Phytoliths reveal the earliest interplay of rice and broomcorn millet at the site of Shuangdun (ca. 7.3–6.8 ka BP) in the middle Huai River valley, China

The middle Huai River valley, located in the climatic, cultural and agricultural transitional zone in the central-eastern China during the Neolithic, occupies an important position in the study of the origins, development and spread of rice and millet farming in China. Previous studies indicate that rice agriculture has extended to the middle Huai River valley since 8.5 ka BP, and rice domestication was in the early stage between 8.5 and 7.5 ka BP, while the earliest evidence of millet remains discovered in this region was as late as ca. 5.0 ka BP. However, the process of rice domestication after 7.5 ka BP in this region, and when and where foxtail millet and broomcorn millet first extended respectively into this valley and interplayed with rice during the Neolithic period are still unknown. In this study, phytolith analysis of the soil samples from the Shuangdun site during the archaeological excavation, which is a representative site of the Shuangdun Culture in the middle Huai River valley dating back to 7.3–6.8 ka BP, reveals evidence of crops in the middle Huai River valley, China. Our results show that rice with japonica characteristics was the dominant crop at the Shuangdun site, which had a higher domestication rate than that at the Shunshanji site during 8.5–7.5 ka BP in terms of the morphological and morphometric analyses of rice bulliform and double-peaked glume cells, while broomcorn millet only occupied a small proportion of the total amount based on analysis of the quantity and ubiquity of phytoliths. Our data extend the record of broomcorn millet use in the middle Huai River valley by nearly 2000 years. The analysis in this paper not only provides significant evidence for illustrating the transformation of crop structure and the domestication process of rice in the middle Huai River valley, but also brings some clues for mapping the spatiotemporal route for the spread of rice and millets in central-eastern China during the Neolithic.