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Reconstructing contact
and a potential interbreeding
geographical zone
between Neanderthals
and anatomically modern humans
Saman H. Guran 1,2*, Masoud Youse 3,4, Anooshe Kafash 5 & Elham Ghasidian 2,3
While the interbreeding of Homo neanderthalensis (hereafter Neanderthal) and Anatomically
modern human (AMH) has been proven, owing to the shortage of fossils and absence of appropriate
DNA, the timing and geography of their interbreeding are not clearly known. In this study, we
applied ecological niche modelling (maximum entropy approach) and GIS to reconstruct the
palaeodistribution of Neanderthals and AMHs in Southwest Asia and Southeast Europe and identify
their contact and potential interbreeding zone during marine isotope stage 5 (MIS 5), when the
second wave of interbreeding occurred. We used climatic variables characterizing the environmental
conditions of MIS 5 ca. 120 to 80 kyr (averaged value) along with the topography and coordinates of
Neanderthal and modern human archaeological sites to characterize the palaeodistribution of each
species. Overlapping the models revealed that the Zagros Mountains were a contact and potential
interbreeding zone for the two human species. We believe that the Zagros Mountains acted as a
corridor connecting the Palearctic/Afrotropical realms, facilitating northwards dispersal of AMHs and
southwards dispersal of Neanderthals during MIS 5. Our analyses are comparable with archaeological
and genetic evidence collected during recent decades.
Keywords Zagros Mountains, Ecological niche, Palaeoenvironment, Neanderthals, Anatomically modern
humans
Following the ground-breaking discovery of biocultural admixture in the Late Pleistocene of dierent early
human groups of Neanderthals, archaic/modern humans, and Denisovans, a large and growing body of research
concerning the nature and evolutionary history of these events is presented. In addition to the signicant con-
sequences that the biological exchanges have had on species e.g.1 and related issues, the time2–4, and geography
of contact and interbreeding are the subject of intense debate5. Neanderthals are an extinct lineage of hominins
that emerged at approximately 400 kyr and died o at approximately 40 kyr.6 Fossil localities and morphological
evidence of Neanderthals indicate that they are companionable with the Palearctic biogeographical realm, which
includes from western Europe to the Altai Mountains in Siberia at 55° latitude and down to approximately 31°
in Western Asia7–10. e chronological settlement patterns of the Neanderthals’ sites indicate their expansion to
the east and southwest Asia from at least 150 kyr11.
On the other hand, Anatomically Modern Humans (AMHs) have evolved in Africa for more than 300 kyr12–14.
e evidence, including physical remains and morphological analyses, suggests that they exited Africa over and
over during a period of at least 200 kyr15–17. AMHs also reached Eastern Asia at approximately 120 kyr18 and
later reached Europe at approximately 60 kyr19,20. Recent accurate archaeological and palaeoenvironmental data
suggest that AMHs rapidly adapted to the new and extreme environments beyond Africa, such as high plateaus,
mountain systems and palearctic ecosystems21. Moreover, archaeological and fossil evidence indicates that AMHs
entered southwestern Asia during MIS 515,22–24.
OPEN
1Institute for Prehistoric Archaeology, University of Cologne, Cologne, Germany. 2DiyarMehr Institute for
Palaeolithic Research, Kermanshah, Iran. 3Stiftung Neanderthal Museum, Mettmann, Germany. 4Department of
Biology, Hakim Sabzevari University, Sabzevar, Iran. 5School of Culture and Society, Department of Archaeology
and Heritage Studies, Aarhus University, Aarhus, Denmark. *email: samanguran@gmail.com
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ere is strong evidence of multiple interbreeding events between two groups of Neanderthals and archaic/
modern humans in western Eurasia e.g. 3,25. Moreover, many attempts have been made to estimate the timing
of this interbreeding, and signicant success has been achieved e.g.26–28 . Palaeogenetic studies have shown that
the second wave of interbreeding occurred during MIS 54,28 . In some studies, researchers have suggested that
the lower latitude regions of southwestern Asia have high potential for the rst overlap between Neanderthals
and AMHs. Sanchez Goñi29 examined the patterns of expansion of Neanderthals and AMHs and showed that
they shared the same ecological niches under certain climatic conditions during the Late Pleistocene. Recently,
Churchill etal.30 reported facial morphological similarities between Neanderthals and AMHs in the Near East,
indicating it could be a key region for interbreeding between the two lineages. However, it is still unclear where
the two species met and interbred.
