Earth and Planetary Science Letters 494 (2018) 148–152

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Earth and Planetary Science Letters

www.elsevier.com/locate/epsl

Hominin expansion into Central Asia during the last interglacial

Yemane Asmerom a,∗, Victor J. Polyak a, Jennifer D.M. Wagner b, P. Jonathan Patchett b

a Department of Earth & Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USAb Department of Geosciences, University of Arizona, Tucson, AZ 85721, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 27 December 2017Received in revised form 16 April 2018Accepted 29 April 2018Available online 17 May 2018Editor: J. Adkins

Keywords:isotope geochemistryU-series chronologypaleoclimategeoarcheology

Central Asia was a likely gateway for hominin expansion. Obi-Rakhmat Grotto, northeastern Uzbekistan, has a deposit that contains the remains of a hominin child (OR-1) and more than 60,000 Paleolithic artifacts, making it one of the most important archaeological sites in Central Asia. Regionally, genetic data suggest hominin presence and human-Neanderthal interbreeding as far back as 100 ka, although, to date, absolute ages of hominin remains that old have not been known in the region. We present new uranium-series dates that constrain the age of the Obi-Rakhmat deposit to 98 ka towards the top and 109 ka for the layer containing OR-1, pushing hominin expansion into Central Asia minimally to 109 ± 2 ka, to a time, based on the genetic clock, of Neanderthal-modern human interbreeding. Our chronology establishes a minimum age of 98 ka for the well-developed lithic industry in Central Asia. The period covering the deposition of the sediments containing the artifacts and specimen OR-1 coincides with a warm and one of the most humid intervals of Marine Isotope Stage (MIS) 5 and last glacial period in the region, providing support for climate-driven hominin expansion.

© 2018 Elsevier B.V. All rights reserved.

1. Introduction

Some of the early hominin finds in Central Asia come from eastern Uzbekistan (Fig. 1) with the discovery of the Teshik-Tash child in 1938 (Okladnikov, 1939) and the discovery of the Obi Rakhmat juvenile hominin (OR-1) in 2003 (Glantz et al., 2008). The Obi-Rakhmat rock shelter is located on the western Tien Shan Mountains of Uzbekistan (41◦34′08.8′′N, 70◦08′00.3′′E) and was first noted as an archaeological site in 1962 (Derevianko et al., 2001). The grotto is formed in a Paleozoic silicified limestone (Mallol et al., 2009). The excavated section consists of 10 m of calcareous and clay-rich sediments interbedded with freshwater tufa/travertine, herein referred to as travertine (Fig. 2).

The sequence is subdivided into 22 stratigraphic layers (L1–L22) (Fig. 2) based on texture and differences in types and abundance of fauna and artifacts (Mallol et al., 2009). The OR-1 remains, con-tained within layer 16 (L16), consist of 6 isolated permanent max-illary teeth and 121 cranial fragments of a child (OR-1) between 9–12 yr old (Bailey et al., 2008; Glantz et al., 2008).

Extensive dental morphometric analysis data on the OR-1 teeth suggest a Neanderthal lineage (Bailey et al., 2008), although the data fall on the periphery of the distribution of primary Euro-pean Neanderthal lines (Glantz et al., 2009). Of the 121 cranial

* Corresponding author.E-mail address: asmerom @unm .edu (Y. Asmerom).

https://doi.org/10.1016/j.epsl.2018.04.0590012-821X/© 2018 Elsevier B.V. All rights reserved.

fragments of OR-1, 27 fragments were able to provide useful di-agnostic information, several fragments of which are suggestive of modern human affinity, making attribution ambiguous. Glantz et al. state, “. . . we are left with either identifying OR-1 as a Nean-dertal or modern human” (Glantz et al., 2008). In addition to OR-1, the site contains a rich collection of Paleolithic artifacts, more than 60,000 in total, along with more than 3000 non-hominin bones (Krivoshapkin et al., 2006; Wrinn et al., 2004a). Our understanding of the modes of lithic industrial development locally and their re-lationship to other lithic industries in other regions hinges heavily on knowing the age of the deposits. Similarly, understanding early hominin population evolution and climate dynamics requires plac-ing precise chronologic constraints on the archaeological materials, given the rapid shifts in climate during the last glacial cycle and earlier (Cheng et al., 2016b).

