Animal fibre use in the Keriya valley (Xinjiang, China) during the Bronze and Iron Ages: A proteomic approach
Introduction
Textile technology played a central role in the development of past societies (Good, 2001; Hardy, 2007). The first archaeological evidence for pelts dates back to 90,000–80,000 years ago, whereas the first evidence for twisted fibres appears as clay impressions 27,000 years ago (Hardy, 2007). The invention of looping and weaving opened up many possibilities for the creation of textile products, from nets to clothing. Starting in the Holocene, domestication and agriculture enabled the increase of textile production by supplying animal or vegetal fibres (Bender Jørgensen et al., 2018; Rast-Eicher, 2005). Therefore, the study of textiles can provide much information about resources, manufacturing techniques, trade, and culture through time. In contrast to other materials, such as ceramics, bone, or metal, textiles are very sensitive to degradation and are rarely preserved in archaeological contexts. Preservation occurs in specific environments, such as deserts, bogs, or permafrost (Gleba, 2011; Good, 2001; Hardy, 2007; Strand et al., 2010).
The preserved fibres are generally studied from the point of view of technology (of weaving, dyes, etc.), but their specific origin (vegetal or animal) is rarely studied in depth. A taxonomic determination of the animal or plant species at the origin of the textile is often made, but the rarity of the remains does not allow questions related to either the production methods of these fibres (organisation of livestock or crops) or the cultural choices underlying the activities to be addressed. Such questions are addressed using information from animal bones and teeth (often more numerous) or plant remains (generally rarer) found in archaeological sites, but it is difficult to use these proxies to define in detail the types of textiles produced and their use. When there are many textiles on a site, we can increase the number of taxonomic identifications and directly acquire information on the animals, plants, herds, and fields that are at their origin.
Generally, fibre identification is undertaken by microscopy. The morphological observation of animal fibres by optical and scanning electron microscopy normally permits specialists to clearly separate animal families. To achieve this, they focus on different parameters, such as the colour and diameter of the fibre, the morphology of the scales, and the structure of the medulla (Houck, 2010; Thomas et al., 2012; Rast-Eicher, 2016). However, alteration of the fibre due to diagenesis can make these techniques inapplicable to archaeological remains. In addition, these techniques cannot always differentiate between certain related species, such as sheep and goat. More importantly, wool characteristics have changed through time, so the general morphology of ancient fibre may not correspond to that of modern wool from the same species (Rast-Eicher, 2016). For any one of these reasons, the differentiation of archaeological fibres can be very challenging.
Palaeoproteomics has recently emerged as a powerful method to characterise the fraction of proteins preserved over time, providing a molecular signature related to the nature and taxonomy of archaeological samples (Cappellini et al., 2014; Hendy et al., 2018). This approach involves the hydrolysis of protein extracts, followed by analysis by mass spectrometry. It can generate diagnostic peptides that help discriminate among animal species, through their m/z value and fragmentation pattern related to their amino acid sequence (Cleland and Schroeter, 2018). The main proteins present in hair and fur are keratins. They constitute a large family of proteins forming heteropolymeric filaments (Plowman, 2007; Schweizer et al., 2006) incrusted into a matrix of keratin-associated proteins (KAPs) that stabilise the filament structure through extensive disulphide bonding (Gong et al., 2012; Marshall et al., 1991). Keratins are classified into two families: type I keratins (40–55 kDa), which are acidic, and type II keratins (55–70 kDa), which are basic or neutral. The revised nomenclature of mammalian keratins names type I keratins from K31 to K40 and type II keratins from K81 to K87 (Schweizer et al., 2006). Keratin K33 generally exists as two isoforms, named K33a and K33b. However, the annotation of keratins in databases is often heterogeneous or related to previous classifications. Thus, wool microfibrillar keratins are often named based on the old nomenclature, which relies on electrophoresis separation: components 8C-1, 8C-2, 8A and 8B for type I and components 5, 7A, 7B, and 7C for type II (Plowman, 2003; Miao et al., 2018). A correspondence between the nomenclatures is provided in the Supplementary Material Table S1. Proteomic analysis of hair, fur, and textiles was first conducted on modern samples to assess the quality of textile products (Hollemeyer et al., 2002; Hollemeyer and Heinzle, 2007; Clerens et al., 2010; Plowman et al., 2012, 2018; Paolella et al., 2013; Vineis et al., 2014; Li et al., 2016). Analysis of ancient samples in order to characterise remains of Neolithic human hair and clothes (Hollemeyer et al., 2008, 2012; Fresnais et al., 2017) and for other, archaeozoological applications (Solazzo, 2017; Solazzo et al., 2017, 2014, 2013, 2011) has permitted researchers to refine taxonomic attributions, in some cases increasing knowledge of the fabrication of ancient textiles (Fresnais et al., 2017; Solazzo et al., 2011). The high sequence similarity between keratins belonging to the same type and between proteins of closely related species makes keratin identification difficult (Plowman, 2007). Nevertheless, diagnostic peptides have been proposed to differentiate between related animal species, such as a fragment of K33, which differs by only one amino acid between goat and sheep (Solazzo et al., 2014, 2013). These markers are not always detected in proteomic studies of archaeological fibres, which leaves many unidentified samples. New markers and identification keys are essential for the improvement of archaeological fibre identification.
