Their migration may have been set in motion by an increase in population caused by a movement of peoples fleeing the desiccation, or drying up, of the Sahara.
But there've been times in the past when monsoon rains soaked the Empty Quarter and turned it from a desert into grassland that was dotted with lakes and home to various animals.
During the dry periods that are common phenomena along the desert margins, the pressure on the land is often far in excess of its diminished capacity, and desertification results.
In many semiarid areas there is also a lower timberline where the forest passes into steppe or desert at its lower edge, usually because of a lack of moisture.
Rock paintings in the Sahara indicate that horses and chariots were used to traverse the desert and that by 300-200 B.C., there were trade routes across the Sahara.
The people who hunted the sparse desert animals responded to drought by managing the wild resources they hunted and gathered, especially wild oxen, which had to have regular water supplies to survive.
A government study recommended a national highway system of 33,920 miles, and Congress passed the Federal-Aid Highway Act of 1944, which called for strict, centrally controlled design criteria.