The Impact of Geomorphological Settings and Environmental Influences on Crop Utilization in the Mid-to-Late Neolithic Period in Shaanxi Province, Northwest China

Abstract

During the Middle-to-Late Neolithic period (7000–3800 BP), Shaanxi Province served as a critical juncture in the transmission of crops. Foxtail millet (Setaria italica), broomcorn millet (Panicum miliaceum), and rice (Oryza sativa) spread westwards into the Gansu–Qinghai region and southwards into the Sichuan basin, whilst wheat (Triticum aestivum) and barley (Hordeum vulgare) were transmitted through the Shaanxi region to the middle and lower Yellow River regions. Neolithic settlements are found in all three of the main geomorphic settings in Shaanxi: the Loess Plateau, plains, and mountainous areas. While the extent to which crop diffusion and distribution were influenced by environmental changes has previously been highlighted, the strategies of crop utilization in different geomorphic contexts have not been specified. Based on crop-remains data from 33 archaeological sites in Shaanxi, this study uses statistical modeling and ArcGIS-based spatial analysis to investigate prehistoric crop utilization in Shaanxi during the Neolithic period and its environmental determinants. Our results indicate the following: (1) The dominant crops in the Neolithic Shaanxi were foxtail millet and broomcorn millet, with the proportion of foxtail millet increasing over time. (2) The Guanzhong Plain was the earliest region in Shaanxi to adopt millet and rice (~7000–3800 BP). Subsequently, millet and rice had influenced the Qinba Mountains by ~5000 BP at the latest. By ~3800 BP, millet had affected the entire northern Shaanxi Plateau, with rice only found at the Shimao site around 4000 BP. Finally, wheat and barley influenced the Guanzhong region and the Qinba region in Shaanxi around 4000 BP. In addition, rice, wheat, and barley mainly enhanced agricultural diversity in the Guanzhong Plain and Qinba Mountains but had limited impact in the Northern Plateau, where cattle and sheep have enriched subsistence strategies since about 4500 BP. (3) Environmental factors affected the distribution of crops to different extents—elevation and river proximity had minimal effects on foxtail millet and broomcorn millet but significantly influenced the presence of rice, wheat, and barley. These factors led to a spatial pattern where millet dominated in the Northern Plateau, while the Guanzhong Plain and Qinba Mountains developed mixed farming systems incorporating all four seed types. This study provides new insights into the environmental mechanisms influencing crop diffusion and prehistoric human adaptation during the Neolithic period in Shaanxi.

1. Introduction

Located at the eastern end of the Hexi corridor, Shaanxi occupies a pivotal location through which to examine cultural exchanges. Set between the East and West, Shaanxi occupies a key crossroads in the prehistoric spread of key crops—such as foxtail millet (Setaria italica), broomcorn millet (Panicum miliaceum), rice (Oryza sativa), and wheat (Triticum aestivum) (Figure 1a)—and domestic animals, including pigs, cattle, and sheep. Shaanxi is an essential point for both cultural transmission and technological exchange across the Eurasian continent during the Middle-to-Late Neolithic period [1,2,3].

Multiple cultural polities have been identified as existing across Northern China during the Middle-to-Late Neolithic period (Figure 1b). In Shaanxi, these include the Laoguantai Culture (9000–7000 BP), Yangshao Culture (7000–4800 BP), and Longshan Culture (4500–3800 BP) (Figure 1c). It was during the times of these cultures that a stable agricultural system was developed, which provided a solid foundation for the subsequent prosperity of the Zhou (1046–256 BC), Qin (221–206 BC), Han (202 BC-220 AD), and Tang (618–907 AD) civilizations [8,9,10,11,12]. During the Early Neolithic, the Laoguantai Culture primarily clustered in the Wei River basin and the upper reaches of the Han River within Shaanxi Province [13,14]. By the Middle-to-Late Neolithic period, agricultural development had promoted population growth and migration, and the distribution of the Yangshao and Longshan Cultures had expanded across the entire region, establishing it as one of the core areas for the diffusion and transmission of millet and rice agriculture [2,15,16,17,18,19].

Over the past two decades, archaeobotanical research has significantly enhanced our understanding of the Neolithic period in Shaanxi Province. Methods such as flotation, phytolith analysis, and carbon–nitrogen isotope analysis have been instrumental in investigating the evolution of agricultural practices and their connection to cultural development in the region during the Early Neolithic period, associated with the Laoguantai Culture period (9000–7000 BP), while direct evidence of carbonized seeds is sparse. However, stable isotope analyses of human and domesticated animal bones, such as those from pigs from the Baijia site, indicate a reliance on C4 plants like foxtail millet and broomcorn millet in the Guanzhong Plain [20,21]. Fortunately, carbonized seeds of broomcorn millet (n = 1805), foxtail millet (n = 24), and rice (n = 3) dating back to 7500 BP were recovered from the flotation at the Beiliu site in the Guanzhong Plain, which provide an insight into the subsistence economy during the Laoguantai Culture [22]. This suggests that the emergence of millet cultivation was a foundational aspect of subsistence strategies. In the Middle-to-Late Neolithic period, during the Yangshao and Longshan Cultures (approximately 7000–3800 BP), archaeobotanical analyses—particularly flotation projects—have uncovered numerous carbonized seeds of foxtail millet and broomcorn millet. These findings underscore the importance of millet-based agriculture in the plains of Shaanxi during this period [23,24,25,26]. Around 6000 BP, rice was also in the predominantly millet-based agricultural system, reflecting a diversification of crops [27]. By approximately 4000 BP, Western-introduced crops such as wheat and barley (Hordeum vulgare) began to integrate into the local farming practices (Figure 1a), further enriching the agricultural framework [28,29,30].

