I would rephrase the question as: What are reasonable bounds on the number of intelligent civilizations, given that we have observed exactly one in the archeolo

I would rephrase the question as: What are reasonable bounds on the number of intelligent civilizations, given that we have observed exactly one in the archeological record of our own planet, and no others elsewhere with current technology? So, tiny sliver of time over billions of years, and communications technology that only reaches to a few nearby stars. Trying to use probability of certain chemistry is, I think, highly subject to errors and failure to consider some alternative possibilities. Life itself, however, is a totally different equation because it’s nearly as old as the earth. This might suggest that there is a lot of it out there.