Investigating the Plant Microremains on Ceramic Li Tripods to Ascertain Their Function and the Plant Resource Exploitation Strategies During the Lower Xiajiadian Culture Period in Chifeng, Northeast China

Introduction

The West Liao River Basin in Northeast China occupies an important role as the birthplace of the Chinese civilization, and it has been the home of several archaeological cultures since the Neolithic Age, including the Xiaohexi, Xinglongwa, Zhaobaogou, Hongshan, and Lower Xiajiadian cultures (Su and Yin, 1981; Xu et al., 2013; Jia et al., 2017a). Among them, the Lower Xiajiadian culture is represented by the Xiajiadian sites in Chifeng city, which ushered in the Bronze Age and spanned the Erlitou culture to the early Shang Dynasty (ca. 3,900–3,400 cal. B.P.) (Zhang et al., 1987; Li, 1990; Wang et al., 1993). The Lower Xiajiadian culture was widely distributed in Chifeng, and 1,321 Lower Xiajiadian culture sites have been discovered in the Aohan Banner alone (Han, 2010). The archaeological sites of this culture surpass those of any other period in terms of their quantity and density, and the stratum associated with the culture is thick and contains widespread ancient urban settlements (Li and Gao, 1984).

Compared with the cylindrical pottery found in Chifeng during the thousands of years between the Xiaohexi culture and the Xiaoheyan culture in the Neolithic Age, the Lower Xiajiadian culture is mainly characterized by ceramic pottery vessels with three feet that are referred to as Li tripods. It has long been acknowledged that Li tripods were used for cooking (Liu, 1989; The Palace Museum, 2014). In recent years, ancient books and records, archaeological typology, and residue analyses have provided important clues about the functions of Li tripods. For instance, according to a typological study of Li tripods in the Yellow River Basin of the Longshan Period, it is believed that the tripod-shaped containers developed in response to practical cooking requirements (Gao, 1996). Moreover, an analysis of the starch grains from the Li tripods of the Bronze Age unearthed at the Dongzhao site in Zhengzhou, Henan province indicates that they were used for cooking rice (Oryza sativa) and other crops (Sun, 2018). Although large numbers of Li tripods from the Lower Xiajiadian cultural period have been unearthed in Chifeng, studies on their functions are still limited. Most studies focus on determining their cultural period and origins (Lin, 1995; Zhao, 2005), e.g., the development of Li tripods in Chifeng is believed to have integrated the Hongshan culture and the Erlitou culture (Liaoning Provincial Museum et al., 1983; Li and Gao, 1984).

In terms of the functions of Li tripods, the carbonized hulled broomcorn millet (Panicum milliaceum) found in a Li tripod unearthed at the Sifendi Dongshanzui sites in Chifeng suggests that they were likely used for cooking. However, this does not exclude the possibility that they were used as storage vessels. Therefore, more direct evidence is required to clarify the functions of these tripods. In this respect, plant microremains (including starch grains and phytoliths) can be analyzed to study the functions of pottery (Yang et al., 2014) and accurately identify crop types (Yang et al., 2006), as they are preserved for extensive periods (Torrence and Barton, 2006) and in large quantities (Wang and Lu, 1993). For instance, identifying the phytolith of foxtail millet (Setaria italica) and broomcorn millet has provided detailed information about the crop species used (Lu et al., 2009a). In addition, it is possible that starch grains and phytoliths produced by different plant organs remain within artifacts that were used to process husked seeds, roots, and tubers, even when residual phytoliths were lacking (Wu et al., 2011). In recent years, microscopic analyses conducted to identify residues on the surface of pottery and determine the morphology and species of plants used have provided direct evidence that can be used to understand the functions of such artifacts, the ingredients used, and how plant resources were utilized (Yang et al., 2014; Wang et al., 2017). Moreover, scholars have also studied the history of the human use of fire and have reconstructed ancient vegetation using the microfossil charcoal method (Patterson et al., 1987; Innes and Simmons, 2000; Li et al., 2010). Furthermore, the surface of most cooking utensils has traces of soot left from burning. As a charcoal residue produced by incomplete combustion of plant tissue, soot provides a reference or indication of the cooking function of Li tripods.

