Evolutionary enigma: Study finds traces of ‘ghost species' in human saliva

BUFFALO, N. Y. — Whoooo are you? Who, who?

Researchers are raising the question about an unknown species that seems to have interbred with humans some 150,000 years ago.

These aren't the dust-covered archeologists one might picture working in the field to unearth physical evidence of such mysterious liaisons. Rather, these scientists have more in common with a CSI forensics team —  performing genetic analyses of living people's saliva to find clues about our enigmatic ancient ancestors.

Working out of the University at Buffalo, they were originally researching the biological importance of the protein found in our spit known as “MUC7.” But then they stumbled across something exciting.

In the MUC7 protein, which gives saliva its slimy consistency and likely confers a host of benefits, they also found a hint about our deep past.

Studying variations in the MUC7 gene across more than 2,500 modern human genomes, the researchers found a surprise: some of the genomes from Sub-Saharan Africa had a version of the protein that was radically different from all the others. Dr. Omer Gokcumen, an assistant professor of biological sciences in the University at Buffalo College of Arts and Sciences, said the most likely explanation was interbreeding with a “ghost species” of ancient people.

“This unknown human relative could be a species that has been discovered, such as a subspecies of Homo erectus, or an undiscovered hominin,” says Gokcumen in a recent press release. “We call it a ‘ghost’ species because we don’t have the fossils.”

These are far from the first findings to suggest interbreeding between known and unknown human ancestors. They are instead the latest in a growing body of evidence pointing to more unknown human relatives.

Indeed, an article published in the journal Nature last year claims that such inter-species encounters were commonplace —  listing five other interbreeding episodes. While four of those were between known species, the fifth is thought to have possibly been with a ghost species as yet unknown to science.

In the Nature article, one evolutionary geneticist is quoted as saying, “We’re looking at a Lord of the Rings-type world, there were many hominid populations.”

This comment is likely in reference to the 2003 discovery of our relative Homo floresiensis, which researchers dubbed a “hobbit” people due to their tiny stature.

While scientists speculate that floresiensis may have been the “ghost species” in other interbreeding episodes with mystery hominids, it is still completely unknown what species was responsible for the MUC7 variations.

To find that out, researchers will have to do more investigating, in both the field and the lab.

The University at Buffalo findings on MUC7 were published in the journal Molecular Biology and Evolution this month.