A new discovery by a team at the University of Buffalo has revealed the genetic trace of a mysterious, and possibly undiscovered, ancient sub-species of human present in the modern-day genome of Sub-Saharan Africans. The discovery also indicates that interbreeding between ancient human species was a lot more widespread than previously thought.

Scientists made the discovery while investigating the origins of the MUC7 protein. This is a mucin protein found in saliva that gives our spit its slimy consistency. A previous study indicated that MUC7 may have conferred an evolutionary advantage in early humans due to its ability in binding to microbes, helping the body rid itself of disease-causing bacteria.

In studying the evolution of the protein, the team looked at the MUC7 gene across 2,500 modern human genomes. In a surprising discovery, a drastically different version of the MUC7 gene was found in the genomes of people from Sub-Saharan Africa.

"When we looked at the history of the gene that codes for the protein, we see the signature of archaic admixture in modern day Sub-Saharan African populations," says Omer Gokcumen assistant professor of biological sciences in the University at Buffalo College of Arts and Sciences.

This Sub-Saharan MUC7 gene variant was so distinctive that it led researchers to conclude that it was the result of interbreeding with another human species as recently as 150,000 years ago.

"This unknown human relative could be a species that has been discovered, such as a subspecies of Homo erectus, or an undiscovered hominin," says Gokcumen. "We call it a 'ghost' species because we don't have the fossils."

This isn't the first time scientists have discovered traces of an undiscovered ancient hominin species, but it is the first instance of a genetic trace being found in our modern human genome. This indicates that there was significantly more interbreeding between ancient human species that previously thought.

"It seems that interbreeding between different early hominin species is not the exception – it's the norm," says Gokcumen.

This discovery adds further complexity to the pathway of human evolution. It seems the modern Homo sapiens may have evolved from a variety of different ancient species, including several mysterious variants we have yet to fully uncover. It's also increasingly clear that our ancient ancestors were having plenty of sex with an assortment of different archaic sub-species.

The new research was published in the journal Molecular Biology and Evolution.