The adaptability of our species, as revealed by the geographic routes and palaeoenvironmental contexts of human dispersal beyond Africa, is a prominent topic in archaeology and palaeoanthropology. Northern and Central Asia have largely been neglected as it has been assumed that the deserts and mountain ranges of these regions acted as ‘barriers’, forcing human populations to arc north into temperate and arctic Siberia. Here, we test this proposition by constructing Least Cost Path models of human dispersal under glacial and interstadial conditions between prominent archaeological sites in Central and East Asia. Incorporating information from palaeoclimatic, palaeolake, and archaeological data, we demonstrate that regions such as the Gobi Desert and the Altai Mountain chains could have periodically acted as corridors and routes for human dispersals and framing biological interactions between hominin populations. Review of the archaeological datasets in these regions indicates the necessity of wide-scale archaeological survey and excavations in many poorly documented parts of Eurasia. We argue that such work is likely to highlight the ‘northern routes’ of human dispersal as variable, yet crucial, foci for understanding the extreme adaptive plasticity characteristic of the emergence of Homo sapiens as a global species, as well as the cultural and biological hybridization of the diverse hominin species present in Asia during the Late Pleistocene.

Funding: This study was funded by Max-Planck-Gesellschaft (DE) to Nicole Boivin, Strategic Priority Research Program of Chinese Academy of Sciences grant XDB26000000 to Feng Li, and Youth Innovation Promotion Association of the Chinese Academy of Sciences grant 2017102 to Feng Li. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Here, we use Geographic Information Systems (GIS) software, alongside the most detailed palaeoclimatic, palaeoglacier, and palaeolake datasets available, to investigate the possible routes of human dispersals between Central and eastern Asia under postulated glacial and interstadial conditions between MIS5 and MIS3. In building the first Least Cost Path models for human dispersals across this part of Asia, we follow the work performed by Field and Lahr [ 35 ] and Field et al. [ 7 ] for southern dispersal routes and Kealy et al. [ 36 ] for corridors extending across Sunda to Sahul. Existing dispersal models for modern human expansions across Eurasia link movements to lithic technocomplexes classed variously as ‘Upper Palaeolithic’, ‘Early Upper Palaeolithic’, and ‘Initial Upper Palaeolithic’ [ 37 – 40 ]. There has been an almost complete lack of systematic overview of the changing potential topographic, climatic, and environmental constraints of the regions studied. Therefore, we seek to provide a new perspective in this regard by producing models that visualize the summary of the effects these variables would have had on human population movements at different points in time, as well as a means of recommending key locations in Mongolia and China for future archaeological survey and excavation.

The last two decades of research into Late Pleistocene (125-12ka) human dispersals has, however, revealed that modern arid environments, such as those found in the Arabian peninsula and the Thar Desert of northwestern India, as well as palaeoarctic and high-altitude regions, traditionally assumed to be dispersal barriers [ 9 , 25 – 27 ], were passable at certain periods in the past [ 10 , 28 – 30 ]. Palaeoclimatic and terrain modeling research has confirmed the appearance of periodic pathways, while new archaeological research has detailed significant hominin presence in the past [ 9 , 25 , 28 – 30 ]. Moreover, it is being increasingly appreciated that our species documents a unique adaptive plasticity in the face of variable habitats and climates when compared to other hominins [ 31 – 34 ]. As a result, it seems likely that renewed investigation and appraisal of the region covered by the varied environments of the Gobi Desert, the Tian Shan, and Altai Mountains, spanning Uzbekistan into northern China, could reveal new potential routes of human dispersal between central and eastern Asia. If this is the case, this would highlight the need to investigate new regions in order to examine the unique ecological capacities of our species and the degree of interaction between Homo sapiens, Homo neanderthalensis, and Denisovans.

