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4.2 Dietary patterns over the life course
The isotopic profiles indicate significant variation in diet across childhood (Figure 3). This is not entirely surprising given the diversity of adulthood diets captured in the same individuals' bone collagen data (and the larger sampling of each population by Dong et al., 2017). The major food groups of millet, wheat, soybean, pig, cattle/ox, dog, horse, sheep, and more, transverse large ranges of δ13C and δ15N, and various combinations of these foods can produce the same isotopic value in skeletal tissues (this is the issue of equifinality, where different isotopic combinations can provide the same result, see King et al., 2018; Quinn, 2019). Therefore, interpreting changes in δ15N and δ13C becomes more challenging as we see the dietary diversity observed in Eastern Zhou adults is also reflected in their childhood diets.
4.2.1 Nitrogen isotopes over the lifetime
For most of the individuals studied, after weaning concludes the δ15N dentin values flatten and remain relatively stable (almost all individuals show this pattern by age 5, therefore we use age 5 as the starting point for where children's diets no longer include breast milk). In most individuals' isotopic profiles (Figure 3), we see that the δ15N values from the later‐forming dentin samples are very close to, if not overlapping with, the δ15N value of the bone collagen sample representing their adulthood diet (average diet for the decade prior to death). This suggests relative stability in dietary protein sources after age ~5 years old through adulthood until death, though there may have been fluctuations during years that our samples do not account for. A few of the individuals from the Changxinyuan cemetery (Cxy M067, Cxy M049, Cxy M045), show more protracted declines in δ15N across their dentin profile, continuing past the estimated age of cessation of breastfeeding. This suggests their dietary protein intake was almost continuously changing across their childhood. However, even these individuals have their final dentin δ15N values approaching their bone collagen δ15N values, potentially suggesting relative stability in dietary protein from that point in later childhood onward in their lifetime.
We calculated the average of the dentin isotope values for each individual after age 5 (Tables 1 and S2) and then grouped the females and males and calculated the isotopic average for each sex during childhood. We found that Xiyasi females average δ15N post‐age 5 years was 7.5‰, and the Xiyasi males average δ15N was slightly higher at 7.8‰ (see loess curves on Figure 2, female curve is slightly lower than males over the entire childhood period covered by these samples). These data indicate protein consumption was relatively similar for both sexes from childhood through adulthood (i.e., no significant difference in protein consumption between the sexes for Xiyasi individuals at any age). The average dentin δ15N after age 5 for Changxinyuan boys was 7.8‰, while the average girl's dentin δ15N after age 5 was 6.3‰. The nitrogen loess curve for Changxinyuan females is notably below the males, and this pattern suggests that Changxinyuan boys may have consumed more animal protein than girls and/or consumed proteins from higher trophic positions (with higher δ15N values). However, we caution that the Changxinyuan group is a small sample and therefore these findings may not be representative of the greater population dietary patterns.
4.2.2 Carbon isotopes over the lifetime
During childhood, we see significant variability in δ13C dentin values, indicating urban Eastern Zhou children were fed across both C3 and C4 plant groups (Figure 3). Importantly, the δ13C values are generally more negative during childhood in comparison to the same individual's later adulthood bone collagen δ13C value. This indicates a greater consumption of C3 foods during childhood, and that as these individuals aged their diets changed and incorporated more C4 foods. We suggest that specific C3 foods such as wheat and soybeans were given to young children as important components of their diet while they were growing and developing, and that in later life their diets shifted to include more C4 foods (millets) which became registered in their bone collagen before death.
We also see that during childhood, the δ13C values of boys are slightly higher than the values of girls, indicating that boys are being fed more C4 foods than their sisters, cousins, and female peers. The average dentin δ13C value of Xiyasi boys (after age 5) was −13.6‰ while the average Xiyasi girl dentin δ13C after age 5 was −14.8‰, indicating greater consumption of C4 foods by Xiyasi boys than girls. We can see this pattern in the loess curve plotted in Figure 2, where the male blue curve is higher after weaning than the female curve, indicating more influence from C4 foods for boys. Within the Changxinyuan group, two of the males (CxyM049, CxyM026) show a different pattern, with dentin δ13C values that are mostly less negative during childhood than their adulthood bone collagen values, indicating greater C4 (millet) consumption during childhood followed by a slightly more mixed diet with more C3 inputs during adulthood. There is a very notable difference in the δ13C values between boys and girls for Changxinyuan: the average δ13C for girl's dentin (after age 5) is −14.2‰, while the average δ13C for boy's dentin (after age 5) is −9.8‰. The carbon loess curves (Figure 2) for these children show a significant divergence between the sexes. This large difference in carbon isotope values indicates significant dietary differences between females and males during childhood for these Changxinyuan individuals, with girls consuming much more C3 food than boys. We again must caveat that the Changxinyuan sample is small, but is suggestive of dramatic dietary differences between boys and girls beginning in early youth.
