Evidence suggests that early humans, Homo sapiens, and Neanderthals, Homo neanderthalensis, engaged in interbreeding across a significant geographical area, extending from Western Europe into Asia.
This ancient interaction is reflected in the genetic makeup of most non-African populations today, who typically carry approximately 2 percent Neanderthal DNA in their genome. The interbreeding also led to the replacement of Neanderthal Y chromosome lineages with those originating from H. sapiens.
Unraveling the Scale and Location of Ancient Hybridization
Despite the established fact of interbreeding, the precise locations and the extent of this phenomenon remained largely unknown for a considerable time, although advancements have begun to clarify the timing.
Neanderthal ancestors departed Africa around 600,000 years ago, migrating into Europe and Western Asia. The earliest indications of H. sapiens venturing out of Africa come from skeletal remains found in modern-day Israel and Greece, dating back approximately 200,000 years.
While genetic contributions from H. sapiens to Neanderthal populations have been observed in the Altai Mountains of present-day Siberia around 100,000 years ago, the primary wave of human migration out of Africa occurred after approximately 60,000 years ago.
Two studies published in 2024, utilizing ancient genome data, proposed that the most substantial gene flow between H. sapiens and Neanderthals took place over a sustained interval of 4,000 to 7,000 years, commencing around 50,000 years ago.
Previously, it was theorized that this genetic exchange likely occurred in the eastern Mediterranean region. However, pinpointing a definitive location has proven challenging.
Mapping the Hybrid Zone Through Genetic Analysis
To investigate further, researchers led by Mathias Currat at the University of Geneva in Switzerland analyzed data from 4,147 ancient genetic samples. The oldest of these samples are approximately 44,000 years old and originate from over 1,200 distinct locations. The team evaluated the prevalence of Neanderthal DNA variants, known as introgressed alleles, that were repeatedly transferred through hybridization.
Currat explained the study’s objective: “The idea was to see whether it is possible using the patterns of Neanderthal DNA integration in past human genomes to see where integration took place.”
The study’s findings indicated a progressive increase in the proportion of transferred DNA as researchers moved further away from the eastern Mediterranean region. This proportion then leveled off at approximately 3,900 kilometers, both westward toward Europe and eastward into Asia.
Currat expressed surprise at the observed trend, noting, “We were quite surprised to see a nice increasing pattern of introgression proportion in human genomes resulting from what we guess is the out-of-Africa human expansion. It’s increasing toward Europe, it’s increasing toward East Asia, and so it allows us to estimate the boundary of this hybrid zone.”
Computer simulations conducted by the researchers suggest a hybrid zone that encompassed a large portion of Europe and the eastern Mediterranean, extending into Western Asia.
“What we see seems to be a single continuous pulse – a continuous series of interbreeding events in space and time,” Currat stated. “However, we don’t know when hybridization took place in the zone.”
Geographic Scope and Regional Exclusions
The identified hybrid zone includes nearly all known sites associated with Neanderthal fossils across Western Eurasia, with the exception of those located in the Altai region.
Leonardo Iasi from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, commented on the significance of the findings: “The finding that the inferred hybrid zone extends broadly into western Eurasia is intriguing and suggests that interactions between populations may have been geographically widespread.”
Notably, the Atlantic fringe, including western France and most of the Iberian Peninsula, falls outside this inferred hybrid zone. This is despite the well-documented presence of Neanderthals in these areas. Currat suggested that either no hybridization occurred in these specific regions, or any interbreeding that did happen was not represented in the analyzed genetic samples.
Iasi summarized the study’s broader implications: “Overall, the study paints a picture of repeated interactions between modern humans and Neanderthals across a broad geographic range and over extended periods of time.” He also pointed out that the hybrid zone might extend further, but limitations in ancient DNA sampling in areas like the Arabian Peninsula hinder a comprehensive assessment of its full extent in that direction.
Challenging Previous Understandings of Ancient Encounters
Chris Stringer of the Natural History Museum in London described the research as “an important paper that challenges the view that there was only one region, probably western Asia, and one Neanderthal population (not represented in the current Neanderthal genetic samples) that hybridised with the Homo sapiens population dispersing from Africa.”
He concluded, “As early sapiens spread out in ever-growing numbers and over an ever-expanding range, it seems they mopped up small Neanderthal populations they encountered along the way, across virtually the whole known Neanderthal range.”