Ancient African ancestors

Researchers studied 121 African populations, four African American populations and 60 non-African populations for patterns of variation at 1327 DNA markers. The study traced the genetic structure of Africans to 14 ancestral population clusters that correlated with ethnicity and shared cultural and/or linguistic properties. The research team demonstrated that there is more genetic diversity in Africa than anywhere else on earth.

They also determined that the ancestral origin of humans was probably located in southern Africa, near the South Africa-Namibian border. Extrapolating the data, scientists were able to map ancient migrations of populations and determined that the exit point of modern humans out of Africa was near the middle of the Red Sea in East Africa. They also provide evidence for ancient common ancestry of geographically diverse hunter-gatherer populations in Africa, including Pygmies from central Africa and click-speaking populations from southern and eastern Africa, suggesting the possibility that the original pygmy language may have contained clicks. Overall, they demonstrate remarkable correspondence between cultural, linguistic, and genetic diversity in Africa.

The San people of southern Africa, who have lived as hunter-gatherers for thousands of years, are likely to be the oldest population of humans on Earth, according to the biggest and most detailed analysis of African DNA. The San, also known as bushmen, are directly descended from the original population of early human ancestors who gave rise to all other groups of Africans and, eventually, to the people who left the continent to populate other parts of the world.

Homo sapiens, known casually as “modern humans”, are thought to have first evolved around 195,000 years ago in east Africa – the earliest remains from that time were uncovered near the Omo River in Ethiopia.

It is thought that by 150,000 years ago these early modern humans had managed to spread to other parts of Africa and fossilised remains have been found on the Cape of Good Hope in South Africa.

The earliest homo sapien remains found outside of Africa were discovered in Israel and are thought to be around 100,000 years old. They are remains of a group that left Africa through what is now the Sahara desert during a brief period when the climate grew wetter, turning the desert green with vegetation. This excursion, however, failed and the population died out when the climate started to dry out again.

While there are 14 ancestral populations in Africa itself, just one seems to have survived outside of the continent.

The latest genetic research has shown that it was not until around 70,000 years ago that humans were able to take advantage of falling sea levels to cross into Arabia at the mouth of the Red Sea, which is now known as the Gate of Grief.

Ethiopian discovery

Kathryn and John Arthur

This rocky area in Mota cave held bones that yielded the first ancient African genome.

In 2011, archaeologists working with Gamo tribesman in the highlands of southwest Ethiopia discovered Mota Cave, 14 metres wide and 9 metres high, overlooking a nearby river. A year later, they excavated a burial of an adult male, his body extended and hands folded below his chin. Radiocarbon dating suggested that the man died around 4,500 years ago — before the proposed time of the Eurasian migrations and the advent of agriculture in eastern Africa.

Advances in ancient DNA technology allow researchers to reap DNA from ever older bones, and the cool, constant temperatures of caves are kind to the molecule. So a team co-led by Ron Pinhasi, an archaeologist at University College Dublin, tested the Mota man’s bones for intact DNA and found enough to sequence his genome 12 times over1.

The man’s genome is, unsurprisingly, more closely related to present-day Ethiopian highlanders known as the Ari than to any other population the team examined, suggesting a clear line of descent for the Ari from ancient human populations living in the area. But further genetic studies show that the Ari also descend from people that lived outside Africa, which chimes with a previous study that discovered a ‘backflow’ of humans into Africa from Eurasia around 3,000 years ago. (Humans first migrated from Africa some 60,000 to 100,000 years ago.)

Eyal Bartov/Alamy

The man found in Mota cave is related to the Ari, modern-day Ethiopian highlanders.

Eurasian influences

Using genetic evidence from Eurasian ancient genomes and present-day populations, the researchers determined that the migrant ancestors of the Ari were closely related to early farmers who moved into Europe from the Near East around 9,000 years ago. Co-author Marcos Gallego Llorente, an evolutionary geneticist at the University of Cambridge, UK, suggests that Middle Eastern farmers later moved south to Africa, bringing new crops to the continent such as wheat, barley and lentils. The team also found vestiges of these migrants’ DNA in people all across sub-Saharan Africa — probably carried by later migrations, such as the expansion of Bantu-speaking groups from West Africa to other parts of the continent around 1,000 years ago.

