Oh, so for whatever reason there was a rift in the home universe they all share, and they each individually found it.

hm...if they all came from the same preexisting universe, then I don't quite see the point of this setup anymore. If they don't come from different worlds...then they should all share the major history of their world. And in that case, there are two options. Either they came from a world without life, in which case the whole "from another world" thing seems largely superfluous; it's just an arbitrary change of scenario for them. Or they came from a world with life, which goes against the basic conceit I had that they were creating life for the first time now.

If we assume that life existed in their past universe, than we need to manufacture a presumably complex past for them instead of just going with the natural progression of an event occurring for the first time.