Of course, the slave trade, which began in West Africa in the late fifteenth century and didn't end until the mid-nineteenth, played a major role in the transformation of West Africa from wealthiest to poorest region. In the begnning, West African kingdoms traded on their own terms with European explorers and found the trade very profitable. Over time, however, Europeans were able to dictate the terms of trade, which went from fairly limited along the coastline to well inside the interior of Africa. By the eighteenth century, and even earlier in many places, Europeans had set up their own slavetrading fortresses on the coast, thus monopolizing the profits for royally chartered companies like the Africa Company in England.
The major change has been the move from independence to colonization and back again. This has changed the face of Africa in tremendous ways. This process created African states in configurations that would not have existed without colonization (ones that, for example, put rival ethnic groups together in one country). This has helped cause the problems that now face African countries.
The Abbasid Empire must have controlled North Africa on the Mediterranean coast in 1000 CE. After the Mongols destroyed Baghdad in the 13th century, at least two other Muslim groups, the Ottoman Turks and the Mameluke's in Egypt, must have competed for power with the Ottomans eventually gaining control.
The printing press in the Muslim empire might be an interesting change to study. As I understand it, Muslim scholars resisted using printed material because they did not want to make printed copies of Quran.
A series of enormously wealthy West African nations, like Mali and Songhai, controlled trade along the Niger River. One king, Mansa Musa, took a pilgrimage to Mecca. He took so much gold that he caused a deflation in the value of gold.
So in the 14th century, West Africa might have been the wealthiest region in the world, now it is among the poorest.
In the 15th century, Europeans and the Moroccans began exploring Africa. By the end of the 19th century, only Ethiopia was still and independent country.
The coming of Europeans to the mouth of the Niger River might be the reason for West African poverty. Trade by ship must have been more efficient than trade by camel caravan.