Hominin interbreeding is an important topic in paleo-anthropological studies but when and where it happened is remained largely unknown. Among the different hominin species interbreeding of Neanderthal and AMH is particularly important as it contributed to genetic of our own species. Here we applied SDMs and GIS and showed that Zagros Mountains of Iran as a potentially highly suitable geographic unit for niche overlap and potential interbreeding zone of the two species.
Our niche models predicted niche overlap for the two species in Zagros Mountains. In support of this finding, it should be noted various research of genetic data (e.g. Lazaridis et al. 2018, Vallini et al. 2024), ecological modelling (e.g. Roberts and Stewart 2018) archaeological and genetic records (e.g. Heydari-Guran and Ghasidian 2021, Vallini et al. 2024) and fossils (Churchill et al 2022) are in concordance with our niche overlap model. The expansion of the Neanderthals to Zagros must have happened in accordance with the Palearctic environment and karstic terrains from both sides of the Black Sea, i.e. the Caucasus and Anatolia crossing towards the southern regions. The latest evidence has shown, the southernmost expanse Neanderthal, until the latitudes of around 31° in an arm shape stretched to the south in two different directions alongside of the Anti-Lebanon and Zagros Mountains (Heydari-Guran et al. 2021). Neanderthals' territories further towards east, such as those found in present-day Uzbekistan (Glantz et al. 2008), Tajikistan (Trinkaus et al. 2000) and Asian Russia (Mednikova 2011, Krause et al. 2004) are known as Central and North Asian Neanderthals. So far, the evidence of the presence of Neanderthal is consistent with Southwestern humid mountainous zones, including Anti-Lebanon in the Levant (Tillier and Arensburg 2016), Anatolia (Harvati and Roksandic 2017), Caucasus (Hajdinjak et al. 2018, Gasparyan and Glauberman 2022) and Zagros (Zanolli et al. 2019, Heydari-Guran et al. 2021).
The data on the MP period in the Zagros Mountains region is rich and more up to date due to the discoveries of stratified sites associated with absolute dates, hominin physical remains, and lithic artefacts. Among the large number of MP sites, four have yielded Neanderthal fossils. The best-known of these is Shanidar Cave, where the remains of ten Neanderthals were discovered (Pomeroy et al 2020). Approximately 350 km to the southeast (around latitude 34˚), Wezmeh and Bisetun caves in the Kermanshah region have also yielded Neanderthal remains (Trinkaus and Biglari 2006, Zanolli et al. 2019). However, the recent discovery of the Neanderthal remains from Bawa Yawan Rockshelter is significant since it yielded an in situ Neanderthal tooth in association with the Zagros Mousterian lithic artefacts (Heydari-Guran et al 2021). The tooth has been dated to between 65 to 60 Kyr, while the age of the Mousterian layer, so far, goes back until 83 Kyr (Heydari et al 2024).
Due to the evidence of various physical remains, it has been determined that the region of Southwest Asia was inhabited by AMHs modern humans in the late Pleistocene. AMHs have inhabited the Levant during at least two periods between 177-194 Kyr, as evidenced at the site of Misliya (Hershkovitz 2018) and between ~120 Kyr and 90 Kyr, as shown at the sites of Skhul and Qafzeh (Mercier et al 1993), before the area was permanently occupied by H. sapiens around 55,000 years (Hershkovitz et al 2015). There is a vast data of hominin (including AMH) occupations from 400 kyr to 50 Kyr in Arabia associated with Eastern African lithic technology (see: Groucutt et al 2021 and references therein) and moreover physical remains including finger bone from Al-Wusta dated to ca. 85 Kyr (Groucutt et al. 2018), all indicate Arabia as a gateway to Eurasia during Middle to Late Pleistocene. There is evidence of the presence of nun-Mousterian MP artifacts dated back to 80 Kyr in the southern regions of the Persian Plateau, both in the Zagros (Heydari et al 2021) and in the southern to central parts of the Persian Plateau (Heydari-Guran and Ghasidian 2021).
There are several reasons that why Zagros Mountains is a suitable place for two species niche overlap and potential interbreeding zone. Firstly, Zagros Mountains is characterised with environmental conditions of the Palearctic realm which is known as a birth place of Neanderthal (references). At the same time surrounding areas of the Zagros are characterised with environmental conditions of Afro-Arabian; the birth place of AMH. Thus, Zagros Mountains could have been visited repeatedly by people who were living in the border areas of the Palaearctic and Afro-Arabian realms during Pleistocene climatic shifts. Therefore, the possibility of interaction between different hominins, including AMHs and Neanderthals, was higher in these areas. Secondly, Zagros is a vast region (overs 1500 km from Lake Van at the Turkish Kurdistan to south-eastern Iran) to support stable human populations. Thirdly, Zagros is exceptionally diverse in terms of topography and biodiversity (Heydari-Guran 2014; Kafash et al 2020, 2021; Yousefi et al., 2022, 2023) making it capable of supporting presence of two sub-species at the same time.
Our initial hypothesis was that climatic factors would be the predominant force in predicting the distribution of both Neanderthal and AMHs. However, our findings revealed a nuanced picture: while climate indeed emerged as the key determinant of Neanderthal habitats, AMHs distribution was significantly influenced by topographical variations. Climate is homogenous but the topography is heterogenous across the AMHs distribution areas. These likely grants topography a more pronounced role in sculpting the distribution patterns of AMH. Our study contributes to the growing body of evidence that underscores the complex interplay between environmental factors in determining species distribution. Our results are in line with prey overlap (Yousefi et al. 2020) showed that annual precipitation and maximum temperature of the warmest month were the most important predictor of Neanderthal distribution on the Persian Plateau. Climate was the most important determinant of the Neanderthal distribution in Europe and the Irano-Turanian region during the Last Interglacial, yet, the influence of topography was confined to local scales (Benito et al. 2017).