Why do you think everybody is just stacking technologies on top of a shared pool? That's a post-globalisation concept.
African people discovered plenty of technologies including metal forging - but they were less technologically advanced than Europeans when they were invaded. Why? Because the material situation in Europe is ideal for technological development. The climate is temperate, food is abundant, the Romans went everywhere dumping their own advanced tech on everybody, probably other factors a historian could elaborate. There are plenty of alternative explanations to the one you're implying, and the one that's been historically taught, which is "black people are not capable of developing civilisation or technology".
Edit given you appended "at any point in time": basically no milestone technological development has happened anywhere for quite some time now, excluding computers and the Internet. Everything's incremental now. Plus for much of that time many African countries have either been trying to recover from colonialism or been caught up in civil war (which should be expected when a power vacuum is created). I would say that mobile money is a good candidate for a meaningful invention though, and that came out of Nigeria.