Figure 2
4.2.3 Dietary differences between females and males over the lifetime
The patterning of δ13C dentin values during childhood relative to adulthood reveal that Eastern Zhou children's diets were distinct from adulthood diets, and that C3 foods were fed to children in greater quantities than what adults consumed (Figure 3). Taken together, these findings are significant because the previous isotopic study of these individuals noted significant dietary differences between males and females during adulthood (Dong et al., 2017), and the dentin data, therefore, suggest that some of these differences are rooted in feeding practices that begin in childhood. Wheat and soybeans may have specifically been used as weaning/childhood foods for Eastern Zhou peoples, with millets being consumed in greater quantities later in life. Comparing the childhood dentin isotope data to each individual's later adulthood bone isotope data show dietary changes occurred over the lifetime and also highlight increasing differentiation between males and females with age, suggesting that food practices were integral to aspects of gender over the lifetime (Appadurai, 1981; Hastorf, 2017; Miller et al., 2018; Richards, 1951; Twiss, 2007; Weismantel, 1988; White, 2005).
Beginning in childhood, boys were fed more millet which then increased significantly at a later point in their lives. One possibility is that dietary changes for males were tied to social age/status changes over their lifetime, such as the coming‐of‐age “capping” ritual for adolescent/young adult men (Chu & Buckley Ebrey, 1991; Hardy, 1993; Marshall, 2003; Rouzer, 2001). This ritual is written about more extensively in the Han Dynasty period but is believed to have had its roots in earlier periods, perhaps beginning in the Bronze Age Eastern Zhou (Hardy, 1993). Males would be ceremoniously capped (in their later teenage years) in a ritual marking their transition into adulthood and heralding their new responsibilities and rights that change in status afforded them (Chu & Buckley Ebrey, 1991; Hardy, 1993; Marshall, 2003; Rouzer, 2001). Perhaps dietary changes accompanied this social age transition to adulthood and Eastern Zhou young men then consumed greater amounts of millet as adults (potentially both as food and in fermented alcoholic beverages). This is a hypothesis that could be tested with a targeted sampling of adult males who died across different age groups using both bone and multiple tooth samples (such as later forming third molars which potentially could capture the period pre‐ and post‐capping).
In childhood, Eastern Zhou girls ate across C3 and C4 food groups but consumed more C3 foods such as wheat and soybean than boys, which continued for these girls into adulthood. One explanation for the retention of more C3 foods in the diets of women may be related to gender divisions of labor, with women responsible for domestic work including household maintenance, food preparation, child care responsibilities, and textile production (Feng, 2013; Hinsch, 2018; Linduff & Sun, 2004; von Falkenhausen, 2006). Archaeological evidence of a gendered division of labor has primarily been inferred from objects found in tombs (Hinsch, 2018; Linduff & Sun, 2004; von Falkenhausen, 2006), however recent bioarchaeological analyses have made significant contributions to our understandings of gendered labor as well (He & Tang, 2015; Linduff & Sun, 2004; Mu & Chen, 2018; Zhang et al., 2017). Additionally, historical Chinese texts suggest that during the Eastern Zhou, women and men's daily lives were increasingly structured along divisions of labor which kept the sexes separated in time and space (“men grow grain and women produce cloth” 男耕女织; Hinsch, 2003, p. 598), and this may have had gendered consequences on the daily dietary practices of each sex (Hinsch, 2018; Linduff & Sun, 2004). If women's roles primarily focused on activities typically relegated to the domestic sphere (Hinsch, 2003, 2018; Linduff & Sun, 2004), and women were the primary cooks for the family, they may have eaten the same meals they fed to their children. Perhaps their husband ate most of these same meals as well, but may have received slightly different portions of particular parts of the meals, such as slightly more meat, and/or more millet, which would cause small but important changes to their bodily isotope chemistry, differentiating Eastern Zhou boys and men from girls and women. Further, it is common for caretakers to demonstrate to young children that a particular meal is good by eating some of it themselves (parental/social modeling), and this may provide one explanation for the retention of more C3 foods in female diets over the lifetime (Birch, 1999; Birch, Savage, & Ventura, 2007; Blissett & Fogel, 2013; Larsen et al., 2015). Taken together, these dietary data indicate that practices of differentiation between boys and girls began early in life through dietary habits where particular foods were associated with gender, and these gendered dietary differences intensified over the lifetime leading to the significant dietary differences noted between urban Eastern Zhou adult men and women (Dong et al., 2017).
