Thursday 10 December 2020

Evidence for two separate dispersals of Neanderthals into southern Siberia

The period of existence of Neanderthals, their geographical range, and the timing of their dispersal and extinction are key issues in the study of human evolution and migration. Most Neanderthal remains and associated artifacts have been reported from Europe and western Asia, where they range in age from about 430 000 to 40 000 years ago. Further east, the unequivocal presence of Neanderthals prior to the last interglacial (which began around 130 000) until about 50 000 is based on Hominin remains and DNA analyses of skeletal remains and sediments at three caves (Okladnikov, Denisova, and Chagyrskaya) in the Altai Mountains of southern Siberia. Additional evidence is required to support suggestions that Neanderthals had reached eastern and northern China by 125 000 to 105 000 and 45 000, respectively. Two genetically distinct Neanderthal populations inhabited the Altai region sometime during the Late Pleistocene, but the geographical origin of these populations and the timing of their migrations into the region remain unclear. On current evidence, Neanderthals were present at Denisova Cave between about 200 000 and 100 000 years ago.

In a paper published in the Proceedings of the National Academy of Sciences of the United States of  America on 27 January 2020, a team of scientists led by Kseniya Kolobova of the Institute of Archaeology and Ethnography of the Siberian Branch of the Russian Academy of Sciences, and Richard Roberts of the Centre for Archaeological Science at the University of Wollongong, and the Australian Research Council Centre of Excellence for Australian Biodiversity and Heritage, present the results of a study which sought to identify the most probable ancestral source region for the Neanderthals of Chagyrskaya Cave in the Altai foothills of southern Siberia.

Chagyrskaya Cave s situated 19 m above the Charysh River in the western piedmont of the Altai Mountains, approximately 100 km west of Denisova Cave. The cave consists of two chambers, with a stratigraphic sequence up to 3.5 m thick. The dense basal deposit (layer 7) is archaeologically sterile and composed mainly of gravel and fine-grained sediments. An erosional contact (unconformity) separates it from overlying layers 6 and 5, which consist of poorly sorted sediments that contain approximately 90 000 Middle Palaeolithic artifacts (including numerous bone tools), 74 Neanderthal specimens, about 250 000 Animal fossils, and a range of Plant remains. The sequence is capped by Bronze Age deposits, with no evidence of Upper Palaeolithic occupation.

Chagyrskaya Cave. (A) Site location in the Altai region of southern Siberia. (B) View of the cave entrance, which faces north. (C) Plan of the cave interior showing the excavated area (in blue). (D) and (E) Stratigraphic profiles along the two transects (A)–(A′) and (B)–(B′), respectively) shown in (C). Kolobova et al. (2020).

Optical ages for 23 sediment samples indicate that layer 7 was deposited 329 000 ± 16 000 years ago (weighted mean age of four samples) and that layers 6 and 5 accumulated sometime between 63 000 ± 4 000 and 48 000 years ago (weighted mean age of 54 000 ± 2500 years ago). The latter are consistent with the mostly infinite radiocarbon ages obtained for 20 Bison, Bison priscus, remains. but are younger than the DNA-based age estimate of 87 000 to 71 000 years for the 'Chagyrskaya Neanderthal' (Chagyrskaya 8), a distal manual phalanx retrieved from the sieved sediments of subunit 6b. This discrepancy may reflect a higher mutation rate in Neanderthals than in modern Humans that has not been taken into account and/or the omission of other uncertainties in the genetic age estimates, such as population size and generation interval. Chagyrskaya 8 and Denisova 3 (the youngest Denisovan fossil) share similar proportions of 'missing' genetic mutations compared to present-day Humans, which suggests that they are similar in age. Denisova 3 has been dated to between 76 200 and 51 600 years ago, an age range compatible with the optical ages for layers 6 and 5 at Chagyrskaya Cave.

(A) Location map of Chagyrskaya Cave, Okladnikov Cave, Denisova Cave, Ust’-Karakol-1 and Kara-Bom in the Altai Mountains. (B) View upstream (i.e., to the southeast) along the Charysh River valley, with the location of Chagyrskaya Cave indicated by the arrow. Kolobova et al. (2020).

