New model sheds new light on the origin of humans in Africa

Two Stellenbosch University (SU) scientists were instrumental in the development of an evolutionary model shedding new light on questions around human origins in Africa.

This new model, recently published in the prominent journal Nature, claims that at least two evolutionary branches of Homo sapiens split (but continued to mix) over hundreds of thousands of years. This so-called “weakly structured stem” (made up by a mixture of these two branches) is proposed to have contributed to the formation of an ancestral African human group, which then branched off into contemporary African populations, as well as populations living outside of Africa.

“While it is widely believed that Homo sapiens originated in Africa, there is uncertainty around how branches of human evolution diverged and migrated across the continent,” says Marlo Möller, a professor of molecular biology and human genetics at SU’s Faculty of Medicine and Health Sciences (FMHS). “This uncertainty is due to limited fossil and ancient genomic data, and the fact that the fossil record does not always align with expectations from models built using modern DNA.”

Möller and Prof Eileen Hoal, both SU scientists affiliated with the DSI-NRF Centre of Excellence in Biomedical Tuberculosis Research and the SAMRC Centre for Tuberculosis Research, collaborated with Prof Brenna Henn, a population geneticist from the University of California, Davis, and evolution geneticist Prof Aaron Ragsdale and colleagues from the University of Wisconsin-Madison on this research.

For the study, they sequenced the genome of 44 modern Nama individuals. (The Nama people are an indigenous southern African population known to carry exceptional levels of genetic diversity compared with other modern groups.) They then tested a range of competing models of evolution and migration across Africa proposed in paleoanthropological and genetics literature, incorporating population genome data from southern, eastern, and western Africa.

The model they developed 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 split, ancient human-like species (hominis) still migrated between the stem populations, creating a weakly structured stem.

“This model offers a better explanation of genetic variation among individual humans and human groups than do previous models,” says Möller.

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.