An article published in the journal “Molecular Biology and Evolution” describes a research that from a protein in human saliva showed that there were interbreedings between Homo sapiens and another unknown hominin species. A team of scientists conducted a study on the gene called MUC7, which encodes a mucin, a protein that determines some of the characteristics of human saliva and tracing its history found traces of these ancient interbreedings between hominins in Sub-Saharan Africa.

This conclusion is the result of a series of researches on the MUC7 gene that had other purposes. Some of the authors had already participated in a previous study on it and its results published in the journal “Nature” concerned the evolution of that gene and considered mammals in general. There are slightly different versions of the MUC7 gene not only in different species but also within a species so their comparison may also be useful to understand the various relationships even between genetic lineages of the same species.

The new research, carried out by researchers at the University of Buffalo, analyzed the MUC7 gene of more than 2,500 modern humans to study their differences. In the past, a connection between the number of sequences repeated in a genetic region and the protection against asthma was suggested, so there was also a possible medical purpose but no correlation was found on this point.

The surprising result of the MUC7 gene analysis was that a group of genomes from Sub-Saharan Africa had a version significantly different from that of all other humans. An expansion of the analysis showed that the MUC7 gene of these individuals was more similar to that of Neanderthals and Denisovans, two species of hominins, which taxonomically constitute a tribe called technically Hominini within the hominid family that includes the various species of the genus Homo.

According to the researchers, the most plausible explanation is an ancient introgression. This is a biological phenomenon resulting from the interbreeding between two different species. In essence, the hybrid interbreeds again with at least one of the two original species forming more hybrids. As time passes, these interbreedings introduce into the species various combinations of genes from the other species. In this specific case, the two species are Homo sapiens and another unknown hominin.

The researchers took into account the speed of genetic mutations to calculate that the interbreeding between Sub-Saharan Homo sapiens and an unknown hominin occurred around 150,000 years ago. Using the same method they also calculated that the separation between these two species occurred between 1.5 and 2 million years ago.

These hominins have been called a ghost species because only their MUC7 gene has been identified so it’s impossible to tell their species. It’s possible that this is an already known species of which we have fossils too ancient to extract DNA or unknown hominins, perhaps a subspecies of Homo erectus. In any case, this is the latest confirmation of the many interbreedings that happened between Homo sapiens and other hominins.