Evidence of enigmatic ancient hominid found in saliva

A specific protein found in saliva suggests that early humans have interbred with an enigmatic ancient hominid species. 

Looking at proteins within saliva, scientists  from the University at Buffalo College of Arts and Sciences found trace evidence that early humans might have interbred with an enigmatic ancient hominid species. The finding is in line with a new body or research that suggests that interbreeding between different early hominids was much more common than previously thought.

Our ancestors in Asia and Europe have interbred with species like the Neanderthals and Denisovans, some recent studies have already concluded. And when looking at the gene coding for a specific saliva protein, the scientists form New York found the signature of archaic admixture of modern day Sub-Saharan African populations.

“It seems that interbreeding between different early hominin species is not the exception — it’s the norm,” says Omer Gokcumen, PhD, an assistant professor of biological sciences in the University at Buffalo College of Arts and Sciences. “Our research traced the evolution of an important mucin protein called MUC7 that is found in saliva,” he says. “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.”

The researchers were actually trying to find the purpose and the origins of the MUC7 protein, responsible for giving spit its slimy consistency. It also helps it bind to microbes, potentially helping to rid the body of disease-causing bacteria.

During their research, the scientists examined the gene in more than 2,500 modern human genomes and they were in for a surprise. When analyzing the data, they found a group of genomes from Sub-Saharan Africa that had a version of the gene that was wildly different from versions found in other modern humans.

How different was the variant? According to the scientists, the variants matched more closely with Neanderthals and Denisovans than with modern Sub-Saharan humans. The researchers say that the only logical explanation is that the variant comes form the introduction of a genetic material pertaining to an unknown species of ancient hominid.

“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 says. “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.”

And looking a the rate of mutation, the study was able to pinpoint that interbreeding came about 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 scientists were interested in MUC7 because in a previous study they showed that the protein likely evolved to serve an important purpose in humans. In some people, the gene that codes the protein holds six copies of genetic instructions that direct the body to build parts of the corresponding protein. In other people, the gene harbors only five sets of these instructions (known as tandem repeats).

Prior studies by other researchers found that the five-copy version of the gene protected against asthma, but Gokcumen and Ruhl did not see this association when they ran a more detailed analysis.

“From what we know of MUC7, it makes sense that people with different versions of the MUC7 gene could have different oral microbiomes,” Ruhl says. “The MUC7 protein is thought to enhance the ability of saliva to bind to microbes, an important task that may help prevent disease by clearing unwanted bacteria or other pathogens from the mouth.”