(A) Map of Initial Upper Palaeolithic (IUP), Middle Palaeolithic (MP) and core and flake archaeological sites, as well as sites containing Homo sapiens fossils across all the countries in Central and Northern Asia. (B) Map displaying the dispersal route suggested by Goebel’s [ 24 ] “Overland” hypothesis together with the all the known IUP sites in the region ( S1 Table ). Sites: 1. Anghilak cave, 2. Teshik Tash, 3. Khudji, 4. Obi-Rakhmat, 5. Shugnou, 6. Chagyrskaya, 7. Okladnikov, 8. Denisova, 9. Ust-Karakol, 10. Kara-Tenesh, 11. Kara-Bom, 12. Luotuoshi, 13. Tongtian Cave, 14. Gouxi, 15. Lenghu, 16. Heimahe 1, 17. Chikhen Agui, 18. Tsagaan Agui, 19. Tolbor 4, 20. Kharganyn Gol 5, 21. Orkhon 1 & 7, 22. Makarovo 4, 23. Kandabaevo, 24. Varvarina Gora, 25. Tolbaga, 26. Temple Canyon 1, 27. Shuidonggou 1, 27. Shuidonggou 2, 27. Shuidonggou 7, 28. Shuidonggou 9, 29. Fanjiagouwan (Salawusu), 29. Yangshugouwan (Salawusu), 30. TX08, 31. TX03, 32. Liujiacha, 33. GY03, 34. ZS08, 35. ZL05, 36. Shuangbuzi, 37. Shixiakou 2, 38. Xujiacheng, 39. Changweigou, 40. Gutougou, 41. Wulanmulun, 42. Yushuwan, 43. Shiyu, 44. Dingcun (7701), 45. Licunxigou, 46. Fuyihe (Xiachuan), 47. Beiyao, 48. Zhiji, 49. Tashuihe, 50. Laonainaimiao, 51. Huangdikou, 52. Longquandong, 53. Xiaonanhai, 54. Dangcheng (Shidie), 55. Xibamaying, 56. Jinsitai, 57. Tianyuandong, 58. Upper cave, 59. Wangfujing, 60. Zhuacun, 61. Huangniliang, 62. Dazhushan, 63. Xiaogushan, 64. Miaohoushan, 65. Xianrendong, 66. Shimenshan, 67. Zhoujiayoufang, 68. Yanjiagang, 69. Guxiangtun, 70. Shibazhan (75075), 71. Salkhit, 72. Ust’-Ishim. Base map raster is from naturalearthdata.com.

For many years, archaeological, biological, and genetic investigations have focused on the ‘southern’ routes of human dispersal throughout the Middle East and Asia [ 7 – 10 ]. Homo sapiens is suggested to have arrived in southern China, following expansion along the Indian Ocean rim [ 11 , 17 , 18 ]. The arrival of humans into the higher latitudes of Asia is seen as a much later phenomenon but has been relatively under-explored until the last few years [ 1 , 19 – 21 ]. The main model in this regard suggests that the distribution of early ‘Upper Palaeolithic’ assemblages in northern Asia indicates that humans moved between central and eastern Asia by travelling north via the Siberian Altai, northern Mongolia, and the Transbaikal region before heading southwards into East Asia [ 22 , 23 ] ( Fig 1 ). The ‘Overland Model’, proposed by Goebel among others [ 24 ] is of great value as a working hypothesis, but this model assumes that west-east movements across the Altai and Tian Shan Mountains, and the modern Gobi and Taklamakan Desert region of southern Mongolia and northern China, were persistently too challenging. These zones are thought to represent cold, high-altitude, and dry desert ‘barriers’, leading archaeologists to largely neglect them in terms of survey and their potential for being important routes of human dispersal.

