we weren't seeing loads of evidence.
Geoffrey Miller has an hypothesis that they just get addicted to computer games. (You can read his piece at seedmagazine.)
However, I suspect the answer is much more mundane. We haven't seen any extraterrestrials because we've vastly overestimated the probability of intelligent life looking and acting like us.
We have a sample population of one: Earth. Because intelligent life did evolve here, we look at the progressions that led to it, and (incorrectly) assume that life evolves along similar paths on every habitable planet. However, that's a case of what statisticians call survivorship bias. Our view is biased because we have only looked at a sample that played out in the way that we expect it to. To really bring our knowledge up to par with reality, we would need to look at a large sampling of habitable planets, and see if and to what extent life evolved. Likely, that would change our ideas considerably about the probability of life, and the paths that lead to intelligent life.
If you look out earth's history, too, you can see ready evidence that life almost got wiped out several times – the most recent 65 million years ago when it appears that a fairly large meteor crashed into the Gulf Sea, and possibly alone or in conjunction with other geologic occurrences (volcanoes) wiped out more than 30% of the known (from fossil records) species. The Permian-Triassic was even worse, with 96% of marine and 70% of vertebrates going extinct. Think of how little more it might have taken to completely wipe all life from planet earth, forcing a restart. Given enough planets, total extinction events have surely occurred. Many unlucky planets may have to restart regularly, every 100 million to 1 billion years. That'd put a crimp on life evolution!
If you look at the diversity of life on our planet, too, you see many, many more species that are just as (and sometimes more) evolved than humans that exhibit no intelligent traits. I know that you are thinking that there is no species more evolved than humans, but that's not true. Viruses are far more evolved, if you look at number of generations and quantity of genetic mutations – the true measure of evolution. Evolution is a game that counts only survivorship – and there is nothing that predicates that intelligence makes a more survivable (or more evolved) species. As another example, take crocodiles: They have been around far longer than humans (84 million years), have developed a cerebral cortex, but are a long way from building rockets or sending radio signals into space.
Our major difficulty when it comes to dealing with questions of this nature is that we find ourselves at the heady top of the life pyramid on this planet, and arrogantly assume that all paths lead to us (for we are here, aren't we?). But that's not properly factoring the role of chance in the outcome. Chance led to those genetic mutations that evolved the primates. And a chance collision with a rather large asteroid wiped out those massive dinosaurs. Chance provided earth with just this temperature and just this quantity of water. And, by chance, each of those other 'habitable' planets that exists in our universe is likely a little different in heat and composition, exhibiting different rates of genetic mutations, different landforms that change migration, dispersion and extinction patterns of any life that does evolve.
Is it a paradox that we haven't encountered any evidence for other intelligent life? We don't have enough evidence, one way or the other, to predict and then label the lack a paradox. The likely answer is that the ways in which life can evolve, and the forms of intelligence that can occur, are far beyond our feeble imaginations.
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