Context: By 3400 BCE, the Sahara was as dry as it is today, due to reduced precipitation and higher temperatures resulting from a shift in the Earth's orbit. As a result of this aridification, it became a largely impenetrable barrier to humans, with the remaining settlements mainly being concentrated around the numerous oases that dot the landscape. Little trade or commerce is known to have passed through the interior in subsequent periods, the only major exception being the Nile Valley. The Nile, however, was impassable at several cataracts, making trade and contact by boat difficult.

Question: By What time period was the Sahara dry like it is today?
Answer: 3400 BCE

Question: What was the main reason that the Sahara became so   dry?
Answer: reduced precipitation and higher temperatures

Question: What area is known for having the majority of the trade?
Answer: the Nile Valley

Question: When did the Sahara become accessible to humans?
Answer: Unanswerable

Question: What features of the Nile made it ideal for trading?
Answer: Unanswerable

Question: Where did people trade in the Sahara before the aridification?
Answer: Unanswerable

Question: What caused the shift in the earth's orbit?
Answer: Unanswerable

Question: How did humans cause the aridification of the Sahara?
Answer: Unanswerable