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I generally agree with you, Niall, that complex life (and hense intelligent life) is very rare in the universe. For a planet to be habitable at all is unlikely enough ... for it to also possess the stunning number of stabilizing factors that the Earth does will probably proove to be several orders of magnitude rarer. The universe is an inherently hostile and indifferent place as far as life is concerned.

However, I think your agrument leans dangerously close to anthropocentrism. Just because the emergence of homo sapiens as a sentient species is fantasically unlikely doesn't mean that the emergence of any sentient species is equally so.

Suppose, for example, that the dinosaurs had not died out. You are correct that their decendents would now dominate the Earth instead of humans ... but they might have well evolved a sentient species, and perhaps done so millions of years sooner than mammals did. Several "raptor" forms were well on their way to developing big brains at the end of the cretaceous; there is no reason that they couldn't have evolved towards intelligence.

You can go back even further: The permian extinction gave vertabrates a leg up on the arthropods, who had dominated the planet for millions of years. Without that event, it is unlikely that our ancestors would have ever developed beyond the level of fish. But the arthopods might well have produced a sentient species. And if you find it hard to imagine an intelligent being without a spinal cord, go to your local aquarium and ask them if they have any good octopus stories. Chances are they do, and you will be amazed.

My position is that we will find simple life - microbes and alge and such - to be fairly common; where a sizable planet exists in a water orbit, there will be icky bluish-green stuff growing on it. But complex organisims will be very, very rare. Perhaps one in a thousand habitable planets will be habitable enough for critters like plants and animals.

But I also think that where complex life develops, it will evolve towards intelligence. Being smart is just too huge of an evolutionary advantage for it not to. Yes, it takes a long time; the energy requirements of a large brain are severe and random chance assures there will always be pitfalls. But once it gets going, intelligence is the mother of all survival traits. Every other species is doomed to adapt to its environment over generations, but intelligent ones adapt the environment to better suit themselves. Barring disaster, a species that can achieve the level of using even the simplest tools has it made.

One unarmed cro-magnon was a joke to the mastadon, an ice-age survival machine if there ever was one. But tribes of cro-magnons with spears and fire hunted those woolly bastards into extinction in just a few thousand years. Intelligence wins ... game, set, and match.

Ultimately it's all a mental exercise until we can build starships, or SETI successfully tunes in to the Zeta Reticulan version of Jeopardy! (I'll take "Delicious Human Recipies" for 1000, Kodos!"). But it's a darn intersting topic, isn't it? A+ for this thread.


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