How would you describe the diffusion of civilizations in Africa and the Near East?

The high antiquity of civilization in the Middle East is largely due to the existence of convenient land bridges and easy sea lanes passable in summer or winter, in dry or wet seasons. Movement of large numbers of people north of the Caspian Sea was virtually impossible in winter, owing to the severity of the climate; central Eurasia was often too dry in summer. Land passage between Asia and Africa was in early times limited to narrow strips of land in the Isthmus of Suez. Large-scale desert travel was limited to special routes in Iran and in North Africa, both east and west of the Nile Valley.