The ZKG section (39°6′N, 110°3′E) located in the southern of the site, which is 155 cm-deep (Figure 1) and contains evidence of human activities, such as pottery and charcoal. According to the color and structural characteristics of the sediments, the section is divided into four layers: 1) 0–40 cm: Light gray sandy silt with abundant charcoal fragments, large calcium carbonate nodules, and small pebbles. 2) 40–70 cm: Light yellow sandy silt with less charcoal than above; abundant larger calcium carbonate nodules, small gravel clasts and gray clay pottery. 3) 70–100 cm: Light gray sandy silt containing occasional charcoal fragments, small calcium carbonate nodules, pebbles and animal remains. 4) 100–155 cm: Pale yellow sandy silt layer containing a small amount of charcoal, fine gravel and gray clay. The natural soil layer is below the depth of 155 cm. In order to determine the age of the sedimentary section, charcoal and grains of millet were selected at the depths of 145 cm, 105 and 60 cm for AMS ¹⁴C dating.

Huang, (1996) suggested that the domesticated animals at the ZKG site consist of pigs, sheep and cattle, and they estimated the age of the remains based on AMS ¹⁴C dating of charcoal recovered from the strata. So far, however, there has been no direct AMS ¹⁴C dating of the remains of domesticated animals at the site, and therefore we selected animal bones from different cultural periods for AMS ¹⁴C dating. The AMS ¹⁴C dating was conducted at the Beta Laboratory, United States. The dated samples included two pig bones, two sheep bones and one cattle bone, from different cultural layers. The AMS ¹⁴C dating results were calibrated using IntCal20 in OxCal 4.3 software (Reimer et al., 2020).

Fifteen pollen samples of ∼30 g weight were analyzed using conventional heavy liquid separation. A known number of Lycopodium spores was added prior to the preparation to calculate pollen concentrations. The identification of pollen and fungal spores was based on atlases of pollen morphology (Taylor, 1988; Xi and Ning, 1994; Wang et al., 1995; Van Geel et al., 2003; Tang et al., 2016). A pollen and fungal spores diagram was drawn and statistical zonation was conducted using CONISS implemented in Tilia 2.0 (https://www.tiliait.com/). The charcoal content was estimated using point counting. Fifteen samples of ∼10 g weight were used for phytolith analysis following the conventional wet oxidation method. Phytoliths were identified with reference to the phytolith key of Wang and Lv (1993), and a phytolith diagram was drawn using Tilia software.

The AMS ¹⁴C dating results for the materials from the ZKG are illustrated in Figure 2 and listed in Table 1. The age of charcoal fragments at the depth of 145 cm in the ZKG section is 3,718–3,975 cal yr BP (OZQ360); and that of the depth of 105 cm is 3,700–3,908 cal yr BP (Beta 563,896); the age of carbonized seeds at the depth of 60 cm is 3,700–3,908 cal yr BP (Beta 563,895); and the age of cattle remains is 3,727–3,972 cal yr BP (median age of 3,870 cal yr BP, the same below). The ages of sheep and pig remains from the second cultural stage are 3,700–3,908 cal yr BP (3,828 cal yr BP) and 3,700–3,894 cal yr BP (3,791 cal yr BP). The age of sheep remains from the third cultural stage is 3,247–3,445 cal yr BP (3,355 cal yr BP). The age of pig remains from the fourth cultural stage is 3,070–3,325 cal yr BP (3,172 cal yr BP). Thus, the age range of the remains of domestic animals at the site is 3,070–3,972 cal yr BP. The published AMS ¹⁴C ages for the ZKG site are between 4,100 and 3,100 cal yr BP, except for two ages that are older than 4,500 years (Figure 2; Table 2).

A total of 4,445 pollen grains, comprising 18 families and genera, were identified from the 15 samples analyzed from the ZKG section (Figure 3). The main tree pollen types are Pinus, Betula, Ulmus and Fagaceae; and the main shrubs and herb pollen types are Artemisia, Chenopodiaceae, Ephedra, Asteraceae, Leguminosae, Tamarix, Polygonum, and Apiaceae. Ferns are rare and include Monoletes, Triletes and Polypodiaceae. The fungal spores are mainly Sporormiella and Srodaria. The pollen diagram (Figure 4) was divided into three zones according to CONISS, and are described below.

