Human ancestors living in Sub-Sahara Africa may have interbred with unkonwn "ghost" species of early hominins, a study on the evolutionary history of a salivary protein has indicated.
"This unknown human relative could be a species that has been discovered, such as a subspecies of Homo erectus, or an undiscovered hominin," Omer Gokcumen, assistant professor at the State University of New York at Buffalo, said in a recent statement.
"We call it a "ghost" species because we don't have the fossils," he added.
The new research is among more recent genetic analyses indicating that ancient Africans also had trysts with other early hominins.
The research team traced the evolution of an important mucin protein called MUC7 that was found in human saliva, examining its gene in more than 2,500 modern human genomes.
"When we looked at the history of the gene that codes for the protein, we see the signture of archaic admixture in modern day Sub-Saharan African populations," Gokcumen said.
The research team found that a group of genomes from Sub-Saharan African populations had a version of the gene that was wildly different from versions found in other modern humans, even beyond the differences between modern humans and the Neanderthals or Denisovans.
The finding, published on Britain's Molecular Biology and Evolution journal, showed that the ancestry of Homo sapiens are more complicated than originally believed.
Given that genes mutate during the course of evolution, the researchers calculated that the ancestors carrying Sub-Saharan MUC7 variant interbred with the "ghost" hominin species as recently as 150,000 years ago, after the two species' evolutionary path diverged from each other some 1.5 to 2 million years ago.
The new studies also found that the MUC7 gene helps give spit its slimy consistency and binds to microbes, potentially helping to rid the body of disease-causing bacteria.