I am with you on this one EWright – and I do recognise that you are more posing a question than stating an opinion. We have a single planet and a single ecosystem on which to base our conclusions: a sample size of one is always a dangerous base for statistics.
Infamous says: “
What about dolphins and chimps?”
As you rightly point out they haven’t shown any signs of attempting interstellar communication.
C1ay says: “
Considering the apparent age of the local universe as we know it, thousands or millions of years older than life on Earth could be very plausible.”
Just for the benefit of those readers new to geologic and astronomic timescales, lets amend that to “tens of millions to hundreds of millions” of years older to make it more realistic. However, life does not equate to intelligent life.
(For those of you with more than a passing interest in this topic may I recommend Rare Earth, by Ward and Brownlee, ISBN 0387987010. The subtitle “Why Complex Life is Uncommon in the Universe”.)
Buffy says: “
Yes, everything depends on the probabilities, but the more we find out about life on earth and how many really hostile places it seems to thrive in, its hard to think the probabilities are that low at all, even for "intelligent" life”
But Buffy there is a world of difference (literally many worlds of difference) between the character of extremophiles and that of metazoans, let alone that of intelligent entities. The acceptable environments for the latter appear much more restricted.
Additionally, consider two factors: a) time b) events.
a) It took almost two billion years to advance from prokaryotes to eukaryotes. It took a further billion and a half years too get to complex life forms, then well over half a billion years too get too intelligence. We don’t know if this was faster or slower than average, but if it was typical we have already eliminated a significant number of stars from consideration – planets will not remain in the Goldilocks zone long enough to get to our ‘status’ if their home star is more massive than the sun.
b) It seems clear that evolution on the Earth has been facilitated by a series of unrelated events that were catastrophic at the time, yet were instrumental in promoting the next wave of evolutionary advancement. Yet these were all chance events. We simply do not know how critical they were in ensuring thee development of intelligence. (To take one case alone, it is possible, even probable that the Cambrian explosion was triggered by the cessation of snowball Earth conditions. No global ice age; no complex life forms.)
Buffy continues: “.
Thus its pretty obvious that its not too difficult to evolve, but it does require an opposable thumb and some environmental shocks to get it going....”
Sorry, it is not at all obvious to me. After 300 million years of large land animals we finally get the right combination of some environmental shocks. That is not easy evolution. If you think it is there are some large bets I should like to place with you.
Boersun says: “I think from a sample of a million planets harbouring life, chances are that you'll find 99% of them similar to Earth in Earth's first couple o' billion year's of life: Algae in shallow ponds and seas. No more.”
I think you are being wildly optimistic and have not nearly enough nines in there.
The arguments against commonplace complex life are presented with much more vigour and detail than I can muster here
http://www.space.com/scienceastronom..._1_020715.html where Ward and Brownlee debate with Drake. )
