Answer: Where these civilizations developed was impacted dramatically by one single geographic event which still dominates Africa: the desertification of the Sahara.
Explanation: Prior to approximately 6,000 B.C., the Sahara got considerably more rainfall that it does today. But at about 6,000 B.C., rainfall tapered and temperatures across the region skyrocketed. It drove the inhabitants of the Sahara, many of whom were practicing sedentary agriculture, north to the Mediterranean and south into Sub-Saharan Africa. By around 3,000 B.C., the Sahara looked as it does today, and it presented a major obstacle to cultural exchange between Northern and Southern Africa.
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