Dietary shift and social hierarchy from the Proto-Shang to Zhou Dynasty in the Central Plains of China

The Proto-Shang, the Shang and the Zhou dynasties (∼2000–221 BCE: Before Common Era) are key periods in the origin and evolution of ancient civilizations in China since the periods include the processes and mechanisms of social development in the Central Plains of China during the Bronze Age. However, human-environment interactions in the context of trans-Eurasia cultural exchange during that time are not well-understood. In this study, isotopic analysis and radiocarbon dating of human and animal bones from Xinancheng cemetery in southeast Shanxi Province are reported. It was deduced that, for the period ∼1000–800 BCE, humans buried in Xinancheng cemetery relied primarily on C4-based foods and upper-status individuals consumed more animal protein and probably C3 crops. Also, considering the paleoclimate and other archaeological data of the Central Plains, the human diet and subsistence strategies changed significantly with more C3 staples such as wheat being consumed during the Eastern Zhou (770–221 BCE), as evidenced by an increased intake of wheat by lower-status individuals and the development of a mixed wheat and millet agricultural system. It is argued that the socio-economic change around the late western Zhou-early eastern Zhou Dynasty occurred as a result of the necessity to adapt to the aggravation caused by climate deterioration and population pressures, factors which profoundly influenced the economic and lifestyle patterns in ancient China. The socio-economic system of the Eastern Zhou Dynasty displayed more resilience to climate change than that of earlier periods.


Introduction
Human-environment interactions were complex in prehistoric and historical periods. Climate change was an important contributing factor to societal change from the third to the first millennium BCE (Before Common Era) throughout the world (e.g. Weiss et al 1993, Staubwasser et al 2003, Wu and Liu 2004, Kuper and Kröpelin 2006. In some cases, however, ancient societies displayed a significant degree of resilience to climate change, such as in northwestern China (Ma et al 2016, Berger and, the Indus river basin of India (Petrie et al 2017), the Maya civilization of Central America (Dunning et al 2012), Southwest Asia (Flohr et al 2016), and areas along the historic Silk Road (Yang et al 2019a). Technological innovation and its widespread impact played an important role in social transformations and human-environment interactions during the period between the third and first millennium BCE (Diamond and Bellwood 2003, Chen et al 2015a, Dong et al 2017a. Ancient civilization in China is one of the oldest civilizations in the world and emerged during the second millennium BCE when the Proto-Shang and Shang states developed in the Central Plains. The territory of ancient China expanded significantly during the subsequent Zhou Dynasty (1046-221 BCE), which included the Western Zhou (1046-771 BCE) and the Eastern Zhou (770-221 BCE) dynasties. The socioeconomic development from ∼2000 to 221 BCE laid the foundation for the formation of imperial China in the Qin-Han era (221 BCE-220 CE). However, the spatio-temporal changes of the economic patterns in the Central Plains, as well as the underpinning mechanisms, have lacked rigorous investigation.
Human subsistence strategies during the Shang and Zhou dynasties can be inferred in part from fragmented records in historical documents, such as the 'Shi Jing' (诗经) and 'Shang Shu' (尚书); however, surviving ancient works are too scarce to reveal the complete picture of human lifestyles in these periods. Archaeometry provides viable approaches for reconstructing human subsistence strategies in both prehistoric and historical periods. For example, archaeobotanical and zoo-archaeological analysis have been used to study human strategies for plant and animal utilization (e.g. Zeder and Hesse 2000, Zhao 2011, Yuan 2015, while stable isotope analysis of human bones unearthed from archaeological sites has been successfully practiced to study human diets and subsistence strategies in the past (Kohn 1999, Hedges and Reynard 2007, Hu et al 2008. These methods have been used increasingly in archaeological studies in the Central Plains to explore the socio-economic systems from ∼2000 to 221 BCE (e.g. Yuan andFlad 2005, Hou et al 2013). Most of these works have focused on local human subsistence strategies which include implications for the interpretation of social structure (Cheung et al 2017a, Zhou et al 2017. However, the spatial and temporal variation of human subsistence strategies and their relationships with climate change have not been discussed in detail, due to a lack of Western Zhou samples and limited interdisciplinary research between archaeologists and earth scientists.
This paper is concerned with the measurement of stable carbon and nitrogen isotopes and radiocarbon dating of human and animal bones unearthed from Xinancheng cemetery, the bones having been identified as relics of the Western Zhou Dynasty by archaeologists. In addition, integration of the results with other archaeological data and paleoclimate records from lacustrine deposits of the Gonghai Lake and Daihai Lake in norther China (Xu et al 2010, Chen et al 2015b, enabled us to examine the spatio-temporal variation of human subsistence strategies and to evaluate the social resilience and adaptive capacity of the societies to respond to climate change in the Central Plains from ∼2000 to 221 BCE.  Detailed information on these sites is listed in supplementary table S1 which is available online at stacks.iop.org/ERL/15/035002/ mmedia.

