Neanderthals and early humans may have interbred over a large area

Homo sapiens and Neanderthals probably interbred in a vast area stretching from Western Europe to Asia.

We have long known that the first humans (Homo sapiens) and Neanderthals (Homo neanderthalensis) interbred, which is why most non-African people today typically have some Neanderthal DNA about 2 percent of their genome. Neanderthals also saw interbreeding Y chromosome lines replaced along the lines of H. sapiens.

But where this interbreeding occurred and on what scale has long been a mystery, although we are now beginning to understand when it occurred. Neanderthal ancestors left Africa about 600,000 years ago and headed for Europe and western Asia. And the oldest evidence H. sapiens migrating out of Africa are skeletal remains from sites in modern Israel and Greece dating back about 200,000 years.

There are signs of this H. sapiens genetically contributed to the Neanderthal populations from the Altai Mountains in present-day Siberia about 100,000 years agobut the main impetus for their migration out of Africa came about 60,000 years ago. Two 2024 studies based on ancient genomes suggested the greatest gene flow between them H. sapiens and Neanderthals happened in a sustained period between ca 4000 and 7000 years, starting about 50,000 years ago.

It was thought that it probably happened in the eastern Mediterranean area, but the location is hard to pinpoint.

explore, Mathias Curratt at the University of Geneva in Switzerland and his colleagues used data from 4,147 ancient genetic samples, the oldest of which are about 44,000 years old and come from more than 1,200 sites. They assessed the proportion of genetic variants from Neanderthal DNA – called introgressed alleles – that were repeatedly passed on through hybridization.

“The goal was to see if it was possible to use the patterns of Neanderthal DNA integration in past human genomes to see where the integration occurred,” says Currat.

The results show a gradual increase in the proportion of DNA transferred the further you go from the eastern Mediterranean region, which emerges both westward into Europe and eastward into Asia after about 3,900 kilometers.

“We were quite surprised to see a nice increasing pattern in the proportion of introgression in human genomes resulting from what we assume is human expansion out of Africa,” says Currat. “Towards Europe it increases, towards East Asia it rises, and so it allows us to estimate the boundary of this hybrid zone.”

The researcher’s computer simulations suggested a hybrid zone that covered most of Europe and the eastern Mediterranean and went into western Asia.

“What we see appears to be a single continuous pulse—a continuous series of intersecting events in space and time,” says Currat. “However, we do not know when hybridization took place in the zone.

The Hybrid Zone includes almost all known sites associated with Neanderthal fossils covering western Eurasia, except for those from the Altai region.

“The finding that the inferred hybrid zone extends widely into western Eurasia is interesting and suggests that interactions between populations may have been geographically widespread,” he says. Leonardo Iasi at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany.

However, the Atlantic margin, including western France and most of the Iberian Peninsula, is not in the hybrid zone, despite the well-documented presence of Neanderthals. It’s possible that no hybridization has occurred in the region, Currat says, or that any hybridization that occurs here is not represented in the 4,147 genetic samples.

“Overall, the study paints a picture of repeated interactions between modern humans and Neanderthals over a wide geographic range and over longer periods of time,” says Iasi, adding that the hybrid zone could extend further, but the limited sampling of ancient DNA in areas such as the Arabian Peninsula makes it difficult to assess how far it went in that direction.

“This is an important paper that challenges the view that there was only one region, probably western Asia, and one Neanderthal population (not represented in contemporary Neanderthal genetic patterns) that hybridized with Homo sapiens population dispersing out of Africa,” he says Chris Stringer at the Natural History Museum in London. “Like soon sapiens spread out in ever-increasing numbers and ever-widening range, they seem to have wiped out the small Neanderthal populations they encountered along the way over virtually the entire known Neanderthal range.”

topics:

• Neanderthals/
• ancient people