Understanding the nature and tempo of Late Pleistocene dispersals of our species, Homo sapiens, across Eurasia is a critical issue in human evolutionary studies [ 1 , 2 ]. Central and eastern Asia are playing an increasingly prominent role in palaeoanthropological investigations. Eastern Asia, and particularly China, has yielded some of the earliest fossil examples of our species anywhere in the world, c. 100ka in the southern, sub-tropical portion of the country [ 3 – 6 ]. Meanwhile, archaeological research in Central Asia, the Siberian Altai, and northern China, suggests that Homo sapiens only extended into and across this part of the world c. 45ka [ 7 – 11 ]. Recent chronological, archaeogenetic, and archaeological research at Denisova Cave in the Altai Mountains has provided high-resolution insights into the cultural and technological changes that followed the arrival of our species, and its interaction and eventual replacement of other hominin species, in northern and Central Asia [ 12 , 13 ]. The Salkhit skullcap, which shows possible archaic features, has been subject to recent dating and mitochondrial DNA analysis, demonstrating that modern human populations were certainly present in Mongolia by c. 34ka [ 14 ]. The earliest modern human fossils found in North China are from Tianyuan Cave directly dated to 40ka [ 15 ] and the Zhoukoudian Upper Cave minimally dated to 35.1–33.5ka by a recently revised dating program [ 16 ]. Yet, despite the growth of detailed investigation of Late Pleistocene archaeological sites in Russia, Mongolia, and China over the past two decades, as well as their rich records of lithic and organic remains, there has been limited, detailed discussion of the temporal and spatial distribution of the potential routes of human migrations and their subsequent adaptations to a range of ecological settings.

The greening of the Taklamakan Desert shown in Fig 3 is based on the investigation of lacustrine sediments of terminal lakes in the Taklamakan Desert; these lakes were re-activated by the inflow of perennial dry-land rivers. It is argued that the latter could have resulted from intensified westerlies and/or increasing glacier melting affecting in particular the desert margins [ 61 ]. Together, these palaeoaenvironmental features were used to test the assumption that aridity would not have been a barrier to human movements in this part of the world during MIS5 and MIS3 interstadials, as per our interstadial model. Similarly, they enable us to further test to the degree to which aridity would have limited movement across Central Asia under glacial conditions in MIS4.

The precipitation records for Central Asia (CA) and the East Asian Monsoon (EAM) shown in Fig 3 have been simulated using the Community Climate System Model version 3. The spatial coverage for the EAM and CA precipitation regimes are 100°E-120°E, 35°N–50°N and 50°E–70°E, 35°N–50°N, respectively [ 59 ]. Reconstructed glacier advances in the Kyrgyz Tian Shan and the Altai rely on surface exposure dating of moraine complexes. The displayed Kyrgyz Tian Shan and the Altai data comprise the mountain ranges of the Kyrgyz Front in the north, the Terskey Ala Tau in the east, the At Bashi to the south of the Kyrgyz Tian Shan [ 49 ], and the Ikh Turgen Mountains in the Altai [ 60 ]. Glacier advances are linked to increasing precipitation.

(A) Orbitally tuned Lake Baikal biogenic silica record as a relative temperature proxy, due to insolation forcing [ 48 ]. (B,C) Modeled annual precipitation for Central Asia (B) and the East Asian Monsoon (C) [ 38 ]. (D,E) Timing of glacier advances for the Kyrgyz Tian Shan (D) [ 76 ] and the Altai (E) [ 50 ]. (F) Timing of wetter periods for the region of the Taklamakan Desert [ 51 ]. (G) Timing of archeological technologies grouped into three major categories: Middle Paleolithic (MP), Initial Upper Paleolithic (IUP), and the core-flake technocomplex.

The climatic records shown in Fig 3 provide a chronological overview of environmental changes across the region to act as a more temporally resolved, detailed points of comparison for the periods under discussion. The environmental data sources selected are 1) all archives representing terrestrial regions, 2) all located within the study area, and 3) cover the Late Pleistocene period from at least 70ka to 12ka. The Lake Baikal biogenic silica dataset represents a relative temperature record for southeastern Siberia. The dataset has been orbitally tuned and suggests that maxima in biogenic silica content (amount of diatoms) coincide with insolation peaks. Hence, the warmest periods during interglacial conditions can be directly inferred from high biogenic silica contents [ 58 ].

