Our spit may reveal the echoes of an ancient “ghost” hominin species. New research has uncovered evidence that suggests the ancestors of sub-Saharan people may have interbred with an as yet unknown ancient hominin.
The researchers stumbled across this latest discovery, published in Molecular Biology and Evolution, while studying something totally different. They were investigating the purpose and origin of a specific protein found within human saliva known as MUC7. This protein is thought to give spit its slick consistency, as well as enable it to bind to microbes in the mouth to possibly aid in ridding the body of disease.
In order to study the history of this particular protein, the researchers inspected the MUC7 gene in over 2,500 people. While most modern humans have a similar MUC7 gene, people from sub-Saharan Africa had a version that was wildly different. In fact, the gene was so distinctive that even the Neanderthal and Denisovan versions of MUC7 were more genetically close to most modern humans than the sub-Saharan variety.
“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,” explains Omer Gokcumen, study author from the University at Buffalo College of Arts and Sciences, in a statement. “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.”
By plotting the evolution of this gene variety amongst modern living humans, they were able to deduce that the ancestors of sub-Saharan people likely interbred with an unidentified archaic hominin species as recently as 150,000 years ago. They were also able to calculate that the last common ancestor between humans and this unknown hominin must have existed some 1.5-2 million years ago.
This is actually not the first time that evidence of a hominin species has been found hidden in our genome. The Denisovans are known from just a few fragments of bone discovered in Siberia, as well as through traces they left in our DNA in Asia. Another study has found that there may have been yet another hominin knocking around in Southeast Asia that contributed to the genetics of Melanesians.
We know that the ancestors of all people outside of Africa mated at some point with Neanderthals and Denisovans. This latest work, however, lends evidence to the idea that mating with other hominins was not as rare of an event for our ancestors as previously thought.