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Ancient boy’s DNA pushes back date of earliest humans

Homo sapiens may have emerged as a genetically distinct species as early as 350,000 years ago

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2:00pm, September 28, 2017
African San people

DEEP PAST  A new comparison of ancient and modern human DNA concludes that Homo sapiens emerged earlier than typically thought, perhaps around 350,000 years ago. African San people, such as these, belong to a genetic line that originated roughly 260,000 years ago, researchers estimate.

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A boy who lived in what’s now South Africa nearly 2,000 years ago has lent a helping genome to science. Using the long-gone youngster’s genetic instruction book, scientists have estimated that humans emerged as a distinct population earlier than typically thought, between 350,000 and 260,000 years ago.

The trick was retrieving a complete version of the ancient boy’s DNA from his skeleton to compare with DNA from people today and from Stone Age Neandertals and Denisovans. Previously documented migrations of West African farmers to East Africa around 2,000 years ago, and then to southern Africa around 1,500 years ago, reshaped Africans’ genetics — and obscured ancient ancestry patterns — more than has been known, the researchers report online September 28 in Science.

The ancient boy’s DNA was not affected by those migrations. As a result, it provides the best benchmark so far for gauging when Homo sapiens originated in Africa, evolutionary geneticist Carina Schlebusch of Uppsala University in Sweden and her colleagues conclude.

In line with the new genetically derived age estimate for human origins, another team has proposed that approximately 300,000-year-old fossils found in northwestern Africa belonged to H. sapiens (SN: 7/8/17, p. 6). Some researchers suspect a skull from South Africa’s Florisbad site, dated to around 260,000 years ago, qualifies as H. sapiens. But investigators often place our species’ origins close to 200,000 years ago (SN: 2/26/05, p. 141). There is broad consensus that several fossils from that time represent H. sapiens.

Debate over the timing of human origins will continue despite the new evidence from the child, whose remains came from previous shoreline excavations near the town of Ballito Bay, says Uppsala University evolutionary geneticist and study coauthor Mattias Jakobsson. “We don’t know if early Homo sapiens fossils or the Florisbad individual were genetically related to the Ballito Bay boy,” he says.

Thus, the precise timing of humankind’s emergence, and exact patterns of divergence among later human populations, remain unclear. Researchers have yet to retrieve DNA from fossils dating between 200,000 and 300,000 years old that either securely or possibly belong to H. sapiens.

Story continues after graphic

Ancient splits

In this diagram based on a new DNA study, Homo sapiens originates as a genetically distinct species between 350,000 and 260,000 years ago. African populations branch in two directions and then further subdivide around 200,000 years ago. Non-African populations, bottom left, appeared shortly after 100,000 years ago.

Diagram of emergence of Homo Sapiens

However early human evolution played out, later mixing and mingling of populations had a big genetic impact. DNA evidence from more recent fossils, including those studied by Schlebusch’s group, increasingly suggests that Stone Age human groups migrated from one part of Africa to another and mated with each other along the way (SN: 10/20/12, p. 9), says Harvard Medical School evolutionary geneticist Pontus Skoglund. In the Sept. 21 Cell, he and his colleagues report that DNA from 16 Africans, whose remains date to between 8,100 and 400 years ago, reveals a shared ancestry among hunter-gatherers from East Africa to South Africa that existed before West African farmers first arrived 2,000 years ago.

That ancient set of common genes still comprises a big, varying chunk of the DNA of present-day Khoisan people in southern Africa, Skoglund’s group found. Earlier studies found that the Khoisan — consisting of related San hunter-gatherer and Khoikhoi herding groups — display more genetic diversity than any other human population.

Schlebusch’s team estimates that a genetic split between the Khoisan and other Africans occurred roughly 260,000 years ago, shortly after humankind’s origins and around the time of the Florisbad individual. Khoisan people then diverged into two genetically distinct populations around 200,000 years ago, the researchers calculate.

Ancient DNA in Schlebusch’s study came from seven individuals unearthed at six South African sites. Three hunter-gatherers, including the Ballito Bay boy, lived about 2,000 years ago. Four farmers lived between 500 and 300 years ago.

Comparisons to DNA from modern populations in Africa and elsewhere indicated that between 9 percent and 30 percent of Khoisan DNA today comes from an East African population that had already interbred with Eurasian people. Those East Africans were likely the much-traveled farmers who started out in West Africa and reached southern Africa around 1,500 years ago, the researchers propose.

Citations

C. Schlebusch et al. Southern African ancient genomes estimate modern human divergence to 350,000 to 260,000 years ago. Science. Published online September 28, 2017. doi:10.1126/science.aao6266.

P. Skoglund et al. Reconstructing prehistoric African population structure. Cell. Vol. 171, September 21, 2017, p. 59. doi:10.1016/j.cell.2017.08.049.

Further Reading

B. Bower. Oldest known Homo sapiens fossils come from northern Africa, studies claim. Science News. Vol. 191, July 8, 2017, p. 6.

E. Wayman. Africans’ genes mute on human birthplace. Science News. Vol. 182, October 20, 2012, p. 9.  

B. Bower. Human fossils are oldest yet. Science News. Vol. 167, February 26, 2005, p. 141.  

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