A number of attempts were made to develop absolute chronol-ogy for the Obi Rakhmat site, both on the bounding travertine units and teeth, with all of those attempts giving disparate results. Radiocarbon dates (Krivoshapkin et al., 2010) on charcoal from upper layers of the site L4–L10 yielded a mixture of ages (Sup-plementary Fig. S1) with poor stratigraphic coherence, from top to bottom (L4 = 19.7 ka, L7 = 36.2 ka, L7(2) = 41.4 ka, L8.1 = 9.7 ka, L8.2 = 44.0 ka, L8.2(2) = 29.3 ka, L9 = 42.1 ka, L10 = 18.6 ka). The bottom layers, including L16 which contains OR-1 were said to be beyond their analytical resolution of about 48 ka (Krivoshapkin et al., 2010). See Supplementary Materials section for further dis-cussion on the radiocarbon ages (Supplementary Fig. S1). Electron

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Fig. 1. Regional setting of Obi-Rakhmat rock shelter, northeastern Uzbekistan. Paleoclimate data for the time of interest come from previous studies (Cheng et al., 2012, 2016b) of two sites bracketing the region, Kesang Cave, China and Tonnel’naya Cave, Uzbekistan.

Fig. 2. The Obi-Rakhmat section [after Mallol et al., 2009], showing the relative po-sitions of the travertine units and the location of the remains of OR-1. Summaries of the uranium-series dates obtained with replicates and their weighted means are shown on the left of the column. A 3 dimensional isochron age based on 16 anal-yses of the top travertine section (Figs. 1 and 3; samples 4B, 5A, 6A, and 7) is 98.1 ± 0.8 ka (2-σ ), within error of the individual dates.

Spin Resonance (ESR) dating of BR-H teeth give a large spread of ages but hints at a combined old >70 ky age (Skinner et al., 2007). Earlier uranium-series data, done at the University of Ari-zona, on travertine between L8 and L12 also suggested much older ages than the radiocarbon results, between 88–97 ka (Wrinn et al., 2004b), even though the authors were tenuous about their results due to high 232Th content of the samples and sensitivity to initial (230Th/232Th) correction.

Given the resurgence in the interest of hominin expansion, especially interaction between Neanderthal and humans and the importance of Central Asia to this narrative, we reanalyzed the travertine samples used for the original uranium-series dating at the Radiogenic Isotope Laboratory, the University of New Mexico.

We were further motivated because of a number of recent tech-nical developments in uranium-series chronology: 1) new gains in efficiency in multi-collector inductively coupled mass spectrometry (MC-ICPMS; Neptune plus), making it possible to use much smaller fractions, allowing for better selection of domain. In addition, re-cent availability of high resolution climate records from the region (Cheng et al., 2012, 2016b), together with our new constraints on timing of hominin occupation, allow us to show the climatic back-drop for hominin presence at this site as well as expansion.

2. Analytical procedures

Purest looking pieces of the travertine samples were trimmed away, surfaces cleaned by etching in 3% nitric acid for 10 s, washed three times with 18 � water, and dried. Pieces larger than ∼200 mg were wrapped in clean room napkins and placed in sandwich-sized plastic bags, then lightly broken into smaller pieces with a mortar and pestle. These smaller pieces were transferred to cleaned 4-ml scintillation tubes for storage until used. Pieces selected for analyses ranging in weight between 20 and 120 mg were dissolved in 15 N nitric acid and spiked with a mixed solu-tion of 229Th–233U–236U. Drops of perchloric acid were added to help equilibrate the spike solution and destroy organic compounds in the sample. The column chemistry used 1 × 8, 400-mesh an-ion exchange resin and is described in Asmerom et al. (2013). All analyses were completed on a Neptune plus MC-ICPMS. All iso-tope signals were measured with Faraday cups except for 230Th and 234U, which were measured using a secondary electron mul-tiplier (SEM). U was separated from Th, and the isotopes of these elements were analyzed in a static mode (all masses of interest run simultaneously) as a U run and a Th run for each sample.

