Eventually she and her colleagues discovered DNA-bearing skeletons as old as 6,000 years in caves in the highlands. Other bones were discovered by archaeologists working in African countries, as well as in museum collections.

David Reich, a geneticist at Harvard Medical School and a co-author of the new study, and his colleagues analyzed DNA from 16 of these fossils, along with the one previously found in Ethiopia, comparing the genetic material to that of living people throughout Africa as well as on other continents.

This analysis allowed them to determine how living Africans descended from ancient populations, which are older in Africa than anywhere else on Earth.

“Africa is now going to be fully included in the ancient genomics revolution,” Dr. Reich said. “We’re going to be able to do a lot of things in Africa that we’ve been able to do in Europe and elsewhere.”

Africa is where our species evolved at least 300,000 years ago. Previous genetic analysis of living Africans had suggested that their ancestors began splitting into distinct groups over 200,000 years ago. Roughly 70,000 years ago some Africans moved out of Africa, becoming the ancestors of non-Africans.

In earlier studies, researchers had concluded that the hunter-gatherers who live today in the Kalahari Desert and other parts of southern Africa descend from the branch believed to be the first to have divided from other Africans.

But the new study suggests that there may be even older branches in the tree. “Something more complicated is going on,” Dr. Reich said.