The Guijiabao site (101.6°E, 27.45°N) (Figure 1) is situated in the Yanyuan County of Sichuan Province and is surrounded by the middle part of the Hengduan Mountains (Zhong et al., 2016; Hao et al., 2022). As a crossroad in the southern part of the Zang-Yi Corridor, this region has played a crucial role in interregional interactions along this route. Guijiabao was first discovered in 2015, and a systematic field survey and small-scale excavation were conducted to confirm the preservation conditions and major remains in the site. Subsequently, excavations were carried out in three seasons from 2016 to 2018.

According to archaeological investigations, the total area of Guijiabao is around 30,000 m², but cultural deposits have been heavily disturbed by modern activities. Even though, a large number of ancient ruins have been unearthed, including house foundations, pits, tombs, and many artifacts such as pottery, stone tools, and spindle whorls (Hao et al., 2022). All of these remains could be grouped into three periods, namely: Neolithic, Bronze Age, and historical period, among which the Neolithic remains could further be divided into two phases. According to the result of systematic radiocarbon dating at the site (Table 1), Phase I of the Neolithic period at this site occurred around 5,000–4,500 cal. BP, and Phase II around 4,500–3,700 cal. BP. Moreover, the beginning of human activities in this period probably could reach 5,300 cal. BP as revealed by radiocarbon dates of foxtail millet grains from context H1 and broomcorn millet grains from context TN22E39②. The date of the Bronze Age remains at the site is roughly 3,200–2,700 cal. BP, where a hiatus of 500 years after the Neolithic period could be observed. Historical relics occurred much later, mainly stretching from the late Northern Song dynasty to the Ming dynasty (ca. 1,200–500 cal. BP).

In order to detect Neolithic agriculture practices at the Guijiabao site, soil samples for phytolith analysis have been collected during the excavations. In total, 154 samples from the Neolithic period were obtained, of which 16 were from Phase I and 138 from Phase II (details in Supplementary Table S1). These sampled contexts cover different types of archaeological features of the site, including cultural layers (sample codes start with T), pits (sample codes start with H), house foundations (sample codes start with F), and ditches (sample codes start with G).

Phytoliths were extracted from soil samples according to established methods (Piperno, 1988; Lu et al., 2002) with minor modifications. Initially, approximately 2 g of each sample was weighed and treated with 30% H₂O₂ and 15% HCl to remove organic matter and carbonate. The samples were then subjected to heavy liquid flotation using ZnBr₂ (density, 2.35 g/cm³) to separate the phytoliths, which were subsequently mounted on a slide using Canada Balsam. After air-drying, the phytoliths on the slide were counted and identified using a Leica microscope at ×400 magnification. More than 400 phytolith particles in each sample were identified and recorded according to previously published references and criteria (Wang and Lu, 1993; Lu et al., 2006; Lu et al., 2007; Lu et al., 2009b; Ge et al., 2018; Ge et al., 2020a; Ge et al., 2020b). In particular, for samples with rice phytoliths, the slides were scanned until 50 rice bulliform phytoliths with clear and countable scales were observed to calculate the proportion of rice bulliform phytoliths with ≥9 scales (Wang and Lu, 2012; Huan et al., 2015; Huan et al., 2020).

The Zang-Yi Corridor is a significant channel for interregional communications and human movements in southwest China during historical and modern times (Fei, 1980). Archaeological investigations in the past decades further suggest that cultural interactions, population migration, and the flow of raw materials and technologies along this route started much earlier than previously thought (Huo, 2005). The Neolithization process of the entire southwest region was based on the introduction of these innovations and possible human migrations, among which the emergence of agriculture was no doubt in the central place.

The new data presented in this study clearly show that abundant rice bulliform phytoliths were present in all samples from Phase I (5,000–4,500 cal. BP) at the Guijiabao site, together with the typical η-type phytoliths of broomcorn millet. A detailed analysis of scales on the edge of rice bulliform phytoliths suggested that the proportion of rice bulliform phytoliths with ≥9 scales in Phase I was 49.40 ± 4.44% (Figure 3), which was close to Phase II level (50.63 ± 4.63%) and modern domesticated rice (Huan et al., 2015; Huan et al., 2020). Considering the current AMS radiocarbon dates of this phase, especially one direct date of rice grain (BA170249, 5,030–4,856 cal. BP, 95.4% probability), these lines of evidence could confirm that rice had already been cultivated together with broomcorn millet in the southern part of Zang-Yi Corridor prior to 5,000 cal. BP.

