A “ghost” species of ancient humans may have left their mark in their saliva of Sub-Saharan populations today.

We know that our ancestors didn’t shy away from mingling with other species of humans. As they migrated to Europe and Asia, they shared their share of genes around — primarily with the Neanderthals and Denisovans. But new genetic evidence shows that ancient Africans also had their fun with a so-far-unknown species of early hominids.

It’s all about mucin

A research team discovered the strange gene while working to understand the role and origin of MUC7, a mucus-like protein which lends saliva its sticky consistency and helps it binds to microbes — which is believed to help the body evict unwanted guests, such as bacteria.

Part of this process was required the team to chart how MUC7 evolved over time in different areas, which they did by analyzing genetic material from over 2,500 people all over the world. To their surprise, they found that one group of genomes had a very different version of the gene encoding MUC7 — all of which were harvested in Sub-Saharan Africa. It’s so different, in fact, that the MUC7 genes from people with Neanderthal and Denisovan heritage are more similar than any of them are to the new variant.

“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,” says Omer Gokcumen, PhD, an assistant professor of biological sciences in the University at Buffalo College of Arts and Sciences.

“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.”

Considering a baseline value of how fast human genes mutate over the generations, the team estimates that the interbreeding happened sometime around 150,000 years ago and that the two groups first grew apart around 1.5 to 2 million years ago.

The team further report that MUC7 seems to influence the makeup of our oral flora — the bacteria that live inside our mouths. MUC7 comes in two flavors: in some people, it’s encoded by five genes; in others, by six. Based on biological samples harvested from 130 different people, the team found that the two different version of MUC7-encoding genes were each strongly associated with a different oral flora makeup.

“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.”

But the biggest find here is that there may be a lost family of hominids just waiting to be discovered. And, yet again, that our ancestors were really horny.

“It seems that interbreeding between different early hominin species is not the exception — it’s the norm,” Ruhl concludes.

The paper “Archaic hominin introgression in Africa contributes to functional salivary MUC7 genetic variation” has been published in the journal Molecular Biology and Evolution.

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