If you consider yourself a diehard Metroid fan, you’ll probably want to check out this lengthy and very thorough recap of the series’ lore over at Neogaf. Normally we don’t go too much into non sanctioned items like this, but the speculation on the history here—not to mention some great artwork—is enough reason to check out this pic heavy page.
Here’s a short selection: “The [Metroid] story really begins tens of thousands of years ago, with an avian race of prophetic poets fighting a war throughout the cosmos. The Chozo – the dominant species in the pre-history of the Metroid universe - gaze into possible futures, invade living worlds, and engage in massively misguided genetic engineering to fight against the horrors of evolution. The primitive human race remains wholly ignorant of this war in heaven, emerging from their caves with no clue that humanity will be the final victors.”
From there it goes pretty in-depth not only with the events of the series, but also the Chozo themselves, the concept of living planets and lots of other stuff. It ain’t a new Samus outing on Wii U, but it’ll be interesting reading for those missing one, regardless.