Shaanxi occupies a strategic geographic position, situated at the intersection of north–south and east–west trade routes, and its diverse natural environment. Shaanxi in the Middle-to-Late Neolithic period fostered a variety of cultural adaptations and subsistence strategies. Archaeological evidence from this era reveals diverse settlement patterns, primarily including highlands, plains, and mountainous regions, which reflect the significant role of topography in shaping settlement choices and agricultural development [18]. The similarities and differences that existed in crop use among Neolithic communities inhabiting diverse geomorphic environments during the Middle-to-Late Neolithic period remain an underexplored area in existing research. Additionally, it is unclear what environmental factors influenced the adoption and diffusion of crops. These questions have not yet been systematically addressed. This study seeks to investigate variations in crop utilization and their environmental determinants across different geomorphic contexts by synthesizing carbonized seed data from archaeological sites in three topographical regions: the Northern Plateau, the Guanzhong Plain, and the Qinba Mountain regions of Shaanxi Province. The aim is to offer practical insights into the trajectory of ancient agricultural development in Shaanxi from a geomorphological perspective by ArcGIS spatial analysis techniques, while also uncovering the underlying environmental factors that drove these changes.

2. Materials and Methods

2.1. Background of the Study Area

Shaanxi Province is located in the middle reaches of the Yellow River basin. It borders Inner Mongolia to the north; Sichuan, Chongqing, and Hubei to the south; Shanxi and Henan to the east; and Gansu and Ningxia to the west. Topographically, Shaanxi can be divided into three distinct regions with significant differences in topography and climate: the Northern Plateau, Guanzhong Plain, and Qinba Mountain regions in southern Shaanxi [31,32]. The Northern Plateau is characterized by the Loess Plateau, which is prone to severe soil erosion and has a relatively fragile ecosystem [33]. The Guanzhong region, dominated by plains, features flat terrain, fertile soils, and relatively abundant water resources, making it a key center for agricultural development [34]. The Qinba Mountain region in southern Shaanxi has a complex topography dominated by mountainous and hilly landscapes, a mild and humid climate, high forest coverage, and rich biodiversity, providing abundant livelihood resources for ancient human populations [35]. In terms of water systems, the region includes rivers from the Yellow River system, such as the Wei, Luo, and Wuding Rivers, as well as those from the Yangtze River system, including the Han and Jialing Rivers (Figure 1b).

Shaanxi Province exhibits significant climate variations across its geography. From north to south, the climate transitions from temperate to warm temperate, and then to the northern subtropical zone [36]. The northern part of Shaanxi, along the Great Wall, is characterized by a temperate climate, with precipitation ranging from 330 to 500 mm. In contrast, most other areas experience a warm temperate climate, with rainfall between 500 and 700 mm. The northern region experiences cold winters, warm summers, and large diurnal temperature variations, with noticeable temperature changes in spring and autumn [37]. The Guanzhong Plain falls under a warm temperate semi-humid climate, with rainfall between 550 and 650 mm. This region experiences distinct seasons, with cold winters, high summer temperatures, and rainfall concentrated in the summer. Spring is relatively dry, while autumn is cool [38]. The Qinba Mountain region in southern Shaanxi is characterized by a northern subtropical climate, with precipitation ranging from 800 to 1000 mm. The climate is mild and humid, with relatively warm winters and cool summers. The region is one of the main precipitation areas in Shaanxi [39], providing rich water resources.

2.2. Analysis Methods for Crop Distribution, Geomorphology, and Hydrological Environmental Factors in Shaanxi

This study systematically organized and analyzed data from published Neolithic sites in the Northern Plateau, Guanzhong Plain, and Qinba Mountain regions of Shaanxi, focusing on the location, elevation, and other data of carbonized crop seeds (foxtail millet, broomcorn millet, rice, barley, and wheat) found at these sites, as well as AMS radiocarbon dating and archaeological typological data (see Table S1 and Figure 1). All data pertaining to carbonized crop seeds and radiocarbon dating were meticulously collated from published research on Neolithic sites in Shaanxi Province. For sites where direct radiocarbon dating was applied to carbonized remains, the data for age were calibrated using the IntCal20 radiocarbon calibration curve, with all age results converted to calendar years (calibrated data available at the following: https://c14.arch.ox.ac.uk; 20 October 2024). For sites lacking radiocarbon dating, the chronological range was determined based on typological analysis of ceramics, using archaeological dating to estimate their chronological span. As for the data of the location, elevation, and distance from the near river for the carbonized crop seeds in the Northern Plateau, Guanzhong Plain, and Qinba Mountain regions of Shaanxi, these were calculated by ArcGIS 10.8 software.

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Crop Proportion Calculation: This study primarily employs two statistical methods for crop analysis—absolute quantity statistics and flotation probability statistics. Actually, both of these methods are commonly used in archaeobotany. When sorting out the references, due to a lack in the literature on relevant methods, we had to recalculate. The absolute quantity method was used to determine the quantity of specific crops, helping to describe and analyze the scale or proportion of a particular crop within the broader agricultural use process. The flotation probability method calculates the proportion of a specific crop species found in the total samples from an archaeological site. This approach does not consider the absolute quantity of plant remains in each flotation sample, thus minimizing the influence of error factors, such as seed fertility, use rates, burial mechanisms, excavation conditions, and other subjective and objective factors that might affect our analyses [40];

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Spatial Distribution Characteristics: The Inverse Distance Weighted (IDW) method was used to calculate the spatial distribution and clustering areas of cereal crops such as foxtail millet, broomcorn millet, rice, barley, and wheat across the Northern Plateau, Guanzhong Plain, and Qinba Mountain regions of Shaanxi, respectively, with the results presented in a visual format. The specific steps were as follows: First, the crops were assigned values based on their chronological order, with earlier samples receiving higher weights and later samples receiving lower weights. Second, the distance between known and unknown points was processed smoothly, and values were assigned to unknown points in Shaanxi, allowing for the temporal and spatial variation characteristics of the cereal remains (foxtail millet, broomcorn millet, rice, barley, and wheat) to be obtained;

