We as humans have put a great deal of effort into exploring the solar system and beyond; and as our technological capability increases rapidly even exponentially, we can contemplate exploring (or even colonizing) the entire galaxy. So the question naturally arises, are there any other intelligent civilizations seeking to explore the galaxy? If there are, why have we apparently not encountered any of them?

The question is usually associated with Enrico Fermi, according to this anecdote:

Originally Posted by Scientific American

On the way to lunch at Los Alamos Scientific Laboratory one day in 1950, Enrico Fermi and three other physicists--Emil Konopinski, Edward Teller and Herbert York--chatted about flying saucers. At lunch, when the talk had turned to other matters, Fermi suddenly said, "Where is everybody?" His companions realized that the talk of flying saucers had turned his mind to the possibility that there is intelligent life elsewhere in the universe and that he was asking why, if there is, we have seen no sign of it. The question encapsulates what is now known as the Fermi paradox.

Fermi's question can be viewed as a simple logical statement: "If there are exploration-minded alien civilizations, then we should have a record of encountering them." Thus, one must either reject some of the premises, or agree with its conclusion. There are several categories of responses to Fermi's Paradox, as listed in the poll.

Note that the Paradox is more than idle speculation: each possible response recommends a very specific course of individual action. Our response to Fermi's Paradox is a good indicator of our entire world view.

The poll responses should be fairly self-explanatory; I'll go into more detail on each, along with my take on the pros and cons of each in later posts.

First, the "aliens are here" options:

Chariots Of The Gods: popularized by the Erich von Daniken book, this view asserts that space travellers visited Earth many millennia ago to establish the human race on this planet. Some see the story of Atlantis as an ancient memory of a more advanced civilization.
Argument for: many see this as a convincing explanation for a number of archaelogical curiosities, as well as an answer to how the human race developed at all.
Argument against: there have been no ancient spacecraft or advanced technologies unearthed.
Action recommended: a supporter of this view should be actively involved in archaelogical research, seeking to uncover the remnants of ancient technologies.

Area 51: popularized by the TV show X-Files, this view asserts that aliens visit our planet regularly, but that the government of the US (and other countries) are actively seeking to suppress this information and keep its benefits to themselves.
Argument for: there have been many, many reports of UFOs in the last 50+ years, and the US government has sought to suppress some of these reports, as well as many other potentially damaging revelations.
Argument against: no widely accepted proof of UFO sightings has been announced, and it is unlikely that even the US government could completely suppress such an important discovery for so long. It would only take a single individual to expose the coverup.
Action recommended: like Fox Mulder, a supporter of this view should be actively involved in attempting to find evidence of the government conspiracy.

Prime Directive: popularized by the TV show Star Trek, this view asserts that aliens are visiting us, but have chosen to keep their presence hidden, perhaps in order to study us and determine whether we are "ready" for their knowledge.
Argument for: we would certainly image that an advanced race would want to interact with us in the least panic-inducing way. This would also reconcile the large number of UFO sightings with the lack of proof.
Argument against: it would only take a single alien race to break the embargo.
Action recommended: a supporter of this view should be actively involved in preparing humanity to accept the inevitable alien contact.

Rick, I like you survey, but it falsely implies that some category other than the last one is worth a dime of consideration, IMHO.

I can stretch my imagination far enough to include microbial life of the DNA/RNA kind on other planets, maybe even in our own solar system. But that is vastly insufficient to allow for any serious possiblilty of ETI. Humans want to believe in ETI, of course, because humans are predisposed to do so. Humans will believe in anything if it serves a human purpose. Frogs are not so gullible.

I have a copy of Asimov's Extraterrestrial Civilizations, wherein he reasons the probability of ETI is 1.0. It's a wonderful book, and he's a wonderful writer, but it is still sci-fi down the line.

Please consider this: It took a billion years after Earth accreted for prokaryotes to evolve on Earth well enough to leave a fossil record. Then it took another billion for eukaryotes to show up. Then they languished in the ancient oceans for another billion years before they invented metazoans. I mean just getting to sponges and jellyfish is so improbably as to render Earth the only home in the universe for these primitive creatures, and they are a long long way from intelligence by any frog or human standard. That kind evolution took another billion years.

What makes anyone believe that this kind of evolutionary careering is likely to happen anywhere else? Hope is the only cause I can think of, and hope is not scientific by any measure. Hope is the province of popular seduction, like religion; it feels good when the odds are bad.