The continuation of this mixed farming strategy could be demonstrated by crop assemblages from Phase II (4,500–3,700 cal. BP) of Guijiabao, when phytoliths of rice, broomcorn millet, and foxtail millet were found together in nearly all samples (Figure 3). A comparison of these phytoliths revealed that their proportions varied greatly among different samples, but generally rice and broomcorn millet were more important than foxtail millet. Nevertheless, given the different preservation conditions of these phytoliths, the specific crop pattern needs to be further confirmed by other evidence, such as macroscopic plant remains and stable isotope analyses of human bones. Another point that needs to be clarified here is the absence of foxtail millet phytoliths in all samples of Phase I in this study, which may have resulted from the relatively poor preservation conditions of foxtail millet phytoliths, as foxtail millet grains have been recovered in this phase and directly dated (Table 1). Another possible reason is that the samples from Phase I in this study were limited.

Overall, the present study clearly suggests that rice was cultivated together with foxtail millet and broomcorn millet from 5,000 cal. BP to 3,700 cal. BP at the Guijiabao site. Along with discoveries from other sites, it could be confirmed that they were introduced simultaneously as a package into the Zang-Yi Corridor and then other parts of southwest China at the very beginning of the Neolithic period in the region, which differs from the previously assumed process of multiple waves of crop introduction (d’Alpoim Guedes, 2011). Actually, the pure millet crop pattern of Yingpanshan, which is the main evidence of the previous model, is also questionable. The absence of rice at the Yingpanshan site might have resulted from insufficient sampling, as only 45 L of soil samples from nine contexts were processed (Zhao and Chen, 2011). Another piece of evidence in support of this query is that stable isotope analysis of human bones from two contexts of Yingpanshan indicated that their long-term diets after childhood consisted of both C₃ and C₄ foods instead of pure C₄ millets (Lee et al., 2020). In this case, these lines of evidence all tend to support a dispersal of crop package including foxtail millet, broomcorn millet and rice into this region.

On the other hand, the hypothesis that the middle Yangtze region as the source of rice for Southwest China is also unreliable, as no evidence supports the early communication between these two regions prior to 4,500 cal. BP. Even around 4,000 cal. BP, except for the sparse traits of pottery styles that are speculated to be influenced by the middle Yangtze region, no other evidence has been found (Jiang et al., 2020). In contrast, the southern part of Gansu Province is not a pure millet farming region as assumed before. Previous studies have shown that mixed farming has been carried out in many regions of Northern China since 7,500 cal. BP (Zhang et al., 2012; Wang et al., 2017; Wang et al., 2018), and the westernmost discovery was at the Xishanping site of Gansu Province, where charred seeds and phytoliths of foxtail millet, broomcorn millet, and rice have been dated back to 5,300–4,800 cal. BP (Li et al., 2007a; Li et al., 2007b). Plant remains from a regional survey in the Li County of southern Gansu also found rice grains in three sites (Ji, 2007). Therefore, it is quite possible that mixed farming was practiced at many sites in the southern part of Gansu province, and the crop package of millets and rice in southwest China was introduced from this region at the same time.

From a broader view, another possible region related to the emergence of mixed farming in Southwest China is the upper Han River valley, where mixed farming should also have been practiced prior to the Yangshao period, referring to the discoveries in the middle Han River Valley (Deng et al., 2015). Typological comparisons of pottery from the Luojiaba site in the Jialing River valley revealed influences of the Yangshao culture in the upper Han River valley (Li, 2018), thus this channel may serve as another parallel dispersal route in the eastern part of the Sichuan Basin. To clarify the emergence of mixed farming in Southwest China, more targeted research in these regions is needed in the future.

With the flow of populations and emergence of innovative technologies, the adoption, adaptation, and integration of exotic elements has been a unique cultural feature of the Zang-Yi Corridor, which could also be observed in the farming strategies. The mixed cropping system has been widely practiced not only in the Neolithic Age, but also in later periods, owing to its complex ground features and environmental conditions. There are seven mountains and six large rivers in the Hengduan Mountainous region, which can be divided into 13 geomorphological zones, covering 19 climatic zones from the tropical zone to plateau frigid zone (Li, 1989; Zhang, 1989). The flexibility of farming practices is clearly reflected in the regional diversity and chronological changes in the cropping systems of this region.