Genetic studies have shown that after the great migration out of Africa, which happened about 60,000 years ago, some people later returned to the continent.

But this study shows that about 3,000 years ago there was a much larger migration than had been thought.

The Neolithic farmers from western Eurasia who, about 8,000 years ago, brought agriculture to Europe then began to return to Africa.

“We know now that they probably corresponded to a quarter of the people that already lived in East Africa (at that time). It was a major backflow, a very sizeable movement of people,” said Dr Manica.

It is unclear what caused this move – potentially changes happening in the Egyptian empire – but it has left a genetic legacy.

“Quite remarkably, we see in Ethiopia about 20% – so a fifth – of the genome of people living there right now is actually of Eurasian origin, it actually comes from these farmers,” explained Dr Manica.

Pontus Skoglund knew there had to be more to the story. So the Harvard University evolutionary geneticist and his colleagues obtained DNA from 15 ancient Africans from between 500 and 6000 years ago, some before the Bantu expansion. In addition, Skoglund’s team got DNA data from 19 modern populations across Africa for comparison, including from large groups like the Bantu and smaller ones like the Khoe-San and the Hadza.

For the most part, the ancient DNA was most similar to that of people living in the same places where the bones were found, Skoglund reported. But some interesting exceptions showed intermingling among various groups. “It’s really exciting to see in Africa that there was already this ancient admixture,” says Simon Aeschbacher, a population geneticist from the University of Bern who was not involved with the work. “There must have been population movements in early Africa.”

The ancient genomes indicate that Southern Africans split off from Western Africans several thousand years ago, and subsequently evolved key adaptations that honed their taste buds and protected them from the sun. Around 3000 years ago, herders—possibly from today’s Tanzania—spread far and wide, reaching Southern Africa centuries before the first farmers. But modern Malawians, who live just south of Tanzania, are likely descended from West African farmers rather than local hunter-gatherers, Skoglund says. Indeed, the analysis suggests that West Africans were early contributors to the DNA of sub-Saharan Africans. But even these DNA donors were a hodgepodge of what are now two modern groups—the Mende and the Yoruba. And one ancient African herder showed influence from even farther abroad, with 38% of their DNA coming from outside Africa.

Another study focused on Southern Africa, where some researchers think modern Homo sapiens evolved. Evolutionary geneticist Carina Schlebusch and her colleagues at Uppsala University in Sweden partially sequenced seven ancient genomes: three from 2000-year-old hunter-gatherers and four from 300- to 500-year-old farmers. They also included modern DNA in their analyses.

The more modern farmers did have Bantu DNA in their genomes, but the ancient hunter-gatherers predated the spread of the Bantu, she and her colleagues reported last month on the preprint server bioRxiv. Their other findings parallel Skoglund’s discoveries: Nine percent to 22% of the DNA of these farmers’ modern descendants—including the southern Khoe-San—comes from East Africans and Eurasian herders.

Schlebusch’s analysis reaches even deeper into human history than does Skoglund’s, as her team used the ancient and modern genomes to estimate that the hunter-gatherers she studied split off from other groups some 260,000 years ago, about the age of the oldest H. sapiens fossil.

Research by geneticists and archaeologists has allowed them to trace the origins of modern homo sapiens back to a single group of people who managed to cross from the Horn of Africa and into Arabia. From there they went on to colonise the rest of the world.

Genetic analysis of modern day human populations in Europe, Asia, Australia, North America and South America have revealed that they are all descended from these common ancestors.

It is thought that changes in the climate between 90,000 and 70,000 years ago caused sea levels to drop dramatically and allowed the crossing of the Red Sea to take place.

The findings are to be revealed in a new BBC Two documentary series,The Incredible Human Journey, that traces the prehistoric origins of the human species.

Dr Peter Forster, a senior lecturer in archaeogenetics at Anglia Ruskin University in Cambridge who carried out some of the genetic work, said: “The founder populations cannot have been very big. We are talking about just a few hundred individuals.”

See also this African history Timeline


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