Scientists have found hints that a “ghost” species of human ancestors may have contributed to the genes of modern humans in Sub-Saharan Africa.
What's interesting about this genetic contribution is that it's unique to modern humans. Our cousins—the Neanderthals and the Denisovans—don't have this same gene. The question, of course, is who contributed this gene to the modern human gene pool?
A mysterious “ghost” species may have given some modern humans a different version of the protein that can be found in our saliva. This protein, named MUC7, is what makes our saliva what it is—thick, sticky, and stringy. MUC7 also attaches itself to microbes, which gives it the potential to help keep harmful bacteria away from our bodies. A variation on the gene that codes for MUC7 came from a distant “ghost” ancestor species.
“Ghost” here simply means that scientists aren't sure which species contributed the gene, and that they don't have fossils of the species. The ancestor in question may be a hominin species that we've already discovered, or one that we have yet to discover.
It's well-known that humans had interbred with older hominin species, like the Neanderthals and Denisovans. There's proof that modern humans from Asia and Europe had interbred with our older genetic cousins, but now it seems that humans in Africa had done so as well. Modern humans split from various hominin relatives to form a new species, but it didn't stop them from having trysts with their Homo erectus cousins. In fact, researchers say that interbreeding between human and hominin species was more of the norm rather than the exception.
Whoever these “ghost” human ancestors may have been, they likely bred with modern humans about 150,000 years ago. Scientists were able to make this estimate by looking at the rate that genes mutate over the course of evolution. Modern humans may have diverged from this “ghost” species around 1.5 to two million years ago.
Another interesting thing to note about this gene is that the variant in Sub-Saharan Africa was very distinctive. In fact, the MUC7 gene in Neanderthals and Denisovans resembled the modern human MUC7 gene more. The Sub-Saharan gene was more distinct.
Thus, whoever contributed to the distinct Sub-Saharan gene, they sure made a mark on the modern human genome. It'll be interesting to find out which one of our human ancestors, known or as of yet unknown, made the gene what it is today.
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