Study
A strict hierarchical structure in society was wellestablished during these periods. For instance, the burial pattern, such as the grave sizes and the type and quality of graves including the burial goods can reflect the socio-economic status of individuals in these ancient societies.

Xinancheng cemetery
Xinancheng cemetery, situated in the southeast margin of the Loess Plateau, is a part of the territory of Zhangzi County, Shanxi Province (figure 1). Between 2012 and 2015, a salvage excavation project for this cemetery was carried out by the Shanxi Provincial Institute of Archaeology. Pottery ware, bronzes, jade and stone artifacts, cowries and other funerary objects were unearthed. According to the characteristics of the mortuary ritual, all the graves were dated as belonging to the mid-late Western Zhou Dynasty

Materials and methods
3.1. Data collection A total of 62 human and three animal bones (two from horses and one from a dog) were collected from Xinancheng cemetery for isotopic analysis. Published isotopic data of human samples (supplementary table S1) were also scrutinized to trace human dietary shifts in the Central Plains from ∼2000 to 221 BCE.
Furthermore, isotopic data for human samples from 13 of these sites (including Xinancheng cemetery) were selected for tracing socio-economic status differences in dietary signatures in the Central Plains for the period ∼2000-221 BCE (supplementary table S1). The status level of individuals was defined as being either upper or lower level based on several criteria: (1) burials with no grave goods were classed as lowerstatus and burials with jade, bronze, shells, etc, were upper-status; (2) occupants of burials classed as human victims including those sacrificed were classed as lower-status otherwise upper-status; (3) burials with no container were classed as lower-status, while burials with containers as upper-status; (4) burials in small graves were classed as lower-status, while those in large/medium graves as upper-status.

Collagen preparation and measurement
Carbon and Nitrogen stable isotope ratios in bone collagen reflect the long-term isotopic composition of an individual's diet, which has been successfully applied to study past human diets and subsistence strategies (e.g. Hedges and Reynard 2007, Hu et al 2008. Feeding experiments (e.g. Ambrose and Norr 1993) show that the collagen δ 15 N values are closely associated with the trophic levels of individuals, while the δ 13 C values are influenced by the ratios of C 3 and C 4 plants in their diet. In this study, we extracted bone collagen samples according to the method of Richards and Hedges (1999) with some modifications . Stable carbon and nitrogen isotopic analyses were conducted in the Key Laboratory of Western China's Environmental System (MOE), Lanzhou University. Collagen samples were processed in an automated carbon and nitrogen analyser linked to a Thermo Finnigan Flash DELTAplus XL mass spectrometer. All carbon and nitrogen isotope ratio analyses were measured relative to V-PDB and AIR standard samples. The measurement analytical precision was better than 0.2‰ for both carbon and nitrogen isotopic ratios.

Radiocarbon dating
We selected eight bone samples with high collagen yields from both upper-and lower-status individuals for accelerator mass spectrometry (AMS) radiocarbon dating at Peking University, Beijing. Conventional ages from 14 C dating values were calibrated to the calendar age based on the IntCal13 calibration curve (Reimer et al 2013) using OxCal v.4.3.2 (Bronk Ramsey 2017). All ages were reported as 'cal BCE'.

