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

In this article we focus on the period between 90 to 50 kyr BP. Throughout this period, tem- peratures and precipitation fluctuated between cold and dry stadials and warm and wet inter- stadials. The climate, which was significantly colder than today, led to the formation of ice caps and glaciers responsible for a fluctuation of sea levels between 40 to 80 meters lower than today [6–9]. This drop in sea level led to the unveiling of land bridges and thus allowed humans to colonize lands that were previously inaccessible on foot, for example between the Italian and the Balkan peninsulas. The presence of extensive ice caps also had an impact on precipitation, which was lower than today. Because of the low precipitation and temperatures, the environment consisted mostly of open grassland, tundra and steppe in Europe and it was semi-arid in the Middle East [10]. During this period, the presence of Neanderthals is attested by numerous sites that have yielded fauna, lithic assemblages and sometimes, Neanderthal remains and it is therefore pos- sible to establish the spatial distribution of this population. Nevertheless, taphonomic factors and the scarcity of excavation in certain areas greatly limits our knowledge of the full extent of the Neanderthal presence. However, it is possible to characterize the environments of the sites that have been investigated and thus identify other areas where Neanderthal presence may have been suitable. We use this approach to investigate the ecological niches that represent the

range of environmental conditions in which the Neanderthals could have survived. It is our intention to examine whether there was a reduction in the areas suitable for Nean- derthal occupation in Europe between 90 to 50 kyr BP, we are therefore interested in the period before the arrival of the Homo sapiens. To address this we divide the timespan 90–50 kyr BP into subperiods, reconstruct the eco- logical niches for each period and examine whether there was a spatial change of these niches over time. Above all, we are interested in quantifying the full extent of the suitable space theoretically available to Neanderthals at the time of their demographic decline. Material and methods To more clearly characterise the stages of environmental change in the area we are studying we divide our study period into 5 subperiods between 90 and 50 kyr BP, each with its own rea- sonably uniform characteristics in the biome reconstructions of Hoogakker [11]: P1 = 90 to 83 kyr, P2 = 83 to 69 kyr, P3 = 69 to 64 kyr P4 = 64 to 51 kyr and finally P5 = 51 to 50 kyr. We focus our analysis on Eurasian territory between 20˚W-100˚E and 20˚N-65˚N, compiling data from 123 Neanderthal fossil sites classified by georeferenced occurrence location and subpe- riod (Fig 1, map made using [12]). We use the date of the layer as indicated in the reference publication (see S1 File). Paleo environmental data To characterize the relevant environmental properties of the territory we select five variables: orographic height, mean annual precipitation, mean temperature of the coldest month, carry-

ing capacity and friction. Values for these environmental variables for our periods are derived from climate model simulations of the last glacial cycle [13,14], and complementary reconstructions of the ecologi- cal biomes implied by these climate simulations. Our biome reconstructions follow the method of Hoogakker [11], where the impact of known biases in the climate simulations is minimised by treating the simulations for each period as anomalies with respect to each model’s modern- day climate; the climate forcing used for our biome reconstructions is obtained by combining the anomalies for each period with modern observational datasets. See [11] for a full discussion PLOS ONE Neanderthal habitability PLOS ONE | https://doi.org/10.1371/journal.pone.0308690 February 26, 2025 2 / 17