o the fact that we have not yet identified alien civilizations does not mean they do not exist, maybe they are just smarter than we think.
Prisco combines the 'they're here but we can't see it' and dark forest answers to [[Fermi Paradox 20201123150738]].
any animal equivalent is going to have to need oxygen — a lot of it.
does this still hold up? the wikipedia lemma points to several more recent sources that seem to offer counter information https://en.wikipedia.org/wiki/Rare_Earth_hypothesis#Free_oxygen_may_be_neither_rare_nor_a_prerequisite_for_multicellular_life The source of the claim seems to be the book itself, so would need to look it up in Ward Brownlee 2000, page 217