Scientist holding DNA gel sample in laboratory.Credit: Andrew Brookes/ Image Source/ Getty Images
A new evolutionary model, published in Nature, examines questions around human origins in Africa. It finds that at least two evolutionary branches of Homo sapiens split, but continued to mix, over hundreds of thousands of years.
The model suggests the earliest detectable population split among early humans occurred 120,000 to 135,000 years ago, following prolonged mixing between two or more weakly genetically differentiated Homo populations.
This weakly structured stem contributed to the formation of an ancestral African human group, which then branched into contemporary African populations, as well as populations outside Africa.
Marlo Möller, co-author and professor in the Division of Molecular Biology and Human Genetics at Stellenbosch University (SU), South Africa, says that “to explain our genetic diversity, some scientists have suggested that early Homo sapiens mated with an undiscovered species. Our models include reticulation and migration between early human populations, rather than archaic hominin admixture from long-isolated branches”. Möller and SU’s Eileen Hoal, in collaboration with Brenna Henn, a population geneticist from the University of California, Davis, evolution geneticist, Aaron Ragsdale, and colleagues from the University of Wisconsin-Madison, sequenced the genome of 44 modern Nama people from an indigenous southern African population, known to carry exceptional levels of genetic diversity compared with other modern groups.
The team 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.
“This model offers a better explanation of genetic variation among individual humans and human groups than previous models,” says Möller.
John Hawks, from the Department of Anthropology at the University of Wisconsin–Madison, says the new research does not totally rule out that such groups were among the ancestors of today’s people, but it says that most of our ancestors probably belonged to a few groups that interacted repeatedly for a very long time, maybe a million years.