Forget the ETIs, Rick. I serious doubt if there is even so much as a sponge anywhere else but on Earth. We are all alone out here on this marvelous rock. We are a one-off, an incredible freak of nature, and there is nohing I know of that suggests any other conclusion.

--Croaker

Our middling size Milky Way galaxy is estimated to have 1 to 200,000,000,000 stars. The Milky Way is but one of an estimated 100,000,000,000 galaxies in the universe.

There are potentially so many different places in the universe that there may well have been enough opportunities for metazoan life to evolve elsewhere.

On the other hand, this and the sheer vastness of the universe would seem to imply that metazoan-bearing worlds are extremely distant.

Second, the "aliens are not here yet" options:

Slow Boat: this view asserts that aliens are in fact exploring the galaxy, but are limited to relatively slow speeds, thus they have not reached us yet.
Argument for: this is consistent with our current understanding of physics, and our own slow progress in exploring even the solar system.
Argument against: we recognize that there are still principles of physics yet to discover; and even at sub-light speeds, it should be possible to explore the entire galaxy in a few million years.
Action recommended: essentially, this is arguing that we are the most advanced race anywhere within range of our sensors, so a supporter of this view would take the same actions as for several other responses, especially the "Naked Ape" below.

Lost In Translation: this view asserts that the aliens are in fact attempting to communicate with us, but we are unable to recognize the signals as communication.
Argument for: it is reasonable to speculate that the aliens may communicate in ways entirely unknown to us.
Argument against: the aliens would surely be aware of simpler, more universal communication mechanisms, such as EM radiation (e.g. light and heat.)
Action recommended: a supporter of this view would be actively involved in the search for alien signals (think SETI@Home.)

Berserkers: popularized by the movie War Of The Worlds as well as the recent book The Forge Of God, this view asserts that we have not seen any aliens, because any civilizations that make themselves known are destroyed by the advanced races (who view them as competition or prey.) Thus, other civilizations are actively hiding their existence.
Argument for: there are certainly rapacious, expansionist civilizations -- just look at our own.
Argument against: it seems reasonable that at least some aliens, assuming they have advanced to the stage of interstellar travel, would have learned to overcome violence and destruction. If even one such civilization existed, they might seek us out as allies rather than enemies.
Action recommended: a supporter of this view would be actively involved in encouraging (or developing) technology to resist the inevitable alien attack.

If aliens are actually common in the galaxy, I think the simplest answer is they have no interest in coming here.

If one assumes that all travel between starts is restricted to sublight speeds, then trave between stars is going to take a long time and require enormous amounts of energy (wealth). Based on our own situation we rarely see people making sacrifices to make enormous investments to accomplish something that will not be achieved in their grandchildren's lifetimes.

Thus I can't see any species making the sort of investment needed to travle to other stars unless they are long-lived. A long-live race would see the reslts of his investment in their own time and if sufficiently wealthy (i.e. able to wield large amounts of energy) would likely be willing to support interstellar travel.

A long-lived species must have reached a zero population growth situation long before attaining their long lifespan. So you would have a long-lived non growing population of beings sort of like Tolkein's elves. Most of them would be old and probably not particularly care for change, sort of like Tolkiens elves who want to preserve all things unchanging.

This is not the sort of society that I see taking on bold expeditions to the stars.

A more youthful, still growing, short-lived race (like men) would be willing to make the trip, but unable to justify the investment.

Thus I would say those who want to go to the stars can't and those who can don't want to.

And last but not least, the "aliens will never get here" options:

Stay-At-Home: this view asserts that alien races who are capable of interstellar travel have no interest in exploring the galaxy.
Argument for: interstellar travel would be enormously expensive and risky. Most civilizations may not be willing to make the investment.
Argument against: if even one civilization chose to explore, it would eventually contact us. Plus, even a civilization that chose not to send manned missions could send automated probes (as we have done.)
Action recommended: like the "Slow Boat" view, a supporter of this view would be actively involved in seeking out signs of other civilizations, but also ensuring that our own planet survives.

