It is unclear if West Africans derived any genetic benefits from this long-ago gene flow. No one should be surprised that mating took place when one population ran into another one, which led to impressive genetic structures. The technique "goes along a person's genome and pulls out chunks of DNA which we think are likely to have come from a population that is not modern human".
Geneticists suspect that the ancestors of modern west Africans interbred with the yet-to-be-discovered archaic humans tens of thousands of years ago, much as ancient Europeans once mated with Neanderthals.
This is in part because of sparse fossil records in Africa and the difficulty of obtaining ancient DNA. More information is required by the researchers to believe that the ghost population descends from the ancestors of Neanderthals.
In the new study, Arun Durvasula and Sriram Sankararaman analyzed the whole-genome sequences of people from four populations living in Nigeria, Sierra Leone, and the Gambia, as well as those from Neanderthal and Denisovan fossils.
Sankararaman later revealed that the interbreeding seems to have left a substantial impact because the mysterious genomes accounted for 2 per cent to 19 per cent of the genetic history. But far from making them a uniform or an inbred population, they are in fact by far the most genetically diverse peoples on Earth. The group of perhaps 20,000 individuals then bred with the ancestors of modern west Africans at some point in the past 124,000 years. "It's nearly certainly the case that the story is incredibly complex and complicated and we have kind of these initial hints about the complexity", says Sriram Sankararaman, a computational biologist at UCLA.
We don't know how these human relatives looked like or how they lived, but their genes paint a compelling story of complex interactions with our ancestors. "It's very likely that the true picture is much more complicated", he said.
The researchers are now keen to delve into the ancient genes and work out what they do. One prospect is that west Africans kept the DNA because it helped them survive and hybridize.
"It is always interesting and useful to see researchers applying new methods to try to get a better idea of what ancient populations might have been like", said John Hawks, an anthropologist at the University of Wisconsin-Madison, who was not involved in the study.
It is theorized that the ancestors of the modern West Africans interacted with the unknown race of archaic humans in a fashion similar to the way in which Europeans mingled with the Neanderthals.
"We do not know what this African population may have been".
The study's authors say that these genetic differences are best explained by hybridisation with an unknown humanoid.