Travertine samples 5A-p1, 5A-p2, and 6A-p1 were sufficiently clean of detrital Th to produce robust uranium-series ages and anchor the age of the L8.1–8.3. Other travertine layers consist of minor to moderate amounts of silt and clay that introduces large signals of detrital 232Th. These large signals of detrital 232Th re-sult in tailing onto adjacent masses (abundance sensitivity) such as 230Th, and 229Th during measurements on the MC-ICPMS, which requires some corrections. These abundance corrections were mea-sured and determined using U or Th standards. To demonstrate the accuracy of our results we heavily overspiked two subsamples to reduce the 232Th/229Th ratio, changing the 232Th/229Th measured values from 1450 to 27 for example, so that any tailing into the 229Th Faraday cup would be miniscule for the latter, and given the abundance corrections are accurate, the ages will match within ab-solute 2-σ error to each other. The same was adjusted for the 235U and 233U signals. 230Th was measured using the SEM with an RPQ filter to eliminate tailing interference from 232Th. Gains between

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the Faraday cups and the SEM were measured using a NBL-112 U standard and an in-house 230Th–229Th Th standard. Procedu-ral blanks are <10 pg for Th and <30 pg for U. Decay constants are from Cheng et al. (2013). We measured a secular equilibrium in-house standard (GnC-SE). The 20 Ma calcite standard yielded a δ234U measured value of 0 ± 1�, and 230Th/238U activity of 0.9983 ± 0.0014.

Applying the typically used bulk silicate earth 230Th/232Th ini-tial atomic ratio value (4.4 ppm) for correction of the uranium-series dates yielded similar ages for the top four samples of ∼99 ka. 232Th/238U–230Th/238U–234U/238U 3-dimensional isochron ages from ISOPLOT (Ludwig, 2001) for each of the four top travertine samples, and from each individual sample (4 subsam-ples/sample) produced similar age results and suggest that the best 230Th/232Th initial atomic ratio value equals 5.7 ± 0.5 ppm (from 15 of 16 subsample results from samples 4B, 5A, 6A, and 7), or 6.7 ± 0.2 ppm (from 11 of 12 subsample results of samples 4B, 5A, and 6A). A value of 6 ±3 ppm was used for all individual dates reported in Supplementary Table S1.

3. Results

The detailed site description (Mallol et al., 2009) shows that travertine layers, syndepositional in the trench stratigraphy and containing bone fragments and lithic debitage, were deposited dur-ing periods of increased spring activity, and as such, accurate dat-ing of the travertine will provide an accurate chronology for the site. Six samples of travertine were reanalyzed. From the upper portion of the sequence, above a distinct breakdown layer (L15.5), we dated four samples, from within L5, L8.1–8.3, L8.3–9–10–11, and L12.1–12.2. We dated four individual subsamples, consisting of single clean chips (Supplementary Fig. S2 A, B, C), from each of the travertine fragments to show reproducibility. Below L15.5 we dated two travertine samples, one from the lower part of the trench in L18-19.1, and a second sample from a travertine within L16, which is the layer that contained OR-1. We dated five sub-samples, here again consisting of a single clean chip, from each of these lower layer samples. Each chip was handpicked under the microscope to select the fragment that was likely to have lower detrital thorium (no inclusions and coloration). Given the high ion-ization efficiency of the Neptune plus MC-ICPMS and the samples’ sufficiently high uranium content, we were able to use a single chip, 20 to 120 mg in weight, per analysis, making it possible to avoid non-pristine or clay-enriched chips (Supplementary Fig. S2 A, B, C). The selected fractions were processed using our standard uranium–thorium chemistry described in our Materials and Meth-ods section. Our results are shown in Fig. 2 and Supplementary Table S1. All the fractions have relatively high uranium concen-trations, between 0.7 and 1.8 ppm, and highly variable detrital thorium (232Th) content, from very low to high values of 0.06 to 1.8 ppm.

All the fractions analyzed have high and near uniform 234U en-richment, with initial δ234U of around 1200� (Table S1). The near-uniform δ234U values would suggest that the material analyzed was uniform as well and is unlikely to have incorporated bedrock material which would have contributed 0 � (secular equilibrium values) and caused more variable δ234U values in the subsamples analyzed. The low and well-behaved initial (230Th/232Th) (note: ra-tios in parentheses are activity ratios) are also consistent with the integrity of the samples analyzed.