To broaden our knowledge of the subsistence strategies and social landscape of people in the early Bronze Age, starch grain, phytolith and microfossil charcoal residues were extracted from the surfaces of Li tripods of the Lower Xiajiadian cultural period unearthed in Chifeng, Northeast China. These residues were then analyzed with the intention of better understanding the functions of Li tripods and how the ancient people of Lower Xiajiadian culture used edible plant resources.

Materials and Methods

Sample Collection

To analyze plant microremains, samples were collected from the feet of 10 Li tripods (Figure 1A) that had been previously unearthed from six sites in Chifeng, Northeast China: Mount Aobao Beiliang, Hadagou Nanliang, Fangshen Xiliang, Tuanjie Yingzi Mount Tuzi, Dujiadi, and Gaojia Wazi. The 10 samples were similar in shape, and residues and soot on the surface were visible to the naked eye (Figure 1B). All samples were from the Lower Xiajiadian cultural period, and they have been preserved at the Wengniute Museum.

FIGURE 1

FIGURE 1. Site locations from which artifacts were obtained and the samples subsequently analyzed. (A) Location of study region and archaeological sites: (a), Mount Aobao Beiliang site; (b), Hadagou Nanliang site; (c), Fangshen Xiliang site; (d), Tuanjie Yingzi Mount Tuzi site; (e), Dujiadi site; (f), Gaojia Wazi site. The sketch is of a basin-shaped Li Tripod, which is typical of an Li Tripod from the Lower Xiajiadian cultural period (Lin, 1995). (B) Tripod foot samples from the Lower Xiajiadian cultural period unearthed from Chifeng, Northeast China at the following sites: (1, 2), Mount Aobao Beiliang site; (3, 4), Hadagou Nanliang site; (5, 6), Fangshen Xiliang site; (7, 8), Tuanjie Yingzi Mount Tuzi site; (9) Dujiadi site, Specimen no. 150426-0230; (10) Gaojia Wazi site, Specimen no. 15046-0231. (I indicates the interior surface of Li tripods, E indicates the exterior surface of Li tripods, and the arrow indicates the sampling location of the residues. Scale bar: 5 cm).

The extraction of residues on the pottery surfaces was undertaken according to the protocols of Yang et al. (2014) and Liu et al. (2014). Twenty samples were analyzed. As the selected samples had been preserved at the Wengniute Museum, the surface was cleaned prior to sampling, that is, both the interior and exterior were rinsed with ultrapure water. To dislodge adhering sediment and starch, the interior surface and exterior surface of each tripod foot were separately shaken in an ultrasonic bath for 10 min, and liquid samples were obtained. To prevent cross-contaminating the samples during sampling process and the experiment, all knives, pipettes, centrifuge tubes, and powder-free gloves used were disposable and none contained starch.

In addition, although previous researchers have taken many images of starch grains from plants commonly used by ancient people, only a few studies have focused on starch grains damaged during processing. Therefore, we collected modern specimens of common Chinese crops (foxtail millet, broomcorn millet, and common wheat), Leguminosae, Liliaceae, and Fagaceae, and conducted processing simulation experiments. These samples were obtained via field acquisition and market purchase.

Starch and Phytolith Extraction

The plant microremains in the residues on the surfaces of tripod foot samples were extracted following steps previously reported (Lu et al., 2009b; Yang et al., 2012a; Yang et al., 2012b). Briefly, 6% H₂O₂ was added to the liquid samples of surface residue to break down some of the larger charred particles via oxidation and to release any starch grains potentially trapped within the particles or adhering to them. HCl (10%) was then added to remove calcium impurities. Then, the heavy liquid part of 1.8 g/cm³ CsCl was added, mixed carefully, and centrifuged for 5 min at 2,500 rpm in order to extract the starch grains. The supernatant containing starch granules was then decanted into a fresh tube. Phytoliths were extracted from sediments remaining following starch grain extraction. For the second float, 2.35 g/cm³ ZnBr₂ heavy liquid was added, mixed thoroughly, and centrifuged, and the supernatant containing the phytoliths was decanted into a fresh tube. Starch granule slides were prepared by mounting the recovered residue onto a slide in a solution of 25% glycerin and 75% ultrapure water. The slide was then sealed with neutral gum. Phytolith slides were prepared by mounting the recovered residue in neutral resin and then fixing it with a cover glass.

