Mid-Pleistocene aridity and landscape shifts promoted Palearctic hominin dispersals

Domain: earth_science
Published: 2024-11-27
Editorial outcome: major_revision

Abstract
Population expansions and contractions out of and into Africa since the early Pleistocene have influenced the course of human evolution. While local- and regional-scale investigations have provided insights into the drivers of Eurasian hominin dispersals, a continental-scale and integrated study of hominin-environmental interactions across Palearctic Eurasia has been lacking. Here, we report high-resolution (up to ∼5-10 kyr sample interval) carbon isotope timeseries of loess deposits in Central Asia and northwest China, a region dominated by westerly winds, providing unique paleoecological and paleoclimatic records for over ~3.6 Ma. These data, combined with further syntheses of Pleistocene paleontological and archaeological records and spatio-temporal distributions of Eurasian eolian deposits and river terraces, demonstrate a pronounced transformation of landscapes around the Mid-Pleistocene Climate Transition. Increased climate amplitude and aridity fluctuations over this period led to the widespread formation of more open habitats, river terraces, and desert-loess landscapes, pushing hominins to range more widely and find solutions to increasingly challenging environments. Mid-Pleistocene climatic and ecological transitions, and the formation of modern desert and loess landscapes and river networks, emerge as critical events during the dispersal of early hominins in Palearctic Eurasia.

Introduction
The Palearctic Realm of Eurasia extends from the Atlantic to the Pacific and includes Europe and Asia north of the Oriental and Sino-Japanese Realms1 (Fig. 1). Its weather system is dominated by westerly winds that bring rain from the Atlantic, and further east, the Mediterranean, Black and Caspian Seas. In China north of the Qinling Mountains, the weather is dominated by the East Asian monsoon. Hominins began to colonize Palearctic Asia before 2 Ma, with the earliest evidence of hominins outside Africa represented by 2.1 Ma-old artifacts from Shangchen in the Chinese Loess Plateau (CLP)2. Fossil evidence from Dmanisi, Georgia, dated at 1.8 Ma, and Gongwangling, China, dated at 1.63 Ma, indicates that the earliest colonizing populations were an early form of H. erectus3,4. Europe appears to have been colonized later: the earliest secure dates from Atapuerca and the Orce Basin (Spain) are ca. 1.4–1.2 Ma5,6; Pirro Nord from Italy may be of comparable age7. By the end of the Mid-Pleistocene Climate Transition (MPT, 1.25-0.7 Ma), hominins had established themselves across the whole of Palearctic Eurasia, up to 40° N in Asia8,9 and 55° N in northwest Europe10.

The climatic and environmental histories of Europe and Asia are usually discussed separately. However, they are parts of the same biological realm, meaning their vertebrate assemblages share a common phylogenetic composition1. As phylogenetic compositions are driven by regional climatic and ecologic factors11, the history of both regions needs to be considered together when examining the drivers of hominin dispersals out of Africa. By integrating new and existing data on the long-term history of the climate, vegetation, and fauna of Palearctic Eurasia, we show how the landscape was dramatically transformed by climatic factors around the MPT, with profound consequences on hominin behavior and evolution.

In Africa, stable carbon isotopic compositions of soil organic matter and paleosol carbonate have been widely used to reconstruct regional vegetation cover and paleoclimate as well as hominin-environmental interactions12,13. However, continuous and high-resolution carbon isotope records since the late Pliocene have rarely been reported from the Westerlies-influenced regions in Europe and Central Asia. To address this deficit, we report carbon isotopic records of soil organic matter (δ13Corg) of loess deposits from the western margin of the Pamir and the southern Tarim Basin, northwest China, respectively (Fig. 1). Both areas are dominated by westerly winds (Supplementary Fig. 1). These records were derived from a 180-m-thick loess sequence in southern Tajikistan (Supplementary Fig. 2), and a 671 m-deep loess core obtained from the highest fan surface of the southern margin of the Tarim Basin and downwind of the Taklimakan Desert. Paleomagnetic dating combined with further analyses of astronomically tuned timescale suggests that loess succession at the southern margin of the Tarim Basin and southern Tajikistan accumulated from ∼3.6 Ma and 2.7 Ma to the present, respectively14,15. Central Asia and NW China are regarded as crucial migration gateways of hominins to disperse west to east (or vice versa) across Eurasia (Fig. 1) and represent the eastern-most boundary of the Westerlies-influenced areas (Supplementary Fig. 1). Thus, δ13Corg records of these loess deposits can provide critical information on precipitation and precipitation-controlled vegetation density as well as their impacts on early hominin dispersals in the Westerlies-influenced regions since the late Pliocence. Moreover, in providing these long paleoclimate records, we can highlight just how unique the changes centered around the MPT are for the Plio-Pleistocene. Our carbon isotope results, combined with syntheses of Pleistocene paleontological and carbon isotope records from SW China and Southeast Asia16,17, provide holistic insights into the regional vegetation cover and paleoclimatic conditions for the northern and southern migration routes of early hominins through Asia.

Results
Eolian dust deposition and pedogenesis in Central Asia and NW China are continuously competing processes, with dust accumulation rates usually showing much higher values in loess units than those in paleosol layers. In southern Tajikistan, paleosols are well developed due to their location on the windward slope of the Westerlies-influenced areas, and lithological characteristics of the loess deposits resemble those of contemporaneous loess-paleosol units in the CLP15. In contrast, paleosols in the Tarim Basin are very weakly developed due to low precipitation, and the carbonate nodular (Bk) horizons are difficult to define at most levels. A total of 286 and 244 loess samples, spanning 2.7 and 3.6 Ma to the present with a temporal resolution of ∼5–10 kyr and ∼10–20 kyr, respectively, were collected from Tajikistan and the Tarim Basins (Supplementary Fig. 2). Samples for the carbon isotope analysis of organic matter were collected from all loess and paleosol units.

The δ13Corg values of the Tajikistan loess and the Tarim Basin loess range from −27.2‰ to −23.87‰ and from −26.78‰ to −23.34‰, respectively. Both records are characterized by long-term increases since the late Pliocene, punctuated by pronounced shifts toward more positive δ13Corg values at the end of the MPT, i.e., ~0.9-0.6 Ma (Fig. 2a, b and Supplementary Fig. 2). Statistical change-point analyses of Mann–Whitney and Ansari–Bradley tests of δ13Corg records can provide crucial constraints on the rhythms, events and trends in paleoclimatic changes across Eurasia (see “Methods” for further discussion). Our results demonstrate that breakpoints in the slopes of both δ13Corg records are evident around the MPT (Supplementary Fig. 3), particularly between 0.9 and 0.6 Ma, indicating a remarkable increase in the carbon isotopic ratios with a larger fluctuation in amplitude corresponding to the shift in Earth’s climate from 41-kyr cyclicity to 100-kyr cyclicity. Specifically, the mean values of δ13Corg in Tajikistan increased from −26.29‰ prior to 0.9 Ma to −25.17‰ after 0.9 Ma, whereas in the Tarim Basin, they increased to −24.42‰ after 0.9-0.6 Ma.

At present, vegetation in Central Asia and NW China is dominated by C3 plants18,19. The δ13Corg value range suggests that the soil organic matter of loess deposits from the Tarim Basin and Tajikistan was sourced mostly from C3 plants since the late Pliocene20. Recently, a series of studies on the relationship between the δ13C data of C3 plants and organic matter from surface soils and climate in NW China and Central Asia have shown that a strong negative correlation exists between δ13Corg values and local mean annual precipitation (MAP)21–23. The δ13Corg records of loess deposits in these areas thus inform paleoprecipitation reconstructions21,23. These observations are consistent with the relationship between climate and δ13Corg of surface soils under dominant C3 vegetation at a global scale that indicates the δ13Corg values are less sensitive to temperature variability, but instead closely related to changes in precipitation18. Planktic foraminiferal δ11B ratios demonstrate that while a significant decrease in atmospheric CO2 occurred mainly after 2.8 and 2 Ma24,25, pCO2 levels were relatively stable or increased slightly since the MPT26,27. Meanwhile, a colder climate is associated with the intensification of Northern Hemisphere glaciations since the MPT28 favors the growth of C3 over C4 plants, thereby leading to a significant reduction in δ13Corg. These observations suggest that changes in temperature and pCO2 levels cannot account for the Mid-Pleistocene increase of δ13Corg records in Central Asia and NW China. Thus, temporal variations in δ13Corg values of loess deposits from the Tarim Basin and Tajikistan mainly reflect the paleoprecipitation variability in the Westerlies-influenced regions (see “Methods” for further discussion).

Using previously established linear correlations between the MAP and the δ13Corg of surface soils in Central Asia and the NW China29,30, paleoprecipitation changes in south Tajikistan and the Tarim Basin can be estimated. These correlations suggest the precipitation in Tajikistan decreased by approximately 100–200 mm after the Mid-Pleistocene. In the Tarim Basin, precipitation decreased from ~200 mm during the late Pliocene to ~100 mm after the MPT. Generally, the absence of paleosols in Chinese eolian deposits indicates that the MAP is less than 200–300 mm31. These findings are consistent with lithological observation that paleosols in the Tarim Basin are very weakly developed since the late Pliocene (see “Methods” for further details).

The relationships between δ13Corg and precipitation vary geographically, and our inferred palaeoprecipitation values require validation from modern datasets from Tajikistan and Tarim Basin, which are currently lacking. Nevertheless, our results indicate that enhanced aridification would have led to a significant increase in δ13Corg values of loess sediments from Central Asia and NW China. The δ13Corg values of modern soil in southern Tajikistan, where MAP varies between 300 and 600 mm, are much lower than those in the Tarim Basin (20–100 mm), providing further evidence that the loess carbon isotope ratios in Central Asia and NW China are controlled mainly by local precipitation.

Methods
Loess successions in the Tarim Basin and Tajikistan
Loess deposits are widely distributed on surfaces of mountain slopes and river terraces in the southern margin of the Tarim Basin and western margin of the Pamir, where the climate is strongly influenced by the Westerlies (Supplementary Fig. 1). These eolian deposits, which are directly located in downwind directions of the Taklimakan Desert and the Karakum Desert, contain critical information on paleoenvironmental and paleohydrological changes of the Westerlies-influenced regions, i.e., in most areas of Europe and Central Asia. On the western margin of the Pamir Plateau, detailed lithologic and magnetostratigraphic investigations suggest that loess deposits in southern Tajikistan began to accumulate since ~2.7 Ma, representing the most complete and oldest loess-paleosol sequence in western Central Asia15. Due to the poor exposure of natural outcrops, a 671 m-deep loess core was recently obtained from the highest fan surface of the southern margin of the Tarim Basin, with a recovery of 96.1%14. Paleomagnetic samples (2×2×2 cm cubes) were taken at 50 cm intervals in the upper 80 m of the Tarim Basin loess core and at 25 cm intervals in the lower 591 m. A total of 2331 samples gave reliable characteristic remanent magnetizations to determine the magnetostratigraphy. Paleomagnetic and optical stimulated luminescence dating analyses suggest that loess succession from the Tarim Basin was accumulated from ∼3.6 Ma to the present14.

Based on chronological constraints of the magnetostratigraphic and OSL analyses, an astronomically tuned age model was established for the 671 m-deep loess core in the Tarim Basin with a temporal resolution of ~0.5 kyr, which has been achieved by tuning the grain size record to theoretical variations in the Earth’s orbital parameters14. In addition, an astronomically tuned age model of the Tajikistan loess has been obtained previously by tuning the magnetic susceptibility record to theoretical variations in the Earth’s orbital parameters71. In our study, the timescale of the Tajikistan loess profile, with a temporal resolution of ~5–10 kyr, was established based on magnetostratigraphic constraints and the correlation of the magnetic susceptibility record with the marine oxygen isotope results (Supplementary Fig. 2). Previous studies confirmed that the grain size records (representing the intensity of westerlies) of the Tarim Basin loess and the magnetic susceptibility records (reflecting the precipitation variability) of the Tajikistan loess both show a dominant periodicity shift from 41-kyr cyclicity to 100-kyr cyclicity between 1.2 Ma and 0.6 Ma14,71. These features are consistent with global ice volume variability during the MPT (Supplementary Fig. 2), highlighting the reliability of the chronological framework.

Carbon isotope analyses
For the organic matter carbon isotope (δ13Corg) analysis, dried loess samples were first sieved through a mesh to remove modern rootlets and then treated with 4 mol/L HCl for 24 h to remove carbonates at room temperature. After washing with distilled water, the samples were dried for over 24 h at 60°C. The dried samples were then weighed and sealed in Sn boats. Finally, the isotopic composition was measured using the Elementar IsoPrime 100 isotope ratio mass spectrometry (IRMS) instrument linked to a Vario EL cube instrument for elemental analysis, with a precision of ±0.2‰. The carbon isotope ratios of organic matter are expressed as permil deviation relative to the Vienna Peedee belemnite (V-PDB) standard.

The δ13Corg values of modern C3 plants and C4 plants respectively vary from −20‰ to −34‰ and from −9‰ to −19‰, with averages of about −27‰ for C3 plants and −13‰ for C4 plants, under modern atmospheric CO2 conditions20,72. The δ13Corg values of ≤−24‰ indicate pure C3 vegetation, and the values varying between −24‰ and −14‰ indicate C3/C4 mixed vegetation18,19. These observations suggest that the organic matter of loess from the Tarim Basin and Tajikistan was sourced mainly from C3 plants since the late Pliocene, which is consistent with the previous studies of the vegetation types in Central Asia and NW China18,19.

The δ13Corg values in sediments are controlled by complex interactions between several environmental factors, including temperature, precipitation, and pCO2 levels. Early studies demonstrated that a substantial decrease in atmospheric pCO2 accounted for the late Miocene increase in soil carbon isotope ratios73. Recent high-resolution reconstructions of pCO2 in South and East Asia indicate that pCO2 levels appear to be rather stable or increased slightly since the MPT26,27. Meanwhile, a colder climate associated with the intensification of the Northern Hemisphere glaciations since the MPT28 would favor the growth of C3 plants over C4 plants, thereby leading to a significant reduction in δ13Corg. This is not the case in our study, where an increasing trend was observed, suggesting that the temperature had a negligible impact on the changes in δ13Corg. Moreover, maximum δ13Corg values of the Tajikistan loess were observed in the last and penultimate glacial loess units (L1 and L2) under cold and dry climate conditions (Supplementary Fig. 2). Combined with further investigations of the close links between δ13C ratios of modern plants and surface soils and precipitation, these observations suggest that the δ13Corg values in Central Asia and NW China are less sensitive to temperature variability, but instead are closely related to changes in precipitation18,23. These results are consistent with global scale investigations that indicate a strong negative correlation between precipitation amount and the δ13C of modern C3 plants74.

