A new study has uncovered evidence for a previously-unknown species of archaic human that may have contributed to the genetic makeup of a group of people living in modern-day Sub-Saharan Africa. Aside from revealing that there may have been even more species of early hominim than those that we know of, this finding is part of a growing body of evidence that suggests that interbreeding between the various early species of humans, such as Neanderthals and Denisovans, was not that uncommon.

"It seems that interbreeding between different early hominin species is not the exception — it’s the norm," explains assistant professor of biological sciences Omer Gokcumen, PhD, with the University at Buffalo College of Arts and Sciences.

Despite the nature of this conclusion, this study wasn’t actually aimed at uncovering hidden ancestors from mankind’s deep past, but rather it was intended to study the evolution and origin of a particular compound found in saliva.

"Our research traced the evolution of an important mucin protein called MUC7 that is found in saliva. 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," continues Gokcumen.

Of the 251 genomes from modern humans that the study used, the MUC7 gene of this one group found in Sub-Saharan Africa, known as haplogroup E, was distinctive enough to make the gene’s Neanderthal and Denisovan counterparts appear to have more in common with those of other modern humans than with haplogroup E itself. The team concluded that this variant of the MUC7 gene must have come from a previously-unknown species, with interbreeding between the two estimated to have happened roughly 148,000 years ago.

"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 elaborates. "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."

The study’s intended purpose — to study the evolution of the MUC7 gene — came about because of earlier studies that found that different variants of the gene instruct the body to produce different types of protein that are found in saliva. Different salivary proteins will affect different microbes that are present in the mouth differently, that in turn can affect the health of the individual.

"From what we know of MUC7, it makes sense that people with different versions of the MUC7 gene could have different oral microbiomes," posits study co-author and professor of oral biology Stefan Ruhl, PhD, with UB’s School of Dental Medicine. "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."