A mysterious sister species to Neanderthals once roamed Africa, interbreeding with human ancestors.
Discovered through a genetic analysis of present-day Africans, the new species appears to have been interbreeding with humans as recently as 20,000 years ago.
The DNA, identified by the University of Washington in Seattle, showed up across the continent, in both pygmies of central Africa and in the Hadza and Sandawe hunter-gatherers of Tanzania. It resembles neither human or Neanderthal DNA.
The lineage of these people appears to have diverged from the modern human lineage several hundred thousand years ago, around the same time that Neanderthals diverged from Homo sapiens.
"Fossils degrade fast in Africa so we don't have a reference genome for this ancestral lineage, but one of the things we're thinking is it could have been a sibling species to Neanderthals," says Joshua Akey of the University of Washington.
Evidence of interbreeding with an archaic lineage, or introgression, was found in all three groups tested.
Indeed, it's already been established that modern humans interbred, not only with Neanderthals but with other ancient hominids too. In 2010, DNA analysis showed that a species known as Denisovans provided as much as six percent of the genome of present-day New Guineans.
"Given that introgression is present in these very diverse groups, I think we can now say that this seems to be a pretty universal aspect of human history," says UW's Benjamin Vernot.
The analysis also threw new light on genetic diversity in modern humans. The team's managed to identify the genes responsible for the small size of pygmies, for example.
""We would have never found these strong candidate genes for short stature, if we hadn't looked at multiple genomic sequences from these isolated populations," says Joseph Lachance from the University of Pennsylvania.
"It hints at the power of what you can do when you sample multiple genomes and compare them at a population scale."