Though the Homo sapiens fossil from Salkhit is not directly associated with archaeological finds, the 34ka age is argued to correspond with Levallois and blade assemblages (or the ‘early Upper Palaeolithic’) in Mongolia [ 14 ]. Therefore, we maintain that the spatial distribution of the IUP likely associates with the widespread dispersal of Homo sapiens populations across North Asia during MIS 3 [ 39 , 40 ], notwithstanding the possibility for interbreeding with indigenous archaic populations. Accordingly, we compiled and mapped the available, well-dated instances of the main technocomplexes for Russian Siberia, Mongolia, and North China. Sites from several other central Asian countries, including Tajikistan and Uzbekistan, representing major potential areas of human dispersal, are also included to provide good comparisons of route trajectories to known archaeological material.

Lithic assemblages were categorized into three major categories: Middle Palaeolithic (MP) with typical Levallois reduction and tool typology, Initial Upper Palaeolithic (IUP) with tool forms and blades produced by a combination of Levallois and volumetric prismatic methods, and core-flake technocomplex typified by more simply-organized flake reduction and lightly retouched tools ( Fig 1 ). The most recent dating program at Denisova Cave convincingly associates the Middle Palaeolithic with Neanderthals (193-97ka) and Denisovans (287-55ka) [ 56 ]. Recent dating of ornaments and the IUP bearing layers at Denisova cannot yet be attributed to a species, although the age range corresponds with the age of the Homo sapiens fossil at Ust-Ishim [ 19 ]. In addition, increasingly frequent use of ornaments, pigments, and formal bone tools is also associated with IUP assemblages in Siberia and Central Asia [ 52 , 57 ], and so far such finds are not found in MP sites.

ArcGIS’s cost distance tool was used to create cumulative cost raster maps from three possible dispersal centers: the Altai Mountains, the Tian Shan Mountains, and the Pamir Mountains ( Fig 2 , See S1 Table for information on the sites). The site of Denisova was used as the arbitrary coordinate for the Altai (51.2°N, 84.7°E), which has IUP assemblages dated up to ~49ka [ 13 ], and several sites in the vicinity, such as Ust-Karakol, Kara-Tenesh and Kara-Bom [ 52 ] all suggest that the Altai may have been a center of a population large enough to encourage dispersals. Similarly, Obi Rakhmat (41.567°N, 70.133°E) in the Tian Shan Mountains, has a large IUP assemblage and it is one of the few stratified sites in the region, and it was therefore selected to be the of the dispersal origin point for this region [ 53 , 54 ]. Shugnou (38.481°N, 71.097°E), in the Pamirs, was selected as the third origin point as it too is one of the few stratified sites with IUP assemblages in that region [ 55 ]. The destination point for the production of all three maps was the Shuidonggou site (38.377°N, 106.512°E), as this is the easternmost known site with IUP material [ 38 ]. We are, of course, not suggesting that all humans dispersing east during MIS3 arrived at Shuidonggou, but that to our current knowledge it is the furthest East that IUP populations reached in northeast Asia. The origins and destination points of the Least Cost Path remained the same for the dry and wet models.

Available raster files of LGM glacial extents from the collection of data from Ehlers et al.’s [ 46 ] LGM extents for Asia and Kuhle's [ 47 ] LGM extents for the Tian Shan were also converted into a 1km 2 raster. The local LGM of northern Asia is around 28-15ka. Although the volume of ice may not have been so large in MIS3, studies [ 48 ] have shown that their geographic extent may have been equal to those of the LGM. Hence, the extents of the glaciers of northern Asia are used as barriers for both the wet and dry models [ 48 , 49 ]. In addition, similar to Field and Lahr [ 35 ] we added both modern lakes and rivers [ 50 , 51 ] as barriers to both models, but with the condition that only river widths larger than 1km were considered impassable. All the impermeable barriers were given the value of NoData. Finally, all of these geographic factors were combined with the slope map to create a base palaeogeographic cost surface for the wet and dry models. The dry model was given the conditions of aridity described above, as an additional barrier, limiting colonization of specific regions with a lack of water (see S1 and S2 Figs for the cost surfaces and cost distance of the wet and dry models, and S2 Table for costs).