Previous studies demonstrate that a linear correlation exists between the mean annual precipitation and the δ13Corg of surface soils in Central Asia (Precipitation=(δ13Corg+22.84)/(-0.0079); n=44, R2=0.44)29 and NW China/Mongolia (Precipitation=–58×δ13Corg–1266.5; n=19, R2=0.9)30. Following these observations, a rough estimation of paleoprecipitation changes in south Tajikistan and the Tarim Basin was conducted based on the temporal variations in δ13Corg values. The results demonstrate that the precipitation in Tajikistan was >500 mm during the early Pleistocene and decreased to ~400-300 mm after the Mid-Pleistocene. In the Tarim Basin, the precipitation decreased from ~200 mm during the late Pliocene to ~100 mm after the MPT.

The δ13C ratios of carbonate from paleosols and fluvial-lacustrine deposits can provide additional information about the paleoenvironmental changes. Different from the δ13C ratios of paleosol carbonate12, carbon isotope values of total carbonate of fluvial-lacustrine deposits in arid regions of Central Asia and NW China are mainly controlled by the carbon isotope compositions and the proportion of detrital and secondary carbonate added to the background signal32,38,75. Increasing evidence suggests that detritus carbonate of fluvial-lacustrine deposits in Central Asia and NW China is mainly sourced from paleo-marine sediments32,38,75. Since the detritus carbonate is highly mixed and well sorted during the sediment transportation, they usually show a constant δ13Ccarb value around 0‰19,32,34,40; whereas in the Qaidam Basin, the background δ13Ccarb signals in detrital carbonates from paleo-marine sediments can be up to 5‰38. As precipitation increases, the δ13Ccarb values of fluvial-lacustrine deposits in arid and semiarid regions of Central Asia and NW China would become progressively negative with increased input of soil organic carbon32,75.

In sum, δ13Corg and δ13Ccarb records of eolian and fluvial-lacustrine sediments in Central Asia and East Asia all show a significant increase since the MPT (Fig. 2 and Supplementary Fig. 3) supporting the view that enhanced aridification, which can be attributed to the intensification of Northern Hemisphere glaciations, played a critical role in the Mid-Pleistocene vegetation degradation on the Eurasian continent.

Change-point analysis and the kernel density estimation diagrams
The change point analysis of Mann–Whitney and Ansari–Bradley tests can provide crucial constraints on the rhythms, events and trends in Plio-Pleistocene climate changes of Asia76,77. These statistical approaches, without requiring an assumption of population normality, are non-parametric hypothesis tests78. In this study, we use the rank sum function embedded in Matlab 2016b to perform the Mann-Whitney and Ansari-Bradley tests79.

The statistical measures of change points in the carbon isotope records and other representative dust and sea surface temperature records in Eurasia and the Pacific and Indian Oceans were performed using the 400-600 kyr window. Prior to the change-point analysis to define environmental episodes, the data were interpolated to an evenly spaced time axis. The plots of the Mann–Whitney and Ansari–Bradley test statistics clearly mark the primary transitions in the original data (Supplementary Figs. 3, 9, and 10).

To avoid the influence of sampling bias/burial loss of older archaeological and fossil hominin sites, we perform change point analysis on the residual value of the site number regressed against age by the SMATR package80. These residual values should be, in a sense, “free” of the tendency for an increase in the preservation and documentation of sites closer in time to the present81. In order to capture the non-linear relationship that exists between age and lithics/hominins sites (this most closely resembles an exponential curve), site number was transformed to non-zero values by adding 1, and a reduced major axis (RMA) regression analysis was performed between log(age) and log(site number+1).

The kernel density estimation diagrams can be used to estimate the population density and spatial distributions of archaeological and fossil hominin sites in a given area based on observed data points. The kernel density estimation diagrams are powerful tools for the visualization of data distribution, providing insights into the structure and characteristics of datasets without imposing strict assumptions about the data distribution. In this study, the bandwidth method of Bivariate Kernel Density Estimator and the density method of Binned Approximate Estimation are employed to show the details in the peaks of the spatial distributions of archaeological and fossil hominin sites, which are performed by the Originpro 2021 software.

Synthesis of archaeological and fossil hominin sites
To examine the spatial and temporal distributions of early hominins in Europe and Asia, the fossil hominin and archaeological records (325 sites, 421 age values) since the Pleistocene were collected across a wider geographic range and shown according to longitude (Fig. 4 and Supplementary Table 3). Considering the potential biases in the dating results and the focus of the Mid-Pleistocene time interval in our research, the paleoanthropological records younger than 100 kyr were excluded and not discussed in the present study. In all of these records, chronological constraints are a critical element in elucidating hominin occupation history in Eurasia. In this study, the early human occupation sites were examined and combined to provide a complete picture of the general patterns and trends in hominin occupation history (Fig. 4 and Supplementary Table 3).

In recent years, the chronological framework of some key archaeological sites has been updated through detailed stratigraphic investigations and the application of new dating technologies. For example, at the ‘Peking Man’ Site in northern China, Zhoukoudian Homo erectus has attributed an age of ~0.78 Ma based on 26Al/10Be burial dating, which is corresponding to a relatively mild glacial period82. In the Lantian area of central China, the age of the Gongwangling cranium was updated from 1.15 Ma to ~1.63 Ma based on recent paleomagnetic dating3. In addition, the age of some key Acheulian sites in Europe, which are characterized by the emergence of bifaces, cleavers, and other Large Cutting Tools, was updated according to new archaeological discoveries55,83,84. New evidence resulting from the rediscovery and the dating of the Moulin Quignon site in France demonstrates that the first Acheulian occupation north of 50° N occurred around 670–650 ka83. At the Notarchirico site in southern Italy, 40Ar/39Ar and ESR dates place Acheulian culture (including bifaces) between 695 and 670 ka, penecontemporaneous with the Moulin-Quignon and la Noira sites in France85.

As paleoanthropological and fieldwork were continuously performed over the last two decades, new archaeological discoveries challenged the timing of the earliest occupation of the Eurasian continent. For example, the magnetostratigraphic results of the Shangchen archaeological site in the CLP have extended the upper age limit of the earliest human occupation in China to 2.1 Ma, which represents the oldest known evidence of hominins in Eurasia2. Developing a complete picture of the spatio-temporal distributions of hominin occupation history is inevitably influenced by dating uncertainties, sampling gaps, and preservational biases in the archaeological and fossil hominin records. It is important to note that taphonomic loss may contribute to the under-representation of older sites. Indeed, increased paleoanthropological records are evident since the last glacial and Holocene periods86,87, which can be attributed to intense investigations on the archaeological sites because of the availability of suitable dating methods, i.e., the radiocarbon (14C) and OSL dates. For the current dataset, the age of paleoanthropological records older than 100 kyr was primarily constrained by paleomagnetic, electron spin resonance (ESR), cosmogenic burial (26Al and 10Be), and U-series dating analyses. In addition, it is important to note that dating errors may emerge in some fossil hominin and archaeological sites in Eurasia. To clarify the potential influences of dating uncertainties on the spatial and temporal distributions of archaeological records, the kernel density estimation diagrams of archaeological and fossil hominin sites with the maximum and minimum (Supplementary Figs. 11, 12 and Supplementary Table 3), and mean (Fig. 4) age estimations derived from the dating errors were investigated. There are no obvious differences in the patterns of the paleoanthropological records, and an expansion in the spatial distribution of fossil hominins and artefacts in Eurasia is evident at ~0.9–0.6 Ma. In contrast, the residual value of the site number of fossil hominins and artefacts in the current dataset does not show a significant increase through time during the Early and Middle Pleistocene. However, marked inflection points in these distributions are consistently observed around the MPT (Fig. 4h–j and Supplementary Fig. 11g–i and p–r). This commonality suggest they aren’t artefacts of any biases present in our dataset but rather represent genuine changes in the distribution of hominin, handaxe, and archaeological sites. However, these data also suggest that these changes may not have occurred synchronously. Determining the relative timings of increases in archaeological sites and/or handaxe occurrences versus records of hominin remains is a geological question that will require much finer-scale considerations of how taphonomic factors control preservation than we currently have available. As such, our results echo the cautionary note of researchers working on the African hominin records regarding the need for better confidence intervals on the stratigraphic ranges of hominins and their proxies, and stronger efforts to identify, discuss, and correct fossil sampling biases88. Nevertheless, the current dataset represent the largest compilation of archaeological records in Europe and Asia that suggests, despite taphonomic and sampling limitations, broadly increasing trends in the distribution of hominin dispersals in Eurasia since the MPT (see Discussion).

Synthesis of carbon isotope and paleontological records
Stable carbon isotopic ratios of sediments are generally regarded as sensitive indicators to unravel the changes in vegetation types (C3, C4, and Crassulacean acid metabolism plants; Crassulacean acid metabolism plants are a group of plants that have adapted to arid environments by opening their stomata at night to take in carbon dioxide) and vegetation cover12,13,73. In this study, carbon isotopic data of organic matter or total carbonate of fluvial-lacustrine and eolian sediments from the Asian monsoon-controlled and Westerlies-influenced areas are compiled for further analysis (Fig. 2). More specifically, lacustrine sediments from the Tarim Basin and Qaidam Basin provide critical stable carbon isotopic records of carbonate in the Westerlies-influenced region. In the Asian monsoon-controlled areas, carbon isotopic records of eolian sediments from the CLP and the adjacent regions are compiled for further synthesis. Paleomagnetic dating results, combined with further stratigraphic correlation of the magnetic susceptibility record and/or calibration of the astronomically tuned timescale, provide reliable age controls on these eolian and fluvial-lacustrine sediment sequences. It is evident that the δ13Corg and δ13Ccarb records all show a prominent increase since the MPT (Fig. 2).

Pollen records, which are distributed from northeastern China throughout southeastern Tibet, cover a wide geographic range in East Asia and Central Asia. Paleomagnetic dating analyses provide reliable age controls for these records. Vegetation types in most areas of East Asia show a remarkable shift from forest to open forest at 1.2-1 Ma and subsequently changed from open forest steppe to steppe at ~0.9-0.6 Ma (Supplementary Fig. 4), which are roughly consistent with the results of carbon isotope analyses and the pollen records in Eurasia (Fig. 2 and Supplementary Fig. 5)42,51,59,89–95. In SE Tibet, glacial periods were longer and dominated by steppe and meadows after the MPT16. These observations are also consistent with the results of vegetation change inferred from large mammal communities in Europe44 (Supplementary Fig. 6).

Spatio-temporal distributions of eolian deposits and river terraces
In recent years, increasing fieldwork and detailed paleomagnetic studies have been carried out to investigate the spatial and temporal distributions of eolian loess deposits in sub-humid and humid regions of China and Europe. A synthesis of the initial timing and spatial distribution of late Cenozoic eolian deposits in Eurasia suggests that, during the pre-MPT, loess deposits are confined to arid and semi-arid regions in Central Asia and northwestern China (Supplementary Fig. 7 and Supplementary Table 1). Whereas during the MPT the Eurasian loess belt expanded significantly and has extended to northeastern and southern China and the Danube River drainage area in Europe. Hence, the Middle Pleistocene is a critical period for the establishment of modern-like loess landscapes in sub-humid and humid regions of China and Europe. Increasing dust fluxes of marine sediments from the Mediterranean Sea, the Arabian Sea, and the North Pacific Ocean provide further support for the occurrence of continental aridification and expansion of the Eurasian loess belt during the MPT, which led to a pronounced increase in eolian sediment supply from the terrestrial dust sources across Eurasia to the downwind oceans96–98 (Supplementary Figs. 8a–e and 9). The formation of continuous loess succession requires a sizeable source region to provide silt materials and a stable and open environment to hold dust particles. We argue that the increased ice volume and climate fluctuation amplitude, as well as lowering sea levels and base levels since the MPT28,99, would lead to a dramatic intensification of the glacial erosion and river incision on the Northern Hemisphere100,101, thereby producing a vast amount of silt materials necessary to form desert-loess landscapes. Moreover, increased ice volume can reduce the sea surface temperatures (SSTs)102–105 (Supplementary Fig. 8g–l) and weaken oceanic water evaporation. Consequently, a substantial decrease in moisture supply would enhance the aridity in Eurasia14,46, which would provide stable and open environments for the accumulation of loess deposits.

