DarkSyd asks a question:

The Fermi Paradox is a conundrum proposed by pioneer physicist Enrico Fermi that questions the likelihood of Intelligent Extraterrestrial life. It begins with the Drake Equation or some derivative which guesstimates the possible number of intelligent civilizations in the universe, and then extrapolates expansion rates into the universe from a point location within the cosmos of that species or culture. The paradox concludes that there should have been enough ET's over the last 14 billion years that even if they moved at velocities achievable by human technology today, they could have swarmed over the galaxy, or even the cluster to which our galaxy belongs, many times over.

…

Remember, all it takes is a single space faring civilization to develop and survive. It would only have to happen once in all the history of the local group of galaxies and they should be here, or we should at least detect signs of them relatively nearby. So the question, naturally, is where are they? Where are the ruins? Even if they're not here on Earth right now in any obvious way, where is the interstellar traffic lights or radio chatter or giant interstellar construction projects, some of which would plausibly be grand enough for us to detect from our earthbound and space based observation platforms? Does this mean we, as intelligent beings, are unique or rare beyond imagination? Is it evidence for a Theistic Creator Entity or entities which created human specifically? Why or why not? How would you address the Fermi Paradox?

I think it's a non-problem and a non-paradox. The simplest explanation for the reason that ET isn't tapping on our shoulder is that the Fermi and Drake assumptions are wrong—the kind of technological intelligence that might build spaceships and radios and harness fire is very rare, and techno-species are spread very thinly over vast and uncrossable tracts of space.

I'm with Ernst Mayr on this one. Read the Planetary Society debate on SETI, in which he took the con side, while Carl Sagan argued for SETI. Mayr has a very dim view of SETI, as do I, and while I think Sagan was a clever man, I think he totally missed the point. There was also some amusing interdisciplinary physicist-bashing.

What Percentage of Planets on Which Life Has Originated Will Produce Intelligent Life?
Physicists, on the whole, will give a different answer to this question than biologists. Physicists still tend to think more deterministically than biologists. They tend to say, if life has originated somewhere, it will also develop intelligence in due time. The biologist, on the other hand, is impressed by the improbability of such a development.

But the gist of his argument is that we do have one fairly substantial body of evidence that illustrates the probability of intelligence evolving, and it's right here in the history of planet earth. We've got about a half-billion years worth of sophisticated multi-cellular animal life on the planet, and our kind of technological intelligence has appeared only once.

After the origin of life, that is, 3.8 billion years ago, life on Earth consisted for 2 billion years only of simple prokaryotes, cells without an organized nucleus. These bacteria and their relatives developed surely 50 to 100 different (some perhaps very different) lineages, but, in this enormously long time, none of them led to intelligence. Owing to an astonishing, unique event that is even today only partially explained, about 1,800 million years ago the first eukaryote originated, a creature with a well organized nucleus and the other characteristics of "higher" organisms. From the rich world of the protists (consisting of only a single cell) there eventually originated three groups of multicellular organisms: fungi, plants and animals. But none of the millions of species of fungi and plants was able to produce intelligence.

The animals (Metazoa) branched out in the Precambrian and Cambrian time periods to about 60 to 80 lineages (phyla). Only a single one of them, that of the chordates, led eventually to genuine intelligence. The chordates are an old and well diversified group, but only one of its numerous lineages, that of the vertebrates, eventually produced intelligence. Among the vertebrates, a whole series of groups evolved--types of fishes, amphibians, reptiles, birds and mammals. Again only a single lineage, that of the mammals, led to high intelligence. The mammals had a long evolutionary history which began in the Triassic Period, more than 200 million years ago, but only in the latter part of the Tertiary Period--that is, some 15 to 20 million years ago--did higher intelligence originate in one of the circa 24 orders of mammals.

The elaboration of the brain of the hominids began less than 3 million years ago, and that of the cortex of Homo sapiens occurred only about 300,000 years ago. Nothing demonstrates the improbability of the origin of high intelligence better than the millions of phyletic lineages that failed to achieve it.

In part, this is a probability argument: it is saying that the relevant parameter in the Drake Equation is very, very small, perhaps much smaller than the SETI devotees were plugging into it. Maybe, if we actually had accurate values for the equation, the expected number of spacefaring civilizations in our galaxy is something less than 1. The 'paradox' isn't.

But there's another, subtler lesson in there. What he's saying is that there doesn't seem to be any evidence for a predisposition to favor intelligence in biology. Features like multicellularity, photoreception, long sharp fangs, flight, etc., pop up in life's history over and over again, independently; but intelligence? Feh. The universe doesn't seem to like smart guys. We happened once, and what's more, we seem to be teetering at the end of one long chain of improbable events in the history of one marginal set of lineages, of which most of its members are in decline.

Carl Sagan didn't get this at all, and actually made a surprisingly foolish comment in his argument:

…it is better to be smart than to be stupid, and an overall trend toward intelligence can be perceived in the fossil record.

That's just wrong! He's done what Gould called "retrospective coronation", standing at the end of a long trajectory of evolutionary events and looking back, and assuming that his path was inevitable and favored. Life has gone in many directions, and intelligence is one of the least used paths. We selectively notice those rare species that show some hint of similarity with us, but honestly, the majority don't swing that way at all. If one looks at the history of the biota here without the usual self-important vanity, it's the bacteria that are the major success story, and the last big innovations that fueled an explosion of new, successful species were the flowering plants and grasses.

There are a couple of other reasons I'd throw out for thinking that extraterrestrial intelligence is not unusual for its absence.

Mortality. There's no reason to assume that intelligence confers longevity on a species…quite the contrary, being highly specialized may well make a species more fragile and sensitive to disturbance. Civilizations may be flickering in and out of existence before they have an opportunity to make themselves known. (Depressing as it may be, Homo sapiens will someday go extinct. Get used to that fact.)

Counterforces. One suggestion is that when a species reaches a certain level of intelligence, it passes a tipping point that drives it towards greater and greater specialization on intelligence. There could also be forces that oppose that trend. Once an intelligence has ensconced itself in a comfortable shell of civilization, there's no further incentive to be smarter, and there's even pressure to be less clever and fit in. Maybe civilizations reach that point where they invent TV, and then everything goes downhill.

Local opportunity. We really haven't reached the level of a spacefaring civilization, so we don't have any idea what it is like to have large numbers of people living off-planet. Maybe once you do reach the level of being able to live comfortably in space for long periods of time, there are new distractions that make haring off to some other star uninteresting. Those rare civilizations that leave their homeworlds may spend millions of years enchanted with and exploiting their local gas giants or asteroids or whatever.

Life can't cope with the Big Empty. We've evolved to live in the thin layer of slime on the surface of a planet with a particular kind of atmosphere, and we're used to thinking of our environment as relatively conducive to our existence. But the rest of the universe isn't like that, and the big message from space is that it doesn't like our kind. Being well adapted to thrive in a biologically rich environment may be what makes intelligence unsuited to thriving in a more sterile, dead environment. Space is for spores, not people.