Big Bang: this view asserts that any civilization sufficiently advanced for space travel also develops the capability to wipe itself out, and quickly does so.
Argument for: we certainly see this possibility in ourselves. Any race capable of understanding space travel would have unlocked the power of the atom, and thus the potential for self-destruction.
Argument against: as with the other scenarios, it only takes one alien race to survive, and eventually travel the galaxy.
Action recommended: a supporter of this view would be actively involved in ensuring that we do not destroy ourselves, at least not before we can move beyond this planet.

Rare Earth: this view asserts that the conditions for life are so rare in this galaxy, this earth may be one of the very few planets in the galaxy capable of sustaining life. Thus, the probability of encountering other life is extremely small.
Argument for: the more we understand about biology and astronomy, the more unusual our planet seems, with its very specific conditions amenable to life.
Argument against: the galaxy is so large, that even the necessary rare conditions should exist in hundred if not thousands of places.
Action recommended: as above, a supporter of this view would be actively involved in ensuring that this planet's fragile ecosystem is maintained.

Naked Ape: this view asserts that even on a planet capable of sustaining life, intelligence and tool-making capability are very unlikely to develop.
Argument for: we know that meteor and comet impacts are fairly frequent, even in our solar system. Perhaps any evolving life is much more likely to be destroyed than to advance sufficiently.
Argument against: again, it would only take a single civilization to reach the point of sending out (automated or manned) probes, even if the civilization were later destroyed.
Action recommended: as above, a supporter of this view would be actively involved in ensuring that this planet's fragile ecosystem is maintained (perhaps also including a system of protection against meteor impacts.)

I meant to post all those explanations together, but it took me a while to get them typed in.

There is one issue that is relevant to all the different options -- that of automated space exploration. A Bracewell-Von Neumann probe is an automated, self-reproducing mechanism with a rudimentary AI. Once launched, it would travel to a target star, and once there, use available raw materials to make copies of itself. Thus, the number of probes would grow exponentially, their reach would eventually expand throughout the galaxy, even assuming substantial losses and slow travel times. We are fairly close to achieving this capability already.

Thus, Fermi's Paradox becomes not only "Where is everybody?" but "Where are their signals and automated probes?" This added possibility substantially weakens the arguments in favor of the "Slow Boat", "Stay-At-Home" and "Big Bang" viewpoints, because if a civilization were capable of launching such a probe at any stage of its existence (even if it destroyed itself or otherwise stopped exploring), then the galaxy would eventually be full of such probes.

Rick: I've said it before, I'll say it again...when you have a BIG post, type it up as a text file, then select all, copy and paste.
Croakmore: It is the next to last option that makes the most sense...just developing a proto-bacterium (or even a super protein or nucleic acid molecule) capable of then evolving into a more sophisticated form is amazingly difficult.

Actually, the aliens are in California, and Texas, and Florida, and everywhere else with a large immigrant population.

Seriously, though, with regards to non-Earthlings. I favor a combo of the last two. I don't think life is that rare (there may well have been life on Mars) but intelligent life certainly is. My opinion is not strongly held though.

Croakmore:

You and I have been over this stuff before, and I think you're assuming something that shouldn't be assumed, namely that evolution is an entirely random process with constant probabilities applied to any given occurrence. Thus, if you see something taking one or two billion years to occur, you assume it to be of a very low probability, thus taking many many trials for a reasonable chance that one will come off.

As a counter-hypothesis, let me suggest that some evolutionary outcomes are condition-dependent. They may have a very low (or even zero) chance of occurrence under some conditions, but a relatively high chance of occurrence under others. What's more, I'm going to suggest that life itself changes the conditions under which life evolves. Neither of these is a radical proposition, of course, but follow this reasoning if you will.

There are several crucial steps to the successful evolution of life intelligent enough to develop interstellar space-travel. These, I submit, are:

1. The emergence of life itself.
2. The development of photosynthesis or some other means of producing nutrients.
3. The development of eukaryotic organisms, or the equivalent.
4. The development of metazoans.
5. The development of semi-intelligent life on the order of crows, monkeys, raccoons, etc. -- animals who rely on the intelligent manipulation of their environment to survive.
6. The emergence of an intelligent species on the order of H. sapiens.
7. The development by this species of civilization.
8. The achievement by this species of an ecologically sustainable, peaceful civilization.

Once these eight steps are successfully taken, and assuming cooperative and currently unknown principles of physics to get around Einstein, interstellar travel becomes merely a matter of time.

