Human Story - How did the Bantu people become so widespread?

In the post on Sunday I remarked in passing how striking it is that such a large part of Africa speaks closely related languages, and offered up the theory that it was the result of a huge population expansion, but then I just dropped it and moved on to talking about Trevor Noah or something.

I mean... it's tough not to.

But let's rewind a second. It's Human Story today. How did Bantu-speaking people get everywhere they are today?

That turns out not to be the easiest question to answer - most of the evidence we have for the expansion at all is just the similarity between the Bantu languages. The thought process goes that if people in Cameroon, Kenya, and Cape Town are all speaking clearly similar languages, they must have been speaking the same language fairly recently. So it goes in historical linguistics; there's simply no other force that could have made them so similar from that distance. Based on linguistic reconstruction, it's pretty clear that the Bantu speakers began about 4,000 years ago in the Niger-Congo heartland, in modern Cameroon, from there expanding south and east into essentially the whole southern half of Africa, absorbing or displacing sparser hunter-gatherer populations like the Congolese pygmies and Khoisan bushmen. Unfortunately, studying the languages has pretty limited ability to fill in any of the juicy details about how or why the expansion happened. This is the point in the discussion where linguists usually turn things over to geneticists and archaeologists, but their findings are... mixed.