The Zhou periods are now viewed as a major cultural shift in early Chinese history, with significant changes to ideology, religion and ritual, and political governing (Feng, 2013; Hinsch, 2018; von Falkenhausen, 2006). Separation of the sexes became increasingly codified in early Chinese society, particularly during the Zhou periods in response to what had been seen as cultural indulgences (such as excessive alcohol consumption) of the earlier Shang Dynasty elites (Feng, 2013; Hinsch, 2003, 2018). Coming‐of‐age rituals, such as capping for men and pinning for women (a ceremony at age 15 involving a change in hairstyle and signifying a social age status change to adulthood for young women), served to reinforce social norms differentiating between the sexes and their roles within the family and the greater social order (Chu & Buckley Ebrey, 1991; Hardy, 1993; Hinsch, 2003, 2018). Hinsch (2003, 2018) explains that the separate spheres of women's and men's daily lives were seen as an important aspect of Chinese cultural identity from this time period (from Eastern Zhou and into the Han Dynasty) and therefore we may be seeing dietary effects of these societal divisions in the isotopic data recorded in the teeth and bones from Xiyasi and Changxinyuan individuals.
Figure 3
4.3 A possible case of early life stress
One male individual, Xiyasi M205, had an exceptionally high dentin nitrogen value which stands out from the rest of the group. The first dentin sample for this individual (estimated median age of sample = 4 months) has a δ15N value of 15.0‰, which is 3‰ higher than the next highest δ15N value in the samples analyzed in this study. The second sample for Xiyasi M205 drops to δ15N = 9.7‰, at estimated age 1.2 years, and then maintains a relatively steady lower δ15N value for the subsequent years sampled (average δ15N value after age 5 years = 8.5‰). The first tooth section analyzed was formed during early infancy and therefore the high nitrogen value is a product of something that occurred in the first months after birth. There are two main theories that are typically proposed to explain high nitrogen values in infants: (a) the mother had a protein‐rich diet, potentially consuming marine food sources (or consumed other high isotope value protein sources), or (b) the infant experienced an intense period of physiological stress (Beaumont & Montgomery, 2016; Halcrow et al., 2018; King et al., 2018; Mbeki, Kootker, Kars, & Davies, 2017; Mekota, Grupe, Ufer, & Cuntz, 2006; Neuberger, Jopp, Graw, Püschel, & Grupe, 2013). We believe that the second option, physiological stress, is likely the cause of Xiyasi M205's high nitrogen value from the first months of life. Although we cannot rule out that this boy's mother may have had a very unusual diet (she would have been an outlier relative to the rest of the population under study to have had δ15N in the range high enough to have a breastfeeding baby reach δ15N = 15‰) we think this is unlikely given that the boy's second dentin sample drops significantly and then continues to slowly decline and stabilize, suggesting that breast milk was still a portion of his diet and therefore his mother's nitrogen values were similar to the rest of the adult female population (around 7–8‰). We believe the high nitrogen isotope value reflects a brief but significant period of stress after birth where the infant was being breast‐fed but also was catabolizing his own bodily tissues to provide him sufficient energy requirements for survival (Beaumont & Montgomery, 2016; King et al., 2018; Mekota et al., 2006; Neuberger et al., 2013). Luckily, this period was short‐term and likely concluded before his first birthday as his second dentin sample shows a much lower nitrogen value, within the range of his peers. Xiyasi M205 survived this early‐life stress event and lived into older age, dying after the age of 50, and was given a moderate level of mortuary treatment. Future study of linear enamel hypoplasia and other pathological skeletal indicators may further illuminate the roles of diet and nutrition on physiological stress for these communities.