Ecological niche models (ENMs) are very practical tools for investigating the geography of two species’
palaeodistribution and potential interbreeding areas31. ENMs have been found to have important applications
in palaeobiogeography, archaeology and palaeoanthropology32–36. ey use occurrence data of a target species,
including ancient humans, as well as palaeoenvironmental variables to calculate the probability of a species or
hominin species’ presence in a dened geographic region37. ese models have successfully been used to recon-
struct the distribution of dierent hominin species35,36,38, identify refugia during the ice ages and reconstruct
dispersal corridors39 , niche overlap among species31 and niche overlap with prey species40 . For example, Ruan
etal.31 successfully used ENMs to identify the contact zones of Neanderthals and Denisovans. In another study,
Benito etal.36 applied the ecological niches to determine the distribution of Neanderthals during the last intergla-
cial period in Europe and in the Irano-Turonian region. us, ENMs can be used to model the palaeodistribution
of Neanderthals and AMHs and locate the geography of their niche overlap31,36.
e aim of the present study was to reconstruct the palaeodistribution of Neanderthals and AMHs dur-
ing MIS 5 to identify the contact and potential interbreeding geographical zones of these two species. We also
estimated the most important predictor of the two species and investigated the responses of the two species to
environmental variables. Previous studies have suggested Southwest Asia as a potential area for the interbreeding
of Neanderthals and AMHs30,41,42 . Notably, this region which is located at the crossroads of the Afrotropical and
Palearctic realms43 , matches the distribution of AMHs and Neanderthals, respectively. us, we hypothesized
that these two species rst met and interbred at the border of these two biogeographic realms where environ-
mental conditions facilitated niche overlap and resource partitioning by providing a highly diverse habitat rich
in resources. Climate is a major determinant of species distributions 44, particularly at large spatial scales, thus
we expect climate to be more eective than topography in shaping the interactions between Neanderthals and
AMHs.
Results
Reconstructing the contact and interbreeding zone
e models developed in this study for Neanderthals (AUC = 941) and AMHs (AUC = 895) performed well
according to the AUC model performance metric. Our model of the palaeodistribution of Neanderthals shows
that north and west of the Mediterranean Sea towards the Levant, vast patches in Turkey, around the Black Sea,
south of the Caspian Sea, Taurus, Caucasus and Zagros Mountains, were highly suitable for this species during
MIS 5 (Fig.1). e AMH palaeodistribution model identied large and continuous suitable patches in Africa,
Arabia and the Iranian Plateau. Our model identied the Zagros Mountains as a contact and potential interbreed-
ing zone in Southwest Asia and Southeast Europe.
Variable importance and response curve
We estimated the relative contributions of the environmental variables to the Maxent model of Neanderthals
and AMHs. We found that the maximum temperature of the warmest month (with 58.5% contribution), the
minimum temperature of the coldest month (with 19.7% contribution), and the annual precipitation (16.5%
contribution) were the most important predictors of the palaeodistribution of Neanderthals36. e maximum
temperature of the warmest month had a negative association with the presence of Neanderthals. Slope (with
35.6% contribution), topographic diversity (with 26% contribution) and precipitation of the warmest quarter
(with 14% contribution) were the most important variables in shaping the palaeodistribution of AMHs. Both
species presented similar responses to decreases in slope and habitat suitability in areas with high slopes. Figure2
shows how each environmental variable aects the Maxent prediction for Neanderthals (a) and AMHs (b). e
curves show how the predicted probability of presence changes as each environmental variable is varied, keeping
all other environmental variables at their average sample value.
Precipitation changes from 140 to 40 kyr
Figure3 shows the changes in precipitation from 140 to 40 kyr, with 10,000 intervals for the Zagros Mountains.
e highest amount of precipitation occurred at 120 kyr, making it a suitable time for range expansion and
interactions between Neanderthals and AMHs.
