The “ghost population” – a group of ancient humans thought to have lived in Africa about half a million years ago – had their genes found by scientists in modern-day humans. Traces of the unknown ancestor emerged when researchers analysed genomes from west African populations and found up to a fifth of their DNA appeared to have come from missing relatives.
Published in the journal Science Advances, geneticists explain how they think the ancestors of modern west Africans interbred with the hitherto undiscovered archaic humans tens of thousands of years ago.
If this is true, it would prove to be similar to the mating of ancient Europeans with Neanderthals.
Sriram Sankararaman, a computational biologist who led the research at the University of California in Los Angeles, revealed the extent of the unknown DNA.
He said: “In the west Africans we looked at, all have ancestry from this unknown archaic population.”
The world was once home to many related species and subspecies of human.
When these different species crossed paths they would sometimes mate.
As a result, many modern Europeans carry a smattering of Neanderthal genes.
Meanwhile, indigenous Australians, Polynesians and Melanesians carry genes from Denisovans, another group of archaic humans.
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The Denisovans are now extinct, yet once called home an area spanning Asia during the Lower and Middle Palaeolithic period.
It is debated whether Denisovans represent a distinct species of Homo or are an archaic subspecies of Homo Sapiens, though they may have interbred with modern humans as recently as 15,000 years in New Guinea.
Previous studies have hinted that other ancient humans once roamed Africa – yet, without any fossils or DNA to back the claims, researchers have struggled to learn any more than the hearsay allows.
Using statistical techniques to figure out whether an influx of genes from interbreeding was likely to have happened in the past, researcher Arun Durvasula and Mr Sankararaman obtained 405 genomes from four west African populations.
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The analysis suggested that it had in every case.
The scientists then went on to scour the African genomes for chunks of DNA that looked different to modern human genes.
This allowed them to pull out sequences that most probably came from an ancient relative.
Then, by comparing these with genes from Neanderthals and Denisovans, the scientists concluded that the DNA had come from an unknown group of archaic humans.
Mr Sankararaman said: “They seem to have made a pretty substantial impact on the genomes of the present day individuals we studied – they account for 2 percent to 19 percent of their genetic ancestry.”
Of the four populations studied, two came from Nigeria, and one each from Sierra Leone and the Gambia.
Despite the findings being far from definitive, the scientist’s best estimates suggest the “ghost population” split from the ancestors of Neanderthals and modern humans between 360,000 and one million years ago.
A group of perhaps 20,000 individuals then bred with the ancestors of modern west Africa at some point in the past 124,000 years.
Some other explanations are possible, according to Mr Sankararaman, for example, there may have been multiple waves of mating over many thousands of years – or even a number of yet-to-be-discovered populations of archaic human relatives.
He said: “It’s very likely that the true picture is much more complicated.”
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