A widespread occurrence of fluvial terraces could exert significant impacts on the availability of fresh water and lithic raw materials (fluvial gravels) for early hominins in Palearctic Eurasia. Exploring the spatio-temporal patterns of river terraces in Eurasia (277 sites, 1520 age values) is inevitably influenced by age uncertainties, preservational biases, and visibility of the river terrace systems. Generally speaking, the river terrace systems in the arid and semi-arid regions of Central Asia and NW China are usually well preserved and discernable due to the weak weathering and sparse vegetation. Meanwhile, OSL, cosmogenic burial, and paleomagnetic dating analyses provide useful methods to constrain the age of the fluvial terraces. Thus, the spatio-temporal patterns of river terraces in Central Asia and NW China are clearly discernable and believable. In contrast, river terrace data in tropical areas of South and Southeast Asia is relatively limited (Fig. 3 and Supplementary Table 2), which can be attributed to poor preservation because of intensive chemical weathering or the problems of site visibility because of dense vegetation. Another prominent feature of the current dataset is the widespread occurrence of fluvial terraces since the last glacial and interglacial periods. The widespread occurrence of river terraces since the last glacial and interglacial periods can be attributed to intense investigations on the river terrace systems because of the availability of suitable dating methods, i.e., the OSL, organic matter 14C, and TL. In contrast, the age of river terrace systems older than 100 kyr in the current dataset was primarily constrained by paleomagnetic, electron spin resonance (ESR), cosmogenic burial (26Al and 10Be), and U-series dating analyses (Supplementary Table 2), which generally show a long timespan from hundreds of thousands of years to millions of years. Hence, potential biases in the dating results of these old river terraces do not disturb the general patterns, and an increasing trend of the number of fluvial terraces in northwestern Europe and northwestern China during the MPT should be discernable and believable.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "The study hinges on two thoughts: 1) the new and 3.6 Myr long paleoprecipitation record (however, rainfall in numeric values like mm/yr with an uncertainty range is not specified) and 2) a comprehensive analysis of known hominin sites in Europe and Asia. However, the link between the two is not that evident: can the hominin site analysis be done without the new record? The discussion essentially focuses on the MPT and younger times (logically since not many hominin sites older than 1.6 Ma are known outside Africa), so what is the merit of the 3.6 Ma to 1.5 Ma portion of the new records (in terms of hominin dispersal routes)? Note that extended data Fig.2 shows the records starting at the Gauss/Matuyama boundary at ~2.6 Ma. The compilation of hominin sites and their environmental frame is a lot of work. The text is well written but seems to report mainly items that are largely known already. The new aspects remain somehow hidden. The change point analysis in this context is shown but its implications are only worded in very general terms while its necessary technical aspects, how robust, level of significance, etc., remain undiscussed.\nBelow are more specific line-by-line comments.\nAbstract. How high is high resolution? Can authors be more specific?\n32-33. “intensive tectonic uplift” reads vague. How much uplift is anticipated/recorded over 0.5-1.0 Myr? Not that much? A fair portion of the Tibetan Plateau was already at significant elevation at ~40 Ma or even before?\n41-43. The Palearctic realm is Europe and most of Asia. The meaning of the red dotted line in figure 1 is the Arctic Sub-Realm. However, that is not mentioned in the main text. What is the point of showing it? All sites discussed are (far) to the south of it. In Asia, they seem to be largely confined to the Sino-Japanese realm and the Saharo-Arabian realm.\n49. typo, take out “ago” after “Ma”\n53. Define MPT again in the main text?\n56. Realm, is capitalization required?\n62. I wonder about the tectonic factors at about 1 Ma: most of the Eurasian continent had already achieved more or less their present-day elevations.\n67. Carbonate-carbon or organic-carbon isotope data? Or both? OK, next sentence organic carbon isotope analysis is done, what about carbonate-carbon isotopes?\n77-81. This is the “northern migration route” through Asia, what about the “southern route” through India?\n85. Is the entire duration from 3.6 Ma (respectively 2.7 Ma) to Recent sampled? Be explicit here.\n86. What is the typical sampling distance? What is the loess/paleosol accumulation rate? And what about the presence of paleosols? There is climatic contrast between them. What if all samples come from loess and none from paleosols? Then bias could creep in.\n94. At other locations the MPT is defined between 1.2 and 0.6 Ma. Something which takes place over 300 kyr is not abrupt.\n95. But this shift in orbital expression cannot be resolved in the carbon isotope data given their resolution.\n105-110. Again, the expression of paleosols in the record may deserve some attention (some information is provided in 366-371, at one sampling location (Tajikistan) paleosols are described as well developed; in the Tarim basin they are nearly absent. Loess is typically associated with drier conditions. What is the inferred range of paleoprecipitation?\n116-121. Nice to know but what is the inferred paleoprecipitation range?\n123-202. Most of the discussion relates to the MPT and younger times. What is the point of providing data from 3.6 Ma (or 2.7 Ma) to Recent? From 1.5 Ma to Recent would suffice? It is a good summary of existing knowledge but at the same time rather long. That central Asian climate became increasingly arid since the Pliocene/Pleistocene boundary with a stepwise increase in aridity during the MPT is well known. What is the new record adding to this knowledge base? This remains unclear.\n124-128. But this is known for while? It is fine that present data concur with known trends, but the inference is not novel.\n139-146. So, the present-day conditions in Tarim Basin prevailed since 0.6 Ma?\n147. Here the MPT is implicitly placed between 1.2 and 0.6 Ma.\n171-173. This pertains to the northern dispersal route in Asia. Up to now no notion of the southern route via India. The northern route relates to the southern boundary of the Palearctic realm. Hominins went further north since the last 0.4-0.5 Ma or so?\n178. Check this tectonic uplift inference. Since 2001 many studies (more than 100?) have returned to the question of when, which parts of, and how much the Tibetan Plateau was uplifted. Current mainstream thinking is that most of the plateau reached more or less its current elevation at the Pliocene/Pleistocene boundary or even before.\n207, 208. Provide the ages for MIS11 and MIS13.\n213. A lower sea level is not “felt” by a river beyond its coastal plain. In the upstream region of interest here the direct impact of a lower sea level on rivers is marginal at best.\n215. Figure 3. What is the basis for allocating a terrace sequence to climate-controlled or tectonic-climate-controlled? How are these two distinguished?\n227. Serbia is more of southeastern Europe; northern Europe is Scandinavia?\n231. And in Atapuerca? There hominin occupation is argued to be continuous since ~1.3 Ma?\n251-265. Reads somehow repetitive?\n266-275. Well-worded text but the statements are generic.\n276. A distinct jump in reasoning? What are “our observations”?\n279-284. Can this be rhymed with younger migration waves as analyzed in Ao et al. (2024) Proceedings of the National Academy of Sciences?\n292. How is the “ready availability of fresh water” to be placed in an environment that features increased aridity?\n296. In the very southern margin of the Palearctic realm.\nFigure 1. The light blue on greenish background does not read well, pick another color combination.\nFigure 3 and 4. The methodology behind the analyses deserves more mention in the main text since this represents a new twist to existing knowledge. How robust are the inferences made?\n358. Why in Europe? That lies up-wind of the Tarim basin loess source areas?\n364-366. So, authors rely on the age model provided by reference 14. How robust is that age model? It is robust, authors should provide some more information concerning sampling resolution and paleomagnetic stability here (is actually done in 374-378).\n372-374. Superfluous, leave out?\n384. Is a winter monsoon/westerlies proxy, specify that?\n385. Is a summer monsoon proxy, specify that?\n392. Strict adherence to GTS2020 nomenclature would imply kyr here.\n393-394. But are they from loess, paleosol, or both per couplet?\n397. At which temperature? Room temperature?\n404-406. Also mentioned in the main text, however without much context over there.\n421. Typos, reconstructions; appear\n425. Numbers become less negative, is that a reduction?\nChange point analysis section.\nIt may be wise to word the concept of this analysis and these tests with reference to a textbook. Extended data Figure 3 (and elsewhere). What is the meaning of the deviation from the zero line (always negative) in the red lines (brown-ish in my print) and green lines? What is the meaning of log(p)? What is implied with Central Tendency and Dispersion?\n459. Asia is probably better here? Not a single record shown in extended data Figure 3 comes from Europe.\n461. Reference for MatLab2016b?\n466-473. These bits are not that helpful for a general readership. What is the outcome of this analysis? Where are the corresponding figures?\n477. Dating values: Is this the number of sites compiled here?\n479-480. What is inferred with this sentence?\n482. No new hominin finds are reported here, what is implied with ‘new discoveries’? How many new entries are included in comparison to previous compilations?\n483. Is all duly referred to, how good are the updates? No new age assignments were obtained in the present study. How many sites/data entries were updated?\n488. How many? Which key sites? Be specific. I presume the examples provided are not exhaustive.\n503-506. Start this section with these notions. They could also be mentioned in the main text.\n509-510. You mean that the methods are suited for the age range considered here? As worded it comes across as not overly informative.\n510. Why ‘residual’? Residual of what?\n510-512. A histogram of the apparently more or less even distribution through time would be insightful.\n512. And distribution through time was more or less even (at least it reads like that to me)? What is the cause of the inflection points?\n513-517. What is the purpose of the notions made here?\n521. How is this done and how is somebody supposed to read the extended data figures 11 and 12?\n530. What are CAM plants? Explain the acronym.\n554-603. How climatically controlled, tectonic-climate, and uplift controlled river terraces are being distinguished and how robust that distinction would be is not discussed.\n564-567. The relation to the present study may become rather thin. What information would be lost if this was omitted?\n566. Presumably dust fluxes are to those seas and oceans, since they cannot deliver dust.\n579. dating values = ages?\n590-593. All correct but what is the relevance since hominin dispersal is being discussed between 0.1 and ~1.3 Ma?\nExtended data Figure 1. Are these figures obtained in this study? If so, provide some technical details in the caption. Which model was used, which years were integrated, etc. If not, properly refer to the source(s).\nExtended data Figure 3. Explain log(p) in the caption. Mean and variance are being mentioned in the caption while central tendency and dispersion are being displayed on the figure panels. What is what?\n878. Are the changes observed significant? At which level?\nExtended data figure 4. How robust are those cluster analysis results? From the text it seems that authors have carried out that cluster analysis. Which input parameters were used and how was the stratigraphic control achieved?\nExtended data figure 6. The age distinction seems rather distinct. How robust are the ages involved in this analysis?\nExtended data 8l. The temperature anomaly is with respect to what?\n917. explain acronym IRM@AF.", "reviewer_2": "The rationale for this study is provided in the second paragraph of the main text:\n“The climatic and environmental histories of Europe and Asia are usually discussed separately. However, they are parts of the same biological Realm…As phylogenetic compositions are driven by regional climatic and ecologic factors 11, the histor[ies] of both regions need to be considered together when examining the drivers of hominin dispersals out of Africa”.\nThis is an important point, but also a substantial undertaking. In this work, the authors start by presenting new δ13C data from loess deposits in Central Asia and northwest China over the last 3.6 Myr. The evidence points to a significant climatic shift during the so-called Middle Pleistocene Transition (MPT; 1.2-0.7 Ma). This is then expanded to a synthesis of palaeoenvironmental and palaeoanthropological/archaeological data to establish the transformation of landscapes across the Eurasian Palaearctic during the MPT, with implications for early human evolution and dispersal.\nThis is an impressive collection of new and published information on a grand scale that is worthy of publication. I have two main general points:\n1. While the MPT ‘sensu lato’ is considered in general terms, the authors tend to focus more specifically on climate changes commencing at 0.9 Ma. Indeed, the so-called ‘0.9 Ma event’ is traditionally taken to represent the first major Northern Hemisphere glaciation of MIS 22 (e.g. Clark et al.., 2006, QSR). However, closer inspection of the Tarim Basin loess δ13C record shows an earlier shift (~1.1 Ma) and this is also observed in certain other records in Fig. 2 and Ext. Data Fig. 8. Margari et al. (2023, Science) presented evidence for an extensive glaciation during the 30kyr-long MIS 34 (possibly associated with the Menapian cold stage of NW Europe), culminating in an extreme terminal stadial in the eastern North Atlantic. Climate envelope model simulations suggest that the intensity of the declines in temperature and net primary productivity would have led to the extirpation of early human populations in Europe and SW Asia. A critical review of archaeological record further suggested a paucity of European and SW Asian early hominin sites after that, possibly until MIS 21 (Margari et al., 2023). This may be worth considering in the present work.\n2. The synthesis part of the paper represents a large collection of information, but space restrictions limit a fuller presentation and discussion of the evidence. For example, the river terraces dataset of Ext Data Table 2 and Fig. 3 are divided into tectonic-climate interactions, climatically-controlled and uplift-control, but their attribution and significance are not explained in the text or in the methods. In the same vein, the archaeological and human fossil evidence of Fig. 4 merit a fuller discussion of the granularity contained therein. More generally, the broad spatio-temporal canvases of Fig. 3 and 4 require a more expanded description.\nIn addition, the following specific points need attention:\nl. 110. “the stable pCO2 levels…associated with the intensification of NH glaciation”: While this may be in line with reconstructions based on carbonated precipitated in palaeosols (Da et al., 2019), it is not in line δ11B reconstructions showing drops in pCO2 after 2.8 and 2 Ma (Bartoli et al., 2011); Martinez-Boti et al., 2015). This may have to be considered in the δ13C changes of the loess records.\nl. 132. “a pronounced shift in dominant periodicity”. Τhis is based on earlier work, but have the authors attempted to explore the periodicities embedded in their new δ13C records?\nl. 166-170. “shift in vegetation from closed forests or woodland to steppe and grasslands is evident at ~0.9-0.6 Ma”: this is not correct. While European pollen records show a serial extinction of moisture-requiring taxa (the so-called Tertiary relicts) over the MPT, interglacial stages were still characterized by forests (though less diverse). In the same vein, the inferences based on mammal evidence of a shift from closed forests to open woodland in Europe (Ext Data Fig. 6) are also tenuous, as they may represent local contexts (e.g. flood plains) and not supported by other lines of evidence (beetles, molluscs).