(3)

Terrain and Hydrological Analyses: The terrain and hydrological conditions of Shaanxi Province were analyzed using the terrain analysis and buffer analysis functions in the spatial analysis module of ArcGIS software. The 90-meter-resolution DEM data, the 1:250,000 river classification dataset, and the 1:100,000 lake database used in this study were all sourced from the International Scientific Data Mirror Site of the Computer Network Information Center of the Chinese Academy of Sciences (http://www.gscloud.cn; 22 October 2024). The coordinates of the archaeological sites provided are based on the coordinate locations provided in the published literature. Terrain analysis is primarily represented by the elevation data of crop remains, including foxtail millet, broomcorn millet, rice, barley, and wheat. In contrast, hydrological analysis is based on the distance between the archaeological sites in which these crop seeds were recovered and nearby rivers. Considering the low probability of ancient river course shifts in the upper and middle reaches of the Yellow River [41,42], the Euclidean distance between the archaeological sites and the nearest river was calculated to explore the relationship between these crops and rivers. The Euclidean distance method was used to calculate the distance between the cereal crop remains (foxtail millet, broomcorn millet, rice, barley, and wheat) and the nearest river, from which the shortest distance to the nearest water source for each sample point is determined. The principle is as follows: let the coordinates of the archaeological site be (x₁, y₁), and the coordinates of the nearest river point be (x₂, y₂). The shortest distance between the archaeological site and the river is represented as $\mathsf{\rho }=\sqrt{{({\mathrm{x}}_2-{\mathrm{x}}_1)}^2+{({\mathrm{y}}_2-{\mathrm{y}}_1)}^2}$.

3. Results

3.1. Crop Types and Distribution Characteristics in the Middle-to-Late Neolithic Period in Shaanxi

Flotation is the most widely used archaeobotanical method in Shaanxi since it is capable of recovering large quantities and diverse types of carbonized crop seeds from archaeological contexts. This study primarily focuses on carbonized crop seeds obtained through flotation to examine the types and proportions of crops in the Middle-to-Late Neolithic period in Shaanxi in order to undertake a quantitative analysis of variations in agricultural practices and diets in three different topographical regions.

Northern Plateau: Abundant carbonized foxtail-millet and broomcorn-millet seeds have been found at all Middle-to-Late Neolithic-period (Yangshao and Longshan Cultures) archaeological sites in the Northern Plateau where flotation has been undertaken. This indicates that these two crops were the primary agricultural staples in the plateau region during the Middle-to-Late Neolithic period. It is not until the Late Longshan period that evidence of rice agriculture is present, which takes the form of 19 carbonized rice seeds and 9 rice rachillas recovered from the Shimao site, a large multiphase settlement that is believed to have been a ”central place” in Neolithic Northern China (Table S1). The presence of rice rachillas suggests that the rice was locally cultivated [26]; however, while this indicates that rice might have been incorporated into the millet-based agricultural system of the Northern Plateau as early as 4000 BP, the proportion of seeds indicates that the influence of rice remained minimally restricted to elite or ritual contexts.

During the Yangshao period, carbonized foxtail millet seeds only represent 12.39% (5801 seeds) of the crop seeds recovered across the sample set, while 87.61% (41,007 seeds) of broomcorn millet seeds were identified at the sites included in this study. In contrast, during the Longshan Cultural period, the total number of carbonized foxtail millet seeds recovered increased to 66.21% (18,301 seeds), while broomcorn millet decreased to 33.79% (9339 seeds). This result indicates a gradual increase in the proportion of foxtail millet and a corresponding decline in broomcorn millet from the Yangshao to the Longshan periods, likely reflecting changes in agricultural practices or crop adaptability over time.

The ubiquity analysis further highlights the dominance of millet in the plateau region of Shaanxi. The average ubiquity of foxtail millet was 83.23%, and broomcorn millet was 92.71% during the Yangshao period. During the Longshan period, these values declined slightly to 78.20% and 71.49%, respectively, while rice appears with a ubiquity of only 8.56%. These results suggest a close relationship between foxtail-millet-based agriculture and ancient human populations in the Northern Plateau. Although a slight decrease in their average ubiquity was observed in the Middle-to-Late Neolithic period, the reliance on these crops remained high, with evidence of rice being found in the region for the first time during the Longshan period.

Guanzhong Plain: At 13 of the 14 Yangshao and Longshan Cultural sites at which flotation has been undertaken, both carbonized foxtail and broomcorn millet seeds were identified; however, in the Late Yangshao period, at the Quanhu site only carbonized broomcorn millet remains were recovered [43]. This strongly suggests that agriculture in the Guanzhong Plain region was millet-based (Table S1).

Carbonized rice remains were found at 8 of the 14 sites in the Guanzhong Plain region, whilst carbonized wheat and barley remains were identified at 8 and 2 of the sites, respectively (Table S1). Notably, at the Dongyang site, carbonized rice seeds and rachillas were recovered, with a radiocarbon date (Beta-454986) of 5000 ± 30 BP, calibrated to 5890–5600 BP (InterCAL20), which is consistent with the timeframe of the Yangshao Culture [2]. The only dated wheat seeds from the Guanzhong Plain, found at the Yuhuazhai site, yielded a radiocarbon date (Beta-376970) of 180 ± 30 BP, calibrated to 295–227 BP; this late date is most likely due to sample contamination or other factors that require further investigation [27]. Although there is clear evidence that rice was incorporated into the millet-based agricultural system of the Guanzhong Plain as early as cal. 5800 BP, further radiocarbon dating will be required before a reliable date for the introduction of wheat and barley to the Guanzhong Plain can be ascertained.

