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.

Around Africa

Linguistic analysis and archeological evidence suggest that the Bantu originated near the border of Nigeria and Cameroon. Their initial expansion appears to have been driven by their development of agriculture, and they later widely adopted ironworking. But the geography around their region of origin—deserts to the north, savannah to the east, and rain forest to the south—would make agricultural practices challenging to move with. In fact, it has been difficult to determine which of the competing ideas about the Bantu's route(s) across Africa made sense.

To get a genetic perspective, a large team of researchers obtained DNA from over 1,300 Africans from 35 different populations in Africa. This DNA was checked for single-base differences that can help identify distinct populations.

The results suggest that the Bantu's first move was south and into the rainforest. Modern Bantu populations in the area have clearly intermingled with the hunter-gatherer groups that still live in the rainforest. But the traces of that population are absent in Bantu people elsewhere in Africa, suggesting it only occurred after the expansion. The DNA data's consistent with that, suggesting the hunter-gatherer DNA wasn't introduced in these populations until about 800 years ago.

From the rainforests of Angola, the population split into two distinct migrations. One swept east to Ethiopia and Somalia and intermingled with the populations there starting about 1500 years ago. About 10 percent of the DNA of Bantu speakers in the area comes from other East African populations. In the south, a separate group intermingled with the San, a population that currently occupies parts of Namibia. The San contributed about 20 percent of the DNA of modern Bantu speakers in South Africa.

Evolving in Africa

The data obtained by the research team also holds out the possibility of identifying the genes that were essential in allowing the Bantu to adapt to radically different environments. But, for the most part, the genes that came out of this analysis were exactly the sort of things we've seen in human populations elsewhere. In Bantu populations in east and west Africa, the biggest signal of evolutionary selection was in a cluster of genes that help the immune system identify pathogens (called the HLA complex).

In western Bantu speakers, the second biggest signal was found in a gene that's been implicated in mediating malarial resistance. In the east, the Bantu speakers came in contact with herding cultures, so there's a strong signal for selection at the gene that mediates lactose tolerance. In fact, it appears that lactose tolerance was introduced to Bantu speakers when they had kids with the pastoralist populations.

Oddly, nothing much—not even the HLA complex—stands out within the southern populations. The authors simply ascribe that to "a different demographic and adaptive history," but it's an odd result that probably deserves more attention.

Finally, the authors compared their data to what we know about Africans who experienced a forced relocation: the ones currently living in North America, who are mostly victims of the slave trade. African Americans typically inherit a genome that's about 75 to 80 percent of African origin. About 30 percent of that, the researchers found, originated near Angola, a core area of early Bantu expansion. As noted above, Bantu in this region have intermingled with populations of rainforest hunter-gatherers, and about four percent of their DNA also shows up in African Americans.

The new work sheds light on one of the largest migrations of humanity's past. One of the clearest messages from it is that we can adapt to things like agriculture and living in radically different environmental conditions without needing much in the way of genetic changes. We primarily adapt with our brains rather than our genes.

But the paper also makes clear how much we still have to learn about our past. Everywhere the Bantu speakers went, they ended up intermingling with pre-existing populations in the area. Each of those, like the Bantu, probably have a history of migration and interbreeding as well. We've only just begun to scratch the surface of our past within Africa.

Science, 2017. DOI: 10.1126/science.aal1988 (About DOIs).