Single-origin theory of humans, challenged

The 2,000-year-old bones of a boy found on a beach in South Africa have provided more grounds to challenge the prevailing theory that modern humans had a single origin in north-eastern Africa.

That fraying theory, based on fossils found at Omo Kibish and elsewhere in Ethiopia, dates the emergence of modern humans to about 180,000 years ago. However, by using DNA analysis as a ‘molecular clock’ to calculate the length of time since the boy’s ancestors diverged genetically from other groups of modern humans, scientists in South Africa and Sweden estimate that modern humans must have existed between 260,000 and 350,000 years ago.

This pushing back of the estimated date of the emergence of modern humans by at least 100,000 years is roughly in line with research published in June that dated human remains and other artefacts found at the Jebel Irhoud archaeological site in Morocco as about 300,000 years old.

The finding lends weight to the hypothesis there was not just a single cradle of modern humankind in north-eastern Africa but, rather, an entire continental nursery.

“It seems that both genetics and archaeology are converging on this point that there might be multiple places in Africa that archaic humans transitioned from Homo erectus to H. heidelbergensis to modern humans,” says Carina Schlebusch of Uppsala University in Sweden, lead author of the new research, published in the journal Science.

170929 khoesan population history
Demographic model of African history and estimated divergences. Vertical coloured lines represent migration. Down-pointing triangles represent admixture into another group. Southern African hunter-gatherers are shown by red symbols, Iron Age farmers by green symbols.
Credit: C. Schlebusch et al. Science (2017)

Scientists have been hesitant about alternatives to the single-origin idea because of the demise of a previous ‘multiregional’ theory that once competed with the ‘Out of Africa’ hypothesis, Schlebusch says. That theory, suggesting separate groups of modern humans evolved from ancient hominin groups around the world, was disproven by DNA analysis showing Homo sapiens fossils throughout the rest of world were much closer genetically to each otherthan those from Africa, and therefore could not have evolved independently.

“The multiregional theory was wrong in terms of how the globe was populated,” Schlebusch agrees, “but it is not necessarily wrong about how humans evolved in Africa.”

It seemed unlikely that such potentially history-changing evidence would come from the bones of the stone-age boy known as Ballito Bay A – named after the place the bones were found on a beach in the KwaZulu-Natal region of South Africa. Exposed to sand, salt, water and other weathering elements in a subtropical climate that is hot and humid, the bones were poor candidates for DNA analysis.

“We had more hope for our other samples, which were from caves,” says Schlebusch. “Caves generally are much better because the climate is very stable and cool, so the DNA doesn’t degrade. But these samples for some reason worked very well.”

Given the lack of supporting archaeological artefacts, the only thing known with certainty about the boy is what his genes tell us: he was a member of the San branch of the Khoe-San peoples of southern Africa. He likely lived a hunter-gatherer lifestyle and spoke with the clicks that linguistically unify the San with the Khoe, who practised a nomadic form of pastoral farming.

The Khoe-San are not only genetically distinct from Europeans and Asians but also from other Africans. Research suggests that they are a branch of modern humans that diverged early from our oldest common H. sapiens ancestors.

What makes Ballito Bay A special, from a contemporary scientific perspective, is his relative genetic “purity’, meaning his ancestry involved fewer procreative liaisons with members of other human groups than the other specimens.

This made it easier for Schlebusch and her colleagues to use his DNA as a ‘molecular clock’, comparing it to the DNA of other specimens and estimating how long it would have taken for various mutations to have evolved from a common ancestor.

DNA dating isn’t infallible. It requires making assumptions about a rate of genetic mutation from one generation to the next, and also about the length of each generation.

“Molecular clocks are very tricky to use, especially when rates are calculated using modern, or near-modern, genetic data only,” notes Alan Cooper at the University of Adelaide in South Australia, who is a world-recognised leader in the field of ancient DNA. “Better estimates of the mutation rate and generation time could bring these dates down by quite a large amount.”

On that basis, though being “somewhat sceptical about the very large dates,” Cooper says the research by Schlebusch and her colleagues is certainly interesting. “Even if we consider the dating as rather ‘aspirational’, they have demonstrated deep genetic splits in human genetic diversity, considerably larger than before, and demonstrated that southern Africa should be considered as playing a more central role in the evolutionary story.”

Schlebusch acknowledges that mutation-rate and generation estimates can be debated, but says the results of DNA dating are still valuable on a relative scale, and in league with other lines of inquiry. “I really think ancient DNA studies in Africa will make a big contribution. We’re at the stage now where we are going to meet up with palaeontological and archaeological estimates to see how archaic humans transitioned to modern humans.”

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