Now for each of these steps, I would suggest the following necessary preconditions:

1. Life cannot emerge unless a proper nutrient environment and chemical conditions exist for it to do so. It is not of low probability in all circumstances; rather it is of zero probability (impossible) in the absence of the proper environment and chemistry, and of unknown probability -- perhaps even a high one -- in the presence of them. On Earth, the emergence of life did not take a billion years as you suggest, because for most of that time the conditions were improper and the emergence of life was impossible. One minimum condition MUST have been the condensation of liquid water oceans, which occurred about 3.8 billion years ago. The first prokaryotes probably emerged about 300 million years after that.

The first living organisms will necessarily propagate by taking advantage of that nutrient environment; they will not produce nutrient, nor will they adopt more complex structures than the very simplest prokaryotic ones.

2. Photosynthesis or a substitute (some means for producing nutrient from available energy) cannot develop until the original nutrient environment is depleted sufficiently to give organisms with this trait a survival advantage over those without it. Thus, the emergence of prokaryotic nutrient-makers must wait until the first forms of life have spread and chemically altered the habitat in which life emerged into something else. On Earth, that took about 700 million years.

3. Eukaryotes, meaning complex one-celled organisms with a nucleus, multiple membranes, and cellular structure, represent an adaptation to an oxidizing environment. Their division of labor provides for more efficient metabolism and better disposal of wastes, as well as better protection against a hostile environment. These adaptations are of little value in a reducing environment but become value in an oxidizing one. The change from a reducing to an oxidizing environment cannot begin until after the development of photosynthesis, which begins releasing free oxygen and maintaining it. On Earth, that chemical transition was complete about 1.6 billion years ago, and the first eukaryotes emerged about 100 million years thereafter.

4. Metazoa are believed to have emerged from colony organisms. Colony organisms cannot develop until eukaryotes have exhausted all or most of the possibilities inherent in single-cell adaptation, because single-cell evolution is easier. The time from the emergence of the earliest eukaryotes to that of the first true metazoa was about 900 million years, but I'm suggesting that most of that time was spent refining and developing the single-cell possibilities, and only when those had achieved a certain high development did cells begin merging into colonies and eventually becoming multicelled life.

Those are the biggest hurdles. Afterwards:

5. The development of what I call "candidate" intelligence requires certain prior evolution: true multicellular life of sufficient complexity and differentiation of the nervous system and the brain to allow it to exist. Once it is allowed, intelligence, like every other survival strategy, inevitably occurs. The time from the emergence of the first metazoans until the emergence of the first candidate intelligence (probably in the Cretaceous) was along the lines of 400 to 500 million years.

6. Human-scale intelligence must emerge from candidate intelligence and therefore cannot precede it. It took 100 to 200 million years to happen on Earth.

7. The development of civilization requires that the intelligent species reach a sufficient population that they are driven to adopt farming. It cannot, of course, happen before there is an intelligent species. From the emergence of H. sapiens until the beginning of civilization required around 100,000 years.

8. The achievement of an ecologically sustainable, peaceful civilization requires the development of a level of technology that renders these changes necessary. We have reached that level of technology, which required about 10,000 years after the dawn of civilization, but have not yet made the changes. If we don't make them, we will at best destroy our civilization and at worst extinguish our species, which will throw the process back either 10,000 years or 100 million years. If we do, then there is nothing left to prevent us from developing interstellar space travel, except perhaps the laws of physics.

Because of all this, I don't agree with you that we should assume the nonexistence or even the scarcity of ETI. Some other explanation for why we don't see Them is more likely, and the simplest one of course is that Einstein was right and FTL travel is impossible.

But that's no fun. I'd prefer to play with alternatives. More later.

Originally Posted by Rick Hirst

Argument against: if even one civilization chose to explore, it would eventually contact us. Plus, even a civilization that chose not to send manned missions could send automated probes (as we have done.

You are assuming that a willingness to send some probes to nearby stars translates into a willingness to send out millions and billions of probes to explore all stars. This is a big assumption. After a while you are going to have seen everything and new stars will just be more of the same.

I can see investigating starts of interest. A star that is emitting radio waves indicative of intelligent life would be interesting. But only starts in the neighborhood of Earth have been exposed to radio transmissions from earth. Assuming 80 years of broadcasting, the farthest our signals could have gone and a response sent is 40 light years. About 750 stars are within that range, maybe 10% of which would be sunlike. That's only 75 candidates for intelligent life to have been able to respond to our radio transmissions if they were listening for them. This is hardly a big sample.