Analysis of Microfossil Charcoal

We detected microfossil charcoal together with the phytoliths and starch grains in the residue on the surface of the Li tripod feet. Next, we counted the total number of microfossil charcoal grains contained in the slides of the starch grains and phytoliths. Subsequently, following the conventions of microfossil charcoal analysis (Wang Z. S. et al., 2020), we randomly extracted 50 grains for each sample. Then we measured the length and width of the microfossil charcoal particles to calculate the length to width ratio (hereinafter referred to as the L/M ratio) for the identification of plant species. Ideally, 50 random grains were measured for the interior and exterior surfaces of each tripod foot sample, starting from the first grain identified. However, if less than 50 grains were identified, all of the grains were measured. Ultimately, we obtained a total of 772 sets of measurement data to determine the L/M ratio range and average L/M ratio for statistical analysis.

Grinding and Cooking Simulation Experiments

The seeds of modern foxtail millet, broomcorn millet, common wheat (Triticum aestivum), and lily (Lilium Brownii) bulbs were selected as samples. They were cleaned and dried after we confirmed that no traces of other species were present. To observe the influence of grinding on starch grains, 0.5 g of each sample was placed into a mortar and ground for 2–5 min, thus obtaining four samples for the grinding simulation experiment. The ground samples were then added to a centrifuge tube with 5 ml of ultrapure water and placed in a low-temperature environment for 1 h. To observe the effect of cooking on the starch grains, 0.5 g of each sample was added to a centrifuge tube with 5 ml of ultrapure water for 5–30 min of cooking in a water bath. The timer was started when the water began to boil, and with intervals of 5 min, 24 samples (six groups) were obtained for the cooking simulation experiment. The appropriate amount of starch suspension generated during the grinding and cooking simulation experiment was extracted with a disposable pipette, and a few drops of it were then added to a glass slide before 25% glycerin was added, and the slide was then sealed with neutral gum.

The prepared slides of starch grains and phytoliths were dried at ≤40°C for 48 h. The samples were then observed, identified, photographed, and counted using a Leica DM 750 microscope at ×400 magnification. The analyses were completed at the Environmental Archaeology Laboratory of Northwest University. The identification and classification of starch grains were mainly conducted by referring to the accumulated modern starch grain database of the laboratory and relevant study results (Yang et al., 2010; Wan et al., 2012; Yang andPerry, 2013; Ma et al., 2014). To identify and describe phytoliths and microfossil charcoal, they were compared with published pictures and materials (Wang and Lu, 1993; Lu and Liu, 2003; Piperno, 2006; Zhang and Lu, 2006; Lu et al., 2009a). All starch grains, phytoliths and microfossil charcoal on glass slides were evaluated and C2 v1.7.3 (Juggins, 2003) software was used to process data and obtain the percentage contents of phytoliths.

Results

Starch Grains From Residues on the Surfaces of Li Tripod Foot Samples

Starch grains were identified from the interior surfaces of all 10 tripod foot samples. Starch grains were identified in only three samples from the exterior surfaces of the tripod feet. A total of 133 starch grains were extracted from the residues found on the interior and exterior surfaces of the tripod feet. Of these, 71 starch grains were seriously damaged and identification was not possible, these were classified as Type IV. The remaining identifiable 62 starch grains were classified into Types I–III based on their shapes and grain sizes. Of the 62 identifiable starch grains, 40 showed slight damage (Table 1). There were 111 damaged starch grains in total, accounting for 83.46% of all extracted starch grains. The definitions of grain types and associated findings are as follows.

TABLE 1

TABLE 1. Data of starch grains obtained from the surface residues of Li tripods of the Lower Xiajiadian cultural period unearthed from the Chifeng area, Northeast China (n).

Type I starch grains (n = 43) were polyhedron-shaped with centric hila and linear, transverse, star, or Y-shaped fissures through the hila of most grains. Extinction crosses were mostly “+“-shaped with straight arms (Figures 2A–D′). The maximum lengths ranged from 9.8 to 20.1 μm, with a mean length of 14.8 ± 2.7 μm. The extinction crosses of 25 starch grains were unclear in polarized light (Figures 2E–F′) and there were cracks on the surfaces of four grains (Figures 2E,F). According to the identification criteria established by Yang et al. (2010), which are based on data analysis of starch grain shapes of Chinese northern modern foxtail and broomcorn millets, and other close wild species, Type I grains were considered to belong to millet, because the surfaces were smooth and the grain sizes were larger than average. The results thus suggest that the starch grains were most likely from foxtail and broomcorn millet crops.

