Earliest domestication of common millet (Panicum miliaceum) in East Asia extended to 10,000 years ago

Results

Phytoliths and biomolecular components have provided substantial empirical evidence demonstrating the considerable antiquity of food production and crop dispersals in many regions of the world (17–20). Based on our observation and statistics of the variation of anatomy and silicon structure patterns in the glumes, lemmas, and paleas occurring in modern cultivated millets and related grass species collected from different regions in China (13), we found that 5 key diagnostic characteristics in phytolith morphology could be used to distinguish between foxtail millet and common millet (13): (i) a cross-shaped type phytolith is formed in the lower lemma and glume of S. italica, whereas a bilobe-shaped type is formed in those of P. miliaceum; (ii) regularly arranged papillae on the surface of the upper lemma and paleas are peculiar to S. italica; (iii) the epidermal long cell walls are Ω-undulated in S. italica (Fig. 2H), and η-undulated in P. miliaceum (Fig. 2 B, D, and F); (iv) the endings structures of epidermal long cells are cross-wavy type in S. italica and cross-finger type in P. miliaceum; the R value (ratio of the width of endings interdigitation to the amplitude of undulations) is higher (0.79 ± 0.12; n = 3,303) in P. miliaceum than in S. italica (0.33 ± 0.11; n = 2,774); and (v) surface ridgy line sculpture of the upper lemma is peculiar to S. italica. These 5 diagnostic characteristics used together give the only reliable way of distinguishing foxtail millet from common millet when only powder remains are available. In addition, a species-specific identification of phytoliths is possible for S. italica and P. miliaceum because they have typically well-defined silica skeletons that are distinguishable from those in Panicum bisulcatum, Setaria viridis, and Setaria plicata, which have no such demonstrable patterns (13).

To determine the taxa of foxtail millet and common millet, we analyzed the phytoliths of 46 grain crop samples stored in 5 newly excavated storage pits (CS-I to CS-V) at the Cishan site, and 1 sample (BWG) preserved in a storage bottle from the Culture Museum of Cishan. These grain crop samples are dated between ca. 10,300 and ca. 7,500 cal yr BP based on new ¹⁴C dating measurements (Fig. 3, Table S1).

All 47 archaeological samples we analyzed contained abundant diagnostic husk phytoliths that can be divided into 2 groups according to their phytolith assemblages and ¹⁴C dating results. The first group, including 27 samples from CS-I, III, and IV, is dated between ca. 10,300 and ca. 8,700 cal yr BP. All of the husk phytoliths present are diagnostic of common millet based on their shapes, η patterns (Fig. 2 A, C, and E), and average R value (>0.7) (Fig. 2I). The second group, including 20 samples from CS-II, V, and BWG, is dated between ca. 8700 and ca. 7,500 cal yr BP. More than 97% of the husk phytoliths are also diagnostic of the common millet (97.2% for CS- II, n = 1,273; 97.5% for CS-V, n = 1,000; 99.6% for BWG, n = 1,000), but a small quantity (0.4–2.8%) of the husk phytoliths in the second group can be attributed to S. italica (Fig. 2 G and I).

Fig. 4 shows that the CS-I storage pit contains ≈1.5-m-thick grain crop remains composed of 3 prominent layers of lemma and palea from common millets alternating with 3 glume plus reed layers. This storage manner indicates that prehistoric humans had known how to preserve large volumes of grains in secure storage. They did so by digging deep storage pits in the dry loess strata and by covering the floors with thick mats of millet glumes and reed (Phragmites australis) leaves.

Identification of biomolecular components was mainly based on our modern reference collection. A recent study has used the biomolecular components of P. miliaceum, the only miliacin-exclusive producer reported, as a basis for identification (20). However, because of the lack of comparable data derived from S. italica, previous investigators are still not clear how to use biomarkers to distinguish P. miliaceum from S. italica. In this study, we examined biomolecular components in 6 samples of modern Paniceae (see Materials and Methods). The results show that biomolecular components can be used to distinguish between P. miliaceum and S. italica based on the presence or absence of 5 biomarkers—miliacin, α-amyrin methyl ether (ME), and 3 pentacyclic triterpene methyl ethers (PTMEs), although the structures of the 3 PTMEs have yet to be confirmed (Fig. 5, Fig. S3). The total ion current trace shows the whole distribution of aromatic hydrocarbons and ethers extracted from P. miliaceum (Fig. 5A) and S. italica (Fig. 5B) to be in the 54- to 62-min analysis time range. The significant relative abundance of miliacin (compound 2) (Fig. 5D) was found in both modern species (89.0 ± 1.64% for P. miliaceum and 33.8 ± 22.2% for S. italica); the miliacin relative abundances in the aromatic hydrocarbons and ethers were estimated by measuring the area of the miliacin peak on the m/z 189 + 204 + 218 ion-specific chromatogram. However, compounds 1, 4, and 5 are peculiar to P. miliaceum, and compound 3 is peculiar to S. italica.

