A genetic study suggests that interbreeding between ancient human species may have given a survival advantage to the groups which migrated to the Americas thousands of years ago.

The study published in Science focused on a gene called MUC19. This gene is involved in the production of proteins which form saliva and mucus barriers in the digestive and respiratory systems.

A variant of the MUC19 present in modern people with Latin American and Indigenous American ancestry didn’t come from ancient Homo sapiens – it came from the archaic human species known as Denisovans.

Denisovans were first discovered in Siberia in 2010. They are most closely related to Neanderthals, having split from that species about 400,000 years ago. Modern humans and Neanderthals diverged at least 500,000 years ago. Genetic evidence shows that all 3 species interbred.

Fossils of Denisovans have been found in Siberia, Tibet, northeastern China and Taiwan.

While Denisovans went extinct about 30,000 years ago, they left their mark on modern Homo sapiens.

Denisovan DNA has been found in modern people from Papua New Guinea. There is even evidence to suggest that Denisovan genetics may have helped strengthen the immune systems of these people.

The new study shows that DNA from archaeological sites of ancient modern humans in North and South America also contained the Denisovan version of the MUC19 gene.

The rate at which the gene appears in modern human populations suggests that it was advantageous in the natural selection of Indigenous Americans’ ancestors.

Denisovan MUC19 may have provided a survival or reproductive advantage.

“From an evolutionary standpoint, this finding shows how ancient interbreeding can have effects that we still see today,” says author Emilia Huerta-Sánchez, a professor at Brown University in the US. “From a biological standpoint, we identify a gene that appears to be adaptive, but whose function hasn’t yet been characterised. We hope that leads to additional study of what this gene is actually doing.”

Huerta- Sánchez and colleagues compared Denisovan DNA with modern genomes through the 1,000 Genomes Project – a global study of human genetic variation.

Denisovan-derived MUC19 is found in high frequencies in populations with Indigenous American ancestry. It is also abundant in the DNA of 23 individuals uncovered at archaeological sites in Alaska, California, Mexico and elsewhere in the Americas.

Statistical analysis showed that the gene was found in unusually high frequencies in ancient Indigenous American populations. It was also in a very long stretch of archaic DNA. Both of these factors signify a gene which is subject to positive natural selection.

“Something about this gene was clearly useful for these populations – and maybe still is or will be in the future,” Huerta-Sánchez comments.

The genetic analysis revealed another surprise. The Denisovan MUC19 gene likely came into modern human DNA through interbreeding with Neanderthals who had previously interbred with Denisovans, rather than direct interbreeding between Denisovans and ancient modern humans.

“Typically, genetic novelty is generated through a very slow process,” Huerta- Sánchez adds.  “But these interbreeding events were a sudden way to introduce a lot of new variation.”

How this sudden new genetic variation helped ancient modern humans who migrated to the Americas will be the subject of future research.