Out of Africa, yes, but all over Africa and not all at once.

New York Times rewrites human prehistory with a genetic study that replaces the tree of life – a diagram of human origins with one trunk growing out of one spot in the continent before sending out different branches around the world – with something a lot more like a grass prairie of life. A new genetic study has found evidence of lots of interlinked human roots, showing that the earliest humans arose all over Africa at different times:

The researchers analyzed DNA from a range of African groups, including the Mende, farmers who live in Sierra Leone in West Africa; the Gumuz, a group descended from hunter-gatherers in Ethiopia; the Amhara, a group of Ethiopian farmers; and the Nama, a group of hunter-gatherers in South Africa.

The researchers compared these Africans’ DNA with the genome of a person from Britain. They also looked at the genome of a 50,000-year-old Neanderthal found in Croatia. Previous research had found that modern humans and Neanderthals shared a common ancestor that lived 600,000 years ago. Neanderthals expanded across Europe and Asia, interbred with modern humans coming out of Africa, and then became extinct about 40,000 years ago.

The researchers concluded that as far back as a million years ago, the ancestors of our species existed in two distinct populations. Dr. [Brenna] Henn [a geneticist at the University of California, Davis] and her colleagues call them Stem1 and Stem2.

About 600,000 years ago, a small group of humans budded off from Stem1 and went on to become the Neanderthals. But Stem1 endured in Africa for hundreds of thousands of years after that, as did Stem2.

If Stem1 and Stem2 had been entirely separate from each other, they would have accumulated a large number of distinct mutations in their DNA. Instead, Dr. Henn and her colleagues found that they had remained only moderately different — about as distinct as living Europeans and West Africans are today. The scientists concluded that people had moved between Stem1 and Stem2, pairing off to have children and mixing their DNA.

The model does not reveal where the Stem1 and Stem2 people lived in Africa. And it’s possible that bands of these two groups moved around a lot over the vast stretches of time during which they existed on the continent. About 120,000 years ago, the model indicates, African history changed dramatically.

In southern Africa, people from Stem1 and Stem2 merged, giving rise to a new lineage that would lead to the Nama and other living humans in that region. Elsewhere in Africa, a separate fusion of Stem1 and Stem2 groups took place. That merger produced a lineage that would give rise to living people in West Africa and East Africa, as well as the people who expanded out of Africa.

You can read more of Henn’s & her colleagues’ research here, in Nature.