Pollen Assemblage Zone I (Artemisia-Chenopodiaceae): 155–130 cm. The zone is dominated by herbaceous pollen which is dominated by Artemisia (53.7–61.2%; average of 57%), followed by Chenopodiaceae (33.3–43.3%; average of 36.7%. Poaceae is relatively poorly represented (1.5–8.8%; average of 4.3%). The content of Sporormiella fungal spores is 0–1.1% (average of 0.6%). The charcoal concentration is low, with an average of 0.9 × 10⁻² cm² g⁻¹.

Pollen Assemblage Zone II (Artemisia-Chenopodiaceae-Poaceae): 130–60 cm. There are substantial changes in the herbaceous taxa compared with Zone I, with a major decrease in Chenopodiaceae (10.4–39.2%; average of 20.8%) and an increase in Poaceae (5.5–49.3%; average of 23.5%). However, Artemisia is still well represented (23.9–63.9%; average of 48.4%). The content of fungal spores is slightly higher than in Zone I, including Sporormiella (0.0–2.7%; average of 0.8%) and Sordaria (0.0–3.6%; average of 0.7%. The concentration of charcoal particles is higher than in the Zone I (1.2–61.8 × 10⁻² cm² g⁻¹; average of 26.9 × 10⁻² cm² g⁻¹).

Pollen Assemblage Zone III (Artemisia-Chenopodiaceae-Poaceae): 60–0 cm. Artemisia (34.7–48.3%, average of 39.6%) and Chenopodiaceae (13.3–26.2%; average of 21.8%) are well represented. The representation of Poaceae (12.5–39%, average of 22.7%) is similar to that of Zone II. There is a substantial increase in the representation of fungal spores, including Sporormiella (0–4.5%; average of 1.8%) and Sordaria (1.3–3.4%; average of 2.5%). The concentration of charcoal fragments is substantial higher than in Zone II (93–968 ×10^–2 cm² g⁻¹).

A total of 9,630 phytoliths were identified in the 15 samples from the ZKG section, comprising 16 taxonomic categories (Figure 3). The main morphotypes are: Elongate psilate, Elongate dendritic, Rectangle, Square, Acicular hair cell, Sponge spicules, Saddle, Cuneiform bulliform, Elongate echinate, Tooth type, “Y” type, Fusiform, Bilobate, Triangular prism, and common millet and foxtail millet. The morphotypes for common millet and foxtail millet are “η” and “Ω”, repectively (Lv et al., 2009). And they come from husks. There are few other Panicoideae phytoliths in the samples. The phytoliths are dominated by Elongate psilate (25.1–49.9%) and Acicular hair cell (11.2–50.6%). The phytoliths of common millet and foxtail millet occur within the depth interval of 70–135 cm; the lowest representation is 0.2% at a depth of 70 cm and the highest is at 105 cm, representing 33.4 and 41.1% of total phytoliths, respectively. Based on the cluster analysis, the phytolith records can also be divided into three zones (Figure 5):

Zone I (155–130 cm). The total numbers of phytoliths in zone I are 1,483. Elongate psilate type (37.8–45%) and Acicular hair cell type (40.1–50.6%) are the most common phytoliths. Common millet (1.2%) occurs in a single sample at the depth of 135 cm.

Zone II (130–60 cm). The total numbers of phytoliths in zone II are 5,571. Elongate psilate type (25.1–49.9%) remains the most common phytolith type, and there is a decrease in the Acicular hair cell type (9.6–38.7%). There are increases in the common millet (0–35.1%) and foxtail millet phytolith types (0–7.9%).

Zone III (60–0 cm). The total numbers of phytoliths in zone III are 2,734. The Elongate psilate type (25.3–41.3%) and Acicular hair cell type (24–27.8%) continue to decrease. Square (14.7–32.3%) and Rectangle (7.4–15.4%) increase substantially, and there is a slight increase in the Cuneiform bulliform type. However, the common millet and foxtail millet types disappear.

Phytoliths are tiny siliceous bodies precipitated in plant cells and deposited in the soil. They have distinct morphological characteristics due to differences in plant cell structure and soil type, and moisture and climatic conditions. As an important biological indicator, phytoliths have been widely used in research in palaeovegetation, palaeoecology and agricultural archaeology (Lv et al., 2009).

