We extracted collagen from bone samples for radiocarbon dating. The bone collagen extraction method adopted in this study was that of Ma et al. (2016), which was modified from Richards and Hedges (1999). In the first step, the bone was scrubbed using a handheld electric grinder to remove contaminants attached to the surface. Approximately 1 g of the dense part of each bone sample was then removed and washed using an ultrasonic bath. In the second step, the samples were demineralized in 0.5 mol/L HCl solution at 4℃. The HCl solution was renewed every 1-2 days until the bone sample was soft and free of air bubbles. Then, the sample was rinsed with Milli-Q water until it was neutral. The third step was to place the sample for 20 h in 0.125 mol/L NaOH solution at 4℃ to remove the humic acid, and then continue to flush the sample with Milli-Q water until it was neutral. Finally, the samples were gelatinized for 48 h in an acidic solution (HCl, pH = 3) at 75℃, and then they were filtered using a filter tube. Finally, the filtrate was frozen for 2 days before being freeze-dried in a lyophilizer to complete the extraction of collagen samples. We measured C% and N% of bone collagen samples on a Flash 1112-Delta V plus Elemental Analyzer-Isotope Ratio Mass Spectrometer (Thermo Finnigan) at the Third Institute of Oceanography, Ministry of Natural Resources. Rate of yield and atomic C/N ratio were then determined to quantify the bone collagen quality.

The Naba sites were dated based on the results of absolute radiocarbon dating and were supplemented by burial forms and unearthed artifacts. A total of 11 bone collagen samples from different prehistoric burials were selected for radiocarbon dating. The samples were tested using accelerator mass spectrometry (AMS) at the Key Laboratory of Western China’s Environmental Systems (Ministry of Education) in Lanzhou University. All age data were calibrated by OxCal 4.4.4 (https://c14.arch.ox.ac.uk/oxcal/OxCal.html) (Bronk Ramsey, 2021) using an IntCal 20 calibration curve (Reimer et al., 2020). All radiocarbon dating results were determined relative to 1950 CE (known as “cal. yr BP”).

During the field excavation, the archaeologists manually collected animal skeletons unearthed at each grave. Prior to formal identification, specimens were cleaned using brushes to ensure that obvious identification features were exposed. We identified faunal remains according to the modern and ancient bone reference samples collected by Lanzhou University and published identification atlases (e.g., Li, 1992; Hillson, 2005; France, 2009). The species, skeletal elements, surface modifications, symmetry, and other information were recorded in detail for all samples. We used two proxies to represent the faunal assemblage. The Number of Identified Specimens (NISP) refers to the total count of faunal bones identifiable to the species or genus level. It is the most commonly used metric in zooarchaeology, but it may lead to potential exaggeration due to the high degree of fragmentation. The Minimum Number of Individuals (MNI) estimates the smallest number of animals represented by the faunal remains, though it can be influenced by the classification units applied during statistical analysis (Grayson, 1984). In addition, we collected all previously published zooarchaeological and archaeobotanical data from the entirety of the IAMC and endeavored to reconstruct the subsistence history in the area as completely as possible.

The selected 11 bone samples are well-preserved, with a very high yield of collagen, allowing for the neglect of contamination during the burial process. The atomic ratios of carbon and nitrogen are extremely stable, both at 3.2 (Table 1), further ensuring the quality of the extracted collagen. Radiocarbon dating results are shown in Table 1. The calibrated dating results indicate that the cemetery dates from 3690 to 2000 cal. yr BP, which is consistent with the initial judgment according to burial form and the assemblage of unearthed artifacts. The chronological order of the cemetery lasted from the Bronze Age to the Yuan Dynasty. This study focuses on faunal remains from the prehistoric (the Bronze and the Early Iron Ages) graves, therefore only Bronze Age and Early Iron Age dates are listed.

A total of 1780 faunal remains were obtained from Naba, with the NISP of 1683 (Figure 1). Among them, 149 animal specimens were unearthed from the graves of the Bronze Age, with the MNI of 26. Sheep/goats dominated these faunal remains with a NISP proportion of 63.7%, while horses accounted for 5.4%. The remaining cattle of the Bronze Age were absent in Naba. There were 1065 samples dating to the Early Iron Age, and 57.1% of these were derived from horses. The proportion of sheep/goat remains decreased to 38.9%, lower than that of the previous stage. In addition, cattle were present in the graves dating to the Early Iron Age, with a NISP proportion of 2.4% (Table 2).

