Re: A Sample of...One
Civilization is probably necessary for an intelligent species to be able to contact humanity or for us to be able to relatively easily find them. Dolphins are reasonably intelligent as far as things go, but they ain't about to send out any radio signals or space probes.
I already mentioned this, but there are two sides to the equation of how common civilizations are (one might call it a... two-sided equation). One is how likely civilizations are to arise. The other is how long civilizations tend to persist.
As I mentioned, if civilization intelligence tends to eventually result in overpopulation, exhaustion of resources or massive pollution/climate change and thus civilizational collapse, and/or catastrophic wars using technologies like nuclear bombs and/or biological agents that create massive pandemics, etc. then that's another reason why the types of intelligent life we would be interested in contacting would be rare. We don't know how much of a tendency this is, or how quickly civilization-building species tend to flame out, but even if the median civilization lasts a million years, that's a very short amount of time on galactic time scales, and so you'd expect nearly all civilizations that ever existed to have died out millions of years ago.
And I think civilizations lasting more than a million years is generous, given how things are going with humans a mere 12,000 years after the development of agriculture. It might be that most intelligent species are less shitty than humans, but I doubt we would be so exceptionally awful that a million years of civilization is typical yet we flame out after less than 20,000 years. Humans (probably) aren't that special.
So I think that even if SETI does discover an unambiguous message from alien intelligent life, the signal would likely be many thousands of years old and there would be a decent chance the civilization that sent it out is long dead. Unless they have figured out how to send faster-than-light communications that nonetheless we are still capable of receiving and decoding.
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