A ever-increasing drought gradually brought about the great Andite exodus from the lands south and east of the Caspian Sea.  An increasing drought caused highland tribes to invade Euphrates valley. 
A great climatic change destroyed the civilization of the Sahara. Northern ice retreated and water-laden winds from the west shifted northward, turning the great open pasture regions of Sahara into barren desert. 
By 8000 B.C. the slowly increasing aridity of the highland regions of central Asia began to drive the Andites to the river bottoms and the seashores. This increasing drought not only drove them to the valleys of the Nile, Euphrates, Indus, and Yellow rivers, but it produced a new development in Andite civilization.  By 3000 B.C. increasing aridity was driving these Andonites back into Turkestan and Europe.  A severe drought meant death to the early agriculturists; weather control was the object of much ancient magic.