Are We Alone in the Universe? - NYTimes.com
So Paul Davies throws (deserved) cold water on the hope that we should encounter intelligent life estimate of how many "Earthlike" planets astronomers say are out there.
As he points out, just because conditions are like they were on Earth at some time in the past doesn't make it inevitable that life will have arisen in a majority of those cases.
But then he makes almost the same error. He says that since intelligent life on Earth has arisen by natural selection, and that life elsewhere must logically evolve as well*, then once life appears, it's only a matter of time before intelligent life evolves.
Sorry. Nope. The only thing that natural selection increases is fitness. If, through some accidental set of circumstances, increased intelligence results in higher fitness, then intelligence might arise. But if anything, I regard that as much less sure than the likelihood that some kind of life will arise.
* Life must be subject to natural selection, because logically, natural selection is inevitable when imperfectly reproducing entities compete for resources. A number of experiments bear this out.
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