Hopefully in my lifetime we will gain the capabillity to detect terrestial-sized planets in other solar systems and we will learn the frequency of such systems.

Originally Posted by Mike Alexander '59

Originally Posted by Rick Hirst

Argument against: if even one civilization chose to explore, it would eventually contact us. Plus, even a civilization that chose not to send manned missions could send automated probes (as we have done.

You are assuming that a willingness to send some probes to nearby stars translates into a willingness to send out millions and billions of probes to explore all stars. This is a big assumption. After a while you are going to have seen everything and new stars will just be more of the same.

See my follow-up post on Von-Neumann probes. We are close to this capability already, and once we have launched the first couple dozen, our interest (or even our continued existence) is irrelevant.

Originally Posted by Croakmore

Rick, I like you survey, but it falsely implies that some category other than the last one is worth a dime of consideration, IMHO.

All categories are worth at least consideration, even if only to eventually reject them. :wink:

Originally Posted by Croakmore

I seriously doubt if there is even so much as a sponge anywhere else but on Earth. We are all alone out here on this marvelous rock. We are a one-off, an incredible freak of nature, and there is nothing I know of that suggests any other conclusion.

--Croaker

This is a reasonably argued position; more importantly, it is consistent with your other statements. I find Fermi's Paradox so intriguing because it has clear implications for our immediate behavior. So many people claim to believe as you do -- that Earth's biosphere is incredibly rare if not unique -- but they do not act accordingly. As you say, they persist in the hope that somebody will get them off this rock before it goes kablooey. They're waiting for their six-eyed, purple-tentacled Prince Charming to sweep them away in his silver saucer, instead of focusing on the mess we have to clean up here.

Now, let me consider some possible explanations for Why We Don't See Them, setting aside for the moment They Can't Get Here.

Mike has pointed out one possibility, namely They Don't Know Anything Interesting Is Here To See. I don't think that should be set aside, but there are other factors to consider, too.

Has anyone considered what "space colonization" would actually entail? It's not like getting in your outrigger canoe and crossing the Pacific to find unknown islands. Those unknown islands were still part of the terrestrial biosphere. They had a breathable atmosphere, and if they were big enough they had drinkable water and probably something edible (fish and seaweed if nothing else).

Intelligent life is a dependent outgrowth of the rest of life on a planet. It is constantly supported by that life, and cannot exist without it. So you cannot simply hop in your spaceship and go settle on another planet. You have to bring along the whole range of terrestrial life, or at least enough to form a self-regulating viable biosphere, and terraform the place. This takes a long time and a lot of effort.

Any species that reaches a successful conclusion to Step 8 above is going to be both pacifist and environmentalist. If they're not pacifist they'll blow themselves up, and if they're not environmentalist they'll exhaust their planet's resources and die out. So they're not going to have a raging military looking for new species to enslave, and they're not going to have an expanding population requiring new colonies to grow into. The long time-frame involved in terraforming new planets to be colonized necessarily makes the expansion slow, and although some colonies will surely be established just to allow their biotype to survive beyond the lifetime of their star, it isn't going to be like humans spreading across the face of the earth.

Planets that already have life may not be all that good candidates for colonization, either. An intelligent species has evolved in symbiosis with all the other species on its home planet. It wouldn't be as well-adapted to survive in a completely alien biosphere, and that biosphere would resist terraforming, struggling against the introduced organisms and making the job much harder. Couple this with the environmentalism that would have to be dominant in the collective ethos of any spacefaring species, and most likely they're not going to bother trying to settle a planet that already has life on it, because what they'd have to do as a precondition is commit biocide.

OTOH, they're certainly likely to study it, and perhaps harvest DNA to expand their own genetic library. So if there's anything in the neighborhood, and if they can get here, they've probably been here in the past.

But remember that "the past" means the past 3.5 billion years, only the last 100,000 years of which have had human beings living in it, and only the past 6,000 years or so have featured human beings with a written language. So it's a lot less likely that they've been here while anyone was around to see them and record the experience.

Now, supposing they did know we were here? In that case, interest would surely increase -- but then again, we don't really have anything we could trade to ETs, except for terrestrial DNA, and they could probably get that for free. And we don't have much to teach them, either, since we're necessarily at a more primitive level than they are. So except for sending out a few observation teams, why should they bother contacting us?

