Improbable Island's history has, for much of its existence, been documented in two main forms: the Messages of the Day, and player word-of-mouth. Improbable Island is also home to people who love to make shit up,1)2) so you probably shouldn't use that second source to pass your history quiz or whatever.
These pages attempt to wrangle the past fifteen-odd years of history and also complete nonsense into plain text form. If you're looking for the fictional history of the game's world, we have a timeline of that under “Canon”.