Neanderthal and Early Human Interbreeding Across Wide Regions: What the Evidence Shows

Homo sapiens and Neanderthals likely interbred across a vast region, extending from Western Europe to Asia.

Modern humans (Homo sapiens) and Neanderthals (Homo neanderthalensis) exhibited mixed ancestry, with most non-Africans today possessing Neanderthal DNA, approximating 2% of their genome. Neanderthals also engaged in interbreeding, leading to a lineage shift in the Y chromosome influenced by Homo sapiens.

Despite increasing knowledge about the timing of this hybridization, the specific regions and scales of these interactions long remained a mystery. Ancestors of Neanderthals departed Africa around 600,000 years ago, migrating toward Europe and Western Asia. The first evidence of Homo sapiens moving from Africa includes skeletal remains from sites in modern-day Israel and Greece, dating to approximately 200,000 years ago.

Evidence suggests that Homo sapiens contributed genetically to the Neanderthal population in the Altai Mountains around 100,000 years ago. However, the primary wave of migration from Africa occurred over 60,000 years ago. Recent studies utilizing ancient genomic data indicate that significant gene flow between Homo sapiens and Neanderthals began around 50,000 years ago, with findings documented in studies of 4000 and 7000 gene transfers.

This interaction is thought to have primarily taken place in the eastern Mediterranean, although pinpointing the exact locations remains challenging.

To investigate, Matthias Karat and his team from the University of Geneva analyzed 4,147 ancient genetic samples from over 1,200 locations, with the oldest dating back approximately 44,000 years. They studied the frequency of genetic mutations (introgression alleles) originating from Neanderthal DNA that were passed down through hybridization.

“Our objective was to use Neanderthal DNA integration patterns in ancient human genomes to determine the sites of hybridization,” Carlat explains.

Findings revealed that the proportion of transferred DNA increased gradually as one moved away from the eastern Mediterranean region, plateauing approximately 3,900 kilometers westward into Europe and eastward into Asia.

“We were surprised to identify a distinct pattern of increasing introgression rates in the human genome, likely linked to human expansion from Africa,” Carlat notes. “This increase toward Europe and East Asia allows us to estimate the parameters of this hybrid zone.”

Computer simulations showed a hybrid zone potentially spanning much of Europe and the eastern Mediterranean, extending into western Asia.

“Our findings suggest a continuous series of interbreeding events across both space and time,” notes Carlat. “However, the specifics of mating occurrences in this hybrid zone remain unknown.”

This hybrid zone encompasses nearly all known Neanderthal remains found across Western Eurasia, with the exception of the Altai region.

“The extensive geographical breadth of the putative hybrid zone suggests widespread interactions among populations,” states Leonard Yasi from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

Notably, the Atlantic periphery—including western France and much of the Iberian Peninsula—was not part of the hybrid zone, despite the established presence of Neanderthals in these regions. Currat suggests that interbreeding may not have occurred here or may not be reflected in the analyzed genetic samples.

“This study reveals ongoing interactions between modern humans and Neanderthals over extensive geographical areas and extended periods,” adds Yasi. The hybrid zone may extend further, though limited ancient DNA sampling in regions like the Arabian Peninsula complicates assessment of its reach.

“This pivotal research challenges the notion that interbreeding occurred only in one area of West Asia with a singular Neanderthal population (not represented in existing genetic samples). Homo sapiens appear to have dispersed from Africa in increasing numbers across expanding territories, likely outcompeting smaller Neanderthal groups they encountered throughout most of the recognized Neanderthal range,” comments Chris Stringer from the Natural History Museum in London.