For tens of thousands of years, two species — Neanderthals and anatomically modern humans — shared vast landscapes. Occasionally, they crossed paths, and when they did, they left traces of their meetings in the DNA of their descendants; evidence of a shared, tangled past.

While scientists have long known that Neanderthals and anatomically modern humans (AMHs) interbred, the precise locations where these encounters occurred remain elusive. A new study, however, has shed light on one possible geographic zone: the Zagros Mountains, a region stretching across present-day Iran, Iraq, and Turkey.

Neanderthal And Homo sapiens Romance

Scientists have struggled for years to pinpoint the exact locations and times where Neanderthals and AMHs might have interbred. Fossil evidence is scarce, and ancient DNA from these populations is often unavailable.

So, researchers led by Saman Guran from the University of Cologne in Germany turned to a clever tool: ecological niche modeling. This method estimates the habitat suitability of species by analyzing environmental factors like temperature and precipitation. By combining this model with geographic information systems (GIS), the researchers reconstructed the potential ranges of both species during a critical period between 120,000 and 80,000 years ago — known as Marine Isotope Stage 5 (MIS 5).

The models showed that during MIS 5, the Zagros Mountains, with their steep cliffs and verdant valleys, served as a crossroads where Neanderthals from the Palearctic region and modern humans migrating out of Africa likely met. This crossroads, with its high biodiversity and varied topography, might have provided the ideal conditions for both species to thrive — and for them to encounter one another.

It’s a theory that places this mountainous region at the heart of one of the most significant moments in human evolution. The researchers believe that during periods of climatic shifts, the environmental conditions in this mountainous area became more favorable, leading to what is known as “niche overlap”.

Neanderthals, accustomed to the cooler, harsher environments of Europe, found in the Zagros a landscape that felt familiar. Meanwhile, modern humans, moving out of Africa, were well-equipped to navigate the region’s challenging terrain. The overlap in their habitats increased the likelihood of contact — and, eventually, interbreeding.

Traces in Stone and Bone

Of course, it’s one thing to build models, but another to find the physical evidence to back them up. Fortunately, the Zagros Mountains are not just a promising location on a map. This region has also yielded some of the most important fossil and archaeological discoveries related to Neanderthals.

In the northern Zagros, at a site known as Shanidar Cave, archaeologists unearthed the remains of ten Neanderthals. The excellent find offered a glimpse into Neanderthal lives and, possibly, their deaths. Some of the Neanderthals were buried with flowers, evidence of funeral rites that extend beyond our species. Further south, in the Kermanshah region of Iran, more Neanderthal fossils have surfaced in the Wezmeh and Bisetun caves, reinforcing the idea that these mountains were a long-term home for our extinct cousins.

But it’s the discovery of a single Neanderthal tooth at the Bawa Yawan rock shelter, found in association with Mousterian stone tools, that has really caught the attention of researchers. The tooth, dated to around 65,000 years ago, provides crucial evidence that Neanderthals were present in the Zagros during the window of time when interbreeding with modern humans likely occurred.

“Our study identified the Persian Plateau, particularly the Zagros Mountains, as a potential interbreeding area for AMHs and Neanderthals. The possibility of attracting different hominin groups in the Zagros Mountains is justified by the geographical conditions of this region, since it is located in two different biogeographical zones, namely, the Palearctic and Afrotropical realms,” the researchers wrote in their study.

“The border areas of two realms are important in biology since they operate as refugia for species from glacial environments. Consequently, some parts of the Zagros Mountains could have been visited repeatedly by people living in the border areas of the Palaearctic and Afrotropical realms during Pleistocene climatic shifts. Therefore, the possibility of interaction between different hominins, including AMHs and Neanderthals, was greater in these areas.”

Why Does This Matter?

The interbreeding of Neanderthals and modern humans has profound implications for our understanding of human evolution. Neanderthal genes make up a small percentage of the DNA of most people of non-African descent today, influencing traits from immune responses to skin pigmentation. Understanding where and when this interbreeding occurred helps scientists piece together the puzzle of human history.

The new findings from the Zagros Mountains add an important piece to that puzzle. They suggest that this rugged, remote region was more than just a place of passage. It was a meeting ground where two human species intertwined. And, as researchers continue to explore the area, they hope to find even more clues about the lives of our ancient ancestors.

These mountains may hold many more surprises — buried in the rocks, hidden in the caves, waiting for the moment when they can tell their stories.

The findings were reported in the journal Scientific Reports.