User blog comment:Warmachine375/Why Marvel Universe is being divided into different alternate universes?/@comment-3008558-20120517114505/@comment-1713281-20120518124324

The way it is commonly recognized is that the universes are on a horizontal plain. They all start at one universe, and branch off at certain points in history, each continuing on it's own path at it's own speed unless destroyed or erased (think like a serious of plant root, but sideways). Earth-0 is considered the original universe, and after a Big Bang-esque event, multiple universes were branched off. Earth-616 is established as the main universe that Marvel focuses on, and for the sake of simplicity, the universe that most other universes branch from. However, some universes branched off at the Big Bang, and not from Earth-616 (Earth-721, home of the Squadron Supreme Tony mentioned above, is a prime example).

Mind you that this is just one of many theories out there. The other often-cited one is that all realities have always existed on a horizontal or vertical plain, running their course indefinetly until destruction or erasing. However, most of the Marvel writers use the branches theory.