Eliezer: Or the first replicator to catch on, if there were failed alternatives lost to history - but this seems unlikely, given the Fermi Paradox; a replicator should be more improbable than that, or the stars would teem with life already.

Why should you assume that they do not? Humans may not have detected other life forms, but you cannot deduce from that that they do not exist. They might not want us to detect them. It might not be possible for a civilisation to extend beyond its solar system.