Could they help us get past Step 8? Maybe, in the sense of providing technical assistance. But that would only be of value once we've achieved the will to get there, and recognized the necessity, and I don't think that's happened yet. Even the most advanced parts of the world aren't yet convinced that we need to give up war or achieve a sustainable relationship to the planet, so very likely ETI technical assistance wouldn't be used properly even if it were provided.

This is really my thinking on Why We Don't See Them, and it doesn't quite match any of the poll possibilities, but it comes closest to the Prime Directive, so that's what I picked.

I chose "Rare Earth" because it seemed less unlikely to me than any of the other possibilities. :wink:

and with the implantation of the "obesity" meme they are fattening us up for harvest when the population gets high enough for it to become worth their while to set up an intergalactic "SPAM" plant.

This is why I try to stay thin and stringy... although those holidays each winter may get me into the production line. There is an interstellar State Fair out there somewhere. And, someone (er, some being) is going to make a killing with a corn battered, deep fried Man-on-a-Stick. ADM will provide the maize and Weyerhauser the sticks. It will be a Fun Food (for Them not so much for us). :shock:


P.S.>There is the problem of the Gourmand alien who will want a "free range" organically fed morsel. Maybe I need some Cool Whip and Cheetos in my diet? :?

My answer to this question is, Naked Ape and Rare Earth to a lesser degree.

The development of humanity as a sentient species was against astronomical odds, so was the development of advanced life on earth. Simple single celled life is probably very common in the galaxy, however complex life is probably very rare. The development of complex life was dependent on Earth having a stable large moon which was formed when another planet smashed into the earth at a certain angle, without that Earth would be one gaint water planet with a axis which tilts widly every so often.

The development of any sentient species to reach the level of technology we have reached now is also against pretty big odds. I would be not surprised if humans were the only sentient species in the galaxy and that planets like Earth that aren't waterworld's are very rare in the galaxy.

Great discussion Rick is putting us through here. He's good at that.

But I must point out that humans are necessarily homo-centric. This is why we want to believe in ETIs. We don't ask if other planets elsewhere support insect societies. So why would should we expect intelligent civilizations? Just because Earth happens to have one or two? The problem here is that many people want to believe in evolutionary progression: life naturally climbs up some kind of "evolutionary ladder." But there simply was no such progression from prokaryotes to eukaryotes to metazoans to deuterostomes to chodates to primates, and so on.

I think this entire discussion assumes that evolutionary progression is a valid concept. Again, there was no such linearity with humans popping out at the top (as the Smithsonian mural depicts). Gould and Dawkins disagreed on a lot of things, but they agreed completely that progression is an artifact of homo-centrism, and it is false. Just because Earth supported the appearance of eukaryotes does not mean that evolution should happen that way anywhere else, no matter how nutritious and bio-friendly another planet might seem to be.

Does anyone here think Buicks are inevitable, too, just because our universe is so vast and aged?

BTW: Brian Rush, I do not think evolution is entirely a random process. I prefer to see it as an opportunitistic process. Genes are deterministic little codes, you know, selfishly bent on their survival. But of course they can randomly drift, too. Natural selection is random only to the extent that it is quirky, and the genes make good use of any opportunities they can find.

--Croaker

Originally Posted by Croakmore

But I must point out that humans are necessarily homo-centric. This is why we want to believe in ETIs. We don't ask if other planets elsewhere support insect societies.

Who's this "we"? I think intelligent insectoids are just as likely as humanoids.

As I see it, while the evolution of intelligent life on Earth was so likely as to be almost inevitable, the evolution of H. sapiens in specific was so unlikely as to be a total fluke, and we shouldn't expect to find it repeated. There are only three requirements for a species to fill our niche: high intelligence, either hands or some substitute for them, and social interaction. Something with radial symmetry, fifteen tentacles, and a dozen eyestalks would fit the bill perfectly.

Disbelieving something because you want it to be true is no more logical than believing it for that reason, Mr. Frog.

The problem here is that many people want to believe in evolutionary progression: life naturally climbs up some kind of "evolutionary ladder." But there simply was no such progression from prokaryotes to eukaryotes to metazoans to deuterostomes to chodates to primates, and so on.

