iron technology
Bantu-speaking peoples migrated from Western Africa (in the area of modern Nigeria) throughout southern parts of Africa starting around 1000 BC and continuing to the 1500s (or perhaps a bit later, by some accounts).
There is some debate among historians as to whether iron metallurgy arose independently in Africa or was learned or borrowed from the Middle East or Europe. But whether independently invented or borrowed, the fact remains the the spread of iron working in Africa and the spread of Bantu languages by Bantu migration across Africa were correlated events.