Genes divide African neighbours

TWO studies exploring the genetics of hunter-gatherers and pastoralists in sub-Saharan Africa, where humans are thought to have originated, reveal that even though the click-language peoples of southern Africa live in close proximity, they belong to two distinct genetic clusters.To assess the degree of genetic difference, both teams looked at single nucleotide polymorphisms (SNPs) ' variations at individual nucleotides between different people ' in the DNA of the various populations.One study, led by Carina Schlebusch at Uppsala University in Sweden and published Monday in Science, analysed SNPs from 220 individuals belonging to 11 different southern African populations. The other, led by Joseph Pickrell at Harvard Medical School in Boston, Massachusetts, and posted to the arXiv preprint server ahead of its publication in Nature Communications, examined SNPs from 187 individuals belonging to 23 southern and eastern African populations.By considering the similarities and differences among the SNPs in the various click-speaking peoples, and comparing them with patterns from other African populations, the teams were able to identify ancestral relationships. Both teams deduced that the southern African click-speaking populations (known generally as the Khoisan) actually belong to two genetically differentiated groups, one in the north and and one in the south of the Kalahari, which went their separate ways around 30,000 years ago. The discovery of this genetic divide is raising numerous questions about how it could have come about.'It makes you wonder if they became isolated from one another for cultural reasons, or if there was some sort of geographical isolation that led to the differentiation,' says Sarah Tishkoff, a human-origins geneticist at the University of Pennsylvania in Philadelphia.Evolutionary geneticist Rasmus Nielsen at the University of California, Berkeley, agrees with the idea of the environment being involved. 'Africa was very dry during the last glacial maximum, and this might have led to an extended period of smaller overall population sizes, more population subdivision and increased fragmentation,' he says.Aside from identifying a key time of separation among southern African peoples, Pickrell and his colleagues also discovered an ancient genetic relationship between the south and east of the continent. They found that the Sandawe and the Hadza, east-African hunter-gatherers who speak a click language but are geographically isolated from the other Khoisan peoples, derive roughly a quarter of their ancestry from the southern African click-speakers. This, Pickrell argues, implies the existence of a link having once existed between these peoples.
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