CSAs are common. Humans, some (though far from all) other animals, and many human-created decision-making programs (game-playing programs, scheduling software, etc.), have CSA-like structure...The ubiquity of approximate CSAs suggests that CSAs are in some sense useful.

I'm not sure this fits. Do the animal examples all share a common ancestor that is also a CSA, or did CSA-ness evolve (and fixate) independantly, on multiple branches of the evolutionary tree?

If it only fixated once, and all other examples stemmed from that (including humans writing programs that are modeled after a part of our own decision-making process) then can it really be said to be common at all? It would still clearly be useful in some sense, or it wouldn't have fixated that one time. But it's not clear to me that it's any more useful than some other algorithm from an unrelated branch.