The “ghost population” of humans, as it has been described, are believed to have lived in Africa around half a million years ago. Their genes were spotted by scientists in modern-day humans, traces that turned up after researchers analysed genomes from West African populations.
They found up to a fifth of their DNA appeared to have come from missing relatives. Published in the journal, Science Advances, geneticists say the presence of the genetical material could reveal some unknown ancestor of Homo sapiens.
They believe ancestors of modern West Africans interbred with the undiscovered archaic human tens of thousands of years ago, something that happened between European Homo sapiens and Neanderthals.
Writing in the paper, Professor Sriram Sankararaman, a computational biologist who led the research at the University of California in Los Angeles, said: “In the west Africans we looked at, all have ancestry from this unknown archaic population.”
Many different species of the Homo genus once roamed the Earth, each wildly different from the other.
Often, when they happened to cross paths, they would mate, something we know happened because of the presence of Neanderthal genes in many modern Europeans.
Similarly, indigenous Australians — Polynesians and Melanesians — carry genes from Denisovans, another group of archaic humans.
The Denisovans called home the area spanning Asia during the Lower and Middle Palaeolithic period, though they disappeared around 50,000 years ago.
Scientists are divided on whether Denisovans represent a distant species of the Homo genus or are merely an archaic subspecies of Homo Sapiens.
Using statistical techniques to assess whether an influx of genes from interbreeding was likely to have occurred, Professors Arun Durvasula and Sankararaman obtained 405 genomes from four West African populations.
The analysis suggested that it had in every case, and so the scientists went on to scour the genomes for chunks of DNA that looked different to modern human genes.
They isolated sequences that likely came from ancient relatives, and compared them with genes from Neanderthals and Denisovans, concluding that the DNA in question had come from an unknown group of archaic humans.
“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,” wrote Prof Sankararaman.
Of the four populations studied, two came from Nigeria, and one each from Sierra Leone and the Gambia.
While the findings were far from definitive, the study’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, perhaps 20,000 strong, then interbred with the ancestors of modern West Africa somewhere in the past 124,000 years.
Prof Sankararaman said other explanations were possible, for example, that multiple waves of mating over many thousands of years had occurred.
However, he added: “It’s very likely that the true picture is much more complicated.”
Dr. Thomas Hughes is a UK-based scientist and science communicator who makes complex topics accessible to readers. His articles explore breakthroughs in various scientific disciplines, from space exploration to cutting-edge research.