Neanderthals and Humans Interbred for 7,000 Years, Study Suggests
Neanderthals and Humans Interbred for 7,000 Years, Study Suggests
Neanderthals and Humans Interbred for 7,000 Years, Study Suggests
Neanderthal genes present in modern humans may have been introduced through an extended period of interbreeding starting around 47,000 years ago and lasting nearly 7,000 years, according to new research.
Ancient Relatives and Their Genetic Legacy
Neanderthals, among the closest extinct relatives of modern humans (Homo sapiens), diverged from a common ancestor approximately 500,000 years ago. Scientific revelations over the past decade have shown that Neanderthals interbred with the ancestors of modern humans who migrated out of Africa. Consequently, the genomes of contemporary human populations outside Africa contain about 1% to 2% Neanderthal DNA.
Researchers continue to investigate when and where this genetic mingling occurred. Questions persist: did Neanderthals and modern humans interbreed at one specific place and time outside Africa, or was this interbreeding more widespread across various regions and periods?
Unraveling the Timeline of Interbreeding
To address these uncertainties, researchers analyzed over 300 modern human genomes spanning the last 45,000 years. This study included samples from 59 individuals who lived between 2,200 and 45,000 years ago and 275 diverse present-day humans. The findings, posted on the BioRxiv1 preprint database, are currently under review for potential publication in a scientific journal.
The research team focused on the amount of Neanderthal DNA present in these samples. By examining variations in Neanderthal ancestry across different times and locations, they estimated the timing and duration of interbreeding events. Their analysis suggests that the most significant period of interbreeding occurred around 47,000 years ago and lasted about 6,800 years.
Where Did the Interbreeding Occur?
As modern humans began leaving Africa at least 194,000 years ago, western Asia, where Africa connects with Eurasia, emerges as a likely site for early encounters with Neanderthals. Chris Stringer, a paleoanthropologist at the Natural History Museum in London, highlighted this possibility, noting that modern humans carrying Neanderthal ancestry could have subsequently spread across the globe.
The Persistence and Impact of Neanderthal DNA
Researchers also examined how Neanderthal DNA persisted in the modern human genome over time. DNA segments that endured likely offered evolutionary advantages, while segments that quickly disappeared may have posed disadvantages. The study found that surviving Neanderthal genes were linked to traits such as skin color, metabolism, and the immune system, providing benefits as modern humans faced new environmental challenges outside Africa.
When the identified interbreeding period ended, over 5% of the modern human genome was Neanderthal in origin. This implies that "about one in 20 parents in our ancestral population was a Neanderthal," according to Fernando Villanea, a population geneticist at the University of Colorado Boulder.
Revisiting Historical Dispersal Timelines
Rajiv McCoy, a population geneticist at Johns Hopkins University, noted that other instances of interbreeding might have occurred without leaving lasting genetic traces. For example, a modern human jaw from 37,000 to 42,000 years ago found in Romania contains Neanderthal DNA not present in other modern genomes, suggesting isolated interbreeding events.
Chris Stringer emphasized that this new estimate of 47,000 years ago for Neanderthal interbreeding influences our understanding of human dispersal from Africa. This timeframe constrains the dispersal of modern human ancestors to less than 47,000 years ago. However, evidence of human occupation in northern Australia around 65,000 years ago poses questions. Stringer suggested that either this evidence is incorrect, those early populations went extinct or were overwhelmed by later dispersals, or they were not Homo sapiens, which is less likely given the complex behavior implied by the evidence.
The One-Way Gene Flow
Curiously, the exchange of DNA appears to have been one-way—modern human DNA does not seem to have entered Neanderthal genomes. Stringer noted,
"There is little evidence of gene flow in the reverse direction at this time—that is, Homo sapiens to Neanderthal."
This absence of reciprocal gene flow could imply behavioral differences between the two populations or possible disadvantages for hybrids, such as reduced health or fertility.
The study's insights into the prolonged period of interbreeding between Neanderthals and modern humans offer a deeper understanding of human evolutionary history and the genetic legacy carried by contemporary populations.
Iasi, L. N. M., Chintalapati, M., Skov, L., Mesa, A. B., Hajdinjak, M., Peter, B. M., & Moorjani, P. (2024). Neandertal ancestry through time: Insights from genomes of ancient and present-day humans. In bioRxiv (p. 2024.05.13.593955). https://doi.org/10.1101/2024.05.13.593955