FIGURE 2

FIGURE 2. Starch grains extracted from the surface residues of Li tripods of the Lower Xiajiadian cultural period unearthed from Chifeng, Northeast China, and starch grain specimens from simulation experiment. (A–D′) Type I intact starch grains and (E–F′) Type I damaged starch grains. (G) Polar view of type II starch grains, (H) lateral view of type II starch grains, and (I–J′) Type II damaged starch grains. (K,K′) Type III starch grains. (L–O′) Type IV starch grains. (P,P′) Thermally damaged lily starch grains after 5 min of heating. (Q–S′) Modern foxtail millet starch damaged by heating: (Q,Q′) heated for 5 min, (R,R′) heated for 15 min, and (S, S′) heated for 30 min (T–V′) Modern broomcorn millet starch damaged by heating: (T,T′) heated for 5 min, (U,U′) heated for 15 min, and (V,V′) heated for 30 min (W–Y′) Common wheat starch damaged by heating: (W,W′) heated for 5 min, (X,X′) heated for 15 min, and (Y–Z′) heated for 30 min (AB,AB’) Common wheat starch damaged by grinding; (AA,AA′) modern foxtail millet starch damaged by grinding; and (AC,AC′) lily starch damaged by grinding. (A,A′–F, F'; I,I′–AC,AC′) Each group of photos was taken separately under bright field (left) and cross-polarized light (right). (Arrows indicate the damage characteristics of starch grains, Scale bar: 10 μm)

Type II starch grains (n = 17) were characterized by biconvex shapes and a centric hilum. The starch grains were olivary and possessed a longitudinal dent when rotated (Figures 2G,H). The extinction crosses of two starch grains were X-shaped in polarized light, and those of the remaining 15 starch grains partially disappeared in polarized light (Figures 2I–J′). The grain lengths ranged from 12.9—35.9 μm with a mean of 23.1 ± 4.6 μm. According to the identification criteria established by Piperno et al. (2004) and Yang and Perry (2013) based on the study of such starch grains, it was determined that Type II starch grains were from the tribe Triticeae.

Type III starch grains (n = 2) were oval with an extremely eccentric hilum. There were no cracks, and the arms of extinction crosses were slightly bent (Figures 2K,K′).The grain lengths were 19.53 and 30.31 μm, respectively. In modern specimen banks, the grain sizes of roots and tubers are relatively large, and the hilum is skewed to one end (Wan et al., 2011). Therefore, it was determined that Type III starch grains originated from roots and tubers.

Finally, the extinction characteristics of Type IV starch grains (n = 71) were not visible. The surfaces of most were unclear, in an aggregated state, overlaid, and lacked an independent outline (Figures 2N–O′). The central parts of some of the starch grains were sunken or hollow, but the basic outline was identifiable (Figures 2L–M′). They were classified as Type IV because the species could not be identified.

Starch Grains Obtained From Grinding and Cooking Simulation Experiments

The results of the grinding simulation experiments of modern foxtail and broomcorn millet, common wheat, and lily starch grains are consistent with that of our previous study (Ma et al., 2019). The observational and statistical results identify the following. 1) Grinding caused physical and mechanical damage to some starch grains, resulting in morphological alterations. The main manifestations were that the relatively smooth surface of the starch grains became coarse, the edges became cracked and broken, and/or the central areas on the extinction crosses became dark (Figures 2AA–AC′). 2) The grinding strength was positively correlated with the content of the damaged starch grains. 3) After grinding, the extinction crosses of the starch grains were distorted, but the position of the hilum remained unchanged.

The results of starch grains from the cooking simulation experiments are consistent with that of previous studies (Ge et al., 2010; Hong et al., 2013; Lu et al., 2014; Chen et al., 2021). The crystal structure of the starch grains was thermally damaged and their central parts became sunken. The starch molecules gradually became transparent as they expanded and became deformed. As the heating duration increased, the number of small visible grains declined and the number of large grains increased. The average size of the foxtail millet starch grains increased to 15.18, 20.57, and 27.85 μm after being heated for 5, 15, and 30 min, respectively (Figures 2Q–S′). This indicates that the grain size of foxtail millet doubled throughout the cooking procedure and became similar to that of broomcorn millet (Figures 2T–V′). The starch grain size of common wheat increased to 21.14 and 41.20 μm after being heated for 5 and 30 min, respectively (Figures 2W–Y′). The starch grain size of lily increased to 40.88 μm after being heated for 5 min, which is close to its size after being heated for 30 min (48.87 μm; Figures 2P,P′).