During the analysis of seed representation in the Guanzhong Plain, it was found that their integration timeline is unclear; future studies with additional dating analyses are necessary to resolve this uncertainty.

In terms of absolute quantities, during the Yangshao Cultural period, foxtail millet made up 77.01% (57,369 seeds); broomcorn millet 21.30% (15,864 seeds); rice 1.41% (1054 seeds); wheat 0.26% (193 seeds); and barley was represented by 57,369 grains (77.01%), 15,864 grains (21.30%), 1054 grains (1.41%), 193 grains (0.26%), and 12 grains (0.02%) of the total number of seeds identified in the region. This contrasts with the Longshan Cultural period, wherein the proportions were markedly different. Foxtail millet dominated with 94.41% (6134 seeds), followed by broomcorn millet at 3.37% (219 seeds), rice at 2.02% (131 seeds), and wheat at 0.20% (13 seeds). Notably, no barley remains were identified from this period. Compared to the Yangshao period, rice, wheat, and barley quantities declined significantly. In addition to these seeds, rice rachillas were also discovered, with two rachillas identified from the Yangshao-period sites and four from Longshan-period sites [27,29,30]. These findings indicate that foxtail millet and broomcorn millet dominated agricultural production in the Guanzhong Plain during the Middle-to-Late Neolithic period, with foxtail millet increasing in proportion and broomcorn millet decreasing from the Yangshao to the Longshan periods. From the Yangshao culture period to the Longshan culture period, the number of carbonized rice seeds decreased from 1054 to 131. Rice usage declined, while wheat and barley remained minor components, with negligible changes over time.

Based on ubiquity analysis, during the Yangshao Cultural period, foxtail millet had an average ubiquity of 92.13%, broomcorn millet—64.81%, and rice—59.10%. In the Longshan period, these values shifted to 92.0% for foxtail millet, 84.5% for broomcorn millet, and 31.5% for rice. Wheat had a ubiquity of 16.65%, and barley 3%, during both the Yangshao and Longshan periods. These figures highlight the central role of millet crops, particularly foxtail millet, in subsistence strategies on the Guanzhong plain, while wheat, barley, and rice played a more limited role. This strong dependence on foxtail millet and broomcorn millet in the Guanzhong Plain continues into the Longshan period, with foxtail millet showing stable ubiquity, broomcorn millet exhibiting a slight increase, rice and wheat use declining over time, and the presence of barley being negligible.

Qinba Mountains in Southern Shaanxi: Archaeobotanical research in this region is significantly less developed than in the Northern Plateau and Guanzhong regions. At present, systematic flotation research has been conducted only at the Longgangsi site, which dates to the transitional period of the Middle-to-Late Neolithic period. At the Longgangsi site, remains of various crops were identified across both Yangshao and Longshan Cultural contexts. The absolute quantities for foxtail millet, broomcorn millet, rice, and wheat were 57.71% (1384 seeds), 41.62% (998 seeds), 0.63% (15 seeds), and 0.04% (1 seed), respectively. The corresponding ubiquity values were 43.70% for foxtail millet, 43.70% for broomcorn millet, 6% for rice, and 0.40% for wheat [25]. Although the number of sites in the Qinba Mountains in which archaeobotanical analyses have been undertaken is limited, the available evidence suggests that foxtail millet and broomcorn millet dominated agricultural production during the Yangshao and Longshan periods. The relatively high ubiquity of rice seeds indicates that rice might have been incorporated into the local agricultural system by this time. Other cereals, such as wheat and barley, remain notably underrepresented, indicating their minimal proportion in the region’s agriculture.

3.2. Spatiotemporal Distribution Characteristics of Crop Remains in Shaanxi

3.2.1. Spatiotemporal Distribution of Crop Remains in Shaanxi

Spatiotemporal distribution of foxtail millet and broomcorn millet: Spatial interpolation analysis of foxtail-millet and broomcorn-millet remains in Shaanxi from the Middle-to-Late Neolithic period sees the Guanzhong Plain emerge as the earliest region to adopt these crops. The earliest traces of foxtail and broomcorn millet identified in the Guanzhong Plain were found at the Zhouyuan Site and are dated to 6500 BP. From there, crop occurrence gradually spreads outward along the Luo, Wei, and Wuding River basins, and then radiates outward from these river systems. These regions likely offered favorable natural conditions that played a key role in the early development of agriculture. Millet then appears to have spread north and south to the Northern Plateau and the Qinba Mountains (Figure 2a), although the rate southwards is notably faster than northwards. By approximately 3850 BP, these crops had reached the Huoshiliang site near Xinji Airport in the Northern Plateau, eventually spreading across the entire Shaanxi region.

Spatiotemporal distribution of rice: Based on the spatial interpolation analysis results, rice remains during the Middle-to-Late Neolithic period were primarily distributed in regions with relatively abundant water and solar intensity, namely the Guanzhong Plain and the Qinba Mountains in southern Shaanxi (Figure 2b). Spatially, rice farming activities were concentrated along the Wei River in the Guanzhong Plain and the Han River in southern Shaanxi, with approximately 80% of rice remains located near or in the Wei River basin (Figure 2b). This distribution pattern highlights the importance of suitable hydrological and climatic conditions for early rice cultivation. Temporally, rice first appeared at the Yuhuazhai site in the Guanzhong Plain around 6000 BP [27]. It subsequently spread along the Wei River basin, reaching the Han River basin in southern Shaanxi by approximately 5000 BP, where carbonized rice remains were discovered for the first time. There is limited evidence for rice agriculture being present in the Northern Plateau during the Middle-to-Late Neolithic period. Although evidence of rice has been discovered in contexts dating to c. 4000 BP at the high-status site of Shimao [26], no evidence of rice use has been identified in any other Northern Plateau Neolithic sites to date.