4. Discussion and conclusion

The samples from the upper travertine section (Fig. 2) have very good spread in (238U/232Th) ratios (Supplementary Fig. S3), and provide an excellent 2-dimensional isochron with an initial

Fig. 3. 3-D, 232Th/238U–234U/238U–230Th/238U, isochrons, projected in 2-D. A. 3-D isochrons where constructed for each sample using the individual analyses of the fragments. Here we show the isochron for sample OBR-5A with an age of 96.03 ± 3.1 ka (2-σ ) within the range of the composite age for the upper layer (Fig. 3B). B. Composite 3-D Sixteen-point isochron for all of the fragments of all the samples from the upper part of the section (Fig. 2). Excluding some of the fractions gives more precise age, but we include all the fractions, giving an age of 98.55 ± 0.84 ka (2-σ ), and initial (230Th/232Th) of 0.89 ± 0.09. The 3-D ages in all cases were calculated and plotted using the computer program Isoplot (Ludwig, 2001).

(230Th/232Th) of 0.89 corresponding to in an initial 230Th/232Th atomic ratio of 5 × 10−6 (5 ppm). The samples from the bot-tom section have very limited (238Th/232Th) spread. We show the data in one diagram to illustrate the differences. Typically, 2-dimensional isochrons are used to constrain initial (230Th/232Th) and is good enough in most cases. But 3-dimensional (232Th/238U–234U/238U–230Th/238U) isochrons are preferred if the system is well-behaved because they take into account initial (234U/238U) variability. Combinations of samples from the upper section, where there is a significant spread in (238U/232Th) (Supplementary Fig. S3), give similar ages with the initial (230Th/232Th) centered around 1.05, corresponding to initial 230Th/232Th atomic ratio of 6 ppm. We have thus used a 230Th/232Th atomic ratio of 6 ±3 ppm for our initial correction. This is validated by a 3-dimensionalisochron (Fig. 3B) of all the samples in the upper section with an

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age of 98.6 ± 0.8 ka which generates an initial ratio corresponding to 230Th/232Th atomic ratio of 5.1 ppm, which is within the er-ror of our chosen initial ratio. Details of the correction procedures we followed are described in the Materials and Methods section. Using an initial 230Th/232Th atomic ratio of 6 ppm, the four sub-sample results from sample 7, the ‘dirtiest’ travertine, shifted from the uncorrected age of 123 ka to a corrected age of 98.72 ± 5.8 ka. The dates for sample 6 shifted from an uncorrected age of 100 ka to a corrected age of 98.3 ± 0.8 ka (Table S1). The salient point about the dates for sample 6 is that because these fractions are clean (contain very little detrital material), they are insensitive to the initial 230Th/232Th correction, within a reasonable range, and thus provide an anchor age of 98 ka for the upper layers.

The similar ages between layers indicate that the sedimenta-tion rate was relatively high in the upper section, at >2.5 m per 1000 yrs (>2.5 mm/yr), but considered a moderate rate of sedimentation for archaeological sites (Ferring, 1986). Sam-ple 2B, which comes from the layer that contains OR-1, L16-17, gives an uncorrected age of 117.05 ka and corrected weight-averaged age of 109.2 ± 1.6 ka. The sample from the lowest unit (L18-19.1), sample 1B, yielded a similar uncorrected 118.7 ka and corrected weight-averaged age of 107.1 ± 2.2 ka age, and both layers have ages within absolute 2σ error of each other. The 232Th/238U–230Th/238U–234U/238U 3-D isochron for the lower sec-tion units (Supplementary Fig. S4) gives a consistent age with larger uncertainty (110 ± 16 ky (2-σ )).