There was a positive correlation between heating duration, moisture content, and gelatinization of the starch grains. Observations of the seed samples showed that the starch grains became transparent and the extinction crosses were unclear after heating for 5 min (Figures 2Q,Q′,T,T′,W,W′). After heating for 15 min, the starch grains were in an aggregated state with increased transparency, and although the extinction crosses had completely disappeared, their basic outlines were maintained (Figures 2R,R′,U,U′,X,X′). After heating for 30 min, the independent outline was lost, the starch grains were almost completely transparent, and the extinction characteristics disappeared (Figures 2S,S′,V,V′,Y,Y′). Lily starch granules were seriously damaged and their identification characteristics were lost after being heated for 5 min (Figures 2P,P′). Finally, grinding damage and cracks were observed in some of the starch grains (Figures 2Z,Z′).

Results of Phytolith Analysis

A total of 3,424 phytoliths were extracted from the study samples, of which 107 were damaged and could not be identified. Figure 3 shows the main phytolith shapes, which include 17 types. There were 692 pointed phytoliths, and they had the highest total phytolith content of 20.21% (Figure 3O), followed by smooth elongates (15.13%; Figure 3M), sinuate elongates (12.44%; Figure 3N), bilobates (11.33%; Figure 3I), rondel-shaped phytoliths (10.86%; Figure 3G), square phytoliths (8.82%; Figure 3P), epidermal cells (3.45%; Figures 3S,T), trapezoid phytoliths (1.31%; Figure 3H), fan-shaped phytoliths (0.50%; Figure 3Q), cross-shaped phytoliths (0.18%; Figure 3L), cylindrical polylobate phytoliths (0.09%; Figure 3K), silicified stomata (0.09%; Figure 3R), and dumbbell-shaped phytoliths arranged vertically (0.06%; Figure 3J). In addition, η-type phytoliths from the husks of broomcorn millet (5.46%; Figures 3B), O-type phytoliths from the husks of foxtail millet (0.73%; Figure 3A), and phytoliths from the husks of the family Poaceae (6.22%; Figures 3C–F) were extracted from the study samples.

FIGURE 3

FIGURE 3. Phytoliths extracted from the surface residues of Li tripods from the Lower Xiajiadian cultural period unearthed in Chifeng, Northeast China. (A) Ω type phytoliths extracted from the husks of foxtail millet, (B) η type phytoliths extracted from husks of broomcorn mille. (C–F) Grain shells of the family Poaceae: (C) Phytoliths that are most likely from the husks of Pooideae; (D–F) Phytoliths that are most likely from the husks of Panicoideae. (G) Rondel, (H) Trapezoid, (I) Bilobate, (J) Dumbbell-shaped phytolith arranged vertically, (K) Cylindrical polylobate, (L) Cross, (M) Smooth-elongate, (N) Sinuate-elongate, (O) Pointed, (P) Square, (Q) Fan, (R) Silicified stomata, and (S,T) Epidermal cells. (Scale bar: 10 μm).

(I indicates the interior surface of Li tripods, E indicates the exterior surface of Li tripods).

Results of Microfossil Charcoal Analysis

Microfossil charcoal grains were identified on both the interior and exterior surfaces of the 10 Li tripod foot samples, and a total of 2,835 grains were detected (Table 2).

TABLE 2

TABLE 2. Quantitative statistics and measurement data of microfossil charcoal.

Microfossil charcoal analysis revealed three main microfossil charcoal shapes: the strip shape, square shape, and approximately circular shape (Figure 4. The longitudinal edge and transverse fracture of the strip-shaped charcoal fossils were relatively straight, with many pore structures (Figures 4A–C). The square and approximately circular shaped charcoal fossils were relatively dense, with few pores, and the edges were irregular (Figures 4D–F). The length of the microfossil charcoal ranged from 5.49 to 148.59 μm, with an average length of 22.26 μm. The L/M ratio ranged from 1 to 5.79 μm, with an average of 1.89 μm (Table 2).

FIGURE 4

FIGURE 4. Morphology of microfossil charcoal. (A–C) Possible microfossil charcoal from grasses, (D–F) Possible charcoal from woody plants. (Scale bar: 10 μm).