Statistical analysis
The independent samples t-test was used to detect differences between two independent groups of samples. Statistical analysis was performed using SPSS 22.0 software.  The δ 13 C value for the dog implies a C 4 foodbased diet, reflecting a relatively large intake of millet and/or human waste and scraps. One horse (sample SX65) with a relatively enriched δ 13 C value (−9.2‰) is likely to have been sustained with C 4 plants (e.g. millet). However, data for another horse sample (sample SX68) suggests it ranged freely and consumed both wild (e.g. grasses and fruits) and domesticated plants (e.g. millet).

Human and animal bone results
The δ 13 C and δ 15 N values for all individuals ranged from −10.8 to −7.4‰ (mean=−8.4‰±0.7‰) and from 7.6 to 10.9‰ (mean=9.0±0.7‰), respectively. These values suggested that all individuals buried in Xinancheng relied on C 4 foods (millet and/or millet-based animal protein). Furthermore, the δ 13 C values for the upper-status individuals were significantly more positive than those of the lowerstatus individuals (mean=−9.1‰ versus −8.2‰, p=0.003, figures 2(b), (c) and table 3), indicating a little more reliance on C 3 foods for upper-status individuals. Upper-status individuals also had higher δ 15 N values than lower-status individuals (mean=9.5‰ versus 8.8‰, p=0.001, figures 2(b), (c) and table 3), indicating a higher protein intake for upper-status individuals.

Socio-economic status differences as reflected in dietary signatures
The carbon and nitrogen isotopic values for human bones in the Central Plains reveal that there were no significant differences between the upper (n=47) and lower (n=102) socio-economic groups during Proto-Shang (supplementary table S3), which implies that there were no dietary differences between the upper and lower-status groups. Significant differences, however, were observed in the δ 13 C and δ 15 N values for humans during the subsequent period, that is, from Shang to Zhou (supplementary table S3). The δ 13 C values of the upper-status individuals (n=64) were significantly more negative than those of the lower-status individuals (n=121) (supplementary table S3) from Shang to Western Zhou, suggesting a little more reliance on C 3 foods for the upper-status group. Furthermore, the upper-status individuals had higher δ 15 N values than the lower-status individuals from Shang to Western Zhou, indicating there was more consumption of animal protein. However, the δ 13 C values for the Eastern Zhou lower-status individuals (n=45) were significantly more negative than those of the upper-status individuals (n=112) (supplementary table S3), indicating more reliance on C 3 foods for the lower-status group. The eastern Zhou lower-status group also had lower δ 15 N values than the upper-status group, suggesting those peoples ate less animal protein.   figure 2(b)), indicating that the upper-status individuals consumed slightly more meat and/or milk. The human δ 13 C values for the upper-status individuals were also significantly more negative than that of the lower-status individuals (p=0.003, table 3 and figure 2(c)), implying that upper-status individuals had more C 3 -food intake (most likely wheat and/or C 3 -based animal protein). The same phenomenon was also reported for sites at Shang and Western Zhou in the adjacent area of Henan Province (Cheung et al 2017b, Dong et al 2017b. This similarity in dietary consumption implies that upperstatus individuals in the Central Plains might have consumed slightly more exotic C 3 staples (i.e. wheat) in the period ∼1600-771 BCE, given that wheat was difficult to procure and/or required higher labor input to produce when it was introduced in China at an early stage (Chen 2016, Long et al 2018.