No, that's not a valid concept. But you're mistaken in thinking that the near-inevitability of the evolution of intelligence depends on it. With regard to the above, I will say that the terms "prokaryote," "eukaryote," and "metazoans" are sufficiently general that this progression really is valid. I'm not so sure about "deuterostomes" (is the separation of food intake from waste disposal basic enough to be inevitable, or is there some other way to do the same thing?). I'm pretty sure "chordates" aren't inevitable, though -- a spinal column is a pretty cool thing, but its functions could be handled by other developments. And I'm absolutely sure there was no particular reason why primates had to evolve, or why they and not some other candidate intelligence produced this planet's current intelligent species. We could just as easily have been intelligent birds or bears or even dinosaurs. But we'd still be here, if not now, then sometime earlier or later.

As you said yourself, evolution is opportunistic. What that means to me is this: If the conditions exist to allow a certain development, it will happen. If a survival strategy works, it will be used; life will seize the opportunity. And that is why intelligence is inevitable: as a survival strategy, it works. So does being really fast and strong, or camouflage, or having lots of offspring most of which become dinner, or being a killing-and-eating machine, or being a plant that grows really tall to reach the sunlight, and all of those are inevitable, too.

This has nothing to do with the anthropic principle, or wishful thinking, or any other such business, Croakmore. It's just a logical outcome from the way evolution operates, if you do the math honestly.

Brian, I am puzzled about the 8th requirement in your list.

Those two parts (ecologic stability and pacifism) would seem like reasons to stay home than to explore. If a civilization can be completely satisfied with their current conditions, then there probably won't be any collective interest in going somewhere else. There has to be an impetus somewhere.

Unless, I would assume the survival of the species were at risk of something whether it be a disease, or more probable, the death of their star.

I could understand that once the problems at home are solved, then most of a civilization's energy can be devoted to space travel, but I wonder where their motivation would come from.

I think that one of the two conditions need not be so perfect. And it could be either one. A society that is ecologically unstable and pacifistic would still probably try and attempt to use space travel as a way to gain resources. And an ecologically stable but war-faring civilization might employ space travel as a form of diaspora. If both are imperfect, we have the civilizations most familiar in those sci-fi alien invasion films. There is a lot of motivation with these scenarios.

If an alien civilization were to encounter ours, I don't think the greeting phrase would always include something along the line of "We come in peace."

Originally Posted by Brian Rush

As you said yourself, evolution is opportunistic. What that means to me is this: If the conditions exist to allow a certain development, it will happen. If a survival strategy works, it will be used; life will seize the opportunity. And that is why intelligence is inevitable: as a survival strategy, it works. So does being really fast and strong, or camouflage, or having lots of offspring most of which become dinner, or being a killing-and-eating machine, or being a plant that grows really tall to reach the sunlight, and all of those are inevitable, too.

This has nothing to do with the anthropic principle, or wishful thinking, or any other such business, Croakmore. It's just a logical outcome from the way evolution operates, if you do the math honestly.

You might be right, but the argument does stumble on the time factor. After the Cambrian Explosion, most of the modern phyla were in place, and many of the smaller divisions emerged fairly early as well. Amid all this, the various survival strategies appeared early on. Fast-and-strong, camoflage, lots of disposable offspring, all appear early, well before the end of the Paleozoic.

Intelligence, as we have it, as far as we know waited through nearly half a billion years to appear, if we start the count at the Cambrian Event. Bipedalism seems to have appeared many times, or approximations of it. Even the dinosaurs' ancestral species might actually have been a small bipedal form. Warm blooded mammals go back to the Triassic (at least).

There were apes by the truckload in the Miocene Period, but they were in decline throughout that epoch relative to the less intelligent, and less manually dextrous monkeys. There is little clear evidence that growing intelligence was a major competitive plus until the advent of Homo habilis, which on this scale was essentially just yesterday.

If sapience emerges in response to opportunity, then the opportunity appears to be an unusual thing, to judge by the fossil record. Why did it take so long for sapience to emerge, compared to the other survival strategies that appear over and over and over?

The poll needs a 'none of the above', and a 'it's a big galaxy' option, too.

Originally Posted by Croakmore

Rick, I like you survey, but it falsely implies that some category other than the last one is worth a dime of consideration, IMHO.