According to criteria for the identification of plant species, based on morphological characteristics and the L/M ratio proposed by Zhang and Lu (2006), strip-shaped microfossil charcoal may originate from grasses, while square-shaped or approximately circular specimens may derive from woody plants. After oscillation and centrifugation in the laboratory, the average L/M ratios of microfossil charcoal for grasses and woody plants were 3.90 ± 0.1 μm and 1.8 ± 0.1 μm, respectively. For the microfossil charcoal residue on the surface of the tripod feet, the average L/M ratio was similar to that of microfossil charcoal from woody plants. In the 772 sets of measured data, only 16 microfossil charcoal grains had a length-width ratio greater than 3.90 μm, which accounted for 2.07% of the total measured grains.

Discussion

The tripod foot samples had been preserved in a museum, and the chance of contamination from deposits in the surrounding environment would have been equal for all the tripod surfaces. We therefore extracted samples from both the interior and exterior surfaces of each tripod foot and compared the quantities and assemblages of starch grains and phytoliths separately to determine the possibility of contamination from surrounding deposits. Table 1 and Figure 5 show the absolute quantity and appearance frequency of starch grains (n = 130; 100%) and phytoliths (n = 2,388; 100%) that were extracted from the interior surfaces of Li tripod foot samples. It is evident that the amounts are considerably greater than those collected from the exterior surfaces (only three starch grains and 1,036 phytoliths were collected). The discrepancies between the numbers thus suggests that most of the starch grains and phytoliths collected from the tripod feet were directly related to the use and behavior of people from the ancient culture.

FIGURE 5

FIGURE 5. Changes in percentages of phytolith types extracted from the surface residues of Li tripods from the Lower Xiajiadian cultural period unearthed in Chifeng, Northeast China.

Functions of Li Tripods

A total of 111 damaged starch grains were identified in the residues on the surfaces of the 10 tripod foot samples (Table 1). They accounted for 83.46% of the total grains, and they appeared on 80% of the interior surfaces. This result suggests that the starch grains were damaged during processing. To explore processing methods and obtain specimen images for comparison with ancient, damaged starch grains, we referred to previous research (Crowther, 2012; Ge et al., 2020; Chen et al., 2021). Ancient food processing methods are classified into two major types: dry cooking (e.g., roasting and baking) and wet cooking (e.g., boiling). Comparative studies indicate that when the dry cooking method was adopted, the extinction cross characteristic of starch granules may have been lost, but that the limited amount of water absorption involved may have restricted their morphological deformation (Bradbury and Holloway, 1988; Babot, 2003; Chandler-Ezell et al., 2006). In extreme circumstances, since no water is involved, starch granules can be heated to 200°C without any measurable change occurring (Wang et al., 1991; Jang and Pyun, 1996). The form of the starch grains found in the residues left on the interior surfaces of the tripod feet examined in our study is closer to that of the contemporary starch grain samples used in our cooking simulation experiment. These results indicate that the starch grains in the residues on the surfaces of the tripod feet were mainly subjected to thermal (boiling) damage (n = 111) and mechanical (grinding) damage (n = 4).

The degree and rate of starch gelatinization mainly depend on the moisture and temperature (Crowther, 2012). On one hand, when the starch moisture content falls below 60%, heating under normal pressure may fail to cause total gelatinization (Wang et al., 1991). Additionally, the water content of different plant storage organs varies considerably. For example, the water content in fresh roots and tubers (approximately 70–80%) is far higher than that of cereals (approximately 20–30%) (Hoover, 2001; Crowther, 2012). If cooked under the same conditions, the gelatinization speed of roots and tubers is considerably faster than that of cereals, so it is less likely that starch granules in roots and tubers survive and recover (Crowther, 2012). On the other hand, the temperature range from the start of gelatinization to its conclusion is usually 5–20°C. Thus, the extinction cross characteristic of starch granules that are not completely gelatinized within this interval may be retained (Lu et al., 2014). These findings indicate that even when the cooking mode is the same, the survival, recovery and recognition probability of starch granules from different plants or the same plant vary.