Spatio-temporal variation of subsistence patterns
To investigate potential dietary changes in the Central Plains from Proto-Shang to Eastern Zhou, the present results were compared with other stable carbon isotope ratio data for human bones unearthed from sites dating to the period between ∼2000 and 221 BCE (figure 3). In general, humans had a C 4 -dominant diet (including millet and C 4 -based animal protein) and were heavily reliant on millet-based agriculture during the period from Proto-Shang to Western Zhou    figure 3(a)), which probably implies an increased consumption of wheat during the Eastern Zhou. An increased consumption of wheat in the Eastern Zhou (Dong et al 2017b, Zhou et al 2017 diet is consistent with the archaeobotanical data, which shows a rapid increase in the frequency and/or ubiquity of wheat in the crop assemblages during this period (e.g. Liu et al 2017, Ma 2017, Deng et al 2019. And historical documentary records also support this increase. For example, wheat cultivation was supported by governments during the Eastern Zhou period according to ancient Chinese books such as 'Zuo Zhuan' (左传) and 'Zhan Guo Ce (战国策).' Human dietary patterns and subsistence strategies, however, were inconsistent in different regions of the Central Plains from Proto-Shang to Eastern Zhou ( figure 3). Humans shifted their diets from predominantly C 4 to mixed C 3 and C 4 in Henan Province during Eastern Zhou ( figure 3(b)), while individuals of Shanxi and Shaanxi Provinces were sustained by a predominantly C 4 diet during Eastern Zhou (figures 3(c), (d)). This suggests that human diets changed in the Central Plains during Eastern Zhou, but the change was more apparent in the core region (i.e. northern Henan Province) of the Central Plains.

Social resilience and adaptive capacity in the context of climate change
The subsistence patterns of humans could shift as a result of changes in climate and society. Climate change has been identified as an important factor in influencing the evolution of human subsistence patterns in the past , Gong et al 2019. When natural resources become unavailable and induced by climate deterioration and/or population pressures, humans may change their subsistence strategies to increase social resilience and adaptive capacity.
Foxtail millet and broomcorn millet, which were domesticated in northern China, were the staple crops in the Central Plains before Eastern Zhou ( figure 3). Furthermore, wheat, an exotic crop, played a limited role in the overall subsistence system before Eastern Zhou in the Central Plains (Chen 2016), although wheat, first domesticated in West Asia, was introduced into the Central Plains around 2000 BCE (Lee et al 2007, Zhao 2014, when trans-Eurasia cultural exchanges gradually intensified. Subsequently, the peoples of the Central Plains consumed more wheat during Eastern Zhou, and spatial differentiation of human dietary changes was observed for different regions of the Central Plains, according to stable carbon and nitrogen isotopic data ( figure 3).
The dietary shift toward more wheat consumption in the Central Plains most likely was associated with the deteriorating climate from the late Shang to the early Eastern Zhou dynasties. An oxygen isotope record of Greenland suggests that globally, the climate was cooling during this period (figure 4(g)). Wang (2011) also shows that dry events increased during 2.8-2.6 ka BP along with a decrease in temperature. Although climate patterns in China were more complex (Wanner et al 2011), its general trend was also similar. The weakening Asian summer monsoon recorded by speleothem δ 18 O from Dongge Cave appears to be consistent with a general cooling and drying climate condition from the late Shang to the early Eastern Zhou dynasties (figure 4(f)). Regionally, three paleoclimatic records from the Gonghai and Daihai Lakes all suggest a declining trend in temperature and precipitation in the Central Plains during this period (figures 4(c)-(e)). Two high-resolution pollen records from the Gonghai and Daihai Lakes further illustrate that an extremely cold episode occurred at the turn of the Western and Eastern Zhou Dynasties, which is consistent with the weakest episode of the Asian summer monsoon (figure 4(f)). The Warring States period (475-221 BCE) chronicle 'Zhushu Jinian' (竹书纪年) recorded that 'Peaches and apricots ripen in September (circa October in Gregorian Calendar)' (九月, 桃杏实) and 'Frost occurred in June (circa July in Gregorian Calendar)' (夏六月, 陨霜) in the Central Plains, also indicating an unusually cold climate during the reign of King You (795-771 BC), the last king of the Western Zhou.
Given that millet was drought-tolerant but not coldresistant (Wang 1994), the grain yield of millet-based agriculture might have started to decrease with the deterioration of climate since the late Shang Dynasty (figure 4). This form of agriculture probably eventually lost its viability during the extremely cold and dry episode that lasted from the late Western Zhou to early Eastern Zhou dynasties. The food supply would have been at risk and could not feed the large population (20 million) because an agricultural system based on only one crop would have been vulnerable to a changing environment. Responding to the climatic pressure, the society of the Central Plains showed its resilience and adaptive capacity by incorporating cold-resistant wheat in the traditional millet-based agricultural system to supplement and vary the agricultural system. Furthermore, wheat is much more high-yielding than millet (Peng 2010), probably facilitating, in part, the rapid increase of population from 10 million to 20 million (Lu and Teng 2000) in the Central Plains from 1046 to 771 BCE, which might in turn have promoted a dependency by society on wheat production, especially in Henan, the core area of the Eastern Zhou government. Even in the Han Dynasty (202 BCE-220 CE), the importance of wheat in the food assemblage slightly exceeded that of millet (Hou et al 2012, Deng et al 2019.
To investigate potential dietary changes between the upper-and lower-status individuals, the δ 13 C and δ 15 N values are plotted for the respective status categories (figures 4(a), (b)). It can be seen that the upper-status individuals ate more C 3 foods than the lower-status individuals from Shang to Western Zhou ( figure 4(b), supplementary table S3), including wheat and C 3 -based animal protein. Given that wheat would have been difficult to procure and/or would have required higher labor input to produce when it was introduced in China at an early stage, upper-status individuals might have consumed more exotic crops (i.e. wheat) from Shang to Western Zhou. With the adoption and expansion of wheat cultivation in the Central Plains Human societies adopted a wide variety of strategies to adapt to environmental changes and extreme climatic events. Migration and subsistence strategy changes are two of them that have received the most attention (Kuper and Kröpelin 2006, Flohr et al 2016, Lespez et al 2016. However, along with the developing civilization, the adaptive capacity of human communities in response to the environmental pressures also developed. Our analysis reveals that the agricultural communities in the Central Plains responded to the environmental stress during the Zhou Dynasty by changing their crop structures rather than radically changing their subsistence strategies. Specifically, they started to plant more wheat to adapt to environmental and population pressure. This implies that collapse of a society was not the only way for ancient societies to respond to climate change, thus, in some instances humans could increase their resilience and adaptive capacity through alteration of their agricultural practices and strategies in response to the environmental pressures. Since then, the rise and fall of civilizations is likely to have been dominated by human society itself instead of climate change as a result of further improvements in the capacity of humans to adapt to environmental change.

