The researchers concluded that a million years ago, our ancestors lived in two different groups. Dr.. Hinn and her colleagues call them Stem1 and Stem2.
About 600,000 years ago, a small group of humans exited Stem1 and went on to become Neanderthals. But Stem1 suffered in Africa for hundreds of thousands of years after that, as did Stem2.
If Stem1 and Stem2 had been completely separated 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 remained somewhat different — and just as different from living Europeans and West Africans today. Scientists have concluded that people have switched between Stem1 and Stem2, setting out to have children and shuffling their DNA.
The model does not reveal where the Stem1 and Stem2 peoples lived in Africa. It is possible that groups of these two groups moved a lot over the vast periods of time that they were present on the continent. The model indicates that African history changed dramatically around 120,000 years ago.
In South Africa, people from Stem1 and Stem2 merged, creating a new lineage that would give rise to the Nama and other living humans in that region. Elsewhere in Africa, separate fusions of the Stem1 and Stem2 groups have occurred. This fusion produced a lineage that would give rise to people living in West Africa and East Africa, as well as people who expanded out of Africa.
It is likely that climatic perturbations drove Stem1 and Stem2 individuals into the same regions, leading them to coalesce into single groups. Some groups of hunter-gatherers may have had to withdraw from the coast as sea levels rose, for example. Some regions of Africa have become barren, which can send people in search of new homes.
Even after these mergers 120,000 years ago, people of either Stem1-only or Stem2-only ancestry seem to have survived. The DNA of the Mende people showed that their ancestors interbred with Stem2 individuals only 25,000 years ago. “This suggests to me that Stem2 was somewhere around West Africa,” said Dr. Henn.
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