Modern humans and Neanderthals interbred in multiple waves over 200,000 years, exchanging genes and influencing each other's evolution. A new study using AI reveals traces of modern human DNA in Neanderthals and revises their population size to 2,400. The findings suggest Neanderthals weren't wiped out but were gradually absorbed into human populations, reshaping our understanding of shared history.
(Image Source: Princeton University)
How Humans and Neanderthals Shaped Each Other: The story of human evolution has long been shrouded in mystery, with questions about our interactions with ancient relatives like Neanderthals and Denisovans sparking debates among scientists and the public alike. A groundbreaking study led by an international team of geneticists and artificial intelligence (AI) experts is shedding new light on the complex relationships between these groups. The research, conducted under the leadership of Joshua Akey, a professor at Princeton University's Lewis-Sigler Institute for Integrative Genomics, offers a comprehensive look at the history of genetic intermingling between modern humans and their ancient relatives, revealing a much more intimate connection than previously believed.
"This is the first time geneticists have identified multiple waves of modern human-Neanderthal admixture," said Liming Li, a professor at Southeast University in Nanjing, China. Li performed much of the research as an associate scholar in Akey's lab. Their findings suggest that for the vast majority of human history, modern humans have maintained contact with Neanderthal populations. This relationship began shortly after the two groups diverged from a common ancestor approximately 600,000 years ago.
Modern humans, who developed the physical characteristics we recognize about 250,000 years ago, did not remain isolated from Neanderthals. Instead, the two groups interacted and interbred for over 200,000 years until the eventual disappearance of Neanderthals roughly 30,000 years ago. The results of this study were published in the journal Science and are poised to reshape our understanding of early human history.
Once caricatured as brutish and intellectually inferior, Neanderthals are now recognized as skilled hunters and toolmakers who developed sophisticated techniques for survival. They lived in cold European climates and exhibited social behaviours such as caring for injured group members. Archaeologists and anthropologists agree that Neanderthals, Denisovans, and modern humans were all distinct groups within the broader category of humans, although modern humans have come to dominate the evolutionary landscape.
To uncover the genetic exchanges between these groups, Akey's team utilized a powerful tool called IBDmix, which employs machine learning to decode ancient genomes. Previous studies relied on comparisons against a "reference population" of modern humans believed to lack Neanderthal or Denisovan DNA. However, Akey's research showed that even these populations carried trace amounts of Neanderthal DNA, likely introduced by ancient voyagers and their descendants.
IBDmix allowed the team to map three significant waves of gene flow between modern humans and Neanderthals. The first occurred about 200,000 to 250,000 years ago, soon after modern humans emerged. A second wave happened around 100,000 to 120,000 years ago, and the most significant interaction occurred 50,000 to 60,000 years ago. This timeline contrasts sharply with previous theories, suggesting a long period of isolation for modern humans in Africa before a major dispersal event 50,000 years ago.
"Our models show that there wasn't a long period of stasis," Akey explained. "Shortly after modern humans arose, we were migrating out of Africa and coming back, encountering Neanderthals and Denisovans much more frequently than previously recognized." This dynamic movement aligns with archaeological evidence of cultural and technological exchanges among these groups.
One of the study's most intriguing insights came from reversing the traditional focus of genetic research. Instead of analyzing modern human genomes for traces of Neanderthal DNA, the researchers examined Neanderthal genomes for evidence of modern human DNA. This approach uncovered the genetic contributions of early modern humans who interbred with Neanderthals but whose offspring remained with Neanderthal populations, leaving no direct trace in living humans.
"The vast majority of genetic work over the last decade has focused on how mating with Neanderthals impacted modern human phenotypes and evolutionary history," Akey noted. "But these questions are just as relevant in the reverse case." This perspective enabled the researchers to identify previously undetected instances of gene flow, revealing a more intricate web of interactions.
Another surprising discovery involved the size of the Neanderthal population. Traditional genetic models estimate population size based on genetic diversity, with greater diversity indicating larger populations. However, Akey's team found that much of the apparent diversity in Neanderthal genomes came from DNA sequences introduced by modern humans, whose populations were significantly larger.
As a result, the estimated effective population of Neanderthals was revised downward from about 3,400 breeding individuals to approximately 2,400. This finding underscores the vulnerability of Neanderthals, who were already at risk due to their small population size.
The new genetic evidence supports the "assimilation model" of Neanderthal disappearance, first proposed by anthropologist Fred Smith in 1989. According to this model, Neanderthals were not entirely wiped out but were gradually absorbed into modern human populations.
"Neanderthals were teetering on the edge of extinction for a long time," Akey explained. “Modern humans acted like waves crashing on a beach, slowly eroding Neanderthal populations. Eventually, we demographically overwhelmed them and incorporated them into our gene pool.”
This nuanced perspective challenges earlier narratives of violent conflict or abrupt extinction. Instead, it highlights a shared history of interconnection and exchange, with Neanderthals leaving a lasting legacy in modern human DNA.
The work of Akey and his team opens up new avenues for understanding the deep connections between modern humans and their ancient relatives. By combining cutting-edge genetic tools with AI-driven analysis, the researchers have uncovered a richer, more dynamic narrative of human evolution. Their findings reveal that migration, interaction, and interbreeding have been fundamental aspects of our shared history, challenging long-held assumptions and offering fresh insights into what it means to be human.
Also Read: Princeton researchers present new insights into Neanderthal and early human evolution
As scientists continue to probe the mysteries of our ancient past, this research underscores the importance of looking beyond traditional perspectives. The story of human evolution is not one of isolation but of connection—a story that continues to unfold with each discovery. Keep reading at Education Post News for more global updates.
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