The phytoliths of common millet and foxtail millet in the section are direct evidence of the cultivation of the parent plants at the site, and are consistent with the discovery of millet seeds and associated agricultural tools at the site (Institute of Cultural relics and Archaeology, Inner Mongolia Autonomous region, 2000; Bao et al., 2018). The AMS ¹⁴C ages of two crop seeds from the ZKG site are 3,700 and 4,000 cal yr BP (median ages of 3,835 and 3,828 cal yr BP, respectively). There is a relatively high abundance of phytoliths in the sediments within this period, indicating that millet was cultivated in the vicinity of the site, and that there was a high intensity of planting activity (Figure 6). Phytoliths are absent from the sediments above 60 cm in the ZKG section, which may have been caused by a reduction in rainfed agriculture at the site.

The life cycle of fungal spores is closely related to the activity of herbivores. After breaking dormancy in the intestinal tract of animals, the spores are excreted and then propagate (Van Geel et al., 2011). Many studies have shown that fecal fungal spores can be used as indicators of domestic herbivores (Van Geel et al., 2003; Zhao et al., 2013; Huang et al., 2020, 2021). Therefore, the occurrence of fungal spores in the sediments of the ZKG site indicates the local presence of herbivorous domestic animals, which is consistent with the large quantity of domestic animal remains excavated from the site (Huang, 1996). The distribution of fungal spores in the strata is characterized by low percentages below the depth of 60 cm, indicating limited cattle and sheep rearing at that time, while the increased percentages above 60 cm indicate that animal husbandry increased (Figure 6).

The carbon isotope composition (δ¹³C) of the bones of domesticated pigs, cattle and sheep can reflect their diet and hence human feeding strategies (Hu S. M. et al., 2008; Chen et al., 2012). The δ¹³C values of carbon isotopes of two pig bones from the ZKG site are −7.1‰ (3,791 cal yr BP) and −7.2‰ (3,172 cal yr BP), which indicates that their food was dominated by C₄ plant types. The δ¹³C values of pigs are similar to those of human bones at the ZKG site (Atahan et al., 2014) (Table 2); moreover, the δ¹³C values of the bones of humans and domestic pigs in North China are similar (Hu Y. W. et al., 2008). The δ¹³C values of cattle and sheep indicate that C₃ plants were the dominant plant types consumed by these animals at the ZKG site. The δ¹³C of cattle is also ∼1‰ more positive than that of sheep, which may be related to the consumption of cultivated millet and millet byproducts. The Taosi site in the Central Plains is an important symbol of Chinese large urban centers (Figure 1; Table 1). The δ¹³C values (average −11.25‰) of cattle bones at the Taosi site are indicative of a diet dominated C₄ plants, which may reflect the supply of fodder by humans (Chen et al., 2012). However, the δ¹³C values of cattle bones at ZKG indicates that a lower proportion of C₄ plants was consumed than at the Taosi site.

The evidence provided by the contents of phytoliths and fungal spores of the section, combined with the archaeological evidence, reveals that a mixed agro-pastoral economy consisting of rainfed millet cultivation and animal husbandry (cattle and sheep) appeared at the ZKG site some 4,000 years ago. However, there were changes in the relative proportions of rainfed millet cultivation and pastoralism over time. Rainfed millet cultivation accounted for a high proportion of the agricultural activity in the early phase, but the proportion of pastoral activity increased subsequently. The nitrogen isotope composition (δ¹⁵N) of two human bones from ZKG were 8.2 and 9.7‰ (median ages of 4,019 cal yr BP and 3,769 cal yr BP, respectively). The 1.5‰ difference between the two indicates an increase of the nitrogen intake and hence an increase in animal husbandry (Atahan et al., 2014). Pig bones dating back 3,200 years have been found at the ZKG site, and pig rearing is direct evidence of settled agriculture. Although there may have been cultural or genetic exchanges with the Andronovo culture (Institute of Cultural relics and Archaeology, Inner Mongolia Autonomous region, 2000; Jeong et al., 2020; Wang et al., 2021), the area remained dominated by a settled agro-pastoral economy from the middle to the late Bronze Age (4,000–5,000 cal yr BP).