During the Neolithic Period (ca. 5,300–3,600 cal. BP), although foxtail millet, broomcorn millet, and rice were widely cultivated in most sites, the proportion of each crop varied considerably among the four sites with systematic archaeobotanical work (Figure 4). Foxtail millet and broomcorn millet were cultivated at the Yingpanshan site and played almost equivalent roles in its farming system (Zhao and Chen, 2011). However, in Phase I of Baiyangcun (4,600–4,300 cal. BP), rice was undoubtedly dominant in the farming system, and foxtail millet was also important. Nevertheless, broomcorn millet only accounted for 2.21% of all crop remains (Dal Martello et al., 2018). This pattern is similar to that of Henglanshan, but plant remains of this site are too limited, and the data are not sufficiently representative (Jiang et al., 2016a; Jiang et al., 2016b). In Phases II and III of Baiyangcun (4,200–4,050 cal. BP), a noticeable change occurred when foxtail millet became the most important crop, followed by rice. The Dadunzi site in Yunnan Province (4,000–3,600 cal. BP) presented a different pattern dominated by foxtail millet, where broomcorn millet was also important. However, the proportion of rice was relatively low (Jin et al., 2014). In addition, wild soybeans were also found at Yingpanshan and Baiyangcun during this period, which was possibly utilized on a small scale.

Archaeobotanical evidence from the early phase of Bronze Age (ca. 3,600–2,800 cal. BP) was more limited as macroscopic plant remains were only recovered from four sites. Unlike the previous period, rice was the main cereal crop in all these sites, accounting for 62.5 to 97.7% of all crop remains. The proportion of foxtail millet in the early phase of Haimenkou was more than 22.88% (Xue, 2010; Xue et al., 2022), but less than 5% in Shifodong and Shapingzhan (Zhao, 2010; Yan et al., 2016b). Broomcorn millet was very rare during this period, with only one grain from Gaopo and two grains from Haimenkou reported (Xue, 2010; Jiang et al., 2013). Another significant change in farming practices of this period is the adoption of wheat and barley, as indicated by the plant remains from Haimenkou, where 261 wheat and seven barley grains have been found. Although wheat and barley were dispersed into the northwestern part of Xinjiang around 5,000 cal. BP (Zhou et al., 2020), the earliest evidence in the Hexi Corridor was only approximately 4,000 cal. BP (Dodson et al., 2013) and 3,600 cal. BP in Central China (Deng et al., 2020). Therefore, the mountainous area of the Zang-Yi Corridor was one of the earliest regions to adopt wheat and barley in southern China. By contrast, no solid evidence of wheat has been reported in the contemporary Chengdu Plain. Cannabis and buckwheat were also found in Haimenkou, revealing more diversified crop patterns since early Bronze Age.

During the late Bronze Age (ca. 2,800–2000 cal. BP), the importance of wheat was greatly improved, which has been found in nearly all sites except for Shilinggang, and is dominant in the crop assemblages of Haimenkou, Guangfentou, and Xueshan (Xue, 2010; Li et al., 2016; Li and Liu, 2016; Wang et al., 2019). Even though, the specific crop patterns at these sites were still different from each other. Foxtail millet was the secondary important crop in the late phase of Haimenkou, with rice and broomcorn millet accounting for a small portion of the entire assemblage (Xue, 2010; Xue et al., 2022). However, in the three phases of Guangfentou and Xueshan, the proportion of rice is much higher than that of foxtail millet (Li and Liu, 2016; Wang et al., 2019). In addition, plant remains from Heposuo and Yubeidi both demonstrated a rice-dominated crop pattern, and the proportions of foxtail millet and rice were slightly different from each other (Yang et al., 2020; Yang et al., 2021). In contrast, broomcorn millet, barley, and soybean were only sparsely found at these sites, which could possibly be used as risk-buffering crops by the locals.

Overall, a comparison of crop patterns from all sites in the different periods of the Zang-Yi Corridor clearly demonstrates that mixed farming and regional diversity are the main characteristics of the agricultural practices in the region. During the Neolithic period, foxtail millet, broomcorn millet, and rice were all staple crops, although their proportions greatly varied in different sites. With the introduction of wheat in approximately 3,600 cal. BP, this exotic crop became the most fundamental crop along with foxtail millet and rice in the Bronze Age, while broomcorn millet was only discovered sparsely and accounted for a low proportion. The conditions were similar for the newly introduced barley. Soybeans, cannabis, and buckwheat have also been utilized on a small scale by local people, forming a more diversified crop pattern.