Ruminant livestock species first appeared in the periphery of the IAMC during the Early Neolithic, approximately 8000 years ago, as revealed by the remains of domestic sheep unearthed in southern Kyrgyzstan (Taylor et al., 2021). The second earliest undoubted presence of sheep remains was from the Altai region of northern IAMC, dating back to 5200 cal. yr BP, which was believed to be introduced by the Yamnaya migrants from the western Eurasian Steppe and managed by the Afanasievo communities in the Altai and its environs (Hermes et al., 2020; Honeychurch et al., 2021). Although certain remain of contemporaneous cattle have not been found thus far, human dental calculus illustrated that cattle and sheep dairy had been consumed by some Altai and Mongolian inhabitants during 5300-4500 cal. yr BP (Wilkin et al., 2020; Miller et al. 2022).

In the Late Chalcolithic Age, as well as in the dawn of the Bronze Age, at least two important ruminant livestock species, cattle, sheep and possibly goats (Hermes et al., 2019), appeared on the periphery of the IAMC, serving humans with both meat and dairy. This phenomenon marked the beginning of a new human-animal relationship in the area. Such practices were restricted to some foothills or river valleys in the western IAMC, and the dominant methods of animal utilization in the rest of the area were still hunting and fishing.

Cattle became ubiquitous in the IAMC during the Bronze Age around 4000-3000 cal. yr BP, as evidenced by the Halehaxite settlement in Xinjiang (54.2%) (Yu et al., 2022) and the Turgen site in Kazakhstan (26.5%) (Haruda, 2018). However, no confirmed cattle remains dating to this period have been found in the Naba cemetery thus far. A similar phenomenon was also found in the Mohuchahan cemetery, located nearly 65 km from Naba (Xinjiang Institute of Cultural Relics and Archaeology, 1999; Zhang et al., 2014). The difference between settlement and cemetery artifacts suggests that, while cattle played a significant role in daily consumption, their importance in ritual activities should be interpreted with caution. The NISPs in this study indicate that sheep/goats significantly outnumbered other sacrificial animals (Figure 3). The exploitation mode of ruminant livestock was more diversified and skilled in this period, as demonstrated by the appearance of kefir dairy products and woollen fabrics that required specialized fermentation and weaving techniques (Yang et al., 2014; Zhang et al., 2021). Additionally, the emergence of wooden wagons in the IAMC hints at the exploitation of another secondary product, animal power. Domestic horses were integrated into the local pastoralism system in this period (Frachetti and Benecke, 2009; Yu et al., 2022). Thus, the Bronze Age marked a transformative period of IAMC, when humans mastered a series of skills related to herding livestock and pastoralism dominated the whole area (Jia et al., 2017; Tian et al., 2021a), thus laying the foundations for social transformation in the following millennium.

The rise of the mobile nomad lifestyle was a key theme of the Early Iron Age. The composition of animal resources remained generally stable during this period. However, the utilization strategies became further skilled. Horseback riding, which was thought to be established in Central Eurasia during the Bronze Age (Chechushkov et al., 2020), spread to the IAMC during this period, endowing pastoral herders with greater mobility (Wertmann et al., 2023). The proportion of horses in Naba was dramatically higher in this period, and corresponding deformations have been discovered on horse teeth and post-cranial bones associated with riding practices (Figure 3). This indicates that the horse, a paramount means of transportation, acquired a higher status in the nomad societies and became the dominant sacrificed animal. This phenomenon has also been found in the nearby Chawuhu cemetery (Xinjiang Institute of Cultural Relics and Archaeology, 1999), where animals buried in the cemetery were strongly influenced by human preference. In the settlement sites, several systematic zooarchaeological case studies have revealed that sheep/goats, cattle and horses were widely used for both primary and secondary products in this period. This also suggests a close relationship between pastoralism and other branches of production department (Li et al., 2020; Ananyevskaya et al., 2021; Doumani Dupuy et al., 2023; Wang et al., 2023).