Spatiotemporal distribution of wheat and barley: Based on the spatial interpolation analysis results, barley and wheat remains had the earliest occurrence in the Guanzhong Plain and the Qinba Mountains in southern Shaanxi, concentrated along the Wei and Han River basins and their surrounding areas (Figure 2c). These regions likely provided favorable climatic conditions for cereal cultivation, although the number of sites with cereal remains was relatively small. Temporally, the earliest evidence of cereals in Shaanxi dates to approximately 6600 BP at the Anban site in the Guanzhong Plain, while the latest evidence in this dataset is from the Wangjiazui locality of the Zhouyuan site, approximately 3600 BP. The results exhibit a tendency of wheat and barley extending from the east to the west; however, this phenomenon is contrary to the fact that wheat crops spread eastward from the Hexi Corridor [29,30,44,45]. Although carbonized cereal seeds have been identified at 10 sites from the Middle-to-Late Neolithic period in Shaanxi, the lack of reliable radiocarbon dates raises questions about their chronology. Based on previous studies and our own data, we support a date of 4000 BP for the introduction of wheat and barley into the Shaanxi region [29,30,44,45].

3.2.2. Environmental Factors Affecting the Distribution of Foxtail Millet, Broomcorn Millet, Rice, Wheat, and Barley in Shaanxi

Although there are multiple factors that affect the viability of crops, one of the most significant and easily traced is elevation. Temperature drops at a rate of 0.6 °C per 100 m increase in elevation [46], which subsequently affects thermal accumulation, leading to variations in growing degree days. The elevation of each of the sites for which archaeobotanical data were available was ascertained using a 90 DEM in ArcGIS to evaluate the adaptation of these crops to thermal accumulation.

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Environmental factors of foxtail millet and broomcorn millet:

Elevation: As shown in Figure 3a, among the 33 sites with foxtail millet and broomcorn millet remains, the highest elevation was at the Longshan Cultural Dayangwa site (n = 1405 m), and the lowest was at the Yangshao Cultural Xinglefeng site (n = 351 m). During the Yangshao Cultural period, the average elevation of foxtail millet and broomcorn millet remains was 700.63 m, while during the Longshan Cultural period, the average elevation increased to 1156 m. These data suggest that, over time, the use of foxtail millet and broomcorn millet in the Middle-to-Late Neolithic periods in Shaanxi expanded from lower to higher elevations.

Distance from Rivers: As shown in Figure 3b, the results from the Euclidean distance analysis in ArcGIS reveal that, during the Yangshao Cultural period, the majority of the foxtail millet and broomcorn millet were distributed along river valleys, exhibiting a linear pattern and clustering along the Wei River. During the Longshan Cultural period, the archaeological sites with foxtail millet and broomcorn millet were also predominantly located near rivers. The number of sites in the Northern Plateau at which millet remains have been identified has gradually increased over time. According to the data presented in

Table 1. Distribution of foxtail millet, broomcorn millet, rice, wheat, and barley remains in relation to river proximity in Shaanxi during the Middle-to-Late Neolithic period.

	Foxtail Millet and Broomcorn Millet		Rice		Wheat and Barley
Distance (km)	(n)	(%)	(n)	(%)	(n)	(%)
0–2	4	11.76	1	10.00	3	30.00
2–5	18	52.94	4	40.00	4	40.00
5–10	5	14.71	3	30.00	1	10.00
10–15	6	20.59	2	20.00	2	20.00
>15	-	-	-	-	-	-

, most of the archaeological sites (~64%) are located within 5 km of rivers. The proportion of sites decreases with distance from a major river, with no sites located more than 15 km from a major river. In terms of chronological distribution, the average distance of foxtail millet and broomcorn millet from rivers during the Yangshao period was 6.5 km, while during the Longshan period it was 5.3 km, indicating a trend of gradually moving closer to rivers by 1.2 km. This trend may be related to the expansion of human activities and the use of water resources.

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Environmental factors of rice remains:

Elevation: As shown in Figure 4a, among the 10 archaeological sites with rice remains in Shaanxi, the highest elevation was at the Shimao site (Longshan Culture, n = 1029 m), while the lowest was at the Xinglefang site (Yangshao Culture, n = 351 m). During the Yangshao period, the average elevation of rice remains was 446.43 m, while in the Longshan period, it increased to 1819.33 m. These data suggest that, over time, rice agricultural activities expanded to higher elevation regions.

Distance from Rivers: Euclidean distance analysis undertaken in ArcGIS indicates that during the Yangshao period, most rice remains were found along river valleys, with a linear distribution mainly along the Wei River in the Guanzhong Plain with a few sites located near the Han River in the Qinba Mountains (Figure 4b). During the Longshan period, sites were still predominantly located near the Wei River, with fewer found near tributaries of the Yellow River in the Northern Plateau. From the Yangshao to the Longshan period, rice agriculture exhibited a gradual shift northward, likely influenced by water resource distribution and expanding human activity. According to Table 1, all rice remains were found within 15 km of rivers, with approximately 80% located within 10 km. As the distance from rivers increased, the proportion of sites decreased. In terms of distance, rice remains in Yangshao-period sites were on average 6.11 km from rivers, while those from Longshan-period sites were 7.3 km away, showing a trend of gradually moving farther from rivers.