In the early mid-Holocene, human activity in the forest and steppe region of northern China mainly took the form of rainfed agriculture and pig rearing. For example, millet cultivation appeared at the Xinglonggou site in the West Liaohe River Basin between 8,000 and 7,500 cal yr BP (Zhao, 2005), and at the Dadiwan site, in the western part of the Loess Plateau, at 7,800–7,350 years cal yr BP (Liu et al., 2004). Pig rearing was also conducted at both sites. Agricultural tools have been excavated from Shihushan (6,700–6,000 cal yr BP), Wangmushan (5,300–4,900 cal yr BP), Xiyuan (5,000–4,800 cal yr BP) and Ashan (4,400–3,800 cal BP) in the northern Loess Plateau and in the Hetao area of the Ordos Plateau (Yang, 1997; Bao et al., 2018; Feng and Wei, 2018). We speculate that they are related to millet agriculture. But usewear and residue analyses on food processing stone tools from these sites have shown that plants including tubers are dominant (Liu et al., 2014; 2016). The seeds have been found from Xiaojiamao, Dakou, Zhaimao, Zhukaigou, Shimao sites (Bao et al., 2018). Millet, a C₄ plant, was an important food source for humans in the steppe region of northern China during 2,500–5,000 cal yr BP, then the ratio of C₃ plants in the diet began to increase after 2,500 cal yr BP (Atahan et al., 2014).

Evidence of cattle and sheep rearing gradually appeared in the region in the middle and late Holocene (from the time of the Longshan culture to the Bronze Age). For example, AMS ¹⁴C ages of the remains of domestic cattle at Houtaomuga of 5,500–5,300 cal yr BP were obtained (Cai et al., 2018). AMS ¹⁴C ages of 4,577–4,454 cal yr BP were obtained for cattle remains at the Ashan site, but it is unclear if they were domesticated (Bao et al., 2018). The latest AMS ¹⁴C dating of cattle bone from the Hongliang site were 4,406–4,186 cal yr BP (Hu, 2021). The first directly cattle bones for the Hexi Corridor region, with the samples dating to 3,850–3,700 cal yr BP (Brunson et al., 2020). The earliest evidence of domestic sheep/goats in China may be at the Shizhaocun and Hetaozhuang sites in the western Loess Plateau; the estimated age range is 5,600–5,000 cal yr BP based on the archaeological context, but no direct AMS ¹⁴C ages were reported (Yuan, 2010).

So far, published AMS ¹⁴C ages for the remains of the earlist domesticated sheep/goats come from the Youyao (4,292–4,029 cal yr BP) site (Dodson et al., 2014). AMS ¹⁴C ages of 4,406–4,151 cal yr BP were obtained for three sheep bones at the Jingbianmiaoliang site (Hu, 2021). An AMS ¹⁴C age of 3,925 cal yr BP was obtained for sheep skin at Gumugou in Xinjiang (Wang, 1983). The dating results for mixed human and sheep bones from Xihe Lanqiao in Gansu Province are 3,450–2,877 cal yr BP (Chinese Archaeological Radiocarbon Collection, 1983). In addition, the AMS ¹⁴C dating results for sheep/goat bones from Hexi Corridor are 4,413–3,073 cal yr BP (Yang et al., 2019). Other dates for sheep or cattle remains have been obtained for the sites of Shimao (4,300–3,800 cal yr BP), Muzhuzhuliang (∼4,000 cal yr BP) and Zhengzemao (∼4,800 cal yr BP) in Shaanxi Province; Yongxingdian in Zhunger Qi in Inner Mongolia Province (4,450–3,950 cal yr BP); Dakou (4,200–3,500 cal yr BP) and Taosi in Shanxi Province (∼4,000 cal yr BP); and Yuanqu Shangcheng (3,300–2,800 cal yr BP), Tongtian cave (5,200–3,200 cal yr BP), Adunqiaolu (3,850–3,200 cal yr BP), Xiaohe (4,000 cal yr BP) and Shirenzigou (2,300 cal yr BP) in Xinjiang. However, no direct AMS ¹⁴C dating has been reported (Figure 6) (Wei, 2000; Cong et al., 2013; You et al., 2014; Yu et al., 2018).

Opposite to the eastern end of the Eurasian steppe (including the steppe region of northern China), the Yamnaya culture (5,300–4,200 cal BP) originated in the western part of the Eurasian steppe and practiced an economy based on hunting and grazing. The Yamnaya people spread eastward and reached Lake Baikal, which was the eastern limit of their extension (Zhou et al., 2020; Wang et al., 2021; Narasimhan, et al., 2019). The Afanasyevo culture (5,000–3,500 cal yr BP), which was believed to have been formed by the eastward spread of the Yamnaya culture (Cong and Jia, 2020; Honeychurch et al., 2021). And the Afanasyevo culture which appeared in the central Eurasian steppe, was characterized by sheep/goat and cattle rearing (Honeychurch et al., 2021). During the middle and late Bronze Age, the Andronovo culture (1900–1,500 cal yr BP) developed in the central Eurasian steppe. Several studies have proposed that practiced a sedentary from of animal grazing, and raised animals such as horses, sheep/goat and cattle (Kuzmina, 2008).