5 CONCLUSIONS
Feeding children goes beyond biological necessity and into the realms of sociopolitical action: what one feeds a developing body needs to be both nutritionally adequate and culturally acceptable (Bentley et al., 1991). Therefore, examining childhood dietary practices provides a unique perspective on a critical period of human development where a body is being fed both to grow healthily and to grow into a body that is socially appropriate. Using isotopic data, we can reconstruct breastfeeding, weaning, and childhood dietary practices for archaeological populations and gain important insights into how ancient peoples raised their families and helped them grow in socially meaningful ways. In this study we have seen how people living in the ancient Zhenghan city during the Eastern Zhou period had access to an array of foods but that some of these foods were consumed in greater proportions during certain periods of their life, and some foods were also variably consumed based on an individual's sex.
We hypothesized that weaning was completed between 2 and 4 years of age, and our dentin isotopic data indicate that most Eastern Zhou children from these urban sites were weaned between 2.5 and 4 years, following the pattern documented in other ancient Chinese populations (Xia et al., 2018; Yi et al., 2018). We also hypothesized that dietary differences between females and males would begin in childhood and would be recorded as isotopically distinct diets, and indeed the isotopic data suggest that boys consumed more millet than girls beginning in childhood. Finally, the comparison of dentin and bone collagen isotope data from the same individuals show that there was significant dietary change over the lifetime for all people, with childhood diet reflecting greater wheat and soybean consumption, and adulthood diet shifting to greater millet consumption.
Many archaeologists are interested in the timing and reasoning for why particular plants are adopted and incorporated into cultures where they are novel and not natively cultivated but over time may become central components of local diet (Boivin, Fuller, & Crowther, 2012; Jones et al., 2011; Liu et al., 2014; Liu & Jones, 2014). Various theories about crop adoption have been proposed ranging from ecological theories (such as drought tolerance, crop yield relative to land usage, human/land energy efficiency models) to more socially‐oriented theories (such as exotic foods as high‐status foods which over time become normalized into general population diet). It is most probable that the incorporation of particular foods that ultimately become dietary staples reflects both ecological and human sociopolitical–ideological factors. During the Bronze Age Eastern Zhou period on the Central Plain of China, wheat and soybean were finding their place within regional Chinese cultures and cuisines, and perhaps for the people living in and around the ancient Zhenghan City, these new plants began to find their place with feeding children.
The isotopic profiles indicate significant variation in diet across childhood (Figure 3). This is not entirely surprising given the diversity of adulthood diets captured in the same individuals' bone collagen data (and the larger sampling of each population by Dong et al., 2017). The major food groups of millet, wheat, soybean, pig, cattle/ox, dog, horse, sheep, and more, transverse large ranges of δ13C and δ15N, and various combinations of these foods can produce the same isotopic value in skeletal tissues (this is the issue of equifinality, where different isotopic combinations can provide the same result, see King et al., 2018; Quinn, 2019). Therefore, interpreting changes in δ15N and δ13C becomes more challenging as we see the dietary diversity observed in Eastern Zhou adults is also reflected in their childhood diets.
4.2.1 Nitrogen isotopes over the lifetime
For most of the individuals studied, after weaning concludes the δ15N dentin values flatten and remain relatively stable (almost all individuals show this pattern by age 5, therefore we use age 5 as the starting point for where children's diets no longer include breast milk). In most individuals' isotopic profiles (Figure 3), we see that the δ15N values from the later‐forming dentin samples are very close to, if not overlapping with, the δ15N value of the bone collagen sample representing their adulthood diet (average diet for the decade prior to death). This suggests relative stability in dietary protein sources after age ~5 years old through adulthood until death, though there may have been fluctuations during years that our samples do not account for. A few of the individuals from the Changxinyuan cemetery (Cxy M067, Cxy M049, Cxy M045), show more protracted declines in δ15N across their dentin profile, continuing past the estimated age of cessation of breastfeeding. This suggests their dietary protein intake was almost continuously changing across their childhood. However, even these individuals have their final dentin δ15N values approaching their bone collagen δ15N values, potentially suggesting relative stability in dietary protein from that point in later childhood onward in their lifetime.
