Climate change in Europe between 90 and 50 kyr BP and Neanderthal territorial habitability

significantly alter the regions our analysis shows were most suitable for Neanderthal occupa- tion. This remains true when considering the envelope of model uncertainty established by our use of two different climate models. The warming in western, central and southern conti- nental Europe has most effect in expanding the zone of habitability during the period 83–69 kyr. However, the reductions in minimum annual temperature after 69 kyr are centered in the north of our domain, from the eastern coast of the British Isles across Scandinavia into Russia, extending south to northern continental Europe. These are not areas that the period 83–69 kyr warming appear to have made suitable for Neanderthal occupation and the major climate changes in our simulation dataset only impact habitability scores around the margins of that favored area. Although we have shown that temperature is the dominant control of the factors we have analysed, it is not the only one; the period 83–69 kyr sees a general increase in precipi- tation and reduction in glacial dry conditions, and although this is partly reversed after 69 kyr the spatial pattern is not as uniform as seen for the cooling. Again, the areas with most drying are largely away from the central zone that is favorable for habitability. The climate model sim- ulations we have used for this analysis are of course not perfect and it is possible that they are not reproducing these spatial patterns correctly. However, repeating our analysis using the

output of Singarayer simulations [13], which used a different climate model run according to a different protocol with different reconstructions of the northern hemisphere ice sheets, pro- duces broadly similar changes in areas suitable for Neanderthal habitability through our cho- sen periods. Results from two models support the idea that large-scale climate change in Europe after 69 kyr is not a simple dominating factor in determining the fate of the Neander- thals. Our results appear to be at odds with recent publications [34,35]. Nevertheless, we have shown that climate change, if considered over time (and not as a contrast between extreme periods), has not significantly altered suitable niches for Neanderthals. If climate change was not the primary driver in the decline of Neanderthal populations in Europe, then what was? During the periods studied here the Neanderthals had a small popula- tion size [36–38], were divided into at least three different populations [39], could have suf- fered a reduction of young female fertility that drove them to extinction [40] and that they may have been susceptible to haemolytic disease of the fetus and newborn (erythroblastosis fetalis), due to maternofetal Rh incompatibility [41]. How can we explain the theoretical contradiction between the decrease of the population size and an increase/stability in the area of suitable territory for Neanderthals? It is possible that the fragmented population found itself restricted to areas that became less suitable (e.g.

southern Europe/ Iberia [42]. This may have had a dual effect: preventing movement to more suitable areas of central Europe and simultaneously reducing or even canceling out migration between groups, which is essential to maintain a certain genetic variability. Each of these effects alone may have been responsible for the disappearance of Neanderthals. If the two effects were associated then the probability of Neanderthal survival was almost null. It should not be forgotten that Homo sapiens began to permanently occupy Euro-Asian ter- ritory in this same period (also supported by genetic result of interbreeding [43–45]). Although the propositions of direct confrontations between Homo sapiens and Neanderthals have not been substantiated, the presence of Homo sapiens alone may have had important consequences on Neanderthals ability to occupy the most suitable niches, or simply to continue to exist [46– 49]. The presence in the same environment of two or more species with the same characteristics is a problem that has been studied extensively in the field of biology since Gause [50]. The con- clusion that two species occupying the same niche in a homogeneous environment cannot coexist and that one excluding the other leads inexorably to the disappearance of one of them PLOS ONE Neanderthal habitability PLOS ONE | https://doi.org/10.1371/journal.pone.0308690 February 26, 2025 13 / 17