Genes of ancient ‘ghost population’ still found in humans

Pablo Tucker
February 14, 2020

We would need a much larger sample to nail down the exact timeline, but Durvasula and Sankararaman estimate the ancestors of modern Africans interbred with the mystery humans at some point in the past 124,000 years.

A research conducted by two biologists from the University of California, Los Angeles, has revealed that a "ghost population" of ancient humans inhabited in West Africa almost half a million years ago. A total of 405 West African genomes were compared with that of Neanderthal and Denisovan genomes to find out if they interbred with the unknown hominin (ghost population).

In ancient times the world was home to many related species or subspecies of human and when these groups came into contact, mating was not out of the question. As such, the surviving DNA of these ancient hominids is sufficiently different from those of the Neanderthals as of the ancient Denisovans to suggest that, in fact, they were a completely different kind of hominid, one that had not been discovered before in fossil form.

The study, by Arun Durvasula and Sriram Sankararaman of the University of California, Los Angeles, published this week in the journal Science Advance, sequenced the genomes of people from four modern West African populations. "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.

The researchers believe that the ancestors of this unknown archaic "ghost" hominin branched off from the modern human family tree before Neanderthals did.

Although there are no bones or DNA to prove the theory, the researches say the evidence is clear in the genes of modern day West Africans. Then, they took the DNA of the Neanderthals to Africa about 20,000 years ago. For example, H. sapiens groups probably crossed with Neanderthals from Europe after leaving Africa 60,000 to 80,000 years ago. 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".

Several sequences stood out in comparison to other modern human genes, and they were isolated for further research. Interestingly, this part of the DNA also doesn't match with that of Neanderthal or Denisovans.

This interbreeding may also have a great impact on the genetic makeup of modern populations: Anywhere from 2 per cent to 19 per cent of their genetic ancestry could be derived from the "ghost population".

"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.

They built "genome-wide maps of archaic ancestry" across four West African ethnic populations living in three countries: Nigeria, Sierra Leone and the Gambia. Sub-Saharan African people's ancestors have, as best we know, never left sub-Saharan Africa. It is tempting to speculate. The fossil record in Africa offers only a few hints.

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