Fig. 5 shows that the prominent compound products of the archaeological samples (BWG, CS-V-03) are compounds 1, 2, 4, and 5, which are also present in P. miliaceum. However, compounds 1, 4, and 5 are notably absent from S. italica, which contain compounds 2 and 3 only. Thus, the distribution pattern of major biomarkers of BWG and CS-V-03 grain crop remains is similar to that of modern common millet (Fig. 5A). In 2 archeological samples, including BWG and CS-V-03, the relative abundance of miliacin reaches 88.5% and 88.2%, respectively, which is also very similar to that of common millet. These results provide further support to our conclusion and suggest that common millet is an important source of grain crops stored in the storage pits.

Discussion

According to archeobotanical research, the early charred grains of common millet occurred during the initial stages of various Early Neolithic sites (Fig. 1), including Dadiwan (ca. 7.8–7.35 cal kyr BP) (21), Xinglonggou (ca. 8.0–7.5 cal kyr BP) (22), and Yuezhuang (ca. 7.87 cal kyr BP) (23) in North China, but foxtail millet was barely present during these stages. Lee et al. (24) have speculated that the Early Neolithic predominance of broomcorn over foxtail millet at Xinglonggou and Yuezhuang ca. 6000 cal B.C. might be a regional phenomenon, implying that broomcorn millet might have been domesticated earlier than foxtail millet. Our analytical results of both phytoliths and biomolecular components have established that the earliest cereal remains stored in the Cishan Neolithic sites, during ca. 10,300–8,700 cal yr BP, are not foxtail millet, but only common millet. After 8,700 cal yr BP, the grain crops gradually contained 0.4–2.8% foxtail millet. Our study also suggests that common millet was used as a staple food significantly earlier than foxtail millet in northern China. It provides direct evidence to show that, by 10,000 cal yr BP, the early people in northern China had developed various methods of maintenance and multiplication of millet seeds for the next generation, and had known how to store crops of staple food in secure, dry places of storage pits during the Early Neolithic epoch.

Common millet has the lowest water requirement among all grain crops; it is also a relatively short-season crop, and could grow well in poor soils (5, 6, 25). The geographical distribution of both foxtail millet and common millet in China (Fig. S4) shows that foxtail millet is more common in the semiwet eastern areas, and its optimal growth occurs at mean annual temperature (MAT) from 8 to 10 °C and mean annual precipitation (MAP) from 450 to 550 mm. However, common millet is more adapted to the drier interior areas, and its optimal growing conditions occur at MAT from 6 to 8 °C and MAP from 350 to 450 mm (5, 6). The origin and dispersal of millet agriculture is a key problem closely related to the history of human impact on the environment and transformation of natural vegetation.

Paleoenvironmental data from the Weinan section (26–29) (Fig. 1) in the southern part of the Loess Plateau between the Cishan and Dadiwan sites are crucial for understanding the early stage of the forager–cultivator transition. The early Holocene was a period of significant environmental change marked by dry climate conditions as inferred from sediment texture (26, 28), magnetic susceptibility (26, 28), pollen (27), phytoliths (28), and mollusk assemblages (29). These proxy records show an environmental transition from cold–dry (ca. 11,000–8,700 cal yr BP) to warm–wet (ca. 8,700–5,500 cal yr BP) conditions. Many lacustrine and loess records from the Chinese Loess Plateau to Central Asia also support the scenario of a dry climate during the early Holocene (30–34). Under the drier climate conditions, soil development was slowed, and the soil developed on the underlying older and coarser loess of the glacial period was poor in nutrients (28). This raises the possibility that common millet was more significant than foxtail millet in the early stages of food production in North China because it was more adaptable than foxtail millet to the dry condition prevailing during the early Holocene. The common millet cultivation may involve complex selection by natural forces and human activities, although no clear evidence has been documented in this region for the transitions from gathering to cultivation and/or from a wild ancestor to domesticated common millet (1, 2, 5).

Conclusions

Our research indicates that the earliest significant common millet cultivation system was established in the semiarid regions of China by 10,000 cal yr BP, and that the relatively dry condition in the early Holocene may have been favorable for the domestication of common millet over foxtail millet. Our study shows that common millet appeared as a staple crop in northern China ≈10,000 years ago, suggesting that common millet might have been domesticated independently in this area and later spread to Russia, India, the Middle East, and Europe. Nevertheless, like Mesopotamia, where the spread of wheat and barley to the fertile floodplains of the Lower Tigris and Euphrates was a key factor in the emergence of civilization, the spread of common millet to the more productive regions of the Yellow River and its tributaries provided the essential food surplus that later permitted the development of social complexity in the Chinese civilization.

Materials and Methods

Supplementary Material