When modern humans travelled from Asia to the Americas, traversing the Bering Strait for the first time, they were well prepared. That’s because these travelers brought along an adaptive variant in their genes — a genetic advantage that they acquired through two separate populations of ancient humans.
Recently published in Science, a new study suggests that the Denisovans passed this variant to the Neanderthals, while the Neanderthals passed it to modern humans, where it is seen in the genomes of past and present individuals with Indigenous American ancestry.
Tracing the origins of this genetic inheritance, the study supports the theory that interbreeding between ancient humans may have helped our species survive and spread.
“From an evolutionary standpoint, this finding shows how ancient interbreeding can have effects that we still see today,” said Emilia Huerta-Sánchez, a study author and a population geneticist at Brown University, according to a press release. “From a biological standpoint, we identify a gene that appears to be adaptive.”
Read More: Who Were the Denisovans?
Fossils with Denisovan DNA
The Denisovans roamed throughout Asia between 300,000 years and 30,000 years ago, though their traces are remarkably rare. Indeed, very few fossils from this population have been found, including a few finger bones and fragments, a few jaw bones, and a recently reassessed skull.
Still, the DNA within these fossils has allowed scientists to search for genetic similarities between the Denisovans, the Neanderthals, and modern humans, revealing the history of interbreeding between these three groups.
Denisovan Gene Variant
To understand this history a bit better, the study authors compared Denisovan DNA against modern human DNA collected from ancient Indigenous Americans and from contemporary Latin Americans with Indigenous American ancestry.
While the ancient American DNA came from a handful of North and South American archeological sites, the contemporary American DNA came from the 1000 Genomes Project, a collection of modern human genomes compiled in the 2000s and 2010s.
Turning specifically to MUC19 — a gene that’s thought to impact the mucosal barriers in the body’s respiratory and digestive systems — the team found that a specific variant was present in the Denisovans and the Indigenous Americans, both ancient and contemporary, suggesting that it was passed from the former to the latter, for whom it served some sort of adaptive purpose.
In fact, the prevalence of this specific version of MUC19 in Indigenous American populations shows that the variant provided some advantage for those individuals who crossed the Bering Strait, whether for survival or reproduction. The specific benefits that it offered aren’t completely clear, though it’s possible that the variant may have protected modern humans from the pathogens that they met in their new environments.
Read More: Who Were the Neanderthals?
Inheritance from Human to Human
So how, exactly, did modern humans acquire this genetic variation? Looking to the location of the MUC19 gene in the modern human genome, the team found that the variant was wedged between two segments of Neanderthal DNA. According to the researchers, this suggests that it was passed from Denisovan to Neanderthal to modern human through interbreeding.
The results solidify the idea that the mingling of ancient human populations has played an important part in the introduction of valuable genetic variants. “Typically, genetic novelty is generated through a very slow process,” Huerta-Sánchez said in the press release. “But these interbreeding events were a sudden way to introduce a lot of new variation.”
Additional research could clarify the specific advantages of this MUC19 variant in North and South America, where modern humans entered environments that they’d never encountered before. While the genetic variation may have helped them fend off disease, it’s impossible to confirm that theory without further study.
“What Indigenous American populations did was really incredible,” said Fernando Villanea, another study author and an anthropologist at the University of Colorado Boulder, according to another press release. “They went from a common ancestor living around the Bering Strait to adapting biologically and culturally to this new continent that has every single type of biome in the world.”
Read More: Did Early Humans Interbreed? These Scientists Made a Map to Prove It
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