A study published in PaleoAnthropology by researchers Chris Stringer and Lucile Crété from the Natural History Museum in London presents a compelling case that the extinction of Neanderthals may have resulted less from conflict and more from interbreeding with Homo sapiens.
The research combines fossil evidence and genetic data to trace long-term contact between the two species. As modern humans spread into Eurasia beginning as early as 200,000 years ago, encounters with Neanderthals weren’t rare—and often turned intimate. Over generations, these sexual interactions may have reduced Neanderthal genetic distinctiveness, ultimately phasing them out not through war or disease, but through biological absorption.
“This behavior could have led to the Neanderthals’ extinction,” said Stringer, “if they were regularly breeding with Homo sapiens, which could have eroded their population until they disappeared.”
DNA Reveals Patterns of Interbreeding—And Its Limits
So far, only 32 Neanderthal genomes have been fully sequenced. They show traces of genetic mixing, particularly around 50,000 to 60,000 years ago, when Homo sapiens were expanding deeper into Europe and Asia. Most modern humans carry about 2% Neanderthal DNA, evidence of these ancient relationships. But tellingly, late-stage Neanderthal remains do not appear to carry Homo sapiens nuclear DNA.
One explanation lies in hybridization barriers. There may have been biological asymmetry—similar to how in some plant species, one direction of cross-breeding produces viable offspring, while the other does not. In this case, successful mating may have only occurred between Neanderthal males and modern human females. Furthermore, evidence suggests male hybrids may have had reduced fertility, a pattern seen in several species today.
This reproductive imbalance, coupled with already small and scattered Neanderthal populations, could have reduced internal breeding rates enough to push them into irreversible decline.
Fossils Show Early and Frequent Contact
For decades, scientists believed Homo sapiens arrived in Europe roughly 40,000 years ago during the Aurignacian period. But newer fossil finds—from sites like Apidima Cave in Greece, Zlatý kůň in Czechia, and Bacho Kiro in Bulgaria—suggest modern humans were already present more than 200,000 years ago in parts of Europe and Western Asia.
These discoveries indicate that early migrations may have brought humans into contact with Neanderthals far earlier and more often than previously thought. Some human populations from this time later vanished, while others contributed genetically to surviving groups. Throughout these overlapping periods, the fossil and genetic records point to repeated bouts of interaction, including interbreeding.
“We now know from a growing body of genetic data that this coexistence… was accompanied by bouts of interbreeding,” the authors wrote. “A continuing absorption of Neanderthal individuals into Homo sapiens groups could have been one of the factors that led to the demise of the Neanderthals.”
Communication Differences May Have Shaped Encounters
Though Neanderthals and modern humans lived alongside each other for thousands of years, their communication styles and physical features likely differed significantly. Neanderthals had prominent brow ridges, possibly used in non-verbal communication. Modern humans, with flatter brows and more mobile eyebrows, may have had greater range in facial expressions.
This contrast could have made mutual understanding more difficult. Anatomical studies also suggest differences in the vocal apparatus and the expression of nearly 600 genes, many linked to facial structure and voice, which might have limited shared language capabilities.
Despite those differences, sexual contact still occurred. But as Stringer points out, whether those encounters were “mutual courtship or less friendly” remains an open question. Studies of chimpanzee groups show that both scenarios are possible in intergroup meetings, making it difficult to generalize about Neanderthal-human interactions.