As regards frequency of civilizations occurring, I've read recently an interesting take: when we look at the Earth, each of the following milestones seems expon

As regards frequency of civilizations occurring, I've read recently an interesting take: when we look at the Earth, each of the following milestones seems exponentially more difficult (i.e., probability density is far smaller for each): - simple unicellular organisms to eukaryotes (~2 billion years) - eukaryotes to multicellular lifeforms (~a few hundred million years) - humans + consciousness + language + prehensile limbs (only a couple of million / a few hundred thousand years ago?) In addition, if panspermia (exchange of biological material via meteorites/comets etc.) is in fact effective, the conclusion was: Earth-like planets may very well be (almost) universally covered (infested?) with one or another kind of unicellular goo; but only some tiny fraction has anything more interesting, and we could be one of the handful - or even the first technical civilization in the Milky Way.