\nL. 230-231. “stable occupation of hominins in Europe was possibly since ~0.9 Ma”. It is worth adding here that Margari et al. (2023) suggested that repopulation of Europe after 0.9 Ma may have been by a more resilient species (e.g. H. antecessor) that was able to withstand the increasing intensity of glacial conditions."}, "author_response": "Reply to Reviewer: 1\nThe study hinges on two thoughts: 1) the new and 3.6 Myr long paleoprecipitation\nrecord (however, rainfall in numeric values like mm/yr with an uncertainty range is not\nspecified) and 2) a comprehensive analysis of known hominin sites in Europe and Asia.\nHowever, the link between the two is not that evident: can the hominin site analysis be\ndone without the new record? The discussion essentially focuses on the MPT and\nyounger times (logically since not many hominin sites older than 1.6 Ma are known\noutside Africa), so what is the merit of the 3.6 Ma to 1.5 Ma portion of the new records\n(in terms of hominin dispersal routes)? Note that extended data Fig.2 shows the records\nstarting at the Gauss/Matuyama boundary at ~2.6 Ma. The compilation of hominin sites\nand their environmental frame is a lot of work. The text is well written but seems to\nreport mainly items that are largely known already. The new aspects remain somehow\nhidden. The change point analysis in this context is shown but its implications are only\nworded in very general terms while its necessary technical aspects, how robust, level\nof significance, etc., remain undiscussed.\nResponse: Thanks for the critical comments. In the revised manuscript, a rough\nestimation of paleoprecipitation changes in south Tajikistan and the Tarim Basin were\nconducted, which was based on the linear correlation between the mean annual\nprecipitation and the δ13C of surface soils in Central Asia and the NW China. Further,\norg\nwe have added several new paragraphs to extend the knowledge about the vegetation\ncover and paleoclimatic changes as well as their implications for hominin-\nenvironmental interactions between 3.6 Ma and 1.5 Ma. Combined with further\nsyntheses of Pleistocene paleontological and carbon isotope records from the SW China\nand the Southeast Asia, we have provided some new discussions about the southern\nmigration routes of early hominins across Asia in the revised manuscript. In addition,\nwe provided more details about the change point analysis and the kernel density\nestimation diagrams in the text and the supplementary information. As outlined by\nReviewer 2, the main contribution of our manuscript is a consideration of Europe and\nAsia together, something that is rarely explicitly considered in studies exploring drivers\nof hominin dispersals, and using both new environmental data and a large compilation\nof archaeological and hominin records to determine how these may be related.\n1. Abstract. How high is high resolution? Can authors be more specific?\nResponse: We have provided more details about the sample resolution in the abstract.\n2. 32-33. “intensive tectonic uplift” reads vague. How much uplift is\nanticipated/recorded over 0.5-1.0 Myr? Not that much? A fair portion of the Tibetan\nPlateau was already at significant elevation at ~40 Ma or even before?\nResponse: Thanks for the critical comments. We agree that the influences of tectonic\nuplift on the Mid-Pleistocene aridity and landscape shifts are overestimated. The\ndiscussions about the tectonic forcing have been abandoned in the revised manuscript.\n3. 41-43. The Palearctic realm is Europe and most of Asia. The meaning of the red\ndotted line in figure 1 is the Arctic Sub-Realm. However, that is not mentioned in the\nmain text. What is the point of showing it? All sites discussed are (far) to the south of\nit. In Asia, they seem to be largely confined to the Sino-Japanese realm and the Saharo-\nArabian realm.\nResponse: We have removed the red dotted line in Figure 1.\n4. 49. typo, take out “ago” after “Ma”\nResponse: Done\n5. 53. Define MPT again in the main text?\nResponse: Done\n6. 56. Realm, is capitalization required?\nResponse: Thanks, we have corrected it.\n7. 62. I wonder about the tectonic factors at about 1 Ma: most of the Eurasian continent\nhad already achieved more or less their present-day elevations.\nResponse: Thanks for the critical comments. We agree that the influences of tectonic\nuplift on the Mid-Pleistocene aridity and landscape shifts are overestimated. The\ndiscussions about the tectonic forcing have been abandoned in the revised manuscript.\n8. 67. Carbonate-carbon or organic-carbon isotope data? Or both? OK, next sentence\norganic carbon isotope analysis is done, what about carbonate-carbon isotopes?\nResponse: Given that the paleosol layers in the loess deposits of the Tarim Basin, NW\nChina, are very weakly developed due to low precipitation, and pedogenic carbonate is\ndifficult to distinguish from the loess layers. Thus, the carbonate-carbon isotopes were\nnot analysed in the present study.\n9. 77-81. This is the “northern migration route” through Asia, what about the “southern\nroute” through India?\nResponse: Thanks for the good suggestions. We have provided more information about\nthis question in lines 82-86:\n“Our novel carbon isotope results, combined with syntheses of Pleistocene\npaleontological and carbon isotope records from the SW China and the Southeast Asia,\nprovide holistic insights into the regional vegetation cover and paleoclimatic conditions\nfor the northern and southern migration routes of early hominins through Asia.”\n10. 85. Is the entire duration from 3.6 Ma (respectively 2.7 Ma) to Recent sampled? Be\nexplicit here.\nResponse: Yes, we have revised the manuscript accordingly in lines 96-99:\n“A total of 286 and 244 loess samples, spanning 3.6 and 2.7 Ma to the present with a\ntemporal resolution of ∼5-10 kyr and ∼10-20 kyr, respectively, were collected from\nTajikistan and the Tarim Basins.”\n11. 86. What is the typical sampling distance? What is the loess/paleosol accumulation\nrate? And what about the presence of paleosols? There is climatic contrast between\nthem. What if all samples come from loess and none from paleosols? Then bias could\ncreep in.\nResponse: Thanks for the critical comments. In the revised manuscript, we added a\nnew paragraph in lines 89-100 to address these issues:\n“Eolian dust deposition and pedogenesis in Central Asia and NW China are\ncontinuously competing processes, with dust accumulation rates usually showing much\nhigher values in loess units than those in paleosol layers. In southern Tajikistan,\npaleosols are well developed due to their location on the windward slope of the\nWesterlies-influenced areas, and lithological characteristics of the loess deposits\nresemble those of contemporaneous loess-paleosol units in the CLP. In contrast,\npaleosols in the Tarim Basin are very weakly developed due to low precipitation, and\nthe carbonate nodular (Bk) horizons are difficult to define in most levels. A total of 286\nand 244 loess samples, spanning 3.6 and 2.7 Ma to the present with a temporal\nresolution of ∼5-10 kyr and ∼10-20 kyr, respectively, were collected from Tajikistan\nand the Tarim Basins. Samples for the carbon isotope analysis of organic matter were\ncollected from all loess and paleosol units (Extended Data Fig. 2).”\n12. 94. At other locations the MPT is defined between 1.2 and 0.6 Ma. Something which\ntakes place over 300 kyr is not abrupt.\nResponse: Thank you for pointing this out. We have reorganized the text accordingly\nand change the word “abrupt” to “pronounced”.\n13. 95. But this shift in orbital expression cannot be resolved in the carbon isotope data\ngiven their resolution.\nResponse: Yes, the shift in orbital expression cannot be resolved in the carbon isotope\ndata. Nevertheless, we provide the grain size evidence of loess deposits in the Tarim\nBasin and Tajikistan to show the orbital shift after 0.9-0.6 Ma in lines 159-162.\n14. 105-110. Again, the expression of paleosols in the record may deserve some\nattention (some information is provided in 366-371, at one sampling location\n(Tajikistan) paleosols are described as well developed; in the Tarim basin they are\nnearly absent. Loess is typically associated with drier conditions. What is the inferred\nrange of paleoprecipitation?\nResponse: Thanks for the critical comments. In the revised manuscript, we have added\ntwo new paragraphs in the main text and the “Methods” to show the inferred range of\npaleoprecipitation.\nLines 136-144:\n“Using previously established linear correlations between the MAP and the δ13C of\norg\nsurface soils in Central Asia and the NW China, paleoprecipitation changes in south\nTajikistan and the Tarim Basin can be estimated. These correlations suggest the\nprecipitation in Tajikistan decreased by approximately 100-200 mm after the Mid-\nPleistocene. In the Tarim Basin, precipitation decreased from ~200 mm during the late\nPliocene to ~100 mm after the MPT. Generally, the absence of paleosols in Chinese\neolian deposits indicates that the MAP is less than 200-300 mm. These findings are\nconsistent with lithological observations that paleosols in the Tarim Basin are very\nweakly developed since the late Pliocene (see Methods for further details).”\nAnd in lines 488-497:\n“Previous studies demonstrate that a linear correlation exists between the mean annual\nprecipitation and the δ13C of surface soils in Central Asia (Precipitation =\norg\n(δ13Corg+22.84)/(-0.0079); n=44, R2=0.44) and the NW China/Mongolia (Precipitation\n= –58×δ13Corg–1266.5; n=19, R2=0.9). Following these observations, a rough\nestimation of paleoprecipitation changes in south Tajikistan and the Tarim Basin was\nconducted based on the temporal variations in δ13C values. The results demonstrate\norg\nthat the precipitation in Tajikistan was >500 mm during the early Pleistocene and\ndecreased to ~400-300 mm after the Mid-Pleistocene. In the Tarim Basin, the\nprecipitation decreased from ~200 mm during the late Pliocene to ~100 mm after the\nMPT.”\n15. 116-121. Nice to know but what is the inferred paleoprecipitation range?\nResponse: Please see the replies to the question 14.\n16. 123-202. Most of the discussion relates to the MPT and younger times. What is the\npoint of providing data from 3.6 Ma (or 2.7 Ma) to Recent? From 1.5 Ma to Recent\nwould suffice? It is a good summary of existing knowledge but at the same time rather\nlong. That central Asian climate became increasingly arid since the\nPliocene/Pleistocene boundary with a stepwise increase in aridity during the MPT is\nwell known. What is the new record adding to this knowledge base? This remains\nunclear.\nResponse: Thanks for the critical comments. While we agree that there has been a\ngeneral appreciation of the increased aridity of the central Asian climate since the\nPliocene, this knowledge has been either broadbrush in approach or rarely focused on\nour area of study. Our results provide a significant new and detailed record of the\ndynamics of this increase in Tajikistan and the Tarim Basin. In providing a longer\n(Pliocene) record, we can highlight just how unique the changes centred around the\nMPT are for the Plio-Pleistocene. In the revised manuscript, we have therefore added a\nnew paragraph to extend the knowledge about the vegetation cover and paleoclimatic\nchanges during the older times.\nIn lines 176-187: “Tajikistan sits on a crucial migration pathway of hominins dispersing\nthrough Eurasia. The Palaeolithic sites in the loess-palaeosol sections of southern\nTajikistan provide the most spectacular picture of the loessic Palaeolithic across Eurasia.\nTo date, little information is known about the regional vegetation cover and\npaleoclimatic changes prior to the Mid-Pleistocene in this area. Our results demonstrate\nthat although vegetation oscillations existed during glacial-interglacial cycles in\nTajikistan, the vegetation community was relatively stable during the late Pliocene and\nearly Pleistocene. A prominent shift in vegetation cover and vegetation communities is\nrecorded only around the MPT. Similar patterns of vegetation changes were also\nobserved in the Tarim Basin (Fig. 2b-c). Collectively, these observations demonstrate\nthat a marked ecological shift towards more open habitats is coeval with the MPT along\nthe northern migration route of early hominins through Eurasia.”\n17. 139-146. So, the present-day conditions in Tarim Basin prevailed since 0.6 Ma?\nResponse: Yes, the δ13C records of the Lop Nor lake and the loess deposits (this study)\norg\nand eolian dust flux records from the Tarim Basin all demonstrate that the present-day\nconditions in Tarim Basin prevailed since the MPT, and a permanent presence of the\nTaklimakan Desert has been identified since 0.7-0.6 Ma (Fang et al., 2020; Liu et al.,\n2020).\n18. 147. Here the MPT is implicitly placed between 1.2 and 0.6 Ma.\nResponse: In fact, a prominent increase in δ13C and δ13C records from the\norg carb\nmonsoon-controlled areas in the Chinese Loess Plateau and the adjacent regions also\nmainly occurred after 0.9-0.6 Ma, which is consistent with the results from the Tarim\nBasin and southern Tajikistan. However, we note that the δ13C records in the Qaidam\ncarb\nBasin showed a slight increase at 1.2 Ma, and thereafter increased significantly at ~0.6\nMa. Thus, here the MPT is placed between 1.2 and 0.6 Ma. Nevertheless, these results\nare not contradictory to our main conclusions.\n19. 171-173. This pertains to the northern dispersal route in Asia. Up to now no notion\nof the southern route via India. The northern route relates to the southern boundary of\nthe Palearctic realm. Hominins went further north since the last 0.4-0.5 Ma or so?\nResponse: Thanks for the critical comments. Combined with further syntheses of\nPleistocene paleontological and carbon isotope records from the SW China and the\nSoutheast Asia, we have provided some new discussions about the southern migration\nroutes of early hominins across Asia in the revised manuscript. In addition, our\nsyntheses of Pleistocene archaeological records suggest that hominins went further\nnorth since the last 0.6-0.5 Ma.\nLines 193-199: “In addition, δ13C of fossil tooth enamel suggests that the spread of\ngrazers and extinction of browsers occurred synchronously in the Oriental Realm since\nthe Mid-Pleistocene due to the ecosystem conversion from forests to savannahs,\nproviding support for reduced precipitation in the Indian summer monsoon-controlled\nareas. This observation suggests that a marked ecological shift towards more open\nhabitats due to decreasing precipitation also occurred along the southern migration\nroute of early hominins in Asia since the Mid-Pleistocene.”\n20. 178. Check this tectonic uplift inference. Since 2001 many studies (more than 100?)\nhave returned to the question of when, which parts of, and how much the Tibetan\nPlateau was uplifted. Current mainstream thinking is that most of the plateau reached\nmore or less its current elevation at the Pliocene/Pleistocene boundary or even before.\nResponse: Thanks for the critical comments. We agree that the influences of tectonic\nuplift on the Mid-Pleistocene aridity and landscape shifts are overestimated. The\ndiscussions about the tectonic forcing have been abandoned in the revised manuscript.\n21. 207, 208. Provide the ages for MIS11 and MIS13.\nResponse: Done.\n22. 213. A lower sea level is not “felt” by a river beyond its coastal plain. In the\nupstream region of interest here the direct impact of a lower sea level on rivers is\nmarginal at best.\nResponse: Thank you for pointing this out. We reorganized the text accordingly in lines\n257-258: “intense river incision along the river margins in the upstream regions\noccurred in response to lowering sea-levels and base level”\n23. 215. Figure 3. What is the basis for allocating a terrace sequence to climate-\ncontrolled or tectonic-climate-controlled? How are these two distinguished?\nResponse: All these climate-controlled or tectonic-climate-controlled terraces are from\nthe previously published papers. The authors distinguished these two terrace sequences\nthrough examining the stratigraphy (layering) of the terraces, studying the regional\ngeology and tectonic history and using radiometric dating techniques to determine the\nages of the terraces, thereby correlating them with known climatic and tectonic events.