Gene Study Challenges Human Origins in Eastern Africa
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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.
Population pinpointed
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
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9 Comments
Add CommentIs this consistent with the August 2010 cover story? What does the timeline look like? Is it possible that the species originated in East Africa, migrated south, and only those who migrated south survived to return to the north and beyond?
Reply | Report Abuse | Link to thisI agree. The current distribution of genetic populations may better represent the location of the largest population of Toba eruption survivors (~70kya), for example, than the origin location of modern humans.
Reply | Report Abuse | Link to thisThose pesky ancestors seem to have been wandering all over the place. Any would think they were itinerant hunter gatherers!!
Reply | Report Abuse | Link to thisThey obviously did not think about the difficulties they were creating for us when we try to track their movements.
You are still wrong, so keep looking. When bipedals migrate, mostly in search of food or water or to get away from danger of some kind, they die along the way. If the bipedals migrated from South Africa and crossed the equator and headed north in search of more food or a warmer climate, you can trace their DNA and it should lead you to a match in the north of the equator. If you can't find a match, less than 1% difference, north of the equator, then the last place you found a match of less than 1%, then that is as far as the African bipedals traveled and the bipedals north of the equator originated from somewhere else. We are not one species and if you keep forcing yourself to believe that we are, then you are going to get more egg on your face.
Reply | Report Abuse | Link to thisWhy do science writers persist in exaggerating the challenges presented by new data? I guarantee the first paragraph of this article will be used by creationists as further evidence of the "disagreement within the science community" regarding evolution, when in fact this data is just one more piece of the evolutionary jigsaw puzzle. And how can a genetic analysis present a major challenge to genetics? It IS genetics!
Reply | Report Abuse | Link to thisAs to this study, I see no incongruity at all. The fact that non-African humans are genetically most similar to East Africans just means the East Africans were the ones who migrated out of Africa. Considering the geography, it would indeed be surprising if this were not the case. This new data simply indicates our East African ancestors weren't also the ancestors of, but were rather the descendants of, a gene pool that also gave rise to the tribes in this study. The fact that these tribes now live in the south might mean that's where they originated, or it could just be that southern Africa is the last refuge of once-widespread tribal groups that have been overrun by their more successful East African descendants.
James,
Reply | Report Abuse | Link to thisI thought the debate had gone way past that a long time ago. You obviously missed, or did not want to see the comment in the article that "humans in the rest of the world all carry a subset of genes found specifically in eastern Africa".
The question is not whether humans originated in Africa, but which part of Africa.
Evolution of humans started when apes who had evolved to hang from trees had to adapt to walking on ground after millions of years of drying out of climate left not many trees to hang from. If the loss of their traditional habitat kept worsening, it would be no surprise if the original human ancestors kept shifting territory to find conditions better suited for survival.
@JamesDavis, what do you mean we are not one species?
Reply | Report Abuse | Link to this"As for why genetic studies show that all modern humans carry eastern African and not southern African DNA..." - but they also show that southern African is more diverse, so presumably east African DNA is a subset of southern? And it is from there that people spread out to the rest of the globe?
Reply | Report Abuse | Link to thisAs it is, the studies on the habitats in the southern Cape region of South Africa are increasingly showing that it is likely that modern humans came from there - which is nice for me, as it means I have simply moved closer to my roots...B-)
Ed Rybicki
Cape Town
It is amazing how this folk lore of Evodelusionism is based on stories that are not ever compared to the other stories. It is one person with fantasy visions who cannot keep up with current events.
Reply | Report Abuse | Link to thisThey recently found MODERN human remains in Israel dated at 400 to 500 thousand years ago.