Published MIS5-MIS3 palaeolake extents were obtained from a variety of sources ( S3 Table ), and were outlined and downscaled to a 1km resolution. A number of geographical investigations into northern Asian palaeolakes suggest large MIS5 lake extents. Many of these were originally radiocarbon dated to MIS3 using radiocarbon methods, but recent OSL dating of the alluvial sediment suggests they could also date to MIS5. Despite the re-dating, some demonstrate high lake stands, but further analysis and discussion is still warranted. Therefore, we used the MIS5 lake stands as a proxy for the wet climate of interstadials between MIS5 and MIS3, potentially enabling hominin dispersal across these arid regions, but also as geographical barriers due to their large size. It is hoped that in the future, more detailed climate simulations of precipitation for MIS3 and MIS5 will enable the production of more comprehensive interstadial base maps for more comprehensive analyses for these periods in the future.

The base topographic map used for both models was the HYDRO1K digital elevation model that has a resolution of 30 arc-seconds (~1km 2 ). The slope of the map was taken to calculate the cost ( S2 Table ) and reclassified to a scale that better reflects terrain difficulty costs for humans based on the assumption that steeper slopes are more difficult to traverse [ 45 ]. To simulate the LGM, regions with an average annual precipitation of less than <250mm, based on world climate models of the LGM, were treated as barriers, and given an appropriate value according to Field and Lahr [ 35 ] that would allow short movements across these regions; we believe this would mirror the ability for populations to traverse arid regions [ 7 ].

Dates from the region suggest the earliest IUP appears during MIS3. These maps provide the ‘cost surfaces’ for the Least Cost Path analysis to determine the most effective route of this arrival. To create these cost surfaces, we first assembled spatial palaeolandscapes using topographic data overlain with modern and published palaeolake extents for the MIS3 model and LGM glacier extents and LGM precipitation estimates for the ‘glacial’ model.

Two palaeogeographic maps were produced as a base for the Least Cost Path. One simulated a dry ‘glacial’ climate using precipitation data from WorldClim’s downscaled Last Glacial Maximum (LGM) simulation from the Community Climate System Model 4.0 (CCSM4). We use the LGM model as it provides the best, most complete framework for estimating the climatic impacts of glacial conditions during MIS4 for the region [ 44 ]. The other model simulated ‘interstadial’ conditions but, due to a lack of suitably detailed precipitation models for MIS5 and MIS3 in the region we focused on relevant, dated topographic and geographic features that may have influenced dispersal during these periods (see below). As a working hypothesis, we assume that the presence of these features, as well as palaeoenvironmental datasets (see below), indicates that aridity did not act as a barrier in these periods.

A GIS approach provides a computational understanding of the best routes to travel across the vast region of Central, northern, and eastern Asia. Specifically, this GIS approach looks to integrate archaeological, environmental, and geographic data in a spatial manner. The Least Cost Path analysis method in ArcGIS 10.5.1 provides an analysis of the route between two points across a rasterized cost surface terrain. Although it has been used extensively in ecology to explore the dispersal corridors and migration routes of animals [ 41 , 42 ], only a few macro-scale archaeological studies have employed this method in a Late Pleistocene context [ 7 , 35 , 36 , 43 ].