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Environmental factors of wheat and barley remains:

Elevation: Using the coordinates of the crop-remains sites in ArcGIS, we extracted the elevation data for each site, as shown in Figure 5a. Among the 10 cereal-remains sites, the highest elevation was at the Zhaolinhetan site in the Guanzhong Plain during the Yangshao Culture (n = 911 m), while the lowest elevation was at the Yangguanzhai site in the Guanzhong Plain during the Late Yangshao Culture (n = 371 m). During the Yangshao period, the average elevation of cereal remains was 528.25 m, and during the Longshan period, it increased to 664 m, a difference of 135.75 m. This suggests that over time, cereal-remains sites expanded from lower-elevation river valley regions to higher elevations.

Distance from Rivers: In terms of proximity to rivers, as shown in Figure 5b, during the Yangshao period, most of the cereal-remains sites were recovered from sites along river valleys, exhibiting a branching pattern, with the majority located near the Wei River in the Guanzhong Plain and a few near the Han River in the Qinba Mountains. During the Longshan period, sites were also mainly located near the Wei River and its tributaries, with others located near the Luo River in the Northern Plateau. From the Yangshao to the Longshan period, cereal agriculture activities gradually spread northward. According to the data in Table 1, most of the sites at which wheat and barley remains were within 5 km of rivers accounted for approximately 70% of the total sites. As the distance from rivers increased, the proportion of sites with wheat and barley remains decreased, with only two sites located between 10 and 15 km from rivers. During the Yangshao period, the average distance of cereal-remains sites from rivers was 3.89 km, while during the Longshan period, the average distance increased to 7.14 km, a difference of 3.25 km, indicating a trend of gradually moving away from rivers.

4. Discussion

4.1. Differentiation in Crop Use Across Various Geomorphic Settings in Shaanxi During the Middle-to-Late Neolithic Period

Located in northwestern China, Shaanxi encompasses diverse landforms, including plateaus, mountains, plains, and basins [31,32]. These varied geological conditions and climatic environments fostered diverse ecosystems and plant resources, influenced prehistoric crop utilization, and shaped distinct adaptive strategies for human survival in different regions. The earliest evidence of agricultural practice in the region is in the Guanzhong Plain, whereafter it expands both north and south to the Northern Plateau and Qingba Mountains, respectively.

The Guanzhong Plain is characterized by fertile soil [34], and, coupled with good drainage, this makes the area eminently suitable for agriculture. During the Early Neolithic period, prehistoric communities utilized local soil and water resources to cultivate crops adapted to the regional climate. Isotopic analyses of human and animal bones, such as pigs, suggest that the Laogantai Culture in the Wei River basin had already begun exploiting C4 crops such as foxtail millet and broomcorn millet during 7709~7389 BP [20,21]. In addition, among the carbonized plant seeds dating back to 7500 years ago discovered at the Beiliu site in the Guanzhong Plain, the seeds of broomcorn millet (n = 1805) and foxtail millet (n = 24) were the most abundant, while those of rice (n = 3) were the scarcest [22]. The development of agriculture stabilized prehistoric human livelihoods, supporting population growth and fostering social progress. Foxtail millet and broomcorn millet appear to have continued to dominate agricultural practice in the region during the Yangshao period, although there is evidence that a mixed dry-rice farming system also began to emerge. Direct radiocarbon dating of carbonized rice at the Dongyang site indicates that rice had persisted in its continuance in the Guanzhong agricultural system by approximately 5800 BP [2]. Notably, rice cultivation expanded significantly from the Late Yangshao to the Early Longshan period but became rare by the Late Longshan period around 4000 BP, while broomcorn millet increased in proportion. This shift may be associated with the global cooling event around 4200 BP [2], as cold and dry conditions likely limited the growth and large-scale use of rice, leading to greater cultivation of more cold-tolerant crops such as broomcorn millet. During this period, the “food globalization” phenomenon also influenced the region [44]. Charred wheat and barley remains were identified at eight sites in the Guanzhong Plain, including Yuhuazhai and Gongbeiya. Although the wheat remains at Yuhuazhai were dated to only 300 BP, there is strong evidence for the presence of both wheat and barley being present in Xinjiang around 5200 BP [47], with significant quantities found in the Hexi Corridor by approximately 4000 BP [44,45]. The Hexi Corridor likely served as a critical conduit linking Shaanxi and other regions, facilitating the westward spread of painted pottery and millets, and the eastward diffusion of wheat, barley, and livestock such as cattle and sheep [44,45]. Therefore, it is reasonable to infer that by approximately 4000 BP, wheat and barley would have become part of the agricultural repertoire of the Guanzhong Plain, further enriching its crop diversity.

The Northern Plateau is formed of aeolian loess deposits, which can in some places be up to 400 m deep. Nominally flat, this plateau is formed of loess remnants and is cut by deep and well-developed gullies that form the loess ridges. Vegetation mainly consists of drought-tolerant plants on loess ridges and herbaceous plants on loess remnants [33]. During the Middle Neolithic period, the expansion of the Yangshao Culture into Northern Shaanxi introduced foxtail millet and broomcorn millet into the region. Evidence from the absolute quantities and ubiquity of millet remains indicate that broomcorn millet dominated the agricultural system during the Yangshao period; however, by the Longshan period, the agricultural structure shifted to being predominantly foxtail millet. This transition may be attributed to the higher drought tolerance of broomcorn millet, which acted as a pioneer crop [48], although it has a lower yield compared to foxtail millet. The higher productivity of foxtail millet may have supported further population growth and facilitated westward expansions. During the Late Longshan period or early Xia Dynasty, rice remains—especially rice rachilla impressions—started to appear in the archaeological record of the Northern Plateau region, suggesting the incorporation of rice into local agricultural systems. However, this phenomenon was rare and restricted to high-complexity sites such as the Shimao settlement. The presence of rice may indicate regional exchanges through which elites accessed rare resources [26]. Additionally, the ubiquity of foxtail millet and broomcorn millet declined during the Longshan period, suggesting a decreasing emphasis on agriculture. This trend aligns with zooarchaeological evidence, indicating a shift from agrarian-dominated subsistence strategies to a mixed farming–pastoral economy, with increasing reliance on livestock such as cattle and sheep around 4500 BP [3]. The loess plateau makes the Northern Plateau a less-than-hospitable environment; this means that prehistoric humans could not rely solely on foxtail millet and broomcorn millet or rice farming, which requires significant water resources. Instead, they incorporated livestock into their lifesystems, which caused a preference for pastoralism around 4500 BP [3], effectively overcoming environmental limitations and balancing their subsistence strategies.

