Even if this first grand civilization is long gone, their initial legacy could live on in the form of a passed-down tradition, or perhaps an artificial life form dedicated to such a goal without the risk of death. Beyond this, it does not even have to be the first civilization, but simply the first to spread its doctrine and control over a large volume of the galaxy. If just one civilization gained this hegemony in the distant past, it could form an unbroken chain of taboo against rapacious colonization in favour of non-interference in those civilizations that follow.