The vast drylands of central Asia are part of a transcontinental network connecting east and west, periodically acting as corridors and passageways for human migration (Li et al., 2021). There has been evidence of human activity in the area without interruption. However, human activity intensity and subsistence patterns have differed significantly in different periods (Figures 2, 5 and S2).

In 6000-4000 cal. yr BP, the intensity of human activities was obviously low and only detectable in some scattered areas even though cattle, sheep and barley had already been introduced into the IAMC area (Figures 3 and 5). Archaeobotanical evidence shows that the crops used by IAMC inhabitants in this period included barley, wheat, foxtail millet and broomcorn, but the proportions of crops varied at the same site across different periods (Figure S1). For example, in the Tongtian Cave in the Altai Region of Xinjiang, barley and broomcorn were the main crops before and after 5000 cal. yr BP (Zhou et al., 2020). In the southern region of the IAMC, ancient humans relied on barley and wheat as staple foods, as evidenced by sites like Sarazm and Chap II (Spengler et al., 2013; Motuzaite et al., 2020) (Figure 4). Zooarchaeological evidence dating from this period is sparse, with only two sites producing evidence. Cattle were the main utilizable animal at the Generalka site (Mileto et al., 2017), and sheep were the main utilizable livestock at the Begash site (Frachetti et al., 2010) (Figure 3). As such, human subsistence patterns differed significantly across different regions of the IAMC during this period. The high proportion of weeds in this period, up to about 70%, indicates that prehistoric humans expressed a low degree of agricultural management, and the faunal remains were mainly herbivorous livestock. This evidence indicates that the people of this period were mainly nomadic, utilizing agriculture as a supplement, which is consistent with knowledge about the Yamnaya/Afanasievo culture.

In 4000-3000 cal. yr BP, as animal and plant resources became utilized more extensively, the intensity of human activities also increased significantly. There is an extensive array of archaeological sites in various regions of the IAMC supporting this, especially in the areas around the Altai and the Tianshan Mountains (Figures 2 and 5). Barley and wheat in this period are the most important crops, and the proportion of them is as high as 15.05% to 83.94% in the sites with existing archaeobotanical evidence (Boivin et al., 2012; Zhao et al., 2013; Matuzeviciute et al., 2017; Tian et al., 2021b). This is followed by foxtail millet and broomcorn millet with a proportion of about 0.08%-17% (Figure 4). Published zooarchaeological evidence from this period shows that cattle, sheep and horses were the main domestic animals during this period, with sheep in particular accounting for the highest proportion of domestic animals at the Dal’verzin site (Figure 3). However, the zooarchaeological data from Naba that is reported in this study shows a similarly high proportion of sheep, up to 63.76%, with a smaller proportion of horses and zero cattle found (Figures 1a and 3). Therefore, the extensive use and spread of livestock, such as cattle, sheep and horses, and of crops, such as barley, wheat, foxtail millet and broomcorn millet, during this period may have promoted the subsequent increase in ancient human population size in the Bronze Age, thus significantly boosting the intensities of human activities.

In 3000-2000 cal. yr BP, it can be seen from previously published site data that the intensity of human activity increased further, now occupying the broader IAMC (Figures 2 and 5). Barley, wheat and broomcorn were the most important crops for ancient humans during this period (Figure 4). According to previously published zooarchaeological data, the herd composition in the region gradually changed from being mainly cattle and sheep to being mainly cattle, sheep, and horses, with the proportion of horses increasing significantly (Figures 3 and S1). The zooarchaeological data from Naba that is reported in this study shows that horses comprised the highest proportion (57.1%), followed by sheep/goats (Figures 1b and 3). The increasingly widespread use of horses contributed to an increase in population mobility during this period, which also led to the intense interconnections between populations in the IAMC (Kumar et al., 2022).

