>unless the probability for evolving a civilization on a habitable-zone planet is less than one in 10 billion trillion, then we are not the first.... the degree

>unless the probability for evolving a civilization on a habitable-zone planet is less than one in 10 billion trillion, then we are not the first.... the degree of pessimism required to doubt the existence... of an advanced extraterrestrial civilization borders on the irrational. After working with probability theory and combinatorics a bit, you realize probabilities much smaller than this occur naturally all the time. It might be hard for people not familiar with that to appreciate this, and be wowed by such a small number. The thing is that independent probabilities multiply together, and so approach 0 exponentially fast. The probability of winning the lottery is of course very small. But there are so many people, that someone wins a lottery every day. But the probability of winning the lottery twice is really small. The set of people that have won twice is a very small set indeed. And the probability of winning 3 times is so small that we should expect it to be a sure sign of corruption of the lottery, or intervention by supernatural forces. It's also about 1 in 10 billion trillion. It seems to me that the evolution of intelligent life from nothing, also requires many things to go right. We don't know what all these things are, and we can only guess what their probability is. But if even a few of them have relatively small probabilities, then the probability of all of them happening could easily be much lower than "1 in 10 billion trillion". There are many candidates for this so called "great filter". Some ideas I think are plausible are the evolution of multicellular life. The Earth was around a very, very long time before multicellular life formed. And it's believed to have required some weird coincidences like a parasite evolving into mitochondria. Another candidate is the evolution of intelligence itself. Most animals aren't intelligent. Intelligence doesn't actually seem to be very correlated with fitness beyond a certain point - and reducing brain size is actually beneficial. Animals with brains were around for a very, very long time before anything like humans appeared. Some scientists believe that humans required a really weird set of conditions to evolve. There are theories that we are a case of accidental runaway sexual selection, like the peacock's feathers. And there are many other factors we can't possibly know, that might be unique to Earth and rare on other worlds. We also don't know a huge amount about the evolution of early life. Getting from RNA self replicators to the complicated machinery of the most recent ancestor was no small feat.