Starch grains with mild gelatinization (Figures 2E–F′,I–J′) were from foxtail millet, broomcorn millet, and the tribe Triticeae, but it was not possible to identify grains that underwent extreme gelatinization (Figures 2L–O′). The shapes and sizes of grains were compared with those from the surface of the tripod, and it was found that those from the tripod feet were closer to the those of the foxtail and broomcorn millets from the cooking simulation specimens (Figures 2Q,Q′,S,S′,T,T′,V,V′). In addition, a handful of starch grains were subjected to mechanical damage (Figure 2Z) during the cooking simulation experiments, even though grinding was not used, and the mechanical force applied was mainly that of stirring. It was thus inferred that the four damaged starch grains (Figures 2E,F) with cracks that were obtained from the residues on the surface of the tripod feet were likely subjected to stirring and squeezing during cooking, or they were from grains that broke during hulling. The identification of gelatinized starch grains in the residues on the surfaces of tripod feet further proves that at least one of the Li tripod functions was cooking plant-based food. It also demonstrates that the starch grains extracted from the experiments were mainly associated with processed cooked food rather than surrounding contamination.

According to previous research (Zhang, 2015) and relevant practical knowledge, if a Li tripod was used as a cooking vessel, there should be traces of incomplete burning of firewood on its surface as a result of cooking, such as common soot or charcoal residue. An analysis of the microfossil charcoal found on both the interior and exterior surfaces of 10 Li tripod foot samples identified different numbers of each. In seven of the samples, the grains detected on the exterior surfaces were far higher than on the interior surfaces (Table 2), indicating that the exterior surfaces had a higher frequency of contact with fire. As for the other three samples, the reasons why the microfossil charcoal grains on the interior surfaces were more numerous than on the exterior surfaces is not clear. However, microfossil charcoal remains were found on the interior surfaces of all the samples, and the corresponding species were likely to include woody plants and grasses. Possible reasons for this phenomenon are as follows. 1. Mostly, a Li tripod without cover was placed on the fire directly for boiling, and the charcoal grains produced by fuel during cooking were very likely to fall into the Li tripod by air. 2, Due to the limited tools available to ancient people for cleaning the tripods, charcoal grains on the outer surface of some Li tripods might have become attached to cleaning tools and thus transferred to the inner surface. The Li tripod foot was likely not easy to clean for space narrowness, and thus, the charcoal grains likely precipitated and became stuck. The analysis results of microfossil charcoal are consistent with the finding that many starch grains were damaged by boiling, as mentioned above. The residue of incompletely burnt charcoal on the surfaces of Li tripods provides further evidence of their cooking function. However, the identification of microfossil charcoal plant species in this study was limited to exploratory experiments, and thus more fundamental experimental studies are required.

In summary, we believe that the Li tripods from the Lower Xiajiadian cultural period unearthed in Chifeng were likely used for cooking foxtail and broomcorn millets as well as plants from the tribe Triticeae. Thus, the food processing methods used by this ancient culture at such sites are revealed. The plant microremains obtained support the use of Li tripods as an important food processing tool during the Lower Xiajiadian cultural period, and it appears likely that they were typically used as cooking utensils.

Plant Resources Revealed by Ancient Plant Microremains

The results suggest that Li tripods of the Lower Xiajiadian cultural period unearthed in Chifeng were used as cooking utensils. The plant microremains on their surfaces signify the food that people used in their recipes. Starch grains of foxtail millet, broomcorn millet, and the tribe Triticeae; roots, and tubers, and phytoliths from the grain husks of foxtail millet and broomcorn millet; and one piece of phytolith that was probably from the husks of Pooideae (Figure 3C) were identified in the surface residues of 10 tripod foot samples. The tribe Triticeae belongs to the Pooideae of the Poaceae subfamily. Furthermore, the phytoliths also included bilobate and cross-shaped phytoliths from the leaves of Panicoideae, trapezoid and rondel-shaped phytoliths from Pooideae leaves, and elongate phytoliths and fan-shaped phytoliths from the family Poaceae (Ge, 2016). Therefore, the analytical results of the starch grains and phytoliths are relatively consistent.

The phytoliths from the husks of foxtail and broomcorn millets and those from the husks of Pooideae were mainly extracted from the interior surfaces of the Li tripods; hence, these crop husks (6.07% of the total) and the phytoliths of the stems and leaves of the Poaceae are quite likely to represent the limited grain processing techniques of the period. The stems, leaves, and husks of foxtail and broomcorn millets and other grains required separation prior to grinding and cooking (Wang Q. et al., 2020). A previous study simulated the processing methods in prehistoric society using modern grain processing tools. The results indicate that using mortars, pestles, slabs, and mullers as dehusking and grinding tools could not fully hull the grains: the highest dehusking rates achieved in the experiments were 99 and 97.9%. Meanwhile, the husked grains were not 100% intact (Wang et al., 2013). In addition, the apparent soot on the outer surface of the tripod feet analyzed likely contained phytoliths that remained following the process of burning plants. The ancients usually used seeds as food and straw as firewood (Wang and Chai, 2010), hence the influence of the residual phytoliths of foxtail and common millet after burning during the heating process cannot be completely ruled out.