Situated in the heart of Eurasia, IAMC’s history of human activities and livelihoods in the IAMC serves as a microcosm of the history of Central Asia, triggered by the ebb and flow of prehistoric East-West exchanges (Dong et al., 2023). Current archaeological studies reveal that there were two significant eastward waves of long-distance cultural exchange originating from western Eurasia that impacted the IAMC after the Chalcolithic Age (Zhang et al., 2021; Kumar et al., 2022). The earlier wave was facilitated by the Yamnaya/Afanasievo culture (approximately 5300-4500 cal. yr BP) bearers, who are archaeologically characterized by the prevalence of the flexed burial position and the presence of sharp or round-based olive-shaped pottery in the western and eastern Eurasian Steppes (Alexandrovich and Zheksenalyevich, 2019; Wang, 2022). Ancient DNA studies further prove the genetic similarities of these two groups (Narasimhan et al., 2019; Wang et al., 2021). Wheat, barley, sheep, and cattle were introduced to the local livelihoods during this period (Hermes et al., 2020; Wilkin et al., 2020; Zhou et al., 2020; Honeychurch et al., 2021). Nevertheless, the exploitation of these exotic species was likely minimal, as evidence primarily comes from cemeteries, with few settlement sites having been systematically excavated thus far (Taylor et al., 2021). The second wave of exchange driven by the Andronovo groups introduced more advanced agro-pastoralism systems involving newer innovations of resource management and additional production technology such as metallurgy and wheel-vehicles approximately 3600-3400 cal. yr BP (Hanks, 2010; Wang et al., 2019). The foreign animal husbandry improved the efficiency of resource utilization in the abundant natural grassland of the IAMC, thus enabling a subsistence mode fitting to the local natural environment. After that, husbandry became the mainstay of the local subsistence economy and continues to this day (Dong et al., 2023). The introduction and development of millet agriculture between 5000 and 3000 cal. yr BP further enriched the subsistence structure of this region. Millet is especially drought-tolerant and barren-tolerant and has become an important staple food alongside barley and wheat in the IAMC region. These waves not only contributed to the primary local activities and subsistence strategies, but they also reshaped the population structure and prehistoric Eurasian economic patterns. Following these waves, the IAMC transformed into a novel cultural hub of Eurasia.

The influence of the climate and environmental backdrop during these long-distance migrations cannot be underestimated. Extreme drought or cold events both have the potential to trigger human migration, a reflection of the early humans’ ability to adapt to changing environmental conditions (Staubwasser et al., 2003; Schroeter et al., 2020; Huang et al., 2021). Climate change is considered a significant and complex factor in social evolution (Tan et al., 2021; Dong et al., 2023). The decreased organic carbon content and magnetic susceptibility of paleosols buried under Yamnaya kurgans and the elevated δ¹⁵N values of bone collagen from pasture-fed sheep of this period illustrate that inhabitants from western Eurasian Steppes migrated eastward approximately 5000 cal. yr BP during a period of aridification (Shishlina et al., 2018; Jeong et al., 2020; Khokhlova et al., 2022). Conversely, the eastern Eurasian Steppes, and especially the Altai area with its warm and humid mid-Holocene climate, became an ideal destination for migrants (Xiang et al., 2023). The decline in Pediastrum simplex suggests that cold events in the Altai region may have triggered southward human migration, as evidenced by the widespread distribution of archaeological sites across the Tianshan Mountains and Tarim Basin (Huang et al., 2021). Climate changes prompted human mobility on one hand, and on the other hand, spurred technological developments, such as the diversification of dairy consumption and the exploitation of new fuel resources (Scott et al., 2022; Qiu et al., 2023). These climate-adapted practices enhanced the social resilience of IAMC inhabitants and underpinned their social development during the Bronze Age. The popularity of horse and horseback riding during the Early Iron Age, as well as the emergence of mobile nomadic lifeways, can also be explained relative to climate changes. Paleo-climate studies reveal that the IAMC and its adjacent areas experienced a relatively cold climate during the late Holocene (Xiang et al., 2023). Unlike the general pattern in monsoon-controlled regions, the colder temperatures in arid central Asia always reduced evaporation, resulting in a wetter environment. This increased biomass production, thereby providing a foundation for the development of the nomadic economy (Taylor et al., 2017; Struck et al., 2021). In addition to driving migration, climate also stimulates technological development and cultural exchange. Technology, in turn, enhances adaptive capacity to climate change, thereby forming a cyclical feedback loop (Dong et al., 2023).

Some endogenous factors could have also contributed to this transformation, including the local development of herding skills, the emergence of nomadic-related implements, and inter-group relationships (Honeychurch et al., 2021). Although the existing evidence is not yet solid enough to confirm these hypotheses, the transformation of prehistoric human activities and subsistence strategies in the IAMC clearly was a systematic process influenced by a complex interplay of natural and cultural factors rather than any single cause.