Europe remains 'barbarian' for longer. When it finally does develop, plague devastates the southern half, shifting power to the north and making Mediterranean Europe prey to Mediterranean Africa and Asia.
West Africa, for some reason or another (perhaps outside influence, perhaps luck, perhaps a virus affecting tse tse flies and another affectng mosquitoes) becomes the hegemon or one of the hegemons of this world. Its premier empire looks to the north.
How do the West Africans colonize Europe?