Thousands of years ago, ancient humans embarked on a perilous journey, traversing the icy expanses of the Bering Strait to reach the uncharted territories of the Americas. In a groundbreaking study led by the University of Colorado Boulder, researchers have uncovered that these early nomads carried with them a surprising genetic legacy—a segment of DNA inherited from the now-extinct Denisovans, which may have been crucial for adapting to their new environment.
The findings, set to be published on August 21 in the journal Science, reveal a fascinating aspect of human evolution. “In terms of evolution, this is an incredible leap,” remarked Fernando Villanea, one of the study’s lead authors and an assistant professor in the Department of Anthropology at CU Boulder. “It shows an amount of adaptation and resilience within a population that is simply amazing.”
Unraveling the Denisovan Legacy
The research revisits the enigmatic Denisovans, ancient human relatives who once roamed regions from modern-day Russia to Oceania and the Tibetan Plateau. Despite their extinction tens of thousands of years ago, Denisovans remain a mystery, with the first known individual identified only 15 years ago from DNA in a bone fragment found in Siberia. Like their Neanderthal cousins, Denisovans likely had distinctive features such as prominent brows and absent chins.
“We know more about their genomes and how their body chemistry behaves than we do about what they looked like,” Villanea explained. A growing body of research indicates that Denisovans interbred with both Neanderthals and humans, significantly influencing the genetic makeup of contemporary populations.
Denisovan DNA in the Americas
To delve deeper into these genetic connections, Villanea and his team, including co-lead author David Peede from Brown University, analyzed genomes from diverse human populations worldwide. They focused on a gene called MUC19, integral to the immune system. Their research revealed that individuals with Indigenous American ancestry are more likely to possess a variant of this gene inherited from Denisovans, suggesting this ancient genetic heritage may have been vital for survival in the Americas’ diverse ecosystems.
“It seems like MUC19 has a lot of functional consequences for health, but we’re only starting to understand these genes,” Villanea noted.
Previous studies have shown that Denisovans carried a unique variant of the MUC19 gene, which they passed on to some humans. This genetic admixture was common in ancient times, with most modern humans carrying some Neanderthal DNA, while Denisovan DNA comprises up to 5% of the genomes of people from Papua New Guinea.
Tracing Genetic Time Capsules
The researchers examined existing data on the genomes of modern humans from Mexico, Peru, Puerto Rico, and Colombia, regions with prevalent Indigenous American ancestry. They discovered that one in three individuals of Mexican descent carries the Denisovan variant of MUC19, particularly in genome segments with Indigenous American origins. In contrast, only 1% of Central Europeans possess this variant.
A surprising find was that the Denisovan gene variant in humans is flanked by Neanderthal DNA. “This DNA is like an Oreo, with a Denisovan center and Neanderthal cookies,” Villanea said, illustrating the complex genetic interplay.
The Journey of DNA
Villanea and his colleagues propose that before humans crossed the Bering Strait, Denisovans interbred with Neanderthals, passing the MUC19 variant to their offspring. Subsequently, Neanderthals interbred with humans, sharing some Denisovan DNA—a rare instance of DNA transferring from Denisovans to Neanderthals and then to humans. As humans migrated to the Americas, natural selection favored the spread of this borrowed MUC19 variant.
The reason for the prevalence of the Denisovan variant in the Americas remains unclear. Villanea suggests that the first Americans encountered unprecedented conditions, including new foods and diseases. Denisovan DNA may have provided additional tools to tackle these challenges.
“All of a sudden, people had to find new ways to hunt, new ways to farm, and they developed really cool technology in response to those challenges,” Villanea said. “But, over 20,000 years, their bodies were also adapting at a biological level.”
Implications and Future Research
To further understand the impact of MUC19 variants on human health, Villanea plans to investigate how these genetic differences affect contemporary populations. The study underscores the remarkable adaptability of human evolution. “What Indigenous American populations did was really incredible,” Villanea stated. “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.”
The study’s co-authors include researchers from Brown University, the University of Washington School of Medicine, Universidad Nacional Autónoma de México, University of Copenhagen, Clemson University, University of Padova, University of Turin, University of California, Berkeley, Université Paris-Saclay, and Trinity College Dublin.