The Qinba Mountains in Southern Shaanxi are characterized by a warm and humid climate, dense vegetation, and complex mountainous terrain [35]. Prehistoric communities in this region likely adapted to varying altitudes and climatic conditions by developing specialized survival strategies. Unfortunately, archaeobotanical research in this region is extremely limited, with the only systematic study conducted to date undertaken at the Longgangsi site [25]. Nevertheless, this research provides preliminary insights into crop use during the Yangshao-to-Longshan transitional period in southern Shaanxi. Crop analysis at the Longgangsi site reveals that foxtail millet and broomcorn millet continued to dominate the agricultural system. However, the proportion of rice was significantly higher than in the Northern Plateau and Guanzhong Plain, likely due to the abundant rainfall and dense river networks in southern Shaanxi, which provided ample water resources to support rice cultivation. In contrast, only a single wheat seed was identified at the site [25], making it challenging to draw definitive conclusions about its role in prehistoric agriculture, particularly given the lack of absolute dates for the site.

In summary, from the Yangshao to the Longshan periods, crop use across different geomorphic settings—the Northern Plateau, Guanzhong Plain, and Qinba Mountains—remained centered on foxtail millet and broomcorn millet. Over time, however, crop diversity gradually increased, transitioning from a millet-based agricultural system to a mixed agricultural economy that included rice, wheat, and barley. However, reliance on foxtail millet and broomcorn millet remained high in the Northern Plateau compared to the other two regions due to the cooler temperatures and lower water resources, which led to lower crop diversity. These changes reflect the evolution of agricultural technology and the influence of environmental conditions and human needs on crop use. Moreover, this agricultural development laid a solid foundation for the diverse agricultural economies that later emerged in these regions.

4.2. Influence of Environmental Factors on Crop Use Differentiation Across Geomorphic Settings in Shaanxi During the Middle-to-Late Neolithic Period

Spatial interpolation analyses reveal notable similarities and differences in the distribution of foxtail millet, broomcorn millet, rice, wheat, and barley across different geomorphic settings.

Foxtail millet and broomcorn millet were consistently the primary crops utilized across the Northern Plateau, Guanzhong Plain, and Qinba Mountains in southern Shaanxi during the Middle-to-Late Neolithic period, with foxtail millet’s proportion steadily increasing over time. Millet agriculture first appeared in the Guanzhong Plain at the Zhouyuan site c. 6500 BP; it is present at the Longgangsi site in the Qinba Mountains c. 5000 BP; and eventually reached the Huoshiliang site near Xinji Airport in the Northern Plateau c. 3800 BP. The faster adoption of foxtail millet and broomcorn millet in the Qinba Mountains region was likely facilitated by its warm and humid climate. Conversely, the sparse vegetation in northern areas of the Loess Plateau likely slowed human migration and crop introduction. The relative reliance on foxtail millet and broomcorn millet varied across regions. While the diffusion of millets to the Northern Plateau was slower, their dominance in the agricultural economy was highest in this region, likely due to the harsher climate and limited availability of alternative plant resources. The introduction of livestock, such as cattle and sheep, during the Longshan period [3] began to offset this dependence.

Rice, wheat, and barley were more prominent in the Guanzhong Plain and Qinba Mountains, with limited representation in the Northern Plateau. Rice appeared in the Guanzhong agricultural system at approximately 5800 BP [2], indicating a longer history in the region in comparison to wheat and barley, which emerged c. 4000 BP. However, the rice was situated at greater distances far away from rivers from the Yangshao Culture period to Longshan Culture period. The underlying cause of this phenomenon might be associated with the migration of ancient human settlements away from rivers [15,16,17,18,19]. Regarding cultivation practices like irrigation, due to the absence of direct evidence, it is currently infeasible for us to conduct a discussion. Anyway, the growth of these crops requires abundant water and favorable temperatures, conditions well-matched to the ecological characteristics of the Guanzhong Plain and Qinba Mountains. As a result, these two regions became the primary areas in Shaanxi province for cultivating rice, wheat, and barley during the Middle-to-Late Neolithic period.

Systematic analysis of the relationship between crop remains and elevation, as well as their proximity to rivers, during the Middle-to-Late Neolithic period in Shaanxi, was analyzed by inputting data on foxtail millet, broomcorn millet, rice, and cereal remains into MATLAB for normal distribution fitting and p-value testing. The results are presented in Figure 6. Figure 6a shows a significant normal distribution relationship between crop remains and elevation. Most crop remains are distributed at elevations between 1000 and 1200 m, with their frequency decreasing as elevation increases. This pattern suggests that settlement location choices were not strongly influenced by elevation but were instead largely determined by suitable environmental conditions for habitation, leading to a normal distribution. Conversely, as illustrated in Figure 6b, there is no significant normal distribution between crop remains and their distance from rivers. The sites at which crop plants have been recovered are mostly located within 5 km of rivers. This finding indicates that prehistoric humans actively selected settlement locations close to rivers, likely to meet water access and agricultural production needs, which disrupted a normal distribution pattern.

From a species-specific perspective, there are varying relationships in terms of significant normal distributions, elevation, and river proximity (

Table 2. p-value results of normal distribution tests for elevation and river proximity of crop remains in Shaanxi during the Early-to-Middle Neolithic period.

