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.