We calculated the average of the dentin isotope values for each individual after age 5 (Tables 1 and S2) and then grouped the females and males and calculated the isotopic average for each sex during childhood. We found that Xiyasi females average δ15N post‐age 5 years was 7.5‰, and the Xiyasi males average δ15N was slightly higher at 7.8‰ (see loess curves on Figure 2, female curve is slightly lower than males over the entire childhood period covered by these samples). These data indicate protein consumption was relatively similar for both sexes from childhood through adulthood (i.e., no significant difference in protein consumption between the sexes for Xiyasi individuals at any age). The average dentin δ15N after age 5 for Changxinyuan boys was 7.8‰, while the average girl's dentin δ15N after age 5 was 6.3‰. The nitrogen loess curve for Changxinyuan females is notably below the males, and this pattern suggests that Changxinyuan boys may have consumed more animal protein than girls and/or consumed proteins from higher trophic positions (with higher δ15N values). However, we caution that the Changxinyuan group is a small sample and therefore these findings may not be representative of the greater population dietary patterns.
4.2.2 Carbon isotopes over the lifetime
During childhood, we see significant variability in δ13C dentin values, indicating urban Eastern Zhou children were fed across both C3 and C4 plant groups (Figure 3). Importantly, the δ13C values are generally more negative during childhood in comparison to the same individual's later adulthood bone collagen δ13C value. This indicates a greater consumption of C3 foods during childhood, and that as these individuals aged their diets changed and incorporated more C4 foods. We suggest that specific C3 foods such as wheat and soybeans were given to young children as important components of their diet while they were growing and developing, and that in later life their diets shifted to include more C4 foods (millets) which became registered in their bone collagen before death.
We also see that during childhood, the δ13C values of boys are slightly higher than the values of girls, indicating that boys are being fed more C4 foods than their sisters, cousins, and female peers. The average dentin δ13C value of Xiyasi boys (after age 5) was −13.6‰ while the average Xiyasi girl dentin δ13C after age 5 was −14.8‰, indicating greater consumption of C4 foods by Xiyasi boys than girls. We can see this pattern in the loess curve plotted in Figure 2, where the male blue curve is higher after weaning than the female curve, indicating more influence from C4 foods for boys. Within the Changxinyuan group, two of the males (CxyM049, CxyM026) show a different pattern, with dentin δ13C values that are mostly less negative during childhood than their adulthood bone collagen values, indicating greater C4 (millet) consumption during childhood followed by a slightly more mixed diet with more C3 inputs during adulthood. There is a very notable difference in the δ13C values between boys and girls for Changxinyuan: the average δ13C for girl's dentin (after age 5) is −14.2‰, while the average δ13C for boy's dentin (after age 5) is −9.8‰. The carbon loess curves (Figure 2) for these children show a significant divergence between the sexes. This large difference in carbon isotope values indicates significant dietary differences between females and males during childhood for these Changxinyuan individuals, with girls consuming much more C3 food than boys. We again must caveat that the Changxinyuan sample is small, but is suggestive of dramatic dietary differences between boys and girls beginning in early youth.
Figure 2
4.2.3 Dietary differences between females and males over the lifetime
The patterning of δ13C dentin values during childhood relative to adulthood reveal that Eastern Zhou children's diets were distinct from adulthood diets, and that C3 foods were fed to children in greater quantities than what adults consumed (Figure 3). Taken together, these findings are significant because the previous isotopic study of these individuals noted significant dietary differences between males and females during adulthood (Dong et al., 2017), and the dentin data, therefore, suggest that some of these differences are rooted in feeding practices that begin in childhood. Wheat and soybeans may have specifically been used as weaning/childhood foods for Eastern Zhou peoples, with millets being consumed in greater quantities later in life. Comparing the childhood dentin isotope data to each individual's later adulthood bone isotope data show dietary changes occurred over the lifetime and also highlight increasing differentiation between males and females with age, suggesting that food practices were integral to aspects of gender over the lifetime (Appadurai, 1981; Hastorf, 2017; Miller et al., 2018; Richards, 1951; Twiss, 2007; Weismantel, 1988; White, 2005).
Beginning in childhood, boys were fed more millet which then increased significantly at a later point in their lives. One possibility is that dietary changes for males were tied to social age/status changes over their lifetime, such as the coming‐of‐age “capping” ritual for adolescent/young adult men (Chu & Buckley Ebrey, 1991; Hardy, 1993; Marshall, 2003; Rouzer, 2001). This ritual is written about more extensively in the Han Dynasty period but is believed to have had its roots in earlier periods, perhaps beginning in the Bronze Age Eastern Zhou (Hardy, 1993). Males would be ceremoniously capped (in their later teenage years) in a ritual marking their transition into adulthood and heralding their new responsibilities and rights that change in status afforded them (Chu & Buckley Ebrey, 1991; Hardy, 1993; Marshall, 2003; Rouzer, 2001). Perhaps dietary changes accompanied this social age transition to adulthood and Eastern Zhou young men then consumed greater amounts of millet as adults (potentially both as food and in fermented alcoholic beverages). This is a hypothesis that could be tested with a targeted sampling of adult males who died across different age groups using both bone and multiple tooth samples (such as later forming third molars which potentially could capture the period pre‐ and post‐capping).
