Although it is unclear when and when these prehistoric encounters took place, the presence of prehistoric hominin genes in the present human genome indicates that Homo sapiens once interbred with other species like Neanderthals and Denisovans. Just where this happened is a mystery.

The authors of a recent study

These matched known locations where prehistoric humans might have resided. This was done by studying the archaeological record. See, stone tools used by Neanderthals typically belong to one of the two technical cultures known as Micoquian or Mousterian. The geographical distance between these lithic traditions raises the possibility that Neanderthals divided into groups and moved eastward via various routes.

The researchers then utilized computer modeling to determine the two most plausible dispersal pathways taken by Neanderthals as they traveled from Europe to Asia based on the distribution of Mousterian and Micoquian archaeological sites as well as information on topography and temperature. One of these routes would have required our prehistoric ancestors to go across Siberia, where the harsh climate would have made survival challenging.

However, the other traveled across the more comfortable Southern Caspian Corridor (SCC) in the direction of the south. The SCC, a thin strip of land in present-day Iran between the Alborz Mountains and the Caspian Sea, served as a warm haven for several plant and animal species through glacial periods between 57,000 and 71,000 years ago. The area served as a biogeographical link at this time, keeping eastern and western populations of flora and fauna together.

The authors hypothesize that this region could have served as a safe haven for Neanderthals, as it has for other species, based on the reconstruction of yearly precipitation and temperature from roughly 57,000 to 71,000 years ago. They continue by speculating that other hominins may have also lived in the area at the same time, creating a meeting spot and the potential for interbreeding… Neanderthals from the west, newly arrived Homo sapiens from the interior of the Iranian Plateau, and other hominins (such as Denisovans) from the east all called the SCC home.

The SCC is thought to be a hotspot for tracking hominin mixing and introgression because of its location between the Caucasus and Central Asia, as was the case for some faunal species. However, the authors of the study point out that unless real hominin remains are found in the region, ancient climate data can only suggest the likelihood of species mixing in northern Iran. Stone tools and other cultural items are the sole indications of past human habitation of the SCC that we now have.

It is too premature to make any more than a conjecture given the current state of our knowledge. To unravel the mysterious picture of hominin settlements in this peculiar refugium, hominin remains from Neanderthals, Homo sapiens, Denisovans, or other ghost lineages are essential.