Conclusion
The isotopic and radiocarbon dating results presented suggest that humans consumed a great amount of C 4 foods (millet and/or C 4 -based animals' protein) during the period between ∼1000 and 800 BCE, while the diets of different hierarchical groups were clearly varied. Upper-status individuals consumed more animal protein and more C 3 crops (most likely wheat) than lower-status individuals. From integration of isotopic data from human bones with archaeobotanical data of the Central Plains, it was confirmed that human diets and subsistence strategies changed significantly such that more wheat was consumed during Eastern Zhou, as evidenced by the increased intake of C 3 staples for lower-status individuals. Black dotted frames represent different archaeological sites with stable isotopic data, the dotted gray line represents the −12‰ value of δ 13 C. The black arrows represent climates that have gradually became cold and dry since the late Shang Dynasty, and the shadows represent extreme dry and cold climatic events. Detail information on these archaeological sites is listed in Supplementary table S1.
The aggravation of survival stress induced by climate deterioration around the late western Zhou-early eastern Zhou Dynasty could have been an important contributing factor to socio-economic change during the Zhou Dynasty (1046-221 BCE). The mixed millet and wheat agricultural system developed in the Central Plains during Eastern Zhou to respond to environmental and population pressures, implied that the socio-economic system of the Eastern Zhou Dynasty displayed a certain degree of resilience to climate change. The combined climate change, human isotopic data and burial patterns presented in this study can be applied more widely to prehistoric and historical periods in China and other countries to shed light on the resilience of human societies and their relationships with environmental change.