The subunits of layer 6 have statistically indistinguishable ages, which indicate that these Neanderthal-associated Middle Palaeolithic deposits accumulated over a few millennia or less, during the final phase of Marine Isotope Stage 4 and/or the start of Marine Isotope Stage 3. This was a period of cold climate (but warmer than during Marine Isotope Stage 4), as indicated by pollen and Mammal and Bird remains from layer 6 compatible with a dry steppe environment and a rarity or lack of tundra species. Layer 5 was deposited during a period of relatively warm and humid climate, characterised by steppe and forest-steppe vegetation. The main Hominin occupation of Chagyrskaya Cave occurred during accumulation of subunit 6c, which represents the primary depositional context of the Middle Palaeolithic assemblage; subunits 6b and 6a and layer 5 include redeposited Middle Palaeolithic artifacts, bones, and sediments. Sedimentology and micromorphology analyses support these interpretations of cave use, site formation, and environmental conditions. Neanderthal hunting activity was focused on bison (juveniles and females in particular) and may have been connected to the seasonal migration of Bison herds to and from the mountain foothills. Other prey hunted to a lesser extent included Horse, Reindeer, Siberian Ibex, and Argali.

(A) Plan map of Chagyrskaya Cave showing excavation squares, locations of optical dating samples collected in 2012, 2014 and 2017 (CHAG12-1 to -10, CHAG14-2 to -12 and CHAG17-3 to -9, shown in orange) and locations of micromorphology samples collected in 2014 (MM2–4, shown in green) and 2017 (2969, 2970, 2984, 2985, 2987–2989, shown in blue). The area shaded in yellow was excavated in 2016 and 2017. (B) Stratigraphic profile along the purple line in panel (A). (C) Stratigraphic profile along the red line in panel (A). Both vertical scales are in cm below reference datum, the horizontal scale is in bottom right-hand corner, and the layer, subunit and sublayer numbers are circled. Kolobova et al. (2020).

A total of 89 539 artifacts have been recovered from layer 6. Detailed lithic analysis of 4249 artifacts from subunits 6a to 6c indicates that the assemblage represents a single technocomplex, with no marked differences between subunits. Subunit 6c consists of two sublayers (6c/2 and 6c/1) with indistinguishable ages. Sublayer 6c/1 contains more artifacts than does sublayer 6c/2, which is preserved in only a few, spatially restricted, parts of the site. Accordingly, we focus here on the technological and typological characteristics of the 3021 artifacts from sublayer 6c/1 and on the morphological variability of plano-convex bifacial tools (commonly retouched using bone) and convergent scrapers in particular.

Stone artifacts from Chagyrskaya Cave, sublayer 6c/1. (A)–(C) Photographs, line drawings, and cross-sectional profiles of three plano-convex bifacial tools diagnostic of Micoquian Bocksteinmesser and Klausennischemesser types. Scale bar is 5 cm. Kolobova et al. (2020).

The lithic assemblage consists of 25 raw materials, including high-quality jaspers, chalcedonites, and porphyrites, which were sourced as pebbles from the nearby riverbed. The assemblage is dominated by debris and chips, with the remaining artifacts characterised by a high proportion of tools and a few cores. Most of the flakes have asymmetrical trapezoidal and rectangular shapes and were manufactured on site using bifacial plano-convex, radial (Levallois centripetal), and orthogonal core-reduction flaking methods; blades occur in low numbers as occasional byproducts. Scrapers dominate the toolkit, with a preference for trapezoidal and leaf shapes.

Photo montage of micromorphology samples collected by Maciej Krajcarz. in 2017. panels (A)–(D), and by Mike Morley in 2014, panels (E)–(G). Kolobova et al. (2020).

The Chagyrskaya toolkit had been previously been grouped with the small artifact assemblage from Okladnikov Cave and named the Sibiryachikha variant, Only the remains of Neanderthals have been found in association with this variant, whereas the assemblages found at Denisova Cave and at the open-air sites of Kara-Bom and Ust’-Karakol-1 cannot be related unambiguously to a specific Hominin species. Both Neanderthals and Denisovans (a genetically related group of archaic Hominins) were present at Denisova Cave during the Middle Palaeolithic, while Kara-Bom and Ust’-Karakol-1 have not yielded any Hominin remains. These assemblages reflect the local development of Levallois-based industries with Mousterian features, and they differ markedly from the Sibiryachikha variant, which is dominated by bifacial plano-convex, radial, and orthogonal flaking methods; bifacial tools; convergent scrapers and points; and the absence of Levallois preferential and Levallois convergent core reductions.