The edible plant resources of the ancient people in the Lower Xiajiadian cultural period included foxtail and broomcorn millets, plants from the tribe Triticeae, roots, and tubers. With respect to their absolute quantities and appearance frequencies, the starch grains and phytoliths from the husks of crops extracted from the residues on the interior surfaces of the tripod feet were mainly millets (starch grains: n = 42; 90%, and phytoliths: n = 208; 40%). There were comparatively fewer starch grains from the tribe Triticeae (n = 16; 70%) and from roots and tubers (n = 2; 20%). These results are consistent with those of previous research that suggest that the dry farming structure during the Lower Xiajiadian cultural period was dominated by foxtail and broomcorn millets (Sun et al., 2014; Jia et al., 2016a; Jia et al., 2016b; Ma et al., 2016; Jia et al., 2017b; Jia et al., 2021).

Starch granules from the tribe Triticeae appeared relatively frequently in the residues from the surfaces of the tripod feet. Triticeae has approximately 20 genera and 330 species globally, of which 13 genera and 175 species are found in China, and they are mainly distributed in northern China throughout diverse ecological environments (Liu Y. P. et al., 2013). In addition, Triticeae has been found in many Neolithic Age sites in Northeast China (Ma, 2014; Wu et al., 2018), which indicates that it was universally bred, cultured, and utilized by the ancient people of this region. Furthermore, recent research shows that Triticeae is closely related to human life and plays an important role in wheat crops and fodder (Culman et al., 2013; Pugliese et al., 2019).

A handful of starch grains from roots and tubers were also identified, which indicates that roots and tubers were also utilized in Chifeng during the Lower Xiajiadian cultural period. Roots and tubers are rich in starch and are abundant in the wild. Furthermore, compared with gramineous plants, they have more edible parts. As such, they have been an important edible plant resource since the late Paleolithic period (Liu L. et al., 2013; Yang et al., 2014). It is evident that the selection of multiple plant resources provided choice for these people, which further reduced risks associated with food scarcity from single crop failures.

Conclusion

We analyzed starch grains, phytoliths, and microfossil charcoal from residues found on the surfaces of 10 Li Tripods from the Lower Xiajiadian cultural period, which were previously unearthed from six sites in Chifeng, Northeast China. In combination with the identification of gelatinized starch grains, the soot traces on Li tripod surfaces, and the evidence indicating that 70% of the exterior surface samples had a higher frequency of contact with fire, our results provide further evidence that the Li tripods were used for cooking foxtail and broomcorn millets and plants from the tribe Triticeae. In addition, the starch grain and phytolith analyses results suggest that the edible plant resources of the ancient people of Chifeng during the Lower Xiajiadian cultural period included foxtail and broomcorn millets, plants from the tribe Triticeae, roots and tubers. Of these, foxtail and broomcorn millets were dominant. This study provides physical evidence of the functions of Li Tripods from the Lower Xiajiadian cultural period, as well as direct evidence of the plant resource exploitation strategies and agricultural patterns of the ancient people of this era, which in turn further promotes knowledge about their subsistence patterns and the social context of early Bronze-Age civilization.

Data Availability Statement

The original contributions presented in the study are included in the article/Supplementary Material, further inquiries can be directed to the corresponding authors.

Author Contributions

ZM and SL designed the study and wrote the manuscript. SL collected the samples and conducted the experiments. SL and ZM analyzed the data. XJ, YS, and ZM contributed to the discussion and approved the final manuscript.

Funding

This work was supported by the National Natural Science Foundation of China (Grant Nos. 41807436 and 41771223).

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s Note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Acknowledgments

We are grateful to Pang Hao and Dao Riji from the Wengniute Museum and Kang Xiaodong from Chifeng University for their assistance in collecting the samples unearthed from the sites. We also thank Ge Yong at the University of Chinese Academy of Sciences for assistance in identifying the ancient phytoliths.

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