\nWe have provided more discussion about this question in lines 388-391.\n24. 227. Serbia is more of southeastern Europe; northern Europe is Scandinavia?\nResponse: Yes, Serbia is more of southeastern Europe and UK is more of western\nEurope. We corrected them accordingly\n25. 231. And in Atapuerca? There hominin occupation is argued to be continuous since\n~1.3 Ma?\nResponse: Yes, the earliest secure dates from Atapuerca and the Orce Basin Spain are\nca. 1.4 – 1.2 Ma (Bermúdez de Castro et al., 2011; Oms et al., 2011). However, Muttoni\net al. (2018) argued that although Europe may have become initially populated during\nthe early Pleistocene, the stable occupation of hominins in Europe was possibly only\nsince ~0.9 Ma.\n26. 276. A distinct jump in reasoning? What are “our observations”?\nResponse: We have reorganized the text accordingly in lines 328-331:\n“Our results indicate that enhanced aridity and pronounced landscape shifts drove\nPalearctic hominin dispersals during the Mid-Pleistocene, suggesting that the\nenvironmental drivers of early hominin dispersals in Eurasia were quite different from\nthose in Africa (deMenocal, 1995; Levin, 2015; Potts et al., 2020).”\n27. 279-284. Can this be rhymed with younger migration waves as analyzed in Ao et al.\n(2024) Proceedings of the National Academy of Sciences?\nResponse: We have provided more discussion about this question in lines 339-341. Our\nresults demonstrate the dominant influences of a prominent shift in vegetation and\nlandscape on Palearctic hominin dispersals at 0.9-0.6 Ma. Nevertheless, it is reasonable\nto expect that hominin dispersals in Eurasia were likely facilitated by humid periods\nwhen arid climate and steppe/grassland prevailed during the late Pleistocene (Ao et al.,\n2024).\n28. 292. How is the “ready availability of fresh water” to be placed in an environment\nthat features increased aridity?\nResponse: Here we focused on the critical role of increasing drainage density in the\navailability of fresh water. We have reorganized the text accordingly in lines 347-348.\n29. 296. In the very southern margin of the Palearctic realm.\nFigure 1. The light blue on greenish background does not read well, pick another color\ncombination.\nResponse: Done.\n30. Figure 3 and 4. The methodology behind the analyses deserves more mention in the\nmain text since this represents a new twist to existing knowledge. How robust are the\ninferences made?\nResponse: We have done so as suggested in lines 401-403 and lines 516-546.\n31. 358. Why in Europe? That lies up-wind of the Tarim basin loess source areas?\nResponse: Yes, it is because the Europe lies up-wind of the loess source areas in\nTajikistan and the Tarim Basin.\n32. 364-366. So, authors rely on the age model provided by reference 14. How robust\nis that age model? It is robust, authors should provide some more information\nconcerning sampling resolution and paleomagnetic stability here (is actually done in\n374-378).\nResponse: We have provided more information about this question in lines 425-428:\n“Paleomagnetic block samples were taken at 50-cm intervals in the upper 80 m of the\nTarim Basin loess core and at 25-cm intervals in the lower 591 m. A total of 2,331\nsamples gave reliable characteristic remanent magnetizations to determine the\nmagnetostratigraphy.”\nAnd in lines 431-446:\n“Based on chronological constraints of the magnetostratigraphic and OSL analyses, an\nastronomically tuned age model was established for the 671 m-deep loess core in the\nTarim Basin with a temporal resolution of ~0.5 kyr, which has been achieved through\ntuning the grain size record to theoretical variations in the Earth's orbital parameters\n(Fang et al., 2020). In addition, an astronomically tuned age model of the Tajikistan\nloess has been obtained previously through tuning the magnetic susceptibility record to\ntheoretical variations in the Earth's orbital parameters (Ding et al., 2002). In this study,\nthe timescale of the Tajikistan loess profile, with a temporal resolution of ~5-10 kyr,\nwas established based on magnetostratigraphic constraints and the correlation of the\nmagnetic susceptibility record with the marine oxygen isotope results (Extended Data\nFig. 2). Previous studies confirmed that the grain size records of the Tarim Basin loess\nand the magnetic susceptibility records of the Tajikistan loess both show a dominant\nperiodicity shift from 41-kyr cyclicity to 100-kyr cyclicity between 1.2 Ma and 0.6 Ma\n(Ding et al., 2002; Fang et al., 2020). These features are consistent with global ice\nvolume variability during the MPT (Extended Data Fig. 2), highlighting the reliability\nof the chronological framework.”\n33. 372-374. Superfluous, leave out?\nResponse: We have done so as suggested.\n34. 384. Is a winter monsoon/westerlies proxy, specify that?\nResponse: We have done so as suggested. That is, the grain size records represent the\nintensity of westerlies.\n35. 385. Is a summer monsoon proxy, specify that?\nResponse: We have done so as suggested. That is, the magnetic susceptibility records\nreflect the precipitation variability.\n36. 392. Strict adherence to GTS2020 nomenclature would imply kyr here.\nResponse: We have done so as suggested.\n37. 393-394. But are they from loess, paleosol, or both per couplet?\nResponse: The samples for the carbon isotope analysis of organic matter were collected\nfrom all the loess and paleosol units. We have reorganized the text accordingly.\n38. 397. At which temperature? Room temperature?\nResponse: Yes, we have provided this information in the text.\n39. 404-406. Also mentioned in the main text, however without much context over there.\nResponse: We have deleted these sentences in the revised text.\n40. 421. Typos, reconstructions; appear\nResponse: We have corrected it.\n41. 425. Numbers become less negative, is that a reduction?\nResponse: We have reorganized the text accordingly.\n42. Change point analysis section. It may be wise to word the concept of this analysis\nand these tests with reference to a textbook. Extended data Figure 3 (and elsewhere).\nWhat is the meaning of the deviation from the zero line (always negative) in the red\nlines (brown-ish in my print) and green lines? What is the meaning of log(p)? What is\nimplied with Central Tendency and Dispersion?\nResponse: We have provided more discussion about these questions in lines 517-522:\n“The change point analysis of Mann–Whitney and Ansari–Bradley tests can provide\ncrucial constraints on the rhythms, events and trends in Plio-Pleistocene climate\nchanges of Asia (Foerster et al., 2022; Trauth et al., 2009). These statistical approaches,\nwithout requiring a normality assumption for the underlying population, are non-\nparametric hypothesis tests (Lepage, 1971). In this study, we use the ranksum function\nembedded in Matlab 2016b to perform the Mann-Whitney and Ansari-Bradley tests\n(Trauth, 2020).”\nAnd in the captions of Extended Data Fig. 3, 9-10, 12:\n“The log(p) denotes the logarithm of the p value of the Mann-Whitney and Ansari-\nBradley tests at the default 5% significance level. The downward prominent peaks\nindicate the position where the central tendency and dispersion change significantly\ndetermined by the Mann-Whitney and Ansari-Bradley tests, respectively.”\nAt the same time, we can combine visual and statistical results to determine whether\nthe central tendency and dispersion are increasing or decreasing. For example:\nLeft picture: paleoclimate time series. Right picture: The Mann-Whitney (middle)\nand Ansari-Bradley test (bottom) of time series.\n43. 459. Asia is probably better here? Not a single record shown in extended data Figure\n3 comes from Europe.\nResponse: We have done so as suggested.\n44. 461. Reference for MatLab2016b?\nResponse: Done.\n45. 466-473. These bits are not that helpful for a general readership. What is the\noutcome of this analysis? Where are the corresponding figures?\nResponse: This analysis can avoid the influence of sampling bias/burial loss of older\narchaeological records on the temporal distributions of fossil hominins. We have\nprovided the corresponding figure in the revised text (Fig. 4h-j).\n46. 477. Dating values: Is this the number of sites compiled here?\nResponse: Not exactly, it represents the age number of Pleistocene archaeological and\nfossil hominin records. For some archaeological sites, fossil hominin records can be\nfound over varying time periods and at different ages.\n47. 479-480. What is inferred with this sentence?\nResponse: We have deleted this sentence in the revised manuscript.\n48. 482. No new hominin finds are reported here, what is implied with ‘new\ndiscoveries’? How many new entries are included in comparison to previous\ncompilations?\nResponse: Thank you for pointing this out. In the revised manuscript, we have deleted\nthe words “new discoveries” and reorganized the text accordingly.\n49. 483. Is all duly referred to, how good are the updates? No new age assignments\nwere obtained in the present study. How many sites/data entries were updated?\nResponse: Thank you for pointing this out. In the revised manuscript, we have\nreorganized the text accordingly.\n50. 488. How many? Which key sites? Be specific. I presume the examples provided\nare not exhaustive.\nResponse: We have provided more discussions about these questions in the section of\n“Synthesis of archaeological and fossil hominin sites”\n51. 503-506. Start this section with these notions. They could also be mentioned in the\nmain text.\nResponse: We have done so as suggested. In addition, these notions were also\nmentioned in the captions of Figure 4.\n52. 509-510. You mean that the methods are suited for the age range considered here?\nAs worded it comes across as not overly informative.\nResponse: We have deleted this sentence in the revised manuscript.\n53. 510. Why ‘residual’? Residual of what?\nResponse: To avoid the influence of sampling bias/burial loss of older archaeological\nand fossil hominin sites, we perform change point analysis on the residual value of the\nsite number regressed against age. These residual values should be, in a sense, “free”\nof the tendency for an increase in the preservation and documentation of sites closer in\ntime to the present (Maxwell et al., 2018). We have provided more information about\nthis question in the section of methods and the main text accordingly.\n54. 510-512. A histogram of the apparently more or less even distribution through time\nwould be insightful.\nResponse: We have done so as suggested and a histogram was added in Figure 4.\n55. 512. And distribution through time was more or less even (at least it reads like that\nto me)? What is the cause of the inflection points?\nResponse: In Fig. 4 and Extended Data Fig. 11, we supplement the histograms of Homo\nfossils, handaxes and core-flakes site numbers over time. The raw histogram data show\nclearly that the site number of Homo fossils, handaxes and core-flakes increased after\nthe Middle Pleistocene. Statistically, the Mann-Whitney test (red arrows) also show that\nthe number of archaeological and Homo fossils sites changed significantly since 0.9-\n0.6 Ma. However, to avoid the influence of sampling bias/burial loss of older\narchaeological and fossil hominin sites, we further performed change point analysis on\nthe residual value of the site number regressed against age (fig. 4h-j). These residual\nvalues should be, in a sense, “free” of the tendency for an increase in the preservation\nand documentation of sites closer in time to the present, which may account for the\nincreased trends observed in the histograms. In Extended Data Fig. 12, the inflection\npoints show downward peaks in both central tendency and dispersion of the data at 0.9-\n0.6 Ma. As a correlated change in the central tendency and dispersion of a dataset is\nrelated to changing the scale of the data, these results suggest that examining the\nresidual values of our dataset against time has scaled but not shifted the significant\nrelationship between variables, thus supporting our inferences based on the raw counts.\n56. 513-517. What is the purpose of the notions made here?\nResponse: Here, these notions are aimed to exclude the sampling bias/burial loss of\nolder archaeological and fossil hominin sites on the spatial and temporal distributions\nof early hominins in Europe and Asia.\n57. 521. How is this done and how is somebody supposed to read the extended data\nfigures 11 and 12?\nResponse: We have provided more details about the change-point analysis and the\nkernel density estimation diagrams in lines 517-547 and lines 592-616.\n58. 530. What are CAM plants? Explain the acronym.\nResponse: Done.\n59. 554-603. How climatically controlled, tectonic-climate, and uplift controlled river\nterraces are being distinguished and how robust that distinction would be is not\ndiscussed.\nResponse: All these climate-controlled or tectonic-climate-controlled terraces are from\nthe previously published papers. The authors distinguished these two terrace sequences\nthrough examining the stratigraphy (layering) of the terraces, studying the regional\ngeology and tectonic history and using radiometric dating techniques to determine the\nages of the terraces, thereby correlating them with known climatic and tectonic events.\nWe have provided more discussion about this question in lines 388-391.\n60. 564-567. The relation to the present study may become rather thin. What\ninformation would be lost if this was omitted?\nResponse: We use this evidence to demonstrate that increasing dust fluxes of marine\nsediments from the Mediterranean Sea, the Arabian Sea, and the North Pacific Ocean\nprovide further supports for the occurrence of continental aridification and expansion\nof the Eurasian loess belt during the MPT, which led to a pronounced increase in eolian\nsediment supply from the terrestrial dust sources to the downwind oceans. We have\nrevised these sentences accordingly.\n61. 566. Presumably dust fluxes are to those seas and oceans, since they cannot deliver\ndust.\nResponse: Yes. We revised the text accordingly.\n62. 579. dating values = ages?\nResponse: Yes. We change the words “dating values” to “ages” in the revised\nmanuscript.\n63. 590-593. All correct but what is the relevance since hominin dispersal is being\ndiscussed between 0.1 and ~1.3 Ma?\nResponse: We deleted these sentences in the revised manuscript.\n64. Extended data Figure 1. Are these figures obtained in this study? If so, provide some\ntechnical details in the caption. Which model was used, which years were integrated,\netc. If not, properly refer to the source(s).\nResponse: Yes, these figures are obtained in this study. We have provided more\ninformation about the technical details in the captions.\n65. Extended data Figure 3. Explain log(p) in the caption. Mean and variance are being\nmentioned in the caption while central tendency and dispersion are being displayed on\nthe figure panels. What is what?\nResponse: We have provided more information about these questions in the revised\nmanuscript. The log(p) denotes the logarithm of p value of the Mann-Whitney and\nAnsari-Bradley tests. When log(p) is less than -1.3 (corresponding to the p value < 0.05),\nthe difference is clear and the changes have statistical significance. In statistics, the\ncentral tendency of time series is usually expressed as mean and median mean, median,\nmode; the dispersion of time series is commonly expressed by range, variance, quantiles\n(Trauth, 2020).\n66. 878. Are the changes observed significant? At which level?\nResponse: The embedded ranksum function in Matlab 2016b were used for Mann-\nWhitney and Ansari-Bradley tests. The default significance level (α) of ranksum\nfunction is 0.05. When log(p) is less than -1.3 (corresponding to the p value < 0.05),\nthe difference is clear and the changes have statistical significance.\n67. Extended data figure 4. How robust are those cluster analysis results? From the text\nit seems that authors have carried out that cluster analysis. Which input parameters were\nused and how was the stratigraphic control achieved?\nResponse: Yes. The cluster analyses have been carried out by the published papers. We\nhave provided more details about these questions in the figure captions.\n68. Extended data figure 6. The age distinction seems rather distinct. How robust are\nthe ages involved in this analysis?