Significance A causality between millennial-scale climate cycles and the replacement of Neanderthals by modern humans in Europe has tentatively been suggested. However, that replacement was diachronous and occurred over several such cycles. A poorly constrained continental paleoclimate framework has hindered identification of any inherent causality. Speleothems from the Carpathians reveal that, between 44,000 and 40,000 years ago, a sequence of stadials with severely cold and arid conditions caused successive regional Neanderthal depopulation intervals across Europe and facilitated staggered repopulation by modern humans. Repetitive depopulation–repopulation cycles may have facilitated multiple genetic turnover in Europe between 44,000 and 34,000 years ago.

Abstract Two speleothem stable isotope records from East-Central Europe demonstrate that Greenland Stadial 12 (GS12) and GS10—at 44.3–43.3 and 40.8–40.2 ka—were prominent intervals of cold and arid conditions. GS12, GS11, and GS10 are coeval with a regional pattern of culturally (near-)sterile layers within Europe’s diachronous archeologic transition from Neanderthals to modern human Aurignacian. Sterile layers coeval with GS12 precede the Aurignacian throughout the middle and upper Danube region. In some records from the northern Iberian Peninsula, such layers are coeval with GS11 and separate the Châtelperronian from the Aurignacian. Sterile layers preceding the Aurignacian in the remaining Châtelperronian domain are coeval with GS10 and the previously reported 40.0- to 40.8-ka cal BP [calendar years before present (1950)] time range of Neanderthals’ disappearance from most of Europe. This suggests that ecologic stress during stadial expansion of steppe landscape caused a diachronous pattern of depopulation of Neanderthals, which facilitated repopulation by modern humans who appear to have been better adapted to this environment. Consecutive depopulation–repopulation cycles during severe stadials of the middle pleniglacial may principally explain the repeated replacement of Europe’s population and its genetic composition.

The replacement of Neanderthals by modern humans is recorded across Europe in a diachronous and culturally complex succession of distinct stone tool assemblages from the Middle–Upper Paleolithic transition (MUPT) roughly between 48 and 36 ka cal BP [calendar years before present (1950)] (1, 2). The succession often includes a regionally distinct “transitional” assemblage of local origin, or an intrusive Initial Upper Paleolithic assemblage between the Neanderthal Mousterian and modern human Aurignacian. The oldest anatomically modern human remains from Europe—found in East-Central Europe and radiocarbon dated to 40.6–38.6 ka cal BP (68% probability) toward the time of Neanderthals’ disappearance from most of continental Europe—carry genetic evidence for species interbreeding four to six generations earlier (3⇓⇓–6). This individual, however, represents a population that did not contribute to the genome of modern humans present in glacial Europe after the MUPT (7), and the archeologic record provides no site with indication of local coexistence. Within a few millennia after the MUPT, at least two other genetically distinct modern human populations came to subsequently dominate Middle Pleniglacial Europe. During the entire interval, northern hemispheric climate went through several millennial-scale Dansgaard–Oeschger (DO) cold cycles (8, 9). A causality between climate change, the archeologic succession, and modern humans’ genetic makeup has been tentatively suggested but not demonstrated (1, 2, 7). Below, we present the climatic history of continental Europe during the MUPT and derive the impact of climate change on MUPT demography, which may have led to the apparent repetitive genome turnover reported for Europe’s human population during the middle pleniglacial.

The MUPT spans five DO cycles approximately between Greenland Interstadial 12 (GI12) and Greenland Stadial 8 (GS8) (9) for which climate change over continental Europe is poorly constrained. The paleoclimatic and environmental context is known with sufficient resolution and age-control only along the continent’s western and southern fringe. In the Aegean and Black Sea region, records of sea surface temperature (10), coastal ice-rafted detritus (IRDc) (11), pollen assemblages (12⇓⇓–15), and stable isotopes in speleothems (16) suggest a DO-type response without the clear prominence of ice-rafting intervals (Heinrich stadials) seen in the Atlantic domain (17). Forest was generally more abundant in Europe during interstadials, while steppe landscape advanced during stadials (12). A taiga and tundra shrub/forest landscape covered the eastern European plains, with some loess deposition east of the Carpathians (12, 18, 19). The middle and lower Danube Plain was a steppe landscape with continuous loess deposition (20). A temperate open forest in the mountains of the southern Balkan passed into a xerophytic steppe toward the Aegean Sea (12, 13, 15). Boreal forest with birch and pine trees was present at 50° N in Western Europe (Eifel maar lakes) but began to degrade after GI12, ∼44.5 ka cal BP (21). Two sparsely dated records from the Western Carpathians (Safarka, Jablunka) suggest a dense taiga forest landscape (14). For the upper Danube Plain, pollen (Füramoos) and loess/paleosol profiles (Willendorf II, Nussloch) suggest a cold steppe environment with few conifers during interstadials, and a tundra landscape with cryosoil formation during stadials (12, 22, 23). During GIs, a temperate forest-steppe prevailed west (marine records) and south of the Alps (Monticchio, Castiglione, and Lagaccione) (12). Dust deposition in the Eifel maar lakes and speleothem carbon isotopes from Villars Cave suggest increasing aridity in Western Europe across the MUPT (24, 25).