Overview of the Human remains from Chagyrskaya Cave. The teeth and postcranial remains are not to scale. (A) Remains from the northern cluster, squares К6, К7 and Л6. (B) Remains from the southern cluster, squares Н10 and Н11. (C) Remains from outside the two clusters. Kolobova et al. (2020).

Similarities between Middle Palaeolithic artifacts and associated Hominin remains in the Altai, central Asia, and eastern Europe have been proposed, but limitations in the archaeological and fossil records have precluded firm conclusions. Kolobova et al. used a set of statistical methods (including hierarchical cluster analysis, nonmetric multidimensional scaling, and principal component analysis) to compare the technological and typological attributes of the Chagyrskaya artifacts with Levallois-Mousterian Middle Palaeolithic and Upper Palaeolithic assemblages in central Asia, and with Micoquian assemblages in central and eastern Europe. The analysis clearly distinguishes the Chagyrskaya assemblage from the central Asian Levallois-Mousterian Middle Palaeolithic and Upper Palaeolithic assemblages. Analysis of debitage that has been examined using a technological approach yields the same outcome. This suggests that the Chagyrskaya assemblage and that from Okladnikov Cave, the age of which is uncertain but is likely also younger than that of the Denisova Neanderthals, constitute a separate and unique regional Middle Palaeolithic variant, technologically and typologically distinct from the Altai Levallois-Mousterian technocomplex.

Location of sites with Levallois-Mousterian and Micoquian assemblages used for statistical comparison with Chagyrskaya Cave artifacts. Kolobova et al. (2020).

There are strong similarities between the Chagyrskaya assemblage and the Micoquian/Keilmessergruppen technocomplex, which is based on non-Levallois corereduction flaking methods and bifacial tool production using plano-convex methods. Micoquian/Keilmessergruppen sites dating to between approximately 130 000 and 30 000 years ago have been found across central and eastern Europe. Statistical analysis of the Micoquian/Keilmessergruppen and Chagyrskaya assemblages (using the same methods as above) demonstrates a uniformity of the European assemblages grouped by 26 variables, the most significant of which are related to the plano-convex technological and typological characteristics. All assemblages from Chagyrskaya and central/eastern Europe contain bifacial tools diagnostic of the Micoquian, such as the Bocksteinmesser and Klausennischemesser types. The Chagyrskaya assemblage fits most closely with the eastern European Micoquian complexes.

Selected isolated teeth from Chagyrskaya Cave. (A), (B) Chagyrskaya 13 I1 in occlusal and labial view. Note the pronounced shoveling and large basal tubercle. (C), (D) Chagyrskaya 12 P3 in occlusal and mesial view. (E), (F) Chagyrskaya 14 P4 in occlusal and distal view. (G), (H) Chagyrskaya 41 P3 in occlusal and distal view. (I), (J) Chagyrskaya 50 P3 in occlusal and mesial view. Kolobova et al. (2020).

The pronounced similarity of European Micoquian and Chagyrskaya bifacial tools is also supported by a geometric morphometric shape analysis of bifaces, including Bocksteinmesser and Klausennischemesser types, from the key Micoquian/Keilmessergruppen site of Sesselfelsgrotte (Germany) and Chagyrskaya Cave. This result suggests the existence of a common design in the technological concept of bifacial production of Micoquian/Keilmessergruppen complexes.

Chagyrskaya 6 mandible fragment preserving right C–M2. (A) Buccal view; note the relatively posterior position of the mental foramen. (B) Lingual view and (C) occlusal view. Kolobova et al. (2020).

Chagyrskaya 8 genetically resembles Neanderthals from northern Croatia and the northern Caucasus, Vindija 33.19 (radiocarbon age approximately 48 000 years before present), and Mezmaiskaya 1 (electron spin resonance ages around 70 000 to 55 000 years). The associated Middle Palaeolithic assemblage at Vindija Cave is in questionable stratigraphic context, but the Micoquian assemblage at Mezmaiskaya Cave is characterized by numerous plano-convex bifacial tools, including Bocksteinmesser bifaces, numerous convergent scrapers, retouched/Mousterian points, and angled scrapers. Kolobova et al. compared the Chagyrskaya assemblage with the combined European Micoquian (including Mezmaiskaya) and Altai/Central Asian Middle Palaeolithic and Upper Palaeolithic datasets using a more limited number of technological and typological variables common to these technocomplexes. The Chagyrskaya and European Micoquian assemblages cluster together, and the Altai/Central Asian assemblages form a separate cluster.

