Earlier studies found evidence that the ancestors of modern humans in Europe and Asia interbred with other hominin species, which include the Denisovans and the Neanderthals. Now, analysis of saliva suggests that ancient Africans had similar trysts with other hominins.

The research, which was published in the journal Molecular Biology and Evolution on July 21, suggests that a "ghost" species of ancient humans may have contributed genetic material to the forebearers of modern-day humans who now live in Sub-Saharan Africa.

MUC7 Protein In Saliva

Study researcher Omer Gokcumen, from the University at Buffalo College of Arts and Sciences, and colleagues traced the evolution of a protein found in saliva known as MUC7.

The protein gives spit a slimy consistency. MUC7 is also believed to enhance saliva's ability to bind to microbes, which is crucial to preventing disease, as this clears away unwanted pathogens from the mouth.

The researchers studied the gene that codes for MUC7 protein by looking at over 2,500 modern human genomes. They found that a group of genomes from Sub-Saharan African contained a gene version that was markedly different from versions present in other modern humans.

The variant was so distinct that the MUC7 genes found in Neanderthals and Denisovans matched more closely with those of other modern humans.

What Is A Human 'Ghost' Species?

Based on their analysis, the researchers think that the extreme variation could be explained by the introduction of genetic material from an unknown human relative. Researchers call this "ghost species" because they do not have the fossils.

"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.

Researchers said that this could possibly be an already-discovered species or a still-undiscovered hominin.

By considering the rate at which genes mutate during evolution, the researchers think that the ancestors of modern humans who carry the distinct variant of MUC7 gene interbred with another species of ancient humans about 150,000 years ago.

"Based on in-depth simulations, we conclude that a divergent MUC7 haplotype likely originated in an unknown African hominin population and introgressed into ancestors of modern Africans," the researchers wrote in their study.

Interbreeding Among Ancient Human Species

The findings offer another evidence that interbreeding between different early hominin species occurred, and this was possibly common. The Neanderthal DNA is in fact believed to influence the health and looks of modern-day humans.

"Hybridization between modern humans and Neanderthals increased genomic complexity," researchers of an earlier study on Neanderthal DNA's contribution to human gene expression said. "Hybridization wasn't just something that happened 50,000 years ago that we don't have to worry about anymore. Those little bits and pieces, our Neanderthal relics, are influencing gene expression in pervasive and important ways."

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