Their bodies were adapting at a biological level over 20,000 years.
Tens of thousands of years before the Americas had a name, the people who first crossed into those continents carried within them a genetic gift from a vanished species — the Denisovans — passed hand to hand through ancient interbreeding across millennia. A new study in Science identifies the MUC19 gene variant, inherited through a chain of Denisovan-to-Neanderthal-to-human encounters, as unusually prevalent among Indigenous American populations, suggesting it conferred a survival advantage in radically unfamiliar lands. In this discovery lies a quiet reminder that human resilience is never purely our own making — it is a collaboration across species and deep time.
- A gene from an extinct human relative, the Denisovans, appears in one out of every three people of Mexican ancestry — compared to just one in a hundred Central Europeans — signaling something far from random.
- The MUC19 variant traveled through a three-step genetic relay: from Denisovans to Neanderthals, then from Neanderthals to modern humans, marking the first documented case of DNA moving across all three lineages.
- The gene sits wrapped in Neanderthal DNA on either side — what researchers describe as an 'Oreo cookie' structure — hinting at the layered, improbable journey this fragment took through prehistoric populations.
- When the first Americans crossed into a continent of entirely new biomes, pathogens, and environments, this ancient immune-related gene may have been a quiet but critical edge in their survival.
- Researchers are now turning toward the living, planning studies on how different MUC19 variants shape modern human health and what this ancient inheritance still means for people today.
When the first humans crossed into the Americas, they carried more than tools and memory — they carried DNA from a species most people have never heard of. A study published in Science reveals that early Americans bore a genetic variant called MUC19, inherited from Denisovans, an extinct human relative identified only fifteen years ago from a bone fragment in a Siberian cave. That variant may have been essential to surviving a continent utterly foreign to them.
Denisovans remain one of prehistory's deepest mysteries. They ranged from Siberia to the Tibetan Plateau and interbred with both Neanderthals and modern humans before disappearing. Most living humans carry traces of Neanderthal DNA; people from Papua New Guinea carry up to 5 percent Denisovan DNA. Anthropologist Fernando Villanea of the University of Colorado Boulder and David Peede of Brown University set out to understand how these ancient exchanges shaped human evolution, focusing on MUC19 — a gene involved in producing mucins, the proteins that form mucus and help defend tissues against pathogens.
What they found was striking. One in three people of Mexican ancestry carry the Denisovan version of MUC19, particularly in the portions of their genome tracing back to Indigenous American ancestors, compared to just 1 percent of Central Europeans. Stranger still, the Denisovan variant appears flanked by Neanderthal DNA on both sides — what Villanea memorably called an Oreo cookie structure, Denisovan center and Neanderthal layers around it.
The researchers believe this DNA moved in a chain: Denisovans first interbred with Neanderthals, passing the variant along; Neanderthals later mated with humans, transferring it again. This is the first documented case of genetic material traveling from Denisovans through Neanderthals to modern humans. When those humans migrated into the Americas, natural selection appears to have favored the spread of this variant among Indigenous populations — possibly because it helped early Americans cope with unfamiliar diseases and environments across every biome the continent offered.
"What Indigenous American populations did was really incredible," Villanea said, pointing to both the biological and cultural adaptation required to populate an entire continent from a single ancestral group near the Bering Strait. Researchers now plan to study how different MUC19 variants affect the health of people alive today, hoping to better understand what this ancient genetic legacy still carries forward.
When the first humans crossed into the Americas tens of thousands of years ago, they brought with them not just tools and knowledge, but a genetic inheritance from a species most of us have never heard of. A new study published in Science reveals that early Americans carried DNA from Denisovans, an extinct human relative who vanished long before the first people set foot on the continents. This ancient genetic legacy, encoded in a gene called MUC19, may have been crucial to their survival in a landscape utterly foreign to them.
Denisovans remain one of prehistory's great mysteries. Scientists only identified them fifteen years ago, extracting DNA from a bone fragment discovered in a Siberian cave. They roamed from Russia through Siberia, across Oceania, and as far west as the Tibetan Plateau before disappearing tens of thousands of years ago. We know more about their genetic code than their appearance—they may have had heavy brow ridges and lacked chins, but the details remain sketchy. What we do know is that they interbred with both Neanderthals and modern humans, leaving traces of their DNA scattered through populations across the world today. Most living humans carry some Neanderthal DNA; people from Papua New Guinea carry as much as 5 percent Denisovan DNA.
