The climatic advantages that mid latitude regions have over tropical regions. The world's largest continuous zone of "temperate" climates lies in a belt stretching across Eurasia from southern Europe in the west to China in the east. Rather persistently neglecting the fact that much of this zone is inhospitable desert and high mountains east-west-trending mid latitude zone of Eurasia as the world region that possessed the very best environment for the invention and development of agriculture and, consequently, for historical dynamism.Its east-west major axis permitted many inventions adopted in one part of Eurasia to spread relatively rapidly to societies at similar latitudes and climates elsewhere in Eurasia...It lacks the severe ecological barriers transecting the major axes of the Americas and Africa. Thus, geographic and ecological barriers to diffusion of technology were less severe in Eurasia than in other continents.
This means that if you migrate east/west, the odds are better that the crops and animals you bring with you will be able to survive in your new home. if you migrate north/south, you've got to rebuild civilization every time you move. this of course gives a huge advantage to continents like Eurasia where the east-west axis is much longer than the north-south access because civilization will be able to develop faster if it doesn't have to keep starting over.