Er... by the way, these are spoilers if anyone hasn't found out already.
Yea, Yoshi's Island DS has Adult Bowser plotting with Present DayKamek to go back in time to steal 8 special souls or something like that. With the power of those 8 souls, he can conquer the world, but they exist only in 8 babies.
I think Bowser is unaware of the specific babies he needs but has an idea, at least. The babies they exist in that Bowser needs to kidnap are: Mario, Luigi, Peach, Donkey Kong, Wario, Bowser (Yep. Remember though that Adult Bowser doesn't know the specific babies. I'm sure he would have objected to the plan if he knew he had to kill himself), and Yoshi (a baby Yoshi, not the one you play as. This Baby Yoshi is possibly the one that grows up to be THE Yoshi Mario would ride in later games, given that the Yoshi you play as might be too old by then and never met Mario before Super Mario World. Hey, Mario even fixes its own plot holes, sometimes!).
Baby Bowser recognizes Bowser and neither one realizes they are one and the same person, which indicates that their meeting in Mario Partners in Time DID happen (take that, all you people who say Mario games have no continuity). This is why Baby Bowser helps you throughout the game (at least, until the end). Past Day Kamek catches on though which is why he's your antagonist throughout the game.
Note that this is all stuff I've seen others say about the game. I haven't gotten it, myself.