\nResponse: We have provided more information about the ages in the caption involved\nin this analysis: The ages of large mammal assemblage were constrained by\npaleomagnetic analyses and the correlations with typical European Mammal Zonation\nduring the late Neogene and Quaternary (MNQ).\n69. Extended data figure 8l. The temperature anomaly is with respect to what?\nResponse: Temperature anomaly indicates the temperature deviation (in °C) from\npresent (average over 0–5 ka). Such information has been provided in the revised\nmanuscript.\n70. 917. explain acronym IRM@AF.\nResponse: Done. It indicates an isothermal remanent magnetization (IRM) that had\nbeen AF (Alternating demagnetization)-demagnetized at 120 mT, representing the\neolian input of hematite.\nReviewer #2 (Remarks to the Author):\nThe rationale for this study is provided in the second paragraph of the main text: “The\nclimatic and environmental histories of Europe and Asia are usually discussed\nseparately. However, they are parts of the same biological Realm…As phylogenetic\ncompositions are driven by regional climatic and ecologic factors 11, the histories of\nboth regions need to be considered together when examining the drivers of hominin\ndispersals out of Africa”.\nThis is an important point, but also a substantial undertaking. In this work, the authors\nstart by presenting new δ13C data from loess deposits in Central Asia and northwest\nChina over the last 3.6 Myr. The evidence points to a significant climatic shift during\nthe so-called Middle Pleistocene Transition (MPT; 1.2-0.7 Ma). This is then expanded\nto a synthesis of palaeoenvironmental and palaeoanthropological/archaeological data\nto establish the transformation of landscapes across the Eurasian Palaearctic during the\nMPT, with implications for early human evolution and dispersal.\nThis is an impressive collection of new and published information on a grand scale that\nis worthy of publication.\nResponse: We greatly appreciate your very positive responses and insightful comments\nto our manuscript. We are very happy to revise our manuscript in light of these helpful\nand insightful comments.\n1. While the MPT ‘sensu lato’ is considered in general terms, the authors tend to focus\nmore specifically on climate changes commencing at 0.9 Ma. Indeed, the so-called ‘0.9\nMa event’ is traditionally taken to represent the first major Northern Hemisphere\nglaciation of MIS 22 (e.g. Clark et al.., 2006, QSR). However, closer inspection of the\nTarim Basin loess δ13C record shows an earlier shift (~1.1 Ma) and this is also observed\nin certain other records in Fig. 2 and Ext. Data Fig. 8. Margari et al. (2023, Science)\npresented evidence for an extensive glaciation during the 30kyr-long MIS 34 (possibly\nassociated with the Menapian cold stage of NW Europe), culminating in an extreme\nterminal stadial in the eastern North Atlantic. Climate envelope model simulations\nsuggest that the intensity of the declines in temperature and net primary productivity\nwould have led to the extirpation of early human populations in Europe and SW Asia.\nA critical review of archaeological record further suggested a paucity of European and\nSW Asian early hominin sites after that, possibly until MIS 21 (Margari et al., 2023).\nThis may be worth considering in the present work.\nResponse: Thanks for the critical comments. We have added these new findings to our\nmanuscript in lines 275-282:\n“Paleoenvironmental reconstruction and climate envelope model simulations\ndemonstrate that an extensive glaciation during MIS 34 (~1.15 to ~1.12 Ma) may have\nresulted in the depopulation of early human populations in Europe and SW Asia due to\nthe declines in temperature and net primary productivity, and that reoccupation of\nEurope may have been delayed until during MIS 21 (~0.81 to ~0.87 Ma) (Margari et\nal., 2023). The repopulation of Europe after 0.9 Ma may have been made only by a\nmore resilient species (e.g. H. antecessor) that was able to withstand the increased\nintensity of glacial conditions (Margari et al., 2023).”\n2. The synthesis part of the paper represents a large collection of information, but space\nrestrictions limit a fuller presentation and discussion of the evidence. For example, the\nriver terraces dataset of Ext Data Table 2 and Fig. 3 are divided into tectonic-climate\ninteractions, climatically-controlled and uplift-control, but their attribution and\nsignificance are not explained in the text or in the methods.\nResponse: Thanks for the critical comments. All these climate-controlled or tectonic-\nclimate-controlled terraces are from the previously published papers. The authors\ndistinguished these two terrace sequences through examining the stratigraphy (layering)\nof the terraces, studying the regional geology and tectonic history and using radiometric\ndating techniques to determine the ages of the terraces, thereby correlating them with\nknown climatic and tectonic events. We have provided more discussion about this\nquestion in lines 388-391.\n3. In the same vein, the archaeological and human fossil evidence of Fig. 4 merit a fuller\ndiscussion of the granularity contained therein. More generally, the broad spatio-\ntemporal canvases of Fig. 3 and 4 require a more expanded description.\nResponse: We have provided a more detailed discussion of the implications of these\nresults in the following lines 600-616:\n“In contrast, the residual value of the site number of fossil hominins and artefacts in the\ncurrent dataset does not show a significant increase through time during the Early and\nMiddle Pleistocene. However, marked inflection points in these distributions are\nconsistently observed around the MPT. This commonality suggests they aren’t artefacts\nof any biases present in our dataset but rather represent genuine changes in the\ndistribution of hominin, handaxe, and archaeological sites. However, these data also\nsuggest that these changes may not have occurred synchronously. Determining the\nrelative timings of increases in archaeological sites and/or handaxe occurrences versus\nrecords of hominin remains is a geological question that will require much finer-scale\nconsiderations of how taphonomic factors control preservation than we currently have\navailable. As such, our results echo the cautionary note of researchers working on the\nAfrican hominin records regarding the need for better confidence intervals on the\nstratigraphic ranges of hominins and their proxies, and stronger efforts to identify,\ndiscuss, and correct fossil sampling biases. Nevertheless, the current dataset represents\nthe largest compilation of archaeological records in Europe and Asia that suggests,\ndespite taphonomic and sampling limitations, broadly increasing trends in the\ndistribution of hominin dispersals in Eurasia since the MPT (see Discussion).”\n4. 110. “the stable pCO2 levels…associated with the intensification of NH glaciation”:\nWhile this may be in line with reconstructions based on carbonated precipitated in\npalaeosols (Da et al., 2019), it is not in line δ11B reconstructions showing drops in\npCO2; Martinez-Boti et al., 2015). This may have to be considered in the δ13C changes\nof the loess records.\nResponse: We have reorganized the text accordingly in lines 126-132:\n“Planktic foraminiferal δ11B ratios demonstrate that while a significant decrease in\natmospheric CO occurred mainly after 2.8 and 2 Ma, pCO levels were relatively stable\n2 2\nor increased slightly since the MPT. Meanwhile, a colder climate associated with the\nintensification of Northern Hemisphere glaciations since the MPT favors the growth of\nC over C plants, thereby leading to a significant reduction in δ13C . These\n3 4 org\nobservations suggest that changes in temperature and pCO levels cannot account for\n2\nthe Mid-Pleistocene increase of δ13C records in Central Asia and NW China.”\norg\n5. 132. “a pronounced shift in dominant periodicity”. Τhis is based on earlier work, but\nhave the authors attempted to explore the periodicities embedded in their new δ13C\nrecords?\nResponse: The shift in orbital expression cannot be examined in the carbon isotope data\ngiven their resolution. Nevertheless, we provide the grain size evidence (representing\nthe intensity of westerlies) of loess deposits in the Tarim Basin and Tajikistan to show\nthe orbital shift after 0.9-0.6 Ma in lines 159-162.\n6. 166-170. “shift in vegetation from closed forests or woodland to steppe and\ngrasslands is evident at ~0.9-0.6 Ma”: this is not correct. While European pollen records\nshow a serial extinction of moisture-requiring taxa (the so-called Tertiary relicts) over\nthe MPT, interglacial stages were still characterized by forests (though less diverse).\nIn the same vein, the inferences based on mammal evidence of a shift from closed forests\nto open woodland in Europe (Ext Data Fig. 6) are also tenuous, as they may represent\nlocal contexts (e.g. flood plains) and not supported by other lines of evidence (beetles,\nmolluscs).\nResponse: Thanks for the critical comments. We have revised the manuscript\naccordingly in lines 209-213:\n“Pollen records from Spain, France, Greece, and Italy suggest that an ecological shift\ntowards more open habitats, together with a serial extinction of moisture-requiring taxa\n(the so-called Tertiary relicts), is evident since ~0.9-0.6 Ma (Extended Data Fig. 5).\nAlthough the principal vegetation biomes in Europe during interglacial stages were still\ncharacterized by forests after the MPT, vegetation nevertheless became less diverse.”\nFor the Extended Data Fig. 6, we provide an additional note that local contexts (e.g.\nflood plains) may lead to some uncertainties on the vegetation reconstruction.\n7. 230-231. “stable occupation of hominins in Europe was possibly since ~0.9 Ma”. It\nis worth adding here that Margari et al. (2023) suggested that repopulation of Europe\nafter 0.9 Ma may have been by a more resilient species (e.g. H. antecessor) that was\nable to withstand the increasing intensity of glacial conditions.\nResponse: Thanks for the critical comments. We have added this finding to the revised\nmanuscript."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The new version rightfully downplays the impact of tectonics. However, the river terraces are still categorized in the same three groups as before. The basis for their allocation is still pretty unclear (it seems to be taken at face value from the respective literature sources). Authors mention some generic geological approaches, but which criteria were used to allocate a river terrace to a certain group is not mentioned. What is the point of making the allocation in the first place since the tectonic impact is argued to be not that relevant?\nTo what extent the present analysis focusing on the MPT adds to the review by Ao et al. (2020) Earth Science Reviews? OK, data from Europe are not considered in that paper but climate change is dealt with in quite some detail. The final sentence of their abstract: “Combined with environmental theories of human evolution, this chronological coincidence suggests a potential link between mid-Pleistocene environmental changes and human evolution.“ A lot seems to have happened during the 400-500 ka age interval, i.e. formally after the MPT range discussed here?\nThe reason for keeping the long paleoclimate records as they are, is explained in the rebuttal letter (providing a kind of base line to better appreciate the changes in the MPT interval); in the revised text this is less clear.\n50. put Spain in between brackets or commas: (Spain) or: , Spain,\n52. Provide the age range of the MPT?\n88-150. OK, the idea is to use 13C for reconstructing a paleoprecipitation record. All is featuring low precipitation but why is the analysis needed? Hominin evolution aspects are introduced before but it seems to disappear somehow here (at least in the text). Perhaps making it a little more specific in 82-86 would be helpful.\n104. is 0.9-0.6 Ma the MPT in its entirety or a part of it?\n158. Were the early Brunhes glaciations the strongest?\n194. Oriental Realm sufficiently clear in the present context?\n247-248. Earlier on emphasis is put on glacial and arid conditions, here interglacials are also stated to be windows of opportunity for hominin migration. This may require some further clarification.\n258. Base level lowering is probably more relevant than sea level lowering for river segments 1000s of km upstream from their respective river mouths. The area is partially an endorheic basin with no outlet to any ocean?\n259. Apparently, we now shift to NW Europe and northern China. Away from Central Asia?\n308-309. Sentence complete?\n339-341. What would be the underlying reason that hominins migrated mainly during humid periods in the Late Pleistocene while dry periods would be required earlier on?\n350. Our analysis, the data comes from various sources.\n376. When is a 0.1 proportion window (meaning?) used and when 0.2?\nFigure 2. In the text mostly 0.9-0.6 Ma is being used for the MPT while in the figure 0.6-1.2 Ma is indicated. Why is that different? Why is the emphasis on the 0.9-0.6 Ma portion of the MPT?\nFigure 3. What is the difference between tectonic-climate interactions (vague wording?) and uplift-controlled? The text in 388-391 is not that helpful in clarifying. Methods used by the references are mentioned but their criteria to allocate terraces to a certain category are not. Tectonic trends typically have time scales of millions of years beyond the age range analyzed here. Climate change may operate at orbital time scales.\nFigures 3 and 4. It looks like that beyond 1.0-1.5 Ma the number of input data is small compared to the 0.0-1.0 Ma interval. How does that impact the analyses performed?\n401-403. This should also be explained in the caption to figure 3.\n406-409. This reads cryptic to me. OK, focus here is on the (younger portion of) the MPT. However, dating is anticipated to be more precise for the most recent 100 kyr? Also the number of sites probably is high in comparison to older hominin sites.\n419. Which regions in Europe? It lies (mostly) upwind of the loess source area? This may need some further clarification.\n425. You mean the 2x2x2cm cubes? A “block sample” typically is a much larger hand sample.\n437. this = the present? (or that?) (this is not ref 70?)\n459. the averaging = averages of\n475. opposing the temperature controls = unclear wording, reword for clarity.\n478-483. Somehow wordy text to argue for a dominant precipitation control on d13C.\n484-485. for the plants under C3 vegetation = unfortunate wording, some rewording is appropriate.\n485-487. We know that already, leave out/merge with foregoing?\n490. take out ‘the’\n498 ff. a jump in the reasoning. Some more lead-in may be required.\n519-520. “without requiring a normality assumption for the underlying population” reads somehow unfortunate, is it the correct statistical terminology?\n525. Why is the age window selected? Why not a shorter or longer window?\n533. Really so? If there are no data the residual is undefined? With few data it is subject to a large uncertainty?\n534-538. With which software package the analysis was performed?\n572. take out ‘ago’\n597. A kind of to be expected? Age uncertainties are probably similar, the minimum and maximum age diagrams would imply shifting the mean diagram downward and upward?\n599. linguistically: can a distribution increase?\n600. How is that residual determined? It is the residual of what (number of sites?) with respect to what (time?)?\n602-603. Refer to the relevant figure(s) here. What is the reader supposed to see?\n605-606. So, how robust are such estimates for data-poor times?\n606-613. So, apparently number of sites and age uncertainty effects play up. Earlier on this was downplayed. This makes me worrying.\n615-616. “broadly increasing trends in the distribution of hominin dispersals” is this wordy for increasing hominin presence?\n620. you may wish to explain concisely ‘Crassulacean acid metabolism plants’\n622. carbonate\n625. organic carbon, carbonate or both? Be more specific?\n629-630. “analysis of the astronomically tuned timescale” reads cryptic, records are tuned to that scale, the scale itself is not analyzed.\n631-632. Nothing new here, at least for the CLP.\n633. take out ‘the’\n637. and when the ‘open forest steppe’ vegetation type did start?\n648. this is textbook? For loess deposits one needs an eolian source, isn’t it?