"It's an exciting moment because these studies open a window showing us that there is much more than we thought to learn about our ancestors". In their study in the journal Science Advances, researchers say they found evidence of what they refer to as an "archaic ghost population".
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.
It cast doubt on the widely held "out of Africa" theory of human migration, meaning that modern humans originated in Africa and dispersed to the rest of the world in a single wave between 60,000 and 80,000 years ago. They even seem to have left their mark in modern populations to an even greater extent than Neanderthals: the amount of "ghost" DNA in modern West African people ranged from 2 to 19 percent. They found DNA segments in the West Africans that could best be explained by ancestral interbreeding with an unknown member of the human family tree that led to what is called genetic "introgression". As a result, modern Europeans carry a smattering of Neanderthal genes, while indigenous Australians, Polynesians and Melanesians carry genes from Denisovans, another group of archaic humans. When they found each other, mating was only a natural process.
The scientists from UCLA overcame these challenges to find the "ghost" of the hominin tribe by using computer modelling techniques on modern DNA. This allowed them to narrow down their search and identify the sequence that most likely belonged to the "ghost population".
Last month, researchers from Princeton's Lewis-Sigler Institute for Integrative Genomics said they also detected Neanderthal ancestry in Africans for the first time.
Although this is a simplified version of events - the separating and admixture of different populations and sub-species can be extremely muddled and complex - this is the most likely scenario that can be pieced together from the limited evidence.
Sankararaman and colleagues sequenced the genomes of 405 individuals from four West African populations: two from Nigeria, one from Sierra Leone, and one from Gambia. Next, the researchers plan to focus on some of these genes in order to tease out what they do.
One of the most remarkable things about the 2014 bombshell was that it proved that the only people on the planet who did not have any Neanderthal DNA were sub-Saharan Africans.
"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 research.
"We do not know what this African population may have been".
The unknown group "appears to have split off from the ancestors of modern humans a little before when Neanderthals split off from our ancestors", he says. The fossil record in Africa offers only a few hints.