Results

‘Glacial’ conditions The Least Cost Path analysis simulation under ‘dry’ conditions (i.e. using Last Glacial Maximum (LGM) climate models produced a ‘Revised Overland’ Route that we expect to hold for conditions during both MIS4 and MIS2 (Fig 2, S3 Fig). The same trajectory Least Cost Path towards Shuidonggou was followed regardless which of the three starting point sites were used. The result largely coincides with the prevalent, previously-published Overland Route [24]. However, the Least Cost Path model produced here integrates Siberian topography and palaeoclimatic barriers to produce a more detailed model of potential human movement. The beginning of this route skirts the westernmost piedmonts of the Gissar Range of the Tian Shan Mountains, rather than directly moving eastwards through river valleys. Upon reaching the piedmonts of the Talas Alatau Range, the Least Cost Path moves east along the range, being channeled between the Karatau Mountains. At the Dzungarian Alatau it continues between Lake Balkash and Lake Alakol through to the Tabagatay Range before reaching the western edge of the Altai Mountains and crossing the Irtysh River. Moving northeast, the route rounds the Altai Mountains and moves into the Siberian Plain, heading towards the Yeniseri River. From this point onwards, the route continues into the plains adjacent to the Eastern Sayan Mountains, before passing the edge of Lake Baikal and the mouth of the Angara River. As the route passes to the eastern side of Lake Baikal, it travels through a number of valleys of the Reka Ingoda River, which channel the route further east than Shuidonggou itself. The Least Cost Path turns south upon reaching Julun Lake and the Ergon River, passing the Buir and Dalinur Lakes before reaching the Taihang Mountains. The ‘Revised Overland’ Route then curves west before crossing the Yellow River and travelling parallel to the Jilantai Basin before it finally reaches the site of Shuidonggou. One of the main factors driving this route to the north, as in the case of the original Overland Route, is the hyper-aridity throughout Central Asia during glacial periods also observed in a number of climate records [44] (Fig 3). As such, the route is driven along a relatively mountainous pathway, with short distances travelled across arid regions, such as the basin of the Tarbagatay Range and the Trans-Baikal. Within this mountainous context, LGM glacial extents present additional challenges, meaning that many valleys were likely inaccessible to hominin groups. The route is therefore dominated by mountain piedmonts, with only one glacial corridor used, that between the northern Tian Shan and Karatay Mountains. In Siberia, the glaciers northwest of Lake Baikal also force the route along the plains of the region, away from potential mountain routes. As this is a dry period, palaeolakes play a minor role in shaping the trajectory of this Least Cost Path. However, palaeolakes at Jinpeng-Dali Nor [62], Daihai Lake [63], Jilantai-Hetao Lake [64], and Salawusu [65] may have represented key sources of water and resources towards the end of this route.

‘Interstadial’ conditions Three Least Cost Path routes were the product of the simulation of “wet routes” (i.e., under postulated wet MIS5 and MIS3 conditions) (S3 Fig). We have named these routes the ‘Altai’ Route (Route I), the ‘Tian Shan’ Route (Route II), and the ‘Tarim’ Route (Route III), respectively. The ‘Altai’ Route simulates dispersal between the Russian Altai Mountains (using the site of Denisova as the center point) towards the Yellow River valley and the location of the Shuidonggou site in China. The ‘Tian Shan’ Route originates in the western Tian Shan Mountains, while the ‘Tarim’ Route demonstrates the best route from the Northwestern Pamir Mountains (Fig 2).

Route I: ‘Altai’ Route This route begins by following the modern day Anuy Valley before joining the tributaries of the Charysh River. None of the rivers are large enough water bodies to be considered barriers to movement in our model. The Least Cost Path then descends into the catchment of the Irtysh River of Kazakhstan (Fig 4A). Once it reaches an impassable width of the river and Zaysan Lake, it skirts the southern end of the Altai Mountain chain before crossing a passable section of the Irtysh and entering northern China and the Junggar Basin. Staying between the Altai Mountains and the Ulungur and Jili lakes, the path descends towards the southeast of the basin and enters the Gobi-Altai region. Before exiting the Junggar Basin, the route passes between the Bogda and Baitag Bogd Mountains. Eventually it passes through the Santanghu Basin between the terminations of the Tian Shan, the Karlik Mountains, as well as the Gobi-Altai, and Gurvan Saikhan Mountains. The Least Cost Path finally curves south to skirt along the boundaries of the northern rim of the Bei and then the Qilian Mountains, traversing through the Yabulai Mountains before ending in the valley of the Yellow River. PPT PowerPoint slide

