[Anthony G Williams]

Travelling on foot out of Africa is an absolute doddle compared with travelling between the stars in any foreseeable future.

Mass migration would only become feasible if something like "stargates" could be established to allow people to step from one planet to another. But that is fantasy rather than science fiction since there is no way that even the most way-out physicists can conceive that it might be even theoretically possible. But still, I notice that you're a fantasy writer so there's no reason why you shouldn't use the idea!