While analysing saliva, scientists have discovered that a "ghost" species of archaic humans may have bred with the ancestors of people currently living in Sub-Saharan Africa.
The research suggests that sexual intercourse between different archaic human species may not have been unusual.
Past studies have concluded that the forebears of modern humans in Asia and Europe interbred with other early hominin species, including Neanderthals and Denisovans.
The new research is among more recent genetic analyses indicating that ancient Africans also had trysts with other early hominins.
"It seems that interbreeding between different early hominin species is not the exception - it's the norm," said Omer Gokcumen, assistant professor at the University at Buffalo in the US.
"Our research traced the evolution of an important mucin protein called MUC7 that is found in saliva," said Gokcumen.
"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 said.
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.
As part of the study, the team examined the MUC7 gene in more than 2,500 modern human genomes.
Researchers found a group of genomes from Sub-Saharan Africa had a version of the gene that was wildly different from versions found in other modern humans.
The Sub-Saharan variant was so distinctive that Neanderthal and Denisovan MUC7 genes matched more closely with those of other modern humans than the Sub-Saharan outlier did.
"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," Gokcumen said.
"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," he said.
Given the rate that genes mutate during the course of evolution, the team calculated that the ancestors of people who carry the Sub-Saharan MUC7 variant interbred with another ancient human 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.
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