By Matt Kaplan
A genetic analysis of modern hunter-gatherer populations in Africa suggests that humans evolved in the south of the continent, rather than the east, as has been thought. The work presents a major challenge to evidence from anthropology, as the earliest anatomically modern human skulls have all been found in eastern Africa; and to genetics, as humans in the rest of the world all carry a subset of genes found specifically in eastern Africa.
The latest study, published today in the Proceedings of the National Academy of Sciences, was led by Brenna Henn, a geneticist at Stanford University in California. She and her colleagues used saliva-sampling kits to collect DNA from rarely studied tribal peoples in Africa. The team worked specifically with the Hadza and Sandawe tribes of Tanzania and the click-speaking ‡Khomani Bushmen of South Africa.
The team compared the collected DNA with samples from more-extensively studied tribal peoples such as the Maasai of Kenya and Tanzania, and the Yoruba of western Africa. The DNA was also compared with that of a group from Tuscany in Italy, for contrast.
Theme and variations
The researchers analysed single-nucleotide polymorphisms (SNPs): minor variations in the placement of the nucleotides (represented by the letters A, C, G or T) in the shared DNA sequences of different human populations. For example, in a specific DNA sequence shared by tribal peoples, an A nucleotide might be present for one tribe where a C is present for others.
These sorts of differences have allowed geneticists to calculate relationships and moments of evolutionary divergence. They also allow them to look at how genetically similar different groups of people actually are, and consider levels of genetic diversity.
Henn and her team report that the genetics of the ‡Khomani and Namibian Bushmen, the Sandawe and the Biaka Pygmies of the Central African Republic seem to be the most diverse, and thus the oldest, of any found among modern humans.
"It is so important to have more information about population genetics in these under-represented populations of Africa," says Sarah Tishkoff, a geneticist at the University of Pennsylvania in Philadelphia.
"These are far more extensive data on single-nucleotide polymorphisms in hunter-gatherer groups than we have ever had before," says Chris Stringer, an anthropologist at the Natural History Museum in London.
The team used the geographic locations of the genetically diverse groups of people to determine where humans might first have emerged.
The researchers noted that whereas the Biaka Pygmies dwell in the rainforests of central Africa and the Sandawe hail from eastern Africa, both the ‡Khomani and the Namibian Bushmen live in southern Africa. This geography, when combined with genetic data from the other populations, led the team to suggest southern Africa as a point of origin.
This suggestion is creating disagreement. "African populations have had complex demographic histories and there is no a priori reason to believe that populations evolved in situ in the regions where they exist today. Some could have migrated from other regions," says Tishkoff.
Different populations in ancient Africa probably contributed various genes and behaviours to modern humans, says Stringer. "I don't think there was a single Garden of Eden where it all happened.""I'd be cautious about localizing origins," says Stringer. "The ranges of the people studied in this group are currently quite limited, but if you look at rock paintings, many are linked to the Bushmen and hint they were once more widespread," he says.
Henn admits that migration could certainly be a possibility, but counters that when a population migrates, typically only a subset moves to a new area, and this subset is less genetically diverse than the parent population. She argues that if a group left eastern African for southern Africa it would be expected to have less diversity in the south. "This is not what we find in the data," she says.
As for why genetic studies show that all modern humans carry eastern African and not southern African DNA, Henn suggests this is just because southern genetics have not had enough attention. "Many African groups, especially those in southern Africa, have been poorly genetically studied," she says