Core, core preparation blanks and tools from Chagyrskaya Cave (subunit 6a): straight ventral scraper with natural back (1), overpassed bifacial thinning flake (2), bifacial thinning flake (3), semi-crescent dorsal scraper (4), sub-trapezoidal alternate scraper (5), semi-leaf dorsal, thinned base point (6), bifacial scraper straight, thinned base, naturally back (7), unidentifiable bifacial fragment (8), radial core (9), semi-leaf dorsal, thinned back scraper (10). Kolobova et al. (2020).

The Chagyrskaya assemblage and the European Micoquian technocomplex overlap chronologically between about 59 000 and 49 000 years ago and have strong technological and morphological similarities. The Chagyrskaya assemblage can therefore be viewed as a southern Siberian variant of the European Micoquian, and the Sibiryachikha variant seen more broadly as an expression of Micoquian variability across Eurasia. Micoquian populations are commonly considered specialised Horse and Bison hunters, adapted to steppe and piedmont environments. Kolobova et al. attribute their presence in the Altai to the eastward migration of Neanderthals from eastern Europe along the Eurasian steppe belt during the cold and arid conditions of Marine Isotope Stage 4. 

Core and tools from Chagyrskaya Cave (subunit 6b): semi-leaf alternate point (1), unidentifiable convergent bifacial scraper (2), semi-crescent dorsal thinned base scraper (3), sub-trapezoidal alternate scraper (4), semi-trapezoidal point (5), semi-triangular dorsal thinned base point (6), semi-triangular dorsal point (7), sub-trapezoidal alternate scraper (8, 11), sub-trapezoidal dorsal thinned base scraper (9), orthogonal core (10). Kolobova et al. (2020).

DNA recovered from Human remains and sediments indicates that Neanderthals first appeared in the Altai before or during Marine Isotope Stage 5. These early populations are not associated with Micoquian artifacts, which appear at Chagyrskaya only toward the end of Marine Isotope Stage 4 or the start of Marine Isotope Stage 3. It is not possible to distinguish Neanderthal from Denisovan technocomplexes in the cultural sequence at Denisova Cave due to the homogeneous technological and typological characteristics of the lithic assemblages. However, the absence of Micoquian-like artifacts at Denisova Cave in deposits dated to between 59 000 and 49 000 years ago indicates that Denisova and Chagyrskaya Caves were occupied by two distinct Neanderthal populations, most likely at different times given current evidence that Neanderthals were present at Denisova Cave much earlier than at Chagyrskaya Cave. Genetic data from Denisova Cave have also revealed several episodes of gene flow between Neanderthals and Modern Humans  and two different Neanderthal components in Denisova 11, the Neanderthal-Denisovan offspring. 

Cores from Chagyrskaya Cave (sublayer 6c/1): radial core (1), orthogonal cores (2), (3). Kolobova et al. (2020).

Kolobova et al. therefore propose that Neanderthals entered southern Siberia on at least two separate occasions, with the most recent incursion originating in eastern Europe and the northern Caucasus, which lie 3000 to 4000 kilometers to the west of Chagyrskaya Cave. The identification of Micoquian assemblages in all three regions is consistent with the genetic similarities between Neanderthal remains at Chagyrskaya, Vindija, and Mezmaiskaya Caves. Kolobova et al.'s archaeological data supports a rarely observed case of long-distance demic dispersal in the Palaeolithic and illustrate that artifacts are culturally informative markers of ancient population movements.

Core preparation blanks from Chagyrskaya Cave (sublayer 6c/1): crested débordant flake (1), débordant flake from radial core (2), bifacial thinning flakes (3, 4), cortical débordant flake (5), technical flake (6), lateral débordant flake (7), débordant flake from radial core/pseudo-Levallois point (8). Kolobova et al. (2020).

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