Study reveals modern humans evolved from two distinct groups in Africa

“At different times, people who embraced the classic model of a single origin for Homo sapiens suggested that humans first emerged in either East or Southern Africa,” says Brenna Henn, a population geneticist and co-lead author of the study, in a statement.

“But it has been difficult to reconcile these theories with the limited fossil and archaeological records of human occupation from sites as far afield as Morocco, Ethiopia, and South Africa which show that Homo sapiens were to be found living across the continent as far back as at least 300,000 years ago,” she added.

So, the team dug deeper

The researchers analyzed 290 genomes of people from Southern, Eastern, and Western Africa, as well as Eurasia. They included such diverse populations across Africa so that the data would reflect more features of genetic diversity than previously reported.

They concluded that contemporary populations descended from two of the earliest human populations in Africa that had been around for hundreds of thousands of years.

The researchers observed two merger events. The first resulted in the formation of an ancestral Khoe-San population in Southern Africa around 120,000 years ago. The second event resulted in the formation of the ancestors of Eastern and Western Africans, including the ancestors of people outside Africa about 100,000 years ago.

The researchers told the New York Times that they are now adding more genomes from people in other parts of Africa to see if they affect the tree models.

The study was published in the journal Nature.

Study abstract:

Despite broad agreement that Homo sapiens originated in Africa, considerable uncertainty surrounds specific models of divergence and migration across the continent1. Progress is hampered by a shortage of fossil and genomic data, as well as variability in previous estimates of divergence times1. Here we seek to discriminate among such models by considering linkage disequilibrium and diversity-based statistics, optimized for rapid, complex demographic inference2. We infer detailed demographic models for populations across Africa, including eastern and western representatives, and newly sequenced whole genomes from 44 Nama (Khoe-San) individuals from southern Africa. We infer a reticulated African population history in which present-day population structure dates back to Marine Isotope Stage 5. The earliest population divergence among contemporary populations occurred 120,000 to 135,000 years ago and was preceded by links between two or more weakly differentiated ancestral Homo populations connected by gene flow over hundreds of thousands of years. Such weakly structured stem models explain patterns of polymorphism that had previously been attributed to contributions from archaic hominins in Africa. In contrast to models with archaic introgression, we predict that fossil remains from coexisting ancestral populations should be genetically and morphologically similar, and that only an inferred 1–4% of genetic differentiation among contemporary human populations can be attributed to genetic drift between stem populations. We show that model misspecification explains the variation in previous estimates of divergence times, and argue that studying a range of models is key to making robust inferences about deep history.