I can stretch my imagination far enough to include microbial life of the DNA/RNA kind on other planets, maybe even in our own solar system. But that is vastly insufficient to allow for any serious possiblilty of ETI. Humans want to believe in ETI, of course, because humans are predisposed to do so. Humans will believe in anything if it serves a human purpose. Frogs are not so gullible.

I have a copy of Asimov's Extraterrestrial Civilizations, wherein he reasons the probability of ETI is 1.0. It's a wonderful book, and he's a wonderful writer, but it is still sci-fi down the line.

Please consider this: It took a billion years after Earth accreted for prokaryotes to evolve on Earth well enough to leave a fossil record. Then it took another billion for eukaryotes to show up. Then they languished in the ancient oceans for another billion years before they invented metazoans. I mean just getting to sponges and jellyfish is so improbably as to render Earth the only home in the universe for these primitive creatures, and they are a long long way from intelligence by any frog or human standard. That kind evolution took another billion years.

What makes anyone believe that this kind of evolutionary careering is likely to happen anywhere else? Hope is the only cause I can think of, and hope is not scientific by any measure. Hope is the province of popular seduction, like religion; it feels good when the odds are bad.

Forget the ETIs, Rick. I serious doubt if there is even so much as a sponge anywhere else but on Earth. We are all alone out here on this marvelous rock. We are a one-off, an incredible freak of nature, and there is nohing I know of that suggests any other conclusion.

--Croaker

We really don't have sufficient information to evaluate the probability of life elsewhere, on any level. We don't even know how life works yet, at its most most basic levels, certainly not sufficiently to make any kind of guess as to how likely origin events are.

But I agree, life is going to be very, very rare, given the enomrous range of astrophysical and geophyical strikes against it, unless our understanding is totally off.

Originally Posted by Tim Walker

Our middling size Milky Way galaxy is estimated to have 1 to 200,000,000,000 stars. The Milky Way is but one of an estimated 100,000,000,000 galaxies in the universe.

There are potentially so many different places in the universe that there may well have been enough opportunities for metazoan life to evolve elsewhere.

True. But let's be fair here, and note that there are a lot of factors that chop the possibilities down. If we assume 'life as we know it, or something close to it' (and we have to, otherwise we have no constraints on pure speculation), we can say with some confidence:

1. Over half of all stars are dinky spectral-class-M 'red dwarfs'. These are by far the most common stars. They are, compared to the Sun, dim, cool, and long-lived. (Dim and cool are relative, of course).

If you replaced Sol with a typical red dwarf, Earth would freeze. Probably even the atmosphere was freeze out over time, and the new 'sun' would be a red spark in the sky. To be close enough for a viable biosphere, a planet of a red dwarf would have to be very close. That, in turn, would mean it would probably experience 'tide lock', it's rotation matching its orbit and keeping one face to the star all the time, the way Luna does to Earth.

That in itself wouldn't necessarily rule out all chance of life, but it probably wouldn't help.

2. Astronomers call all elements heavier than helium 'metals' (much to the disgust of chemists), and old stars tend to have low 'metalicity'. What that means is that they are short-changed on heavy elements, since at the time of their stellar formation, those heavy elements weren't around, not yet having been 'cooked' by nucleosynthesis inside big stars. Younger stars tend to have more heavy elements, since they incorporate more 'ash' from previous star generations.

Now, odds are good that a low-metalicity star will have star systems made up of gas giants and/or small worlds, not large planets like Earth. The Sun is a 2nd or maybe even 3rd generation star, and IIRC it's got a higher level of heavy elements than most of its generation and type. So the first-generation stars that still survive (mostly red dwarfs) are probably not likely abodes for life.

3. There's a fair chance that if a star is too close to the inner regions of the galaxy, local conditions will not be very tolerant toward life. If, as is now suspected, inner regions of galaxies are sometimes flooded with radiation from the nucleus, that alone is a problem. Another is that the high stellar density in inner regions makes regular supernovae exposure more likely.

4. This is iffier, but the star systems we've discovered so far haven't been terribly stable-looking places. Often, we find Jovian-sized worlds in the inner reaches of the system, which is very odd on the face of it. If Jupiter or Saturn had come wandering inward in the past, it's a fairly safe bet that Earth would have paid a high price. If this sort of situation is actually common, it cuts down the possibilities again.

There are a lot of other potential limiting factors. But it's true that given the total stellar population, that still leaves lots of possible sites. But they may be spread out very, very thinly.