Discussion
Hominin interbreeding is an important topic in palaeoanthropological studies, but when and where it occurred
remain largely unknown. Among the dierent hominin species, the interbreeding of Neanderthals and AMHs
is particularly important, as it contributes to the genetics of our own species. Here, we applied ENMs and GIS
and revealed that the Zagros Mountains of Iran is a potentially highly suitable geographic unit for niche overlap
and a potential interbreeding zone of these two species.
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Our niche models predicted niche overlap for the two species in the Zagros Mountains. In support of this
nding, various studies of genetic data e.g. 45,46, ecological modelling e.g.21 , archaeological and genetic records
e.g.24,46 and fossils30,47 are in concordance with our niche overlap model. e expansion of Neanderthals to Zagros
must have occurred 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. e latest evidence has shown
that the southernmost expanse of Neanderthals was up to the latitudes of approximately 31° in an arm-shaped
area that stretched to the south in two dierent directions alongside the Anti-Lebanon and Zagros Mountains9.
e Neanderthals in territories further east, such as those found in present-day Uzbekistan47 , Tajikistan48 and
Asian Russia26,49 , are known as Central and North Asian Neanderthals. To date, evidence of the presence of
Neanderthals is consistent with southwestern humid mountainous zones, including Anti-Lebanon in the Levant50
, Anatolia51, the Caucasus52,53 and Zagros9,54.
e data on the MP period in the Zagros Mountains region are rich and more up-to-date due to the discov-
eries of stratied sites associated with absolute dates, hominin fossil records, and lithic artefacts. Among the
large number of MP sites, four yielded Neanderthal fossils. e best-known of these is Shanidar Cave, where the
remains of ten Neanderthals were discovered55. Approximately 350 km southeast (approximately 34° latitude),
the Wezmeh and Bisetun caves in the Kermanshah region also yielded Neanderthal remains9,54,56. However, the
recent discovery of Neanderthal remains from the Bawa Yawan Rock Shelter is signicant since it yielded an
Figure1. Habitat suitability models of the two Homo species and their potential contact and interbreeding
zones in Southwest Asia and Southeast Europe. is gure was generated in QGIS 3.14.1 (www. qgis. org). e
gures of the Neanderthal (le) and modern human (right) are adapted from www. demor gen. be.
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insitu Neanderthal tooth in association with the Zagros Mousterian lithic artefacts9 . e tooth has been dated
to around 65 kyr, whereas the age of the Mousterian layer, dates back to 83 kyr57.
Owing to the evidence of various hominin fossil remains, it has been determined that the region of Southwest
Asia was inhabited by AMH modern humans in the Late Pleistocene. AMHs have inhabited the Levant during
at least two periods between 177 and 194 kyr, as evidenced at the site of Misliya16 , and between ~ 120 kyr and
Figure2. Response curves showing how the presence of Neanderthals (a) and AMHs (b) is related to the
environmental variables (Bio5: maximum temperature of the warmest month, Bio6: minimum temperature of
the coldest month, Bio12: annual precipitation and Bio18: precipitation of the warmest quarter) (https:// biodi
versi tyinf ormat ics. amnh. org/ open_ source/ maxent/).
Figure3. Precipitation changes from 140 to 40 kyr at 10,000 intervals.
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90 kyr, as shown at the sites of Skhul and Qafzeh58, before the area was permanently occupied by H. sapiens
approximately 55,000 years ago59. ere is a vast amount of data on hominin (including AMHs) occupations
from 400 to 50 kyr in Arabia associated with Eastern African lithic technology see 42 and references therein,
and moreover, physical remains, including AMH nger bones from Al-Wusta dated to ca. 85 kyr15, all indicate
that Arabia was a gateway to Eurasia during the middle to late Pleistocene. ere is evidence of the presence of
nun-Mousterian MP artefacts dating back to 80 kyr in the southern regions of the Persian Plateau, both in the
Zagros60 and in the southern to central parts of the Persian Plateau24.