Here we present the results of a proteomic study aimed at identifying ancient fibres from the Xinjiang region at the species level. Xinjiang is an arid region of northwestern China particularly rich in well-preserved organic remains, including textiles and furs (Wang, B., 1999; Good, 1998; Keller et al., 2001; Liu et al., 2011; Mallory and Mair, 2000; Zhao, 2004, 2002, 2001, 1999).
Located in the eastern part of Central Asia, Xinjiang was a nodal point within the large Eurasian sphere of contact and interaction known as the Silk Roads, and within their forerunners. Many sites from the Bronze Age and Iron Age have been discovered so far (Chen and Hiebert, 1995; Jia et al., 2009; 2010), the oldest dating to around 2500-2000 BCE. Isotopic analyses on vegetal remains made it possible to enlighten us on human diet, subsistence strategies, and cross-cultural contacts from the Late Bronze Age to early historic times, both in northern and in southern Xinjiang (Wang et al., 2017a . Wang et al., 2017b). However, the textile remains found to date come mainly from eastern Xinjiang (the Wupu cemetery, near Hami); from the Lopnor region (including the Xiaohe and Gumugou cemeteries) (Li, 2007; Liang et al., 2012; Qiu et al., 2014; Qu et al., 2017; Wang, 2014; Yang et al., 2014a) and from the Tarim Basin, in both the Turfan region (including the Yanghai and Subeixi cemeteries) and the southern part of the Taklamakan desert (notably the Keriya Valley and the Chärchan area) (Wang, B., 1999; Good, 1998; Wang et al., 2016).
We focused on two sites in the Tarim basin (southern Xinjiang), where large corpuses of textiles and furs have been unearthed: Djoumboulak Koum (also called Yuansha gucheng), which is an Iron Age fortified settlement, and the Northern Cemetery, which dates from the Bronze Age (Fig. 1) (Debaine-Francfort and Idriss, 2001; Debaine-Francfort, 2013). Discovered by the Sino-French Archaeological Mission in Xinjiang, both are located in the now-dry protohistoric delta of the Keriya River, which crosses the Taklamakan desert from south to north. Both have yielded large corpuses of textiles and furs that are being studied in a multidisciplinary way. Our research not only aims to discriminate between native and exogenous fauna, it also focusses on the methods used to process the fibres and initiates wider research on the evolution of fleeces and textiles from the Bronze Age to the Iron Age and a comparison with contemporaneous fleeces and textiles found in Europe. This information is cross-referenced with that provided by archaeozoology to supplement our knowledge on the history of breeding, transhumance practices, and hunting. This work, which has already revealed the complexity and diversity of weaving techniques and dying practices, suggests that the textile industry had a major importance in the ancient societies of the Keriya valley (and beyond) (Cardon et al., 2013; Debaine-Francfort and Idriss, 2001). The often poor state of preservation of the fibres, which is sometimes insufficient to enable their taxonomic determination using microscopy, has led us to adopt non-zooarchaeological analytical methods to overcome this taphonomic handicap and to thus address unanswered questions about the raw materials used in the manufacture of these textiles.
In this article, we report on how a proteomics approach was used on raw fibres and textiles in order to identify the origin of animal fibres from these two archaeological sites. The taxonomic specificity of keratins was evaluated on a large corpus of samples, with the aim to define new peptide markers for species differentiation. The fibre identification was used to figure out the relation between domestic animal species and type of textile produced and to document the diachronic development of textile use in Xinjiang from the Bronze Age to the Iron Age.
Section snippets
Samples
The samples come from Djoumboulak Koum and the Northern Cemetery site. The Sino-French archaeological mission in Xinjiang found both sites, located in the protohistoric delta of the Keriya River, in the Taklamakan desert.
Djoumboulak Koum is an Iron Age settlement and the Northern Cemetery is a cemetery dating from the Bronze Age. Djoumboulak Koum (mid-1st millennium BCE), was found in 1994. The area enclosed by the walls covers 10 ha and is surrounded by various cemeteries. The sedentary
Results
Protein inference is the process used to assemble identified peptides into a list of proteins that are believed to be present in a sample (He et al., 2016). This process is challenging in the case of keratins, given their high intra- and inter-specific sequence similarities. First, a systematic analysis of the best protein hits obtained for each sample was carried out (Supplementary Material Table S3). Although a specific peptide can scarcely be assigned unambiguously to a specific keratin
Keratin identification, variation and degradation
A high percentage of archaeozoological identifications was obtained due to good sample preservation, owing to the dry environment at the two sites. In total, 97% of the samples were identified to the tribe level or lower, and 85% of caprines were identified to the genus level. This result confirms that proteomics, when allied with detection of appropriate makers, is a powerful technique to differentiate ancient animal fibres. One limitation of this technique is that it requires complete and
Conclusion
The dry climate of the Tarim Basin and the Taklamakan desert is highly favourable to the preservation of animal fibre. The results of this study confirm the heuristic potential of the proteomic approach for the determination of archaeological fibres and for textiles studies in general, especially when the fibres are degraded and their distinctive features are no longer observable or interpretable by other methods. In combination with other analytical approaches, such as microscopy, as well as
Acknowledgments
We are grateful to theLabEx BCDiv for financial support (see reference in the funding session). We thank the members of the Sino-French archaeological Mission in Xinjiang who provided the samples. All proteomics experiments were conducted in the bioorganic mass spectrometry platform at the National Natural History Museum in Paris. We are also thankful to the three reviewers for their critical comments, which greatly helped to improve this paper, and to Suzanne Needs-Howarth for the
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