Enigmatic Archaic Human Relative Interbred with Ancestors of Modern Africans, Study Suggests

A mysterious hominin species mated with the ancestors of modern-day Sub-Saharan Africans, according to an analysis of modern human genomes published this month in the journal Molecular Biology and Evolution.

Xu et al find genetic proof of interbreeding between an unknown ancient hominin and the ancestors of modern-day Africans. Image credit: Bob Wilder, University at Buffalo.

Previous studies have concluded that the forebears of modern humans in Asia and Europe interbred with other early hominin species, including Neanderthals and Denisovans.

The new research is among more recent genetic analyses indicating that ancient Africans also had trysts with other early hominins.

“It seems that interbreeding between different early hominin species is not the exception — it’s the norm,” said co-lead author Dr. Omer Gokcumen, of the State University of New York at Buffalo.

“Our research traced the evolution of one of the most abundant proteins in human saliva, mucin-7, which helps give spit its slimy consistency and binds to microbes, potentially helping to rid the body of disease-causing bacteria.”

“When we looked at the history of MUC7, the gene that codes for mucin-7, we see the signature of archaic admixture in modern-day Sub-Saharan African populations.”

Dr. Gokcumen and his colleagues from the Foundation for Research and Technology-Hellas in Heraklion, Crete, Greece, Pennsylvania State University, the University of Minnesota, Twin Cities, and the State University of New York at Buffalo examined the MUC7 gene in more than 2,500 modern human genomes.

The analysis yielded a surprise: a group of genomes from Sub-Saharan Africa had a version of the gene that was wildly different from versions found in other modern humans.

The Sub-Saharan variant was so distinctive that Neanderthal and Denisovan MUC7 genes matched more closely with those of other modern humans than the Sub-Saharan outlier did.

“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,” Dr. Gokcumen said.

“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.”

Given the rate that genes mutate during the course of evolution, Dr. Gokcumen and co-authors calculated that the ancestors of Sub-Saharan Africans interbred with the ‘ghost’ hominin species as recently as 150,000 years ago, after the two species’ evolutionary path diverged from each other some 1.5 to 2 million years ago.

_____

Duo Xu et al. Archaic hominin introgression in Africa contributes to functional salivary MUC7 genetic variation. Mol Biol Evol, published online July 21, 2017; doi: 10.1093/molbev/msx206