Paleoclimatic Context of the Middle Pleniglacial in Europe A reduction of forest and expansion of steppe biomes occurred during all stadials (12⇓⇓–15, 21), but climate records between the Atlantic and the Black Sea show a spatially heterogeneous response to DO cycles (Fig. 2). GS13-H5 is apparent in the TC speleothem δ18O record but unlike in the Atlantic domain, its amplitude is small compared with subsequent stadials. Southern Black Sea IRDc data (Fig. 1) indicate less sea ice than during subsequent stadials despite apparent colder annual sea surface temperature (Fig. 2). This suggests a different seasonality with less severe winters. Thus, extreme cold was unlikely in East-Central Europe. The extreme aridity apparent in the Aegean region (13) may not have been as severe in the Balkan and Black Sea region (15, 16). GS12, from 43.4–44.3 ka GICC05 or 43.3–44.0 ka U-Th (AC), is a very prominent stadial in Greenland and Central Europe (Fig. 2), where permafrost changed the upper Danube lowland cold steppe into a tundra (see above). Malacological data from Willendorf II and the Eifel maar dust record suggests dryer conditions also in Western and Central Europe (22, 24) (Fig. 2). A cooling step likely occurred over Western Europe, supported by evidence of deep frost in the loess/paleosol record of northern France at ∼43–44 ka cal BP (39). However, this stadial is less prominent in the Atlantic record (17). GS11 is present in all continental records, but not as pronounced as GS12. A common cooling trend superimposed over DO cycles 12–9 is apparent in all records (Fig. 2). GS10—from 40.1–40.9 ka GICC05 and 39.7–40.7 ka U-Th (TC)—is a very prominent stadial over continental Europe, coeval with another cooling step in Western Europe (Fig. 2), but also a reduction in moisture and reduced speleothem growth (25). The first occurrence of loess in the Willendorf II profile (C7-2) around that time (22) and another strong dust peak in the Eifel maar record suggest stronger aridity than during GS12. Although present in all continental records, this stadial is unremarkable in the Atlantic and in the Black Sea (Fig. 2). However, annual sea surface temperature and winter coastal ice abundance in the Black Sea show conflicting results for GS10, which could again indicate a seasonality change. GS9-H4 is a long-lasting stadial that was less prominent in the Central European record but coeval with significant cooling and aridity in Western Europe (Fig. 2). Over the Atlantic and the Black Sea, GS9-H4 is a much more significant stadial. Subsequent GI8 is as warm as GI12 in Central Europe and the Black Sea region, with soil formation in the Danube region and East Carpathians (19, 20, 22), but colder in Western Europe and the Atlantic. Dust deposition in the Eifel maar records was also high during GI8, unlike GI13. The causes to this regionally heterogeneous response to DO climate cycles remain uncertain, but a variable influence of the Siberian high has been suggested (20). Following the MUPT, GS8 from 36.6 to 35.5 ka GICC05 and GI7 from 35.5 to 34.7 ka GICC05 are difficult to separate from each other in the speleothem records. The Black Sea record shows severe cooling during GS8 (Fig. 2). The AC-δ18O record suggests a significant fluctuation of the regional hydrologic condition during GS8 and GI7 in agreement with extended soil formation in the Danube Valley and the East Carpathians (19, 22). Higher moisture availability may have masked the temperature signal in AC-δ13C. After GI7, a long-term cooling trend is superimposed on DO variability throughout Europe (Fig. 2), in many places coincident with a long drying trend (20, 25).

Acknowledgments This research was supported by Deutsche Forschungsgemeinschaft Funding (SFB 806, TP B2) (to M.S.). V.D. acknowledges support by the European Social Fund, Sectoral Operational Programme Human Resources Development, Contract POSDRU 6/1.5/S/3—“Doctoral Studies: Through Science Towards Society,” PCE-2016-0179 Grant CARPATHEMS, and IFA-CEA C4-08 (FREem). Part of the isotopic analysis were funded by the University of South Florida via an internal grant (to B.P.O.).

Footnotes Author contributions: M.S., V.D., and B.P.O. designed research; M.S. and V.D. performed research; V.D., B.P.O., S.A., and D.L.H. analyzed data; and M.S., V.D., B.P.O., V.E., and D.V. wrote the paper.

The authors declare no conflict of interest.

This article is a PNAS Direct Submission.

This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1808647115/-/DCSupplemental.