In Jack L. Chalker's excellent Rings of the Master series, the Master System was humanity's first sentient AI. Although no date is given, it is implied that it is created during one of the low points of the Cold War, and given the task by its creators of preserving humanity.
The machine thought about this for a moment, and then decides that the most logical way to preserve humanity is to split the species up, with a population of maybe half a billion on Earth. The rest are sent into space to colonise other planets.
Rather than wasting time and resources in Terraforming the other planets so that they are fit for human habitation, the Master System modifies the humans instead. This has the added bonus of preventing rebellion; with so few on each planet, and there being no way that humans from one planet to survive on another, it is supposedly impossible to form any kind of organised resistance against the machine.
Of course, the fact that the machine controls all access to any technology that is post-Industrial-Revolution helps keep the humans in check as well.
I was going through a dearth of good science fiction phase before a friend recommended me these books: they are refreshing, thought-provoking and above all, you can't put them down.