\n649. take out ‘the’\n651. were these regions also sub-humid (correct term?) and humid at the time of loess expansion? From the text further on it does not seem that way. So, why the emphasis on humid is required?\n667-669. This pertains to glacial times! In the current interglacial in by far most of Europe there is no active loess accumulation. Loess-paleosol profiles are typically featuring ‘telescoping’, the downward migration of soil development with time.\n670 ff. A jump in reasoning, suddenly a switch to river terraces is made. What is the point of this paragraph? Older river terraces have much more potential to be eroded since river terraces are something dynamic (for example meandering rivers cut away riverine deposits down-stream with time).\n971-972. “Average wind data at 850 hPa is reached from ERA5 dataset” reads cumbersome. Did authors download the data and plot them with their favorite plotting package?\n972. insert ‘the’ before ‘European’\nExtended data Fig. 2 here the MPT is indicated from 1.2 to 0.8 Ma. Make consistent throughout text and figures.\n991-1001. Apparently shifts in central tendency and dispersion are something independent since they change at different times.\n994. I can’t see those indicated.\n994-997. OK but what is new here? Many things have changed during the MPT.\n1008-1010. Was this cluster analysis carried out by the present authors or by the studies referred to? Be explicit here.\nExtended data figures 5 and 6 seem compilations of literature data. Is that sufficiently clear from the wording?\n1010-1011. What variables are clustered? Some words on the premises and difference/coherence measure of that cluster analysis type would add.\n1019. typo, take out ‘the’ at the end of the sentence.\nExtended data figure 8. Note that the MPT (yellow shading) is differently indicated as appears from the main text.\nExtended data Figures 9 and 10. These are all literature data. Refer to the sources explicitly.\nWhat is the point of extended data figure 11? The caption is not overly helpful.\nThe extended data tables 1, 2 and 3, duly refer each entry. Where are these references provided in full? Do they correspond to the references in the main text? Probably only in part. There is some file named supp_refs but it is not specified in table headings.", "reviewer_2": "I thank the authors for addressing the majority of comments/suggestions by reviewers. Only a few matters remain outstanding.\n1. In response to a question by Reviewer 1on the inferred range of palaeoprecipitation, the authors have attempted to do this:\n“Using previously established linear correlations between the MAP and the δ13Corg of surface soils in Central Asia and the NW China, paleoprecipitation changes in south Tajikistan and the Tarim Basin can be estimated. These correlations suggest the precipitation in Tajikistan decreased by approximately 100-200 mm after the Mid-Pleistocene. In the Tarim Basin, precipitation decreased from ~200 mm during the late Pliocene to ~100 mm after the MPT”\nHowever, closer inspection shows that the data set used for the transfer function applied to the Tajikistan record has been developed from surface soil samples in northern Iran, in the western part of arid central Asia (Wang et al, 2019), while that applied to the Tarim Basin record is based on a surface soil samples from a transect in central East Asia from the northern slopes of Qinling Mountain to near the border of Mongolia and Russia (Lee et al., 2006). Since the relationship between δ13Corg and precipitation appears to vary geographically, the lack of modern datasets for Tajikistan and Tarim Basin suggest that the inferred palaeoprecipitation values need to be treated with caution. This uncertainty should be discussed more clearly in the text.\n2. lines 204-213. I still disagree with the statement “the replacement of woodland and closed forest by steppe and woodland-steppe environments across most of Palearctic Eurasia during the Mid-Pleistocene”. Forest biomes occurred less frequently, because of the shift from 41-kyr to ~100-kyr glacial cycles, but interglacial forests did not turn to steppe-woodland or open forests in most places, only in the lower precipitation areas. Some of the moisture-requiring tree taxa disappeared from interglacial forests, but what mainly changed was the character of the glacial periods with some tree presence during the Early Pleistocene, but then mostly absence later on (see for example Zhao et al 2020 Science Advances; Donders et al. 2021 PNAS). The same correction is needed for the caption of Ext. Fig. 5, line 1020, the herb values (pollen percentages not pollen concentrations) increased during the glacial periods (not the interglacials).\n(Incidentally, the attribution of the serial extinction of Tertiary relicts (line 211, ref 42) is incorrect, it should be Tzedakis et al. 2006 QSR.)\n3. Fig. 4b,c,d: the density diagrams appear to show a low value at ~2.4 Ma near 0° longitude. Since the data refer only to Europe, is this an artefact of the analysis?\n4. Extended Data Table 2:\n“Loir” should be “Loire”\nThe Voidomatis River in Greece is in the NW not NE. Also, while I agree that both tectonic and climate influences are present in that basin, the reference provided (Hughes 2010) does not discuss that. So where was the information taken from?\n5. Finally, a more general comment: the main thrust of this work is that Mid-Pleistocene enhanced aridity and landscape shifts “drove” Palaearctic hominin dispersals. This is plausible, but the confluence of the climate shift over the MPT and human dispersals does not necessarily mean a direct causal relationship (since other factors are also important). I wonder, therefore, whether this could be phrased in a more circumspect manner (e.g. may have “promoted” or “influenced”).\nProvided that that the authors address/consider these comments, I am happy recommend publication."}, "author_response": "Reviewer #1:\nWe are extremely grateful that you could take the time to conduct the second review of\nour paper. Based on these insightful comments and suggestions, we revise the\nmanuscript carefully. In the revised manuscript, we abandon the river terrace\nclassification and put all the terrace data together. We also provide more details about\nthe statistical analysis software in the methods. In addition, we have carefully proofread\nthe manuscript and corrected all grammatical errors as suggested by you.\n1. The new version rightfully downplays the impact of tectonics. However, the river\nterraces are still categorized in the same three groups as before. The basis for their\nallocation is still pretty unclear (it seems to be taken at face value from the respective\nliterature sources). Authors mention some generic geological approaches, but which\ncriteria were used to allocate a river terrace to a certain group is not mentioned. What\nis the point of making the allocation in the first place since the tectonic impact is argued\nto be not that relevant?\nResponse: We thank the reviewer for pointing out this issue. We agree that there is no\nneed to categorize the river terraces into three groups as before. We abandon the river\nterrace classification in the revised manuscript and put all the terrace data together.\n2. To what extent the present analysis focusing on the MPT adds to the review by Ao et\nal. (2020) Earth Science Reviews? OK, data from Europe are not considered in that\npaper but climate change is dealt with in quite some detail. The final sentence of their\nabstract: “Combined with environmental theories of human evolution, this\nchronological coincidence suggests a potential link between mid-Pleistocene\nenvironmental changes and human evolution.“ A lot seems to have happened during\nthe 400-500 ka age interval, i.e. formally after the MPT range discussed here?\nResponse: In fact, the review paper by Ao et al. (2020) was focused on the precipitation\nreconstructions in East Asia (mainly in the Chinese Loess Plateau) over the past 0.8 Ma.\nThe paleoclimatic records are mostly derived from the rock magnetic results. In our\nstudy, we report high-resolution (up to ∼5-10 kyr sample interval) carbon isotope time\nseries of loess deposits in Central Asia and northwest China, providing unique\npaleoecological and paleoclimatic records spanning the past 3.6 Ma. These isotopic data,\ncombined with novel syntheses of Pleistocene paleontological and archaeological\nrecords and spatio-temporal distributions of Eurasian eolian deposits and river terraces,\nprovide a comprehensive picture about the critical role of aridity and landscape shifts\nin promoting Palearctic hominin dispersals.\n3. The reason for keeping the long paleoclimate records as they are, is explained in the\nrebuttal letter (providing a kind of base line to better appreciate the changes in the MPT\ninterval); in the revised text this is less clear.\nResponse: In the revised manuscript, the reason for keeping the long paleoclimate\nrecords was explained in the introduction in lines 80-85:\n“Thus, δ13C records of these loess deposits can provide critical information on\norg\nprecipitation and precipitation-controlled vegetation density as well as their impacts on\nearly hominin dispersals in the Westerlies-influenced regions since the late Pliocence.\nMoreover, in providing these long paleoclimate records, we can highlight just how\nunique the changes centred around the MPT are for the Plio-Pleistocene.”\n4. 88-150. OK, the idea is to use 13C for reconstructing a paleoprecipitation record. All\nis featuring low precipitation but why is the analysis needed? Hominin evolution\naspects are introduced before but it seems to disappear somehow here (at least in the\ntext). Perhaps making it a little more specific in 82-86 would be helpful.\nResponse: We have provided more information about this question in lines 80-83:\n“Thus, δ13C records of these loess deposits can provide critical information on\norg\nprecipitation and precipitation-controlled vegetation density as well as their impacts on\nearly hominin dispersals in the Westerlies-influenced regions since the late Pliocence.”\n5. 104. is 0.9-0.6 Ma the MPT in its entirety or a part of it?\nResponse: Yes, the MPT covers the time interval between 1.2-0.7 Ma. Here, we\nemphasized that the pronounced shifts towards more positive δ 13 Corg values primarily\noccurred at the end of the MPT, i.e. ~ 0.9-0.6 Ma. We reorganized the text accordingly.\n6. 158. Were the early Brunhes glaciations the strongest?\nResponse: In fact, global ice volume began to increase significantly between 0.9 and\n0.6 Ma, and thereafter a further increase occurred during the early Brunhes. We\nreorganized the text accordingly.\n7. 194. Oriental Realm sufficiently clear in the present context?\nResponse: The term \"Oriental Realm\" has been replaced with \" Southeast Asia\".\n8. 247-248. Earlier on emphasis is put on glacial and arid conditions, here interglacials\nare also stated to be windows of opportunity for hominin migration. This may require\nsome further clarification.\nResponse: We refer here specifically to periods in interglacials that have been identified\nbased on local geological information as particularly humid. We thus further argue that\nit was not just the increased aridity that drove dispersals, but that establishment of river\nterraces was essential to dispersals under drier conditions (as opposed to sporadic\nmigrations during interglacials prior to the establishment of river terraces). This has\nnow been made explicit in the text.\n9. 258. Base level lowering is probably more relevant than sea level lowering for river\nsegments 1000s of km upstream from their respective river mouths. The area is partially\nan endorheic basin with no outlet to any ocean?\nResponse: We reorganized the text accordingly and emphasized the influences of\nlowering Base level.\n10. 259. Apparently, we now shift to NW Europe and northern China. Away from\nCentral Asia?\nResponse: Given that the river terrace data in Central Asia is relatively limited, here we\nhave to shift to NW Europe and northern China.\n11. 339-341. What would be the underlying reason that hominins migrated mainly\nduring humid periods in the Late Pleistocene while dry periods would be required\nearlier on?\nResponse: We emphasized that the climate in most areas of Eurasia switched from\nhumid to arid conditions at the end of the MPT. During this transition period, dry\nclimate led to remarkable landscape shifts (including the ecosystem conversion),\nthereby promoting Palearctic hominin dispersals. In contrast, a typical arid climate and\nsteppe/grasslands prevailed in most areas of Eurasia during the Late Pleistocene.\nThus,\nhominin dispersals in Eurasia during the Late Pleistocene may have been better\nfacilitated by humid periods like in northern Africa.\n12. 376. When is a 0.1 proportion window (meaning?) used and when 0.2?\nResponse: The length of sliding window was determined according to the data\nresolution. When the data resolution is high, a longer window was used; when the data\nresolution is low, a shorter window was used.\n13. Figure 2. In the text mostly 0.9-0.6 Ma is being used for the MPT while in the figure\n0.6-1.2 Ma is indicated. Why is that different? Why is the emphasis on the 0.9-0.6\nMa portion of the MPT?\nResponse: Yes, the MPT covers the time interval between 1.2-0.7 Ma. Here, we\nemphasized that the pronounced aridity and landscape shifts primarily occurred at the\nend of the MPT, i.e. ~ 0.9-0.6 Ma. We reorganized the text accordingly.\n14. Figure 3. What is the difference between tectonic-climate interactions (vague\nwording?) and uplift-controlled? The text in 388-391 is not that helpful in clarifying.\nMethods used by the references are mentioned but their criteria to allocate terraces to a\ncertain category are not. Tectonic trends typically have time scales of millions of years\nbeyond the age range analyzed here. Climate change may operate at orbital time scales.\nResponse: We thank the reviewer for pointing out this issue. We agree that there is no\nneed to categorize the river terraces into three groups as before. We abandon the river\nterrace classification in the revised manuscript and put all the terrace data together.\n15. Figures 3 and 4. It looks like that beyond 1.0-1.5 Ma the number of input data is\nsmall compared to the 0.0-1.0 Ma interval. How does that impact the analyses\nperformed?\nResponse: To avoid the influence of sampling bias/burial loss of archaeological and\nfossil hominin sites, we perform change point analysis on the residual value of the site\nnumber regressed against age (Fig. 4h-j). These residual values should be, in a sense,\n“free”of the tendency for an increase in the preservation and documentation of sites\ncloser in time to the present.\n16. 406-409. This reads cryptic to me. OK, focus here is on the (younger portion of)\nthe MPT. However, dating is anticipated to be more precise for the most recent 100 kyr?\nAlso the number of sites probably is high in comparison to older hominin sites.\nResponse: The reviewer is correct, and these properties mean that if sites younger than\n100 ka are added to the dataset, the relationship between site number and date becomes\neven more exponential than currently modelled, with the asymptote at the young end\nof the scale. Such a relationship is heavily skewed towards the youngest sites and may\nobscure any patterns in the flatter (i.e. mid-Pleistocene) part of the exponential, even\nwhen using log-transformed values.\n17. 478-483. Somehow wordy text to argue for a dominant precipitation control on d13C.\nResponse: We reorganized the text accordingly in lines 488-494.\n18. 498 ff. a jump in the reasoning. Some more lead-in may be required.\nResponse: A new sentence has been added at the beginning of this paragraph: “The δ13C\nratios of carbonate from paleosols and fluvial-lacustrine deposits can provide additional\ninformation about the paleoenvironmental changes.”\n19. 525. Why is the age window selected? Why not a shorter or longer window?\nResponse: In most cases, trends and events in the time and frequency domain are\ndetected by computing the statistical parameters of the data (e.g., measures of central\ntendency and dispersion). The precision of these parameters depends on the length of\nthe window. According to the data resolution and research purpose, the appropriate\nsliding window are selected.\n20. 533. Really so? If there are no data the residual is undefined? With few data it is\nsubject to a large uncertainty?