Fernando Villanea, an anthropologist at the University of Colorado Boulder, and David Peede of Brown University set out to understand how these ancient genetic exchanges shaped human evolution. They focused on MUC19, a gene that plays a central role in immune defense by producing mucins—proteins that form mucus and protect tissues from pathogens. The function of MUC19 remains poorly understood; it is one of 22 genes in mammals that produce mucins, and scientists are only beginning to grasp its full consequences for human health. Yet when Villanea and his team analyzed genomes from populations worldwide, they found something striking: people with Indigenous American ancestry carried the Denisovan variant of MUC19 at far higher rates than other groups.
The numbers tell a compelling story. One in three modern people of Mexican ancestry carry a copy of the Denisovan version of MUC19, particularly in the portions of their genome inherited from Indigenous American ancestors. By contrast, only 1 percent of people of Central European ancestry carry this variant. The researchers discovered something even more unexpected: the Denisovan gene variant appears to be surrounded by DNA from Neanderthals, as if the ancient genetic material had been wrapped in a protective layer. Villanea described it with a vivid metaphor—an Oreo cookie, with a Denisovan center and Neanderthal cookies on either side.
The story of how this DNA traveled through time is itself a chain of ancient encounters. The researchers believe Denisovans first interbred with Neanderthals, passing along their MUC19 variant to Neanderthal descendants. Later, Neanderthals mated with humans, transferring that same genetic fragment once more. This marks the first documented case of DNA moving from Denisovans to Neanderthals and then to humans—a three-step genetic relay across tens of thousands of years. When humans eventually migrated into the Americas, natural selection appears to have favored the spread of this inherited variant among Indigenous populations.
Why this particular gene became widespread in the Americas but not elsewhere remains uncertain. The first peoples to cross into North and South America encountered environments radically different from anything their ancestors had known. They faced unfamiliar plants, animals, and diseases. Over more than twenty thousand years, they adapted not only through cultural innovation—developing new hunting techniques, farming methods, and technologies—but also at a biological level. The Denisovan variant of MUC19 may have offered a survival advantage in these novel conditions, helping early Americans cope with pathogens and environmental stresses their bodies had never encountered.
Villanea emphasizes the broader significance of this discovery. "What Indigenous American populations did was really incredible," he said. "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 stands as evidence of human resilience and the power of evolution to shape populations across generations. The researchers now plan to investigate how different MUC19 variants affect the health of people living today, hoping to unlock more of the mysteries surrounding this ancient gene and what it reveals about our capacity to survive and thrive in new worlds.
Notable Quotes
In terms of evolution, this is an incredible leap. It shows an amount of adaptation and resilience within a population that is simply amazing.— Fernando Villanea, University of Colorado Boulder
What Indigenous American populations did was really incredible. 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.— Fernando Villanea
The Hearth Conversation Another angle on the story
How do we know this Denisovan DNA actually helped early Americans survive, rather than just being along for the ride?
That's the honest answer—we don't know for certain yet. What we see is that the gene became much more common in Indigenous American populations than elsewhere, which suggests natural selection favored it. But the mechanism, the actual biological advantage it provided, that's still being worked out.
So this gene makes mucus. How does that help you survive in a new continent?
Mucus is more important than it sounds. It protects tissues from pathogens. When you're exposed to entirely new diseases, new plants, new animals—your immune system is essentially meeting them for the first time. A variant that helps your body mount a better defense could make the difference between thriving and struggling.
Why would Denisovans have a version that was better suited to the Americas? They never went there.
They didn't, but they lived in diverse environments across Siberia, Russia, and Asia. Their version of the gene may have been useful in cold climates or in dealing with unfamiliar pathogens in those regions. When humans migrated to the Americas, that same adaptation turned out to be valuable again.
This seems like an incredibly specific piece of DNA to track. How did they even find it?
They looked at published genome data from Indigenous populations in Mexico, Peru, Colombia, and Puerto Rico, then compared it to other populations worldwide. The pattern emerged—one in three people of Mexican ancestry carried it, versus one in a hundred Europeans. That disparity was the signal.
What does it mean that the Denisovan gene is surrounded by Neanderthal DNA?
It's a record of that relay I mentioned. The Denisovan variant got wrapped up in Neanderthal DNA when the two species interbred, and that package stayed intact when Neanderthals passed it to humans. It's like finding a letter inside an envelope inside another envelope—each layer tells you something about the journey.