Homo sapiens may be the only hominin alive today but go back tens of thousands of years ago and the planet was a hodgepodge of various human and protohuman species, including the Neanderthals and Denisovans.

As the result of some interspecies mingling, some of their DNA has been passed down to modern humans: Traces of Neanderthal DNA are still found in people of non-African descent, Denisovan DNA lives on in people of Asian heritage, and researchers recently learned that the DNA of an unknown population of archaic hominins continues to exist in Melanesians. 

Now, Arun Durvasula and Sriram Sankararaman from the University of California in Los Angeles believe they have discovered remnants of DNA from an as yet unknown species of ancient hominin in the genomes of the Yoruba people, in West Africa. The find, published in the online archive bioRxiv, is currently awaiting peer review.

Because DNA is easily damaged by weather that is hot and humid, we do not have DNA from any African-dwelling ancient hominins, whereas we do have DNA from those living on other continents. This makes it hard to identify any ancient genes in modern-day African populations.

To skirt around the problem, Durvasula and Sankararaman came up with a statistical method able to highlight any abnormal genetic code without needing the genome of the species it was inherited from. The technique was applied to the DNA of 50 people who had had their DNA sequenced as part of the 1000 Genomes Project.

It turned out that roughly 8 percent of their DNA comes from a "ghost" species – but who are they?

The Neanderthals and Denisovans can be ruled out – we already have their DNA and there is no evidence to suggest they lived in Africa. And it's not the modern-day pygmies. Their DNA has been sequenced and it is not a match. 

Homo naledi, a small-brained hominin that could be found roaming around the South African plains 250,000 years ago, is a possible but unlikely contender. Researchers believe they were too different from us genetically to be able to mate and reproduce successfully. As Mark Thomas from University College London, UK, said to New Scientist, “I would be amazed if there was anything of them in us.” 

Homo heidelbergensis was a more advanced hominin living in Africa circa 200,000 years ago and a more probable contestant. It could also be that the mystery DNA came from an isolated group of Homo sapiens or population of hominins that are as yet unknown to researchers.

Whatever the answer turns out to be, the study is a reminder that our species did not emerge from a single founding population, Thomas told New Scientist. 

[H/T: New Scientist]