Migration is generally seen a result of push and pull factors, though, so that is a place to start. Push factors relate to the circumstances that can make someone's homeland an unpleasant enough place for them to ditch it entirely in favour of something new. With regard to these early human migrations, of course 'no jobs' or 'terrible political circumstances' do not apply; rather, think of stuff like the climate taking a turn for the worse and turning places into huge ovens or freezers where barely anything can live or grow, natural disasters, competition with hostile neighbouring groups, food and other resources running too low to support the amount of people within an area, or the more mobile type of food (herds of herbivores) migrating away.Pull factors, on the other hand, involve the draw of new possibilities and rewards; basically, the more favourable side of the things mentioned in the 'push' section, such as greener lands with better climates and luscious amounts of food and resources. Of course, this is a bit of a simplification, and it will be hard to track down the exact combination of factors that led to each individual instance of early human migration.