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larger image TIFF original image Download: Fig 4. (A) Map of geographic features that guide or border the ‘Altai’ Route. (B) Map of geographic features that guide or border the ‘Tian Shan’ Route. The eastern part of the route after the Junggar Basin connects with the ‘Altai’ Route. (C) Map of geographic features that guide or border the ‘Tarim’ Route. The eastern part of the route after the Bei Mountains connects with the two other routes. Sites: 4. Obi-Rakhmat, 5. Shugnou, 8. Denisova, 9. Ust-Karakol, 10. Kara-Tenesh, 11. Kara-Bom, 12. Luotuoshi, 14. Gouxi, 15. Lenghu 1, 17. Chikhen Agui, 18. Tsagaan Agui, 19. Tolbor 4, 20. Kharganyn Gol 5, 21. Orkhon 1 & 7, 22. Makarovo 4, 23. Kandabaevo, 24. Varvarina Gora, 25. Tolbaga, 27. Shuidonggou 1, 28. Shuidonggou 9. Base map is HYDRO1K. https://doi.org/10.1371/journal.pone.0216433.g004 In addition to the basic topographic constraints, certain palaeoclimatic barriers guide the ‘Altai’ Route. For example, glacier extents during the LGM [46] (Fig 2) were at their largest, which had increased since MIS4 and MIS3 [48, 60] implying that ice and glacial dammed lakes, such as at Kanas River [66], Hoton Nuur and the Chuja Basin, would have covered the majority of the Altai Central Mountain complex [67, 68]. As these features would have been largely impassable, these features force the model to bypass around to the south. Glacial features also prevent this route ‘contacting’ the large palaeolake water bodies in Mongolia. Towards the end of the route, the palaeolakes of Balikun [69], the Ejina Basin [70], and Yabrai and Zhuye [71], would have represented potential corridors for movement. Indeed, the Jilantai-Hetao megalake [64], would have provided a key final conduit for final movement through a relatively arid region to the site of Shuidonggou. A lack of archaeological survey and excavation along much of this route means that confirming humans may have taken such a path will require future work. At the beginning of the route, there are a series of significant Late Pleistocene sites in the Altai region, including Denisova cave, Kara-Tesh, Kara Bom, and Ust-Karakol [72, 73]. All of these sites have IUP lithic material in them, and some, such as Kara Bom and Denisova, also possess Middle Palaeolithic artefacts [3]. Overall, however, there is little known IUP archaeological evidence along most of the remainder of the Altai Route. One exception is the surface site of Luotuoshi [72], which contains typologically IUP assemblages, and is situated close to Manas Lake. Two additional archaeological sites, Chikhen Agui and Tsaagan Agui, within 400-600km of the Least Cost Path, and on the Altai Mountain chain, have IUP lithic assemblages [38, 73, 74].

Route II: ‘Tian Shan’ Route This route begins along the Charvak Basin of the Talas Alatau range (Fig 4B). Its topographic path then descends along the Chirchiq River Valley, eventually crossing the river itself before curving northeast around the Talas Alatau. The Least Cost Path is constrained between the Karatau Mountains and the Kyrgyz Alatau before crossing the modern day Betpaqdala Desert. Continuing east, this model crosses the Ili River, south of Lake Balkhash and east of the Junggar Alatau. Much of this region is flat with limited elevation changes leading to a linear trajectory towards, and then around, the Junggar Alatau. At the western edge of Lake Alakol the pathway shifts to the north of this water body before heading through the valley between the Tarbagatay Range and Junggar Alatau, avoiding the Dzungarian Gate. When the model enters the Junggar Basin and reaches the Ailike and Manas Lakes, it takes a linear route to Shuiddongou. It is at this point, where the Chinese provinces of Xinjiang and Inner Mongolia intersect with Mongolia, that this model meets and follows the Altai Route. In the context of palaeoclimatic features, the direction of the model is reinforced by the fact that the steep slopes of the high altitude Tian Shan Mountains would have been glaciated through the Late Pleistocene, with an equilibrium line (ELA) of ~1300m a.s.l [47] (Fig 4). Similarly, glaciation extents within the Dzungarian Alatau would have further encouraged the valley trajectories of the Tian Shan Route described here. The initial part of this route only passes one potential palaeolake at Manas Lake [75]. While no published MIS3 extents of this lake are currently available, the Least Cost Path does pass by this water body, and would have its direction modified should the lake have been larger during the Late Pleistocene. The majority of the Dzungarian Basin covered by this route is currently desert or hyper-arid, suggesting that most modern lakes are a fraction of the size that they might have been at various points in the past.