Evolution: Modelling the origins of humans in Africa(Nature)

Human origins in Africa may be best described using a model in which at least two evolutionary branches split (but continued to mix) over hundreds of thousands of years, according to a paper in Nature. This so-called weakly structured stem is proposed to have contributed to the formation of an ancestral African human group that then diverged into contemporary African populations, as well as all populations living outside of Africa.

Although it is widely understood that Homo sapiens originated in Africa, uncertainty surrounds how branches of human evolution diverged and people migrated across the continent. This uncertainty is due to limited fossil and ancient genomic data, and to the fact that the fossil record does not always align with expectations from models built using modern DNA.

Aaron Ragsdale, Brenna Henn, Simon Gravel and colleagues tested a range of competing models of evolution and migration across Africa proposed in the palaeoanthropological and genetics literature, incorporating population genome data from southern, eastern and western Africa. The authors included newly sequenced genomes from 44 modern Nama individuals from southern Africa, an Indigenous population known to carry exceptional levels of genetic diversity compared with other modern groups. The model suggests the earliest population split among early humans that is detectable in contemporary populations occurred 120,000 to 135,000 years ago, after two or more weakly genetically differentiated Homo populations had been mixing for hundreds of thousands of years. After the population split, ancient hominins still migrated between the stem populations, creating a weakly structured stem — this offers a better explanation of genetic variation among individual humans and human groups than do previous models, the authors suggest.

The authors predict that according to this model, 1–4% of genetic differentiation among contemporary human populations can be attributed to variation in the stem populations. This model may have important consequences for the interpretation of the fossil record — owing to migration between the branches, these multiple lineages were probably morphologically similar, which means morphologically divergent hominid fossils (such as Homo naledi) are unlikely to represent branches that contributed to the evolution of Homo sapiens.

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