The travertine growth intervals represented by the new ura-nium-series ages coincide with a prominent moist interval of the Tien Shan (Fig. 4A) based on recent speleothem climate recon-struction from Kesang Cave, eastern Tien Shan, China (Cheng et al., 2012) and Tonnel’naya Cave, western Tien Shan, Uzbekistan (Cheng et al., 2016b). The study area lies in between these two caves and is thus represented well by the two climate records. There are four pluvial intervals in the region between the beginning of the last in-terglacial and 40 ka, and the interval associated with Obi-Rakhmat travertine growth falls within one of the longest and wettest in-tervals (Fig. 4A, B). These pluvial intervals coincide with deep pen-etration of the summer Asian monsoon in the western Tien Shan (Fig. 4C) (Cheng et al., 2012) and strengthening of the Westerly-derived winter precipitation in Central Asia (Cheng et al., 2016b;Kutzbach et al., 2014), associated with Northern Hemisphere sum-mer insolation peaks. The near-uniform initial δ234U of about 1200� is suggestive of a uniform moisture regime (Polyak et al., 2012) during the deposition of the entire section. Although the uranium-series chronology is robust, having the climatic coinci-dence that would explain high spring activity and travertine/sed-iment deposition rates provides an additional validation to our interpretation. Conversely, the combined data show that hominin expansion represented by the Obi Rakhmat site into Central Asia likely took place during a peak warm and wet interval of MIS-5. It is unlikely that this deposit captured the initial penetration of hominins into Central Asia, but it provides evidence that hominins were present in this part of the world by the middle of MIS-5, assigning the initial hominin biological and cultural evolution re-flected in this deposit minimally to older than 109 ± 2 kr BP. The interval of the deposit was one of the most humid intervals of the last glacial and MIS-5 periods in this region of Central Asia (Cheng et al., 2016b) (Fig. 4). To the extent that travertines are deposited during humid intervals, our data don’t rule out the pres-ence of hominins during the prior and subsequent arid intervals. But the presence of extensive artifacts and hominin and other an-imal remains during this prominent humid interval is consistent with the idea of climate modulated hominin expansion (Parton et al., 2015). The 60,000 lithic artifacts excavated from the 10 m deep trench (Glantz et al., 2008; Krivoshapkin et al., 2006) are similar to other lithic artifacts found in the Denisova and Ok-

Fig. 4. Climatic backdrop for the hominin expansion into Central Asia. A) Oxygen isotopic record (δ18O) for the last glacial and interglacial periods from Tonnel’-naya cave, Uzbekistan (Cheng et al., 2016b) to the west of Obi-Rakhmat (Fig. 1), composed of two samples (light and dark green) from the same cave. B) δ18O record from Kesang Cave (Cheng et al., 2012), the Tien Shan of China to the east of the study area. In both settings lighter δ18O values represent wetter conditions. Obi-Rakhmat travertine growth occurred during one of the most prominent humid intervals of the last glacial and MIS-5 periods. C) The humid intervals are associated with Northern Hemisphere insolation highs (shown here maximum summer insola-tion (July 1 65◦N). Variations in the Asian monsoon intensity (Cheng et al., 2016a)(black line) are shown (strengthening of the Asian monsoon indicated by more neg-ative δ18O values). (For interpretation of the colors in the figure(s), the reader is referred to the web version of this article.)

ladnikov caves of the Altai Mountains, Siberia (Derevianko et al., 2005). The Denisova Cave is an important site from which the bones that represent human introgression into the Neanderthal genome were discovered (Kuhlwilm et al., 2016; Prüfer et al., 2014). The correspondence between the extensive lithic indus-tries at the Obi Rakhmat settlement (Krivoshapkin et al., 2006;Derevianko et al., 2005), which we now have shown to be be-tween 98–109 ka, and the settlements in the Altai Mountains, suggests regional scale hominin interactions around the time of Neanderthal-human interbreeding suggested by the genetic clock.

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Recent findings indicating anatomically modern human occu-pation of southern China (Liu et al., 2015) and genetic data from modern Papuans showing incorporation of genome from extinct modern humans (Pagani et al., 2016) in the time-frame of OR-1, suggest anatomically modern human migration from Africa, pre-dating the later successful expansion, most likely through Central Asia during a period of ‘greening’.

Combined, these findings provide more strength to the possibil-ity of interaction between Neanderthal and modern humans in the Central Asia corridor at or before 109 ka. Our data provide abso-lute chronology placing hominins, in this case one with ambiguous lineage, as far east as Central Asia during a period of what looks like a time of early human penetration (Liu et al., 2015) and inter-action between humans and Neanderthals (Kuhlwilm et al., 2016;Prüfer et al., 2014).

Acknowledgements

This work was in part supported by grants from the US Na-tional Science Foundation (EAR-0326902 to Y. Asmerom) and a Leaky Foundation grant to J. Olsen, University of Arizona for field work. Support was also provided by the University of New Mexico and the University of Arizona.

Appendix A. Supplementary material

Supplementary material related to this article can be found on-line at https://doi .org /10 .1016 /j .epsl .2018 .04 .059.

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