But Since Asauchi didn't exist from the beginning it would mean that first shinigamis weren't shinigamis at all, also kido has nothing to do with zanpaktou since creator of zanpaktou didn't mention it and we saw only shinigamis using it with means that this power only belongs to shnigami. it looks like there is even more to the shinigami then just his sword.
I must say that Zanpakutō being "invented" makes shinigami power hard to understand, it's hard to find line where shinigami NATURAL powers end and artifical powers start(for example: zanpaktou). Of course it's possible that shnigami powers as a whole are artifical but that would create different problem: why shinigami and quincy powers are EXACTLY opposite, it COULDN'T be accident chances for that are EXTREMLY small, it can mean one of two things:
1)they are part of something greater(somekind of system or something created by higher being like God).
2)Not sure which power was first(shinigami or quincy) but the second power that was created was created on PURPOSE as OPPOSITE of first power.