Researchers stumbled upon the unknown species after studying the genomes of West African populations. They found that up to one-fifth of the population's DNA could not be accounted for in previously discovered species.
Sriram Sankararaman led the research at the University of California in Los Angeles and commented that the West Africans his team studied all had ancestry from an archaic human population. In fact, the DNA from the unknown group made up a substantial part of the genomes of the present day individuals studied.
Geneticists believe that modern West African individuals bred with members of the unknown archaic group tens of thousands of years ago, which is why their DNA is found in modern people. At one point, several species and subspecies of humans coexisted at the same time, leading them to mate with one another. Neanderthal genes have been found in the genome of modern-day Europeans while DNA from another group of archaic humans, the Denisovans, has been found in indigenous Australians, Polynesians, and Melanesians.
For years, scientists have known that other groups of ancient humans lived in Africa but have had a difficult time studying them due to a lack of fossils and DNA. This led Sankararaman and his team to look for DNA in West African individuals that differed from modern human genes.
The scientists obtained 405 genomes from four West African populations -- two from Nigeria, and one each from Sierra Leone and the Gambia -- and used statistical techniques to discover whether there was evidence of interbreeding genes that likely happened thousands of years ago.
When the researchers found chunks of DNA sequences that differed from modern human DNA, they compared them to genes from Neanderthals and Denisovans, arriving at the conclusion that the DNA came from an unknown group of ancient humans.
Sankararaman warns that the results are far from definitive and adds that it's likely that the true picture is much more complicated.
John Hawks, an anthropologist at the University of Wisconsin-Madison, commented on the results of the study, agreeing with Sankararaman's warning.
"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. But actually knowing who those ancestors were, how they interacted, and where they existed is going to take fieldwork to find their fossil and archaeological remains."