Over the past few decades, advances in DNA technology have completely changed our view of our species’ past. We can now trace our ancestors out of Africa, into North America, and across the vastness of the Pacific. What often goes missing within this big picture of continual expansion is that our ancestors didn’t sit still once they had settled an area. Technological changes like agriculture led to additional migrations and population disruptions.

One of the biggest of these is known as the Bantu expansion. There are over a billion people in Africa, and nearly a quarter of them speak languages from the Bantu family. Bantu speakers occupy regions from the rainforests of central Africa to the savannah of East Africa and dry climates of the south. And they occupied all of that territory in less than 4,000 years despite the fact that Africa had been teeming with humans for tens of thousands of years.

Now, researchers have turned to DNA to trace the route of the Bantu expansion across Africa. In the process, they’ve discovered that the Bantu ended up as part of the slave trade, contributing to the African American population as well.