The "Green Sahara":

The Neolithic Subpluvial... was an extended period (from about 7,000 BC to about 3,000 BC) of wet and rainy conditions in the climate history of northern Africa. It was both preceded and followed by much drier periods. The Neolithic Subpluvial was the most recent of a number of periods of "Wet Sahara" or "Green Sahara" during which the region was much moister and supported a richer biota and human population than the present-day desert.

The Big Picture just posted an excellent series of photos of an archeological dig in this area. The giraffe petroglyph is awesome.