New York, Jul 23 (IANS): In a salivary protein, researchers have found hints that a "ghost" species of archaic humans may have contributed genetic material to ancestors of people living in Sub-Saharan Africa.

The research, by a team from the University of Buffalo and published in journal Molecular Biology and Evolution, adds to a growing body of evidence suggesting that sexual rendezvous between different archaic human species may not have been unusual.

"It seems that interbreeding between different early hominin species is not the exception -- it's the norm," said Omer Gokcumen from the university.

"Our research traced the evolution of an important mucin protein called MUC7 that is found in saliva. 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," he added.

The scientists came upon their findings while researching the purpose and origins of the MUC7 protein, which helps give spit its slimy consistency and binds to microbes, potentially helping to rid the body of disease-causing bacteria.

"Based on our analysis, the most plausible explanation for this extreme variation is archaic introgression -- the introduction of genetic material from a 'ghost' species of ancient hominins," added Gokcumen.

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