Posted 10 January 2011 - 02:24 AM
One problem that I have with folks like Sagan is that in his earlier calculations of the probability of life he neglected the fact that the center of our galaxy is filled with so much synchnotron radiation that it's probably lethal. I don't think it's a coincidence that Earth is on one of the outer arms of the Milky Way a safe distance from that deadly black hole. And if this requirement is true for most life, and if it's true that probes will have a hard time 'cutting through the center' it implies that any life which does exist may have evolved fairly recently (as the outer arms formed) and will have to go 'the long way round' to get to us.
Also remember, there wasn't even any helium when the universe started. It had to be formed in stars which went supernova. The helium had to fuse to form more complex elements with greater atomic numbers. And so on. So when does the clock start ticking on a universe that's actually capable of supporting life, as opposed to just a bunch of elementary gasses and stars? It would start a long long time after the big bang. You'd need lots of carbon, at the very least in addition to hydrogen, oxygen and nitrogen. So atoms with atomic #s of 8 or less, at the bare minimum. Many complex structures like hemoglobin or chlorophyll require iron or magnesium which would only develop much much later.
And while the harsher environment at the center of our galaxy may eventually support more heat resistant silicon based life, silicon only comes into existence a long time after the existence of C, N, O, H etc. Which means that more durable silicon based life would have far less time to evolve than carbon based life.
Also, abiogenesis, the creation of life from non-life, is a far far stickier problem than most high school biology textbooks let on. The results of the Urey-Miller experiment are greatly overplayed. The problem of getting a string of nucleotides that are capable of self-replicating is hard enough. Then add in the fact that they have to be chiral, which doesn't happen naturally in non-life and you'll probably need some kind of external structure which helps the first RNA strand form. The hypothesis that life started on chiral clay is probably bogus since you probably need life to create chiral clay. And such a self replicating strand of nucleic acids, even if it formed by chanc cannot even produce its own nucleotides. It cannot protect itself. It can't even re-create the support structure that created it. There are a lot of unsolved problems, and they all point to the difficulty of abiogenesis.
Add to this the fact that, through all of Earth's evolution, homonids are the only species which, as far as we know, have developed any kind advanced technology and we see that intelligent life is something of a rarity. What did it require? Jared Diamond suggests that it required a huge east-west land mass and the existence of domesticatable megafauna. And maybe it also required a long stretch of temperate weather resulting from Antarctica moving over the South Pole.
There are probably quite a few issues not even considered here. But my take is that intelligent life itself is a pretty rare thing. And any other intelligent life that did evolve probably didn't come into existence too long before us and is probably very far away, on another outer arm of our Milky Way galaxy at the very nearest.
I'm very much interested in the question of how probable it was that intelligent life came to exist. The more that I look at the issue, the more difficult it seems.