1. Look at the contributions of African-Americans in the United States as it pertains to the arts, technology, and agriculture even when fettered by chattel slavery, then post-reconstruction, then Jim Crow, then de facto racism...now take those SAME innovations and imagine a people (or many peoples) with NONE of those roadblocks. No colonialism then you have no basis for skin based racism, African people establish colleges (or hell even attend prestigious european/asian ones).
Remember, Colonialism brought virtually NO advancements to the continent of Africa for the gain and benifit of AFRICANS. There is a difference between Belgium going to the Congo and saying "Hey, guys! They invented this dope thing called electricity! Lets work out a deal so ya'll can have some." as opposed to "Hey, guys the Congo's got TONES of diamonds lets go down there and get them!" and they JUST SO HAPPENED to leave behind their crumbling infrastructure for the natives to learn and build upon. Again is a by PROXY benefit, that was never meant for Africans to enjoy.
2. One of the first things Caesar did when he overthrew the Ptolemaic dynasty in Egypt (who were Greeks) was burn down the Library of Alexandria. Why? Because even back then Europeans knew the power of culture. It was why Europeans wanted to wipe out not only the Moorish controlled Cordoba in Spain but get them out of ALL of North Africa, destroying countless seats of power, libraries, scrolls and the sort. When your CULTURE is destroyed you have no common links to create NEW culture....which severely stunts a civilizations ability to innovate. So when slaves were brought from the motherland, people with shared languages were IMMEDIATELY broken apart...WHY? To destroy their spirits AND their culture.
Expand that by an ENTIRE continent, sow the seeds of discord between rival tribes and you create 3 centuries of infighting and war that doesn't' lend itself to innovation beyond how to more effectively kill your enemy. And even THEN Europeans demanded that Africans buy THEIR technology so that they didn't create their own...