Book excerpt: This history relates the human consequences of the remorseless spread of the Great Desert that now stretches in almost unbroken continuity from Mauritania's Atlantic seaboard through the Middle East and Central Asia to the Great Wall of China. The author seeks to understand how the great civilizations in the original green lands of North Africa, Ancient Egypt, the Middle East, South Asia and China responded and changed under the pressure of invaders fleeing growing environmental degradation in the surrounding deserts.