Interestingly in the Pokemon Mystery Dungeon games, particularly Explorers of Time, Darkness and Sky, there's an actual record of the creation of the world, contained in murals.
While Arceus isn't shown in this mural Mew seems to be first, so perhaps everything stems from it. It seems curious it would be first, even before the expansion of time and space. But that of course is dependant on this mural being in the right order, or even being a record of the creation of the world at all...