In childhood, Eastern Zhou girls ate across C3 and C4 food groups but consumed more C3 foods such as wheat and soybean than boys, which continued for these girls into adulthood. One explanation for the retention of more C3 foods in the diets of women may be related to gender divisions of labor, with women responsible for domestic work including household maintenance, food preparation, child care responsibilities, and textile production (Feng, 2013; Hinsch, 2018; Linduff & Sun, 2004; von Falkenhausen, 2006). Archaeological evidence of a gendered division of labor has primarily been inferred from objects found in tombs (Hinsch, 2018; Linduff & Sun, 2004; von Falkenhausen, 2006), however recent bioarchaeological analyses have made significant contributions to our understandings of gendered labor as well (He & Tang, 2015; Linduff & Sun, 2004; Mu & Chen, 2018; Zhang et al., 2017). Additionally, historical Chinese texts suggest that during the Eastern Zhou, women and men's daily lives were increasingly structured along divisions of labor which kept the sexes separated in time and space (“men grow grain and women produce cloth” 男耕女织; Hinsch, 2003, p. 598), and this may have had gendered consequences on the daily dietary practices of each sex (Hinsch, 2018; Linduff & Sun, 2004). If women's roles primarily focused on activities typically relegated to the domestic sphere (Hinsch, 2003, 2018; Linduff & Sun, 2004), and women were the primary cooks for the family, they may have eaten the same meals they fed to their children. Perhaps their husband ate most of these same meals as well, but may have received slightly different portions of particular parts of the meals, such as slightly more meat, and/or more millet, which would cause small but important changes to their bodily isotope chemistry, differentiating Eastern Zhou boys and men from girls and women. Further, it is common for caretakers to demonstrate to young children that a particular meal is good by eating some of it themselves (parental/social modeling), and this may provide one explanation for the retention of more C3 foods in female diets over the lifetime (Birch, 1999; Birch, Savage, & Ventura, 2007; Blissett & Fogel, 2013; Larsen et al., 2015). Taken together, these dietary data indicate that practices of differentiation between boys and girls began early in life through dietary habits where particular foods were associated with gender, and these gendered dietary differences intensified over the lifetime leading to the significant dietary differences noted between urban Eastern Zhou adult men and women (Dong et al., 2017).
The Zhou periods are now viewed as a major cultural shift in early Chinese history, with significant changes to ideology, religion and ritual, and political governing (Feng, 2013; Hinsch, 2018; von Falkenhausen, 2006). Separation of the sexes became increasingly codified in early Chinese society, particularly during the Zhou periods in response to what had been seen as cultural indulgences (such as excessive alcohol consumption) of the earlier Shang Dynasty elites (Feng, 2013; Hinsch, 2003, 2018). Coming‐of‐age rituals, such as capping for men and pinning for women (a ceremony at age 15 involving a change in hairstyle and signifying a social age status change to adulthood for young women), served to reinforce social norms differentiating between the sexes and their roles within the family and the greater social order (Chu & Buckley Ebrey, 1991; Hardy, 1993; Hinsch, 2003, 2018). Hinsch (2003, 2018) explains that the separate spheres of women's and men's daily lives were seen as an important aspect of Chinese cultural identity from this time period (from Eastern Zhou and into the Han Dynasty) and therefore we may be seeing dietary effects of these societal divisions in the isotopic data recorded in the teeth and bones from Xiyasi and Changxinyuan individuals.