\nResponse: The residual values are calculated by subtracting the predicted value from\nthe observed value, so if there are no values at all then there are no residuals. However,\nif the observed value is zero but this is drawn from a valid statistical sample, then a\nresidual will exist. The degree of certainty attached to the residual is only as good as\nthe underlying dataset describing the phenomena of interest. We have compiled as\nextensive a dataset as possible, thereby seeking to minimize uncertainties.\n21. 534-538. With which software package the analysis was performed?\nResponse: The SMATR package was used for the data analysis. We added this\ninformation in the revised manuscript.\n22. 597. A kind of to be expected? Age uncertainties are probably similar, the minimum\nand maximum age diagrams would imply shifting the mean diagram downward and\nupward?\nResponse: As commented by the reviewer, the minimum and maximum age diagrams\nrespectively shift the mean diagram downward and upward. Nevertheless, an expansion\nin the spatial distribution of fossil hominins and artefacts in Eurasia is evident at ~0.9–\n0.6 Ma. Moreover, marked inflection points in these distributions are consistently\nobserved around the MPT.\n23. 600. How is that residual determined? It is the residual of what (number of sites?)\nwith respect to what (time?)?\nResponse: The residual value of the site number regressed against time is obtained for\nfurther change point analysis. To obtain the residual value, site number was\ntransformed to non-zero values by adding 1, and a reduced major axis (RMA)\nregression analysis was performed between log (age) and log (site number + 1).\n24. 605-606. So, how robust are such estimates for data-poor times?\nResponse: Indeed, the residual value of the site number primarily reflects the long-term\ntrends in the distribution of hominin dispersals rather than fine-scale spatial patterns.\n25. 615-616. “broadly increasing trends in the distribution of hominin dispersals” is this\nwordy for increasing hominin presence?\nResponse: Here we focused on the expansion in spatial distribution of hominins rather\nthan the increase in hominin populations. We reorganized the text accordingly.\n26. 637. and when the ‘open forest steppe’ vegetation type did start?\nResponse: According to our novel syntheses of Pleistocene paleontological records, the\nvegetation type of open forest steppe in East Asia appeared around 1.2-1 Ma.\n27 651. were these regions also sub-humid (correct term?) and humid at the time of\nloess expansion? From the text further on it does not seem that way. So, why the\nemphasis on humid is required?\nResponse: Thanks for pointing out this issue. In the revised manuscript, we deleted\nthese words.\n28. 667-669. This pertains to glacial times! In the current interglacial in by far most of\nEurope there is no active loess accumulation. Loess-paleosol profiles are typically\nfeaturing ‘telescoping’, the downward migration of soil development with time.\nResponse: We have deleted the word “continuous”.\n29. 670 ff. A jump in reasoning, suddenly a switch to river terraces is made. What is the\npoint of this paragraph? Older river terraces have much more potential to be eroded\nsince river terraces are something dynamic (for example meandering rivers cut away\nriverine deposits down-stream with time).\nResponse: In this paragraph, we aim to provide some background information for the\nnovel syntheses of river terraces. A new sentence has been added at the beginning of\nthis paragraph: “A widespread occurrence of fluvial terraces could exert significant\nimpacts on the availability of fresh water and lithic raw materials (fluvial gravels) for\nearly hominins in Palearctic Eurasia.”\nIn addition, we agree that older river terraces have much more potential to be eroded in\nthe down-stream areas, especially for tectonically active regions. Nevertheless, growing\nevidence suggest that river terraces in Central Asia and NW China are preserved well\ndue to the arid climate and relatively stable tectonic setting during the Pleistocene. Thus,\nthe spatio-temporal patterns of river terraces in these regions are discernable and\nbelievable.\n30. Extended data Fig. 2 here the MPT is indicated from 1.2 to 0.8 Ma. Make consistent\nthroughout text and figures.\nResponse: We have done so as suggested.\n31. 991-1001. Apparently shifts in central tendency and dispersion are something\nindependent since they change at different times.\nResponse: In statistics, the central tendency of time series is usually expressed as mean\nand median mean, median, mode; the dispersion of time series is commonly expressed\nby range, variance, quantiles (Trauth, 2020). Thus, the shifts in central tendency and\ndispersion are something independent.\n32. 1008-1010. Was this cluster analysis carried out by the present authors or by the\nstudies referred to? Be explicit here.\nResponse: The cluster analysis was carried out by the previous studies. We provided\nthis information in the revised manuscript.\n33. Extended data figures 5 and 6 seem compilations of literature data. Is that\nsufficiently clear from the wording?\nResponse: We have provided more information in the revised manuscript.\n34. 1010-1011. What variables are clustered? Some words on the premises and\ndifference/coherence measure of that cluster analysis type would add.\nResponse: The relative abundance and pollen assemblage were used for cluster analysis.\nWe have added some worlds on the premises and difference/coherence measure: “Prior\nto pollen cluster analysis, the premises and difference/coherence measure, including\npollen representativeness and source stability, statistical independence and\nenvironmental coherence, have been evaluated.”\n35. Extended data figure 8. Note that the MPT (yellow shading) is differently indicated\nas appears from the main text.\nResponse: We revised the figure accordingly.\n36. Extended data Figures 9 and 10. These are all literature data. Refer to the sources\nexplicitly.\nResponse: We have done so as suggested.\n37. What is the point of extended data figure 11? The caption is not overly helpful.\nResponse: To clarify the potential influences of dating uncertainties on the spatial and\ntemporal distributions of archaeological records, the kernel density estimation diagrams\nand the breakpoints of archaeological and fossil hominin sites with the maximum and\nminimum age estimations were investigated (Extended Data Fig. 11). The figure\ncaption has been revised accordingly in lines 1121-1126:\n“To clarify the potential influences of dating uncertainties on the spatial and temporal\ndistributions of archaeological records, the kernel density estimation diagrams and the\nbreakpoints of archaeological and fossil hominin sites with the maximum and minimum\nage estimations were investigated. An expansion in the spatial distribution of fossil\nhominins and artefacts in Eurasia are consistently observed around the MPT.”\n38. The extended data tables 1, 2 and 3, duly refer each entry. Where are these\nreferences provided in full? Do they correspond to the references in the main text?\nProbably only in part. There is some file named supp refs but it is not specified in table\nheadings.\nResponse: Given that a vast amount of literature is available, we thus provided these\nreferences in the supplementary information. In the supplementary file, table headings\nare specified.\nReviewer #2:\nI thank the authors for addressing the majority of comments/suggestions by reviewers.\nOnly a few matters remain outstanding.\nWe are extremely grateful that you could take the time to conduct the second review of\nour paper. Based on these insightful comments and suggestions, we revise the\nmanuscript carefully.\n1. In response to a question by Reviewer 1on the inferred range of palaeoprecipitation,\nthe authors have attempted to do this: “Using previously established linear correlations\nbetween the MAP and the δ13Corg of surface soils in Central Asia and the NW China,\npaleoprecipitation changes in south Tajikistan and the Tarim Basin can be estimated.\nThese correlations suggest the precipitation in Tajikistan decreased by approximately\n100-200 mm after the Mid-Pleistocene. In the Tarim Basin, precipitation decreased\nfrom ~200 mm during the late Pliocene to ~100 mm after the MPT”. However, closer\ninspection shows that the data set used for the transfer function applied to the Tajikistan\nrecord has been developed from surface soil samples in northern Iran, in the western\npart of arid central Asia (Wang et al, 2019), while that applied to the Tarim Basin record\nis based on a surface soil samples from a transect in central East Asia from the northern\nslopes of Qinling Mountain to near the border of Mongolia and Russia (Lee et al., 2006).\nSince the relationship between δ13Corg and precipitation appears to vary\ngeographically, the lack of modern datasets for Tajikistan and Tarim Basin suggest that\nthe inferred palaeoprecipitation values need to be treated with caution. This uncertainty\nshould be discussed more clearly in the text.\nResponse: Thanks for the critical comments. We agree that the relationships between\nδ13Corg and precipitation vary geographically, and the lack of modern datasets for\nTajikistan and Tarim Basin suggest that the inferred palaeoprecipitation values need to\nbe treated with caution. In the revised manuscript, we have provided more discussion\nabout this question in lines 148-150:\n“The relationships between δ13C and precipitation vary geographically, and our\norg\ninferred palaeoprecipitation values require validation from modern datasets from\nTajikistan and Tarim Basin, which are currently lacking.”\n2. lines 204-213. I still disagree with the statement “the replacement of woodland and\nclosed forest by steppe and woodland-steppe environments across most of Palearctic\nEurasia during the Mid-Pleistocene”. Forest biomes occurred less frequently, because\nof the shift from 41-kyr to ~100-kyr glacial cycles, but interglacial forests did not turn\nto steppe-woodland or open forests in most places, only in the lower precipitation areas.\nSome of the moisture-requiring tree taxa disappeared from interglacial forests, but what\nmainly changed was the character of the glacial periods with some tree presence during\nthe Early Pleistocene, but then mostly absence later on (see for example Zhao et al 2020\nScience Advances; Donders et al. 2021 PNAS). The same correction is needed for the\ncaption of Ext. Fig. 5, line 1020, the herb values (pollen percentages not pollen\nconcentrations) increased during the glacial periods (not the interglacials). (Incidentally,\nthe attribution of the serial extinction of Tertiary relicts (line 211, ref 42) is incorrect, it\nshould be Tzedakis et al. 2006 QSR.)\nResponse: Thanks for the critical comments. We agree that the replacement of\nwoodland and closed forest by steppe and woodland-steppe environments mainly\noccurred during the glacial periods. We reorganized the text accordingly in lines 209-\n218 and in the caption of Ext. Fig. 5:\n“The first was the replacement of woodland and closed forest by steppe and woodland-\nsteppe environments across most of Palearctic Eurasia during the glacial periods of the\nMid-Pleistocene. In northern China and southeast Tibet, the principal vegetation biomes\nduring the early Pleistocene glacial periods were characterized by coniferous or\nbroadleaf forests, and then show a glacial change towards open forest steppe at the onset\nof the MPT (Extended Data Fig. 4). Pollen records from Spain, France, Greece, and\nItaly suggest that an ecological shift towards more open habitats, together with a serial\nextinction of moisture-requiring taxa (the so-called Tertiary relicts), is evident during\nthe glacial periods since ~0.9-0.6 Ma (Tzedakis et al., 2006).”\n3. Fig. 4b,c,d: the density diagrams appear to show a low value at ~2.4 Ma near 0°\nlongitude. Since the data refer only to Europe, is this an artefact of the analysis?\nResponse: We thank the reviewer for pointing out this issue. Yes, this data is an artefact\nof the analysis, which is aimed to set a suitable scale to show the density diagrams.\nWe\nare really sorry for the mistake and remove this data in the revised figures.\n4. Extended Data Table 2:“Loir” should be “Loire”. The Voidomatis River in Greece is\nin the NW not NE. Also, while I agree that both tectonic and climate influences are\npresent in that basin, the reference provided (Hughes 2010) does not discuss that. So\nwhere was the information taken from?\nResponse: Thanks for pointing out this. We correct these mistakes accordingly. The\ninformation about the river terrace classification is from two additional references by\nLewin et al. (1991, Quat. Res.) and Hamlin et al. (2000). As commented by the reviewer\n1, there is no need to categorize the river terraces into three groups as before. Hence,\nwe abandon the river terrace classification in the revised manuscript and put all\nthe terrace data together.\n5. Finally, a more general comment: the main thrust of this work is that Mid-Pleistocene\nenhanced aridity and landscape shifts “drove” Palaearctic hominin dispersals. This is\nplausible, but the confluence of the climate shift over the MPT and human dispersals\ndoes not necessarily mean a direct causal relationship (since other factors are also\nimportant). I wonder, therefore, whether this could be phrased in a more circumspect\nmanner (e.g. may have “promoted” or “influenced”).\nResponse: We thank the reviewer for pointing out this issue. In the revised\nmanuscript, the main thrust of this work has been phrased in a more circumspect\nmanner and we also change the word \"drove\" to \"promoted\" in the title."}, {"round": 3, "reviewer_comments": {"reviewer_1": "The re-revision has addressed all my comments to a sufficient extent. I recommend acceptance for publication. Usage of articles may require some attention (a few times 'the' seems superfluous). AF in line 1096 stands for Alternating Field; inconsistencies in the reference list can be remedied at the proof-reading stage.", "reviewer_2": "The authors have addressed the points I raised and I am happy to recommend publications.\nIn the final MS, two more points require minor modification:\nl. 218 After reference “42” please add the following reference: MAgri et al., Quaternary disappearance of tree taxa from Southern Europe: Timing and trends. Quaternary Science Reviews 163 23-55 (2017).\nl. 648-650 and Extended Data Fig. 4: “Vegetation types in East Asia show a remarkable shift from forest to open forest at 1.2-1 Ma, and subsequently changed from open forest steppe to steppe at ~0.9-0.6 Ma (Extended Data Fig. 4)”\nWith respect to Zoige Basin, this sentence is not correct: after 0.62 Ma, interglacials were still characterized by tree population expansions (mainly Picea), but with presence of alpine meadows; glacial periods were longer and dominated by steppe and meadows. Please correct this in the text and also add some spruce trees in the Ext Fig 4 in the interval 0-0.62 Ma and even more for the interval 0.62-0.9 Ma"}, "author_response": "Reviewer #1:\nWe are extremely grateful that you could take the time to conduct the third review of\nour paper. In the revised manuscript, we have carefully proofread the\nmanuscript and corrected all grammatical errors as suggested by you.\nReviewer #2\nResponse: We are extremely grateful that you could take the time to conduct the third\nreview of our paper. In the revised manuscript, we have reorganized the text accordingly.\nl. 218 After reference “42” please add the following reference: MAgri et al., Quaternary\ndisappearance of tree taxa from Southern Europe: Timing and trends. Quaternary Science Reviews 163 23-55 (2017).\nResponse: We have done so as suggested.\nl. 648-650 and Extended Data Fig. 4: “Vegetation types in East Asia show a remarkable\nshift from forest to open forest at 1.2-1 Ma, and subsequently changed from open forest\nsteppe to steppe at ~0.9-0.6 Ma (Extended Data Fig. 4)” With respect to Zoige Basin,\nthis sentence is not correct: after 0.62 Ma, interglacials were still characterized by tree\npopulation expansions (mainly Picea), but with presence of alpine meadows; glacial\nperiods were longer and dominated by steppe and meadows. Please correct this in the\ntext and also add some spruce trees in the Ext Fig 4 in the interval 0-0.62 Ma and even\nmore for the interval 0.62-0.9 Ma\nResponse: We have done so as suggested."}]

Decision letter