It turns out Alduin actually ruled Skyrim in the early days of men aided by dragon priests. It is somewhat amusing to see that Alduin preferred to rule over men than do his job as a world eater.



Speaking of world eating. something does not hang together about how they say it works:

First world eating is supposed to be literal. Second Alduin is supposed to feed on human souls and continue feeding with a hunger that never gets satisfied.

However that story does not jive with the glaring fact that he appears too small in the game to consume a large boulder. We also don't see him grow in size when he is eating souls in Sovngarde



What was Alduin doing in the world when he was overthrown? My guess is he had just finished eating the last world, and refused to leave. Preferring to rule over men instead. However this is just a guess.


It also makes clear the Fact that Alduin is not the same as Akatosh