In accordance with our initial expectation, the interaction and potential interbreeding zone of Neander-
thals and AMHs was located in the contact zone of the Afrotropical and Palearctic realms, namely, the Zagros
Mountains. ere are several reasons why the Zagros Mountains are a suitable location for the niche overlap
and potential interbreeding zone of two species. First, the Zagros Mountains are characterized by the envi-
ronmental conditions of the Palearctic realm, which is the birthplace of Neanderthals61. At the same time, the
areas surrounding Zagros are characterized by the environmental conditions of the Afrotropical realm, which
is the birthplace of AMHs. us, the Zagros Mountains could have been visited repeatedly by people living in
the border areas of the Palaearctic and Afrotropical realms during Pleistocene climatic shis. erefore, the
possibility of interactions between dierent hominins, including AMHs and Neanderthals, was greater in these
areas. Second, Zagros covers a vast geographical region (over 1500 km from Lake Van in Turkish Kurdistan to
southeastern Iran) capable of supporting large stable human populations. ird, Zagros is exceptionally diverse
in terms of topography and biodiversity40,62–66, making it capable of supporting the presence of two species at
the same time. ese mountains facilitate the niche overlap of some animal species with similar niches within
the same habitat40,67. ese mountains are known to play a very signicant role in species distribution by acting
as a dispersal barrier or as a dispersal corridor63,65. ese ndings support the results of our study.
Our ndings are further supported by new fossil discoveries in the Zagros Mountains and new genetic
data46. We assumea migration route into the Central Plateau from other directions, including the southern
region via Arabia, the Persian Gulf, and the Oman Sea, is plausible46 . is route might have followed the coastal
lines towards the north and eventually into the inner parts of the Persian Plateau. Recent evidence of hominin
occupations scattered on the surface in areas located in the southernmost part of the Persian Plateau supports
our hypothesis24,60,68.
Our initial suppositionwas that climatic factors would be the predominant force in predicting the distribu-
tions of both Neanderthals and AMHs. However, our ndings revealed a nuanced picture: while climate emerged
as the key determinant of the Neanderthal habitat, AMH distribution was signicantly inuenced by topographi-
cal variations. e climate was homogenous, but the topography was heterogeneous across the AMH distribution
areas. ese ndings likely suggest that topography played a more pronounced role in sculpting the distribution
pattern of AMHs. Our study contributes to the growing body of evidence that underscores the complex interplay
between environmental factors in determining species distributions. Our results are in line with prey overlap40,
showing that the annual precipitation and maximum temperature of the warmest month were the most important
predictors of Neanderthal distribution on the Persian Plateau. Climate was the most important determinant of
Neanderthal distribution in Europe and the Iran–Turanian region during the last interglacial period; however,
the inuence of topography was conned to local scales36.
One particular application of ecological niche models (ENMs) is to identify suitable areas for the presence
of target species where no observations have been made69,70. Field surveys guided by ENMs have led to the dis-
covery of new populations and rare species69,70, thereby proving the utility of ENMs in this context. Our model,
which predicts the interbreeding areas of Neanderthals and AMHs, is assigned a very high priority for future
eld investigations and excavations. Although eld testing of ENMs in archaeological studies is limited40, we
encourage Iranian archaeologists to conduct eld excavations in this potential interbreeding area to evaluate
the practicality of the models in archaeological research. Moreover, the use of ENMs can guide the allocation of
resources for archaeological excavations, ensuring that eorts are concentrated in areas with the highest potential
for signicant ndings. By prioritizing these high-probability locations, researchers can maximize the eciency
of their eldwork, leading to more targeted and fruitful excavations.
Conclusions
Before this study, our understanding of the interbreeding of AMHs and Neanderthals was based on genetic and
morphology data alone71,72. For the rst time, we applied ENMs as additional and independent lines of informa-
tion to locate possible geographic locations where the two species interbred. Our study identied the Persian
Plateau, particularly the Zagros Mountains, as a potential interbreeding area for AMHs and Neanderthals.
e possibility of attracting dierent hominin groups in the Zagros Mountains is justied by the geographical
conditions of this region, since it is located in two dierent biogeographical zones, namely, the Palearctic and
Afrotropical realms. e 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 shis. erefore, the possibility of interaction between dierent hominins, including AMHs and Nean-
derthals, was greater in these areas.
In addition to our ndings that the Persian Plateau served as a hub for Homo sapiens aer dispersal from
Africa46, we conclude that this plateau contributed signicantly to hominin distribution40,62,73 , dispersal24,74,75
and evolution46,76, and we await many exciting discoveries that will shed light on human evolution and dispersal.