Figure 3
4.3 A possible case of early life stress
One male individual, Xiyasi M205, had an exceptionally high dentin nitrogen value which stands out from the rest of the group. The first dentin sample for this individual (estimated median age of sample = 4 months) has a δ15N value of 15.0‰, which is 3‰ higher than the next highest δ15N value in the samples analyzed in this study. The second sample for Xiyasi M205 drops to δ15N = 9.7‰, at estimated age 1.2 years, and then maintains a relatively steady lower δ15N value for the subsequent years sampled (average δ15N value after age 5 years = 8.5‰). The first tooth section analyzed was formed during early infancy and therefore the high nitrogen value is a product of something that occurred in the first months after birth. There are two main theories that are typically proposed to explain high nitrogen values in infants: (a) the mother had a protein‐rich diet, potentially consuming marine food sources (or consumed other high isotope value protein sources), or (b) the infant experienced an intense period of physiological stress (Beaumont & Montgomery, 2016; Halcrow et al., 2018; King et al., 2018; Mbeki, Kootker, Kars, & Davies, 2017; Mekota, Grupe, Ufer, & Cuntz, 2006; Neuberger, Jopp, Graw, Püschel, & Grupe, 2013). We believe that the second option, physiological stress, is likely the cause of Xiyasi M205's high nitrogen value from the first months of life. Although we cannot rule out that this boy's mother may have had a very unusual diet (she would have been an outlier relative to the rest of the population under study to have had δ15N in the range high enough to have a breastfeeding baby reach δ15N = 15‰) we think this is unlikely given that the boy's second dentin sample drops significantly and then continues to slowly decline and stabilize, suggesting that breast milk was still a portion of his diet and therefore his mother's nitrogen values were similar to the rest of the adult female population (around 7–8‰). We believe the high nitrogen isotope value reflects a brief but significant period of stress after birth where the infant was being breast‐fed but also was catabolizing his own bodily tissues to provide him sufficient energy requirements for survival (Beaumont & Montgomery, 2016; King et al., 2018; Mekota et al., 2006; Neuberger et al., 2013). Luckily, this period was short‐term and likely concluded before his first birthday as his second dentin sample shows a much lower nitrogen value, within the range of his peers. Xiyasi M205 survived this early‐life stress event and lived into older age, dying after the age of 50, and was given a moderate level of mortuary treatment. Future study of linear enamel hypoplasia and other pathological skeletal indicators may further illuminate the roles of diet and nutrition on physiological stress for these communities.
5 CONCLUSIONS
Feeding children goes beyond biological necessity and into the realms of sociopolitical action: what one feeds a developing body needs to be both nutritionally adequate and culturally acceptable (Bentley et al., 1991). Therefore, examining childhood dietary practices provides a unique perspective on a critical period of human development where a body is being fed both to grow healthily and to grow into a body that is socially appropriate. Using isotopic data, we can reconstruct breastfeeding, weaning, and childhood dietary practices for archaeological populations and gain important insights into how ancient peoples raised their families and helped them grow in socially meaningful ways. In this study we have seen how people living in the ancient Zhenghan city during the Eastern Zhou period had access to an array of foods but that some of these foods were consumed in greater proportions during certain periods of their life, and some foods were also variably consumed based on an individual's sex.
We hypothesized that weaning was completed between 2 and 4 years of age, and our dentin isotopic data indicate that most Eastern Zhou children from these urban sites were weaned between 2.5 and 4 years, following the pattern documented in other ancient Chinese populations (Xia et al., 2018; Yi et al., 2018). We also hypothesized that dietary differences between females and males would begin in childhood and would be recorded as isotopically distinct diets, and indeed the isotopic data suggest that boys consumed more millet than girls beginning in childhood. Finally, the comparison of dentin and bone collagen isotope data from the same individuals show that there was significant dietary change over the lifetime for all people, with childhood diet reflecting greater wheat and soybean consumption, and adulthood diet shifting to greater millet consumption.
Many archaeologists are interested in the timing and reasoning for why particular plants are adopted and incorporated into cultures where they are novel and not natively cultivated but over time may become central components of local diet (Boivin, Fuller, & Crowther, 2012; Jones et al., 2011; Liu et al., 2014; Liu & Jones, 2014). Various theories about crop adoption have been proposed ranging from ecological theories (such as drought tolerance, crop yield relative to land usage, human/land energy efficiency models) to more socially‐oriented theories (such as exotic foods as high‐status foods which over time become normalized into general population diet). It is most probable that the incorporation of particular foods that ultimately become dietary staples reflects both ecological and human sociopolitical–ideological factors. During the Bronze Age Eastern Zhou period on the Central Plain of China, wheat and soybean were finding their place within regional Chinese cultures and cuisines, and perhaps for the people living in and around the ancient Zhenghan City, these new plants began to find their place with feeding children.