According to recent ancient DNA research, during the early Bronze Age there was a possible cultural connection between the populations of Yamnaya and the Eastern Eurasian steppe (Wang et al., 2021). The economy of the steppe of northern China at this time was dominated by rainfed agriculture and pig rearing. Cattle and sheep/goat were also raised sporadically, but did not play the dominant role. By the middle to late Bronze Age, cattle and sheep/goat rearing had increased substantially, similar to the subsistence economy of the Andronovo culture, but horse raising was uncommon. According to this study, the mixed economy of rainfed agriculture and cattle and sheep/goat rearing in the steppe region of northern China was practiced as early as 4,000 years ago. In addition to cultural influences from the central and western steppe areas, climatic and environmental changes were an important factor that must be considered when interpreting patterns of subsistence activity, which is discussed below.

The steppe region of northern China, dominated by arid and semiarid ecosystems, is located on the margin of the zone of influence of the Asian summer monsoon. A shift in the East Asian monsoon occurred some 4,000 years ago and it had an important impact on the environment of East Asia. Stalagmite records from Dongge Cave show that the intensity of the Asian summer monsoon weakened from 4,400 years BP to 3,900 years BP corresponding to a decrease in North Atlantic sea surface temperatures (Bond et al., 1997; Demenocal et al., 2000; Wang, 2017) (Figure 7). Pollen records and quantitative palaeoclimatic reconstructions from Gonghai in Taihang Mountain and Tianchi in Liupan Mountain indicate a substantial decrease in precipitation in the monsoon margin zone at ∼4,000 years BP (Zhang et al., 2010; Chen, 2015; Li et al., 2017). Additionally, the pollen percentages of Chenopodiaceae increased significantly during 4,400–3,350 cal yr BP at Hulun Lake, indicating arid climatic conditions (Wen et al., 2010).

According to a comprehensive study of environmental and archaeological records (Cui et al., 2019; Sun et al., 2019), the decline of major Neolithic cultures in China was closely related to the weakening of the monsoon some 4,000 years ago. Neolithic cultures, such as the Qijia, Laohushan, Hongshan, Longshan and Liangzhu, all declined and ended in or after this period (Wu and Liu, 2001; Mo et al., 2003; An et al., 2005; Yang et al., 2015).

Major drought events at ∼4,000 cal yr BP and their impacts may have been of global extent. The drought event during 4,200–3,900 years BP caused the abrupt decline of the Akkadian empire in Mesopotamia; and large urban centers in Egypt, on the Nile, and in India, on the Indus, also declined at the same time. Moreover, large urban centers in Anatolia, Aegean and Levant also developed during the Bronze Age (Weiss et al., 1993; Weiss and Bradley, 2001). The occurrence of severe droughts is suggested to have been a major cause of the declines of these large urban centers, on the one hand, and on the rise of a nomadic culture in steppe regions (Weiss and Bradley, 2001).

Tree pollen percentages in the Daihai DH99 drill core decreased substantially during 4,450–3,950 cal yr BP, while herb pollen types increased, indicating a pronounced cold and dry interval in the marginal area of the summer monsoon zone (Xiao et al., 2004) (Figure 7). The pollen record from the Zhukaigou site also suggests typical steppe vegetation, dominated by Artemisia and Chenopodiaceae, that would have favored the development of cattle and sheep rearing. Sheep/goats and cattle may have appeared in northern China some 5,000 years ago, but they were not universally raised. The period after the 4,200 years BP drought event, with substantially decreased precipitation and the expansion of steppe vegetation, coincided with a substantial increase in the numbers of domestic cattle and sheep (Figure 7). The low crop yields during this arid period may have been insufficient to support the population, while on the other hand the steppe vegetation benefited a pastoral economy. It is evident that the inhabitants of the region were able to adapt to these major environmental changes and adopt a modified economic structure which enabled the population carrying capacity of the land to be maintained. Therefore, we suggest that the combination of rainfed agriculture and pastoralism increased the adaptability of the inhabitants of northern China to arid conditions and increased their socioeconomic resilience.