A recent study indicates that multiple instances of gene flow occurring between 250,000 and 200,000 years ago impacted the genomes and biology of both modern humans and Neanderthals, who are believed to share 2.5 to 3.7 percent of human ancestry.
Li othersIt provides insight into the history of modern-human Neanderthal admixture, shows that gene flow has significantly influenced patterns of genomic variation in modern and Neanderthals, and suggests that taking into account human-derived sequences in Neanderthals allows for more precise inferences about admixture and its consequences in both Neanderthals and modern humans. Image courtesy of the Neanderthal Museum.
“For the first time, geneticists have identified multiple instances of interbreeding between modern humans and Neanderthals,” stated Professor Li Ming from Southeast University.
“It is now evident that throughout most of human history, there was interaction between modern humans and Neanderthals,” added Professor Joshua Akey from Princeton University.
“Our direct ancestors, hominins, diverged from the Neanderthal lineage approximately 600,000 years ago and acquired modern physical characteristics around 250,000 years ago.”
“Subsequently, modern humans continued to engage with Neanderthals for around 200,000 years until the extinction of Neanderthals.”
The researchers utilized the genomes of 2,000 modern humans, three Neanderthals, and one Denisovan to track gene flow between human populations over the past 250,000 years.
They employed a genetic tool called IBDmix, developed several years ago, which utilizes machine learning techniques for sequencing genomes.
Previously, scientists relied on comparing the human genome to reference populations of modern individuals with minimal or no Neanderthal or Denisovan DNA.
The study authors discovered traces of Neanderthal DNA even in populations residing thousands of miles south of Neanderthal caves, suggesting that the DNA might have been transmitted southward by travelers or their descendants.
Using IBDmix, they identified a first contact wave around 200,000-250,000 years ago, a second contact wave around 100,000-120,000 years ago, and a peak contact wave around 50,000-60,000 years ago, deviating from previous genetic data.
“Most genetic data indicates that modern humans originated in Africa 250,000 years ago, persisted there for another 200,000 years, and only around 50,000 years ago dispersed from Africa to populate other regions as humans,” said Prof Akey.
“Our model suggests that there wasn’t a prolonged period of stasis, but soon after the emergence of modern humans, we migrated out of Africa and eventually returned.”
“To me, the narrative revolves around dispersal, highlighting that modern humans have been more mobile than previously assumed, encountering Neanderthals and Denisovans,” added Prof Akey.
This portrayal of migrating humans aligns with archaeological and paleoanthropological evidence indicating cultural and tool exchanges among human populations.
A crucial insight was to search for modern human DNA in the Neanderthal genome, rather than vice versa.
“While much genetic research in the past decade focused on how interbreeding with Neanderthals influenced the evolution and phenotype of modern humans, these questions also hold importance and interest in the opposite direction,” noted Professor Akey.
They realized that the descendants of the initial interbreeding event between Neanderthals and modern humans likely stayed with the Neanderthals and thus left no genetic trace in modern humans.
“By incorporating Neanderthal elements into genetic studies, we can analyze these early migrations in a new light,” Prof Akey mentioned.
The final revelation was that the Neanderthal population was smaller than previously estimated.
Traditional genetic modeling used diversity as an indicator of population size: greater genetic diversity implied a larger population.
However, using IBDmix, the team showed that most diversity came from DNA sequences originating from a larger modern human population, leading to a reduction in the effective Neanderthal population from around 3,400 breeding individuals to approximately 2,400.
Collectively, these new findings provide insights into the disappearance of Neanderthals from the record roughly 30,000 years ago.
“I prefer not to use the term ‘extinction’ because I believe Neanderthals were mostly assimilated,” mentioned Prof Akey.
It is theorized that the Neanderthal population gradually dwindled, with the last survivors merging into modern human communities.
“The assimilation model was first proposed in 1989 by anthropologist Fred Smith from Illinois State University, and our results offer compelling genetic evidence supporting Fred’s hypothesis,” Prof Akey stated.
“Neanderthals likely faced prolonged near-extinction.”
“Our estimates suggest that even a slight decrease of 10 to 20 percent in the population size would have a significant impact on an already vulnerable population,” Prof Akey added.
“Modern humans can be likened to waves gradually eroding the shoreline, eventually overwhelming Neanderthals demographically and integrating them into the modern human population.”
Read the full research findings published in the journal Science.
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Li-Ming Lee others2024. Recurrent gene flow between Neanderthals and modern humans over the past 200,000 years. Science 385(6705); doi:10.1126/science.adi1768
Source: www.sci.news