Crop Type	Elevation (p-Value)	River Proximity (p-Value)
Foxtail millet and broomcorn millet remains	0.0014	0.0027
Rice remains	0.0306	0.6483
Wheat and barley remains	0.4187	0.0513

). Foxtail and broomcorn millet remains have significant normal distributions with elevation (p = 0.0014 < 0.05) and river proximity (p = 0.0027 < 0.05), indicating their broad environmental adaptability and minimal dependency on specific elevation or proximity to rivers for cultivation and diffusion. Rice continues to conform to a normal distribution with elevation (p = 0.0306 < 0.05) but not with river proximity (p = 0.6483 > 0.05), indicating a strong dependence on water resources, likely due to the crop’s need for wet environments during its growth cycle [49]. Wheat and barley remains, including wheat and barley, do not conform to normal distributions in terms of either elevation (p = 0.4187 > 0.05) or river proximity (p = 0.0513 > 0.05). This may reflect the specific climatic requirements of cereal crops, such as synchronized heat and water availability during the growing season, which limited their distribution to regions with the most favorable environmental conditions [50,51].

The above results indicate that rice, wheat, and barley were primarily concentrated in the Guanzhong Plain and Qinba Mountains in southern Shaanxi, a phenomenon likely influenced by accumulated growing degree days (GDD). Based on previous studies of climatic changes during the Middle-to-Late Neolithic period in Shaanxi, the Middle Neolithic (7000–5000 BP) was characterized by a warm period, whereas the Late Neolithic (4200–4000 BP) experienced a cooling phase [19,52]. Using GDD modeling methods [53] and radiocarbon dating of foxtail millet and broomcorn millet, we selected 6500 BP as the starting point and 4000 BP as the endpoint for our analysis. Tree-ring temperature reconstructions from junipers in the Delingha region of the eastern Tibetan Plateau [54,55] revealed that temperatures between 6500 and 6300 BP and 4200 and 4000 BP exhibited similar patterns to those observed between 1900 and 2000 CE. Using the CRU TS4.05 climate dataset (https://catalogue.ceda.ac.uk/uuid/715abce1604a42f396f81db83aeb2a4b/; 25 October 2024), GDD conditions for different crops in Shaanxi were simulated and evaluated. Importantly, these simulations incorporated calibrated climate curves to account for temperature fluctuations, enabling the construction of suitability maps for foxtail millet, broomcorn millet, rice, wheat, and barley under both warm and cold scenarios (Figure 7). As shown in Figure 7, the growth ranges of foxtail millet and broomcorn millet remained largely unaffected by temperature fluctuations. In contrast, rice, wheat, and barley were consistently concentrated in the Guanzhong Plain and Qinba Mountains. During the cold period, their suitable growth zones shifted further south, corroborating evidence for the decline in the frequency of rice, wheat, and barely in the Guanzhong Plain during the Longshan period.

In conclusion, the results indicate significant differences in the distribution patterns of crops in Shaanxi during the Middle-to-Late Neolithic period. Among foxtail millet, broomcorn millet, rice, wheat, and barley, foxtail millet, and broomcorn millet exhibited notable environmental adaptability, allowing them to be widely distributed across the Northern Plateau, Guanzhong Plain, and Qinba Mountains in southern Shaanxi. In contrast, rice and cereal crops were more dependent on specific environmental conditions, primarily occurring in more suitable climatic regions such as the Guanzhong Plain and Qinba Mountains.

5. Conclusions

During the Middle-to-Late Neolithic period (7000–3800 BP), Shaanxi Province served as a central hub for the spread of foxtail millet, broomcorn millet, and rice, as well as a critical corridor for the diffusion of wheat- and barley-based agriculture. Its diverse geomorphic settings, including mountains, hills, plains, and the Loess Plateau, created varying environmental conditions that shaped crop adaptation and influenced prehistoric agricultural practices. By employing statistical analyses and spatial tools such as ArcGIS, this study provides the following key findings:

(1)

Introduction and Subsequent Domination of Specific Crops: Despite regional differences in geomorphic environments, foxtail millet and broomcorn millet consistently present as staple crops in Shaanxi during the Middle-to-Late Neolithic period. Rice was integrated into local agricultural systems c. 6000 BP, while wheat and barley were introduced c. 4000 BP. This progression resulted in a mixed farming economy that incorporated multiple crop types;

(2)

Regional Variations in Crop Utilization: Significant differences in crop utilization were observed across geomorphic regions. Foxtail millet and broomcorn millet were first adopted in the Guanzhong Plain c. 6500 BP, appearing in the archaeological record later in the Northern Plateau and Qinba Mountains. Rice, wheat, and barley were adopted later than millet and appear in the archaeological record in the Guanzhong Plain and Qinba Mountains, where water and thermal conditions were more favorable. In contrast, these crops were rarely found in the arid Northern Plateau. Spatial analyses indicated that geomorphic environments influenced the suitability of specific crops, thereby shaping the diversity of crops available to prehistoric communities;

(3)

Adaptation Strategies of Prehistoric Humans: Prehistoric humans actively adapted their subsistence strategies to different geomorphic environments. Under the influence of “food globalization,” the Northern Plateau incorporated livestock such as cattle and sheep during the Late Neolithic to enrich its millet-dominated economy. In contrast, the Guanzhong Plain and Qinba Mountains developed more diversified mixed agricultural economies, integrating rice, wheat, barley, foxtail millet, and broomcorn millet, alongside limited animal husbandry.

In summary, the distribution and utilization of prehistoric crops in Shaanxi were both diverse and regionally specific, reflecting the complex interplay between environmental conditions and human adaptation. This study highlights the significance of geomorphic settings in shaping agricultural practices and provides critical insights into prehistoric human survival strategies and the development of early civilizations.