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Methods
Archaeological sites
We obtained 38 occurrence points for Neanderthals and 45 for AMHs (Fig.4), extracted from multiple sources,
including the “Role of Culture in Early Expansions of Humans Out of Africa” (ROCEEH: http:// www. roceeh.
net) Database (ROAD30,31) and Appendix S1 in Benito etal.36. Each archaeological site was associated with
one or two species on the basis of fossil records and lithic artefacts. Since our research focuses on the time
frame MIS 5 (e.g., 120–80 kyr), we used only the archaeological sites during this period for southwest Asia and
Figure4. Distribution of key archaeological sites dating between MIS 5 to 3across southwest Asia and
southeast Europe. Map data acquired from http:// www. roceeh. org and created in www. qgis. org.
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southeast Europe. We carefully examined each coordinate and removed the duplicates. Since the environmental
data were at a spatial resolution of ~ 5km (4.65km at the equator), we thinned the occurrence data to 5 km to
avoid pseudoreplication40. is time frame was selected because it is suggested that interbreeding events take
place during three dierent periods28. e initial wave of interbreeding occurred ~ 250 to 200 kyr, the second
wave of interbreeding occurred ~ 100 to 120 kyr and the third and last interbreeding occurred ~ 60 to 50 kyr.
We were unable to nd enough archaeological sites associated with the presence of the two species for the rst
interbreeding event to construct robust niche models; thus, we focused on the second interbreeding event that
occurred during MIS 528.
Environmental predictors
We considered environmental variables related to past climate and topography to reconstruct the AMH and
Neanderthal niches during MIS 5. As palaeoclimatic variables, we added the maximum temperature of the
warmest month, the minimum temperature of the coldest month, the annual precipitation and precipitation
of the warmest quarter to the niche models for the MIS 5-time span. Palaeoclimatic data were obtained from
Oscillayers, which is a dataset of climatic oscillations over Plio–Pleistocene time scales at high spatial–temporal
resolution77. We estimated the average values for each of the abovementioned variables during MIS 5 via the
raster package v. 3.4–1378 implemented in the R environment79. To consider topography, we included the slope
and topographic heterogeneity36,40, which were downloaded from EarthEnv80. To avoid multicollinearity among
the predictors, we calculated a variance ination factor (VIF;81) via the “vifstep” function in the “usdm” package82
and ensured that the collinearity among the predictors was low (VIF < 10).
Ecological niche modelling
In this study, we used the maximum entropy modelling approach83 to reconstruct ecological niche models of
Neanderthals and AMHs during MIS 5. Maxent version 3.4.4 was used to build the niche models84. We used the
Maxent model because it has been shown to perform better than other niche modelling methods37,84. We then
overlapped the two palaeodistribution models to identify potential areas for their contact zones in the QGIS
(www. qgis. org). e performance of the niche models was assessed via the area under the curve (AUC) metric
of the receiving operator characteristic (ROC) curve83. An AUC value of 0.5 indicates that the performance of
the model is not better than random, whereas values closer to 1.0 indicate better model performance85. e ROC
curves were created by selecting 80% of the data for training and 20% for testing.
Data availability
All data needed to evaluate the conclusions in the paper are present in the paper or the references cited here
within. We obtained archaeological sites data from the ROCEEH Out of Africa Database (ROAD) (http:// www.
roceeh. org) and references cited in the manuscript.
Received: 20 April 2024; Accepted: 13 August 2024
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Acknowledgements
All data needed to evaluate the conclusions in the paper are present in the paper or the references cited here
within. We obtained archaeological sites data from the ROCEEH Out of Africa Database (ROAD) (http:// www.
roceeh. org) and references cited in the manuscript.
Author contributions
"S.H.G., M.Y., and contributed equally to the design and conceptualization and writing and A.K. in performance
and analysis of the data. E. G. in review & editing. All authors reviewed the manuscript."
Funding
Open Access funding enabled and organized by Projekt DEAL. e research was funded by the Deutsche
Forschungsgemeinscha (DFG), Priority program 2176: e Iranian highlands: Resiliencies and integration in
premodern societies, “Last Neanderthal and early Homo sapiens occupations in the Bawa Yawan Rockshelter,
Kermanshah, West-Central Zagros Mountains (Iran), under project number 402379177.
Competing interests
e authors declare no competing interests.
Additional information
Correspondence and requests for materials should be addressed to S.H.G.
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