A new study published this week rejects the long-held argument that modern humans arose from one place in Africa during one period in time.
Instead, researchers concluded that modern humans descended from at least two populations — referred to as Stem1 and Stem2 — that coexisted in Africa for a million years before merging in several independent events across the continent.
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, by analyzing the genomes of 290 living people, the scientists found that the two populations 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.
Unknowns: 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.
