A new high-resolution climate study from China's Loess Plateau suggests that soil temperature, not just air temperature or rainfall, played a decisive role in shaping the spread of millet farming during the Neolithic period.
Published earlier this month in the journal Proceedings of the National Academy of Sciences, the study offers new insight into a longstanding question surrounding early Chinese civilization: why millet agriculture, which emerged about 8,000 years ago, remained limited for nearly two millennia before expanding rapidly around 6,000 years ago.
Researchers from the Institute of Earth Environment of the Chinese Academy of Sciences, in collaboration with Lanzhou University, the Institute of Geology and Geophysics, the British Museum, Washington University, and South Korea's Institute for Basic Science, reconstructed soil temperature fluctuations from 12,300 to 2,800 years ago at Longgugou, a fossil site with a typical loess deposit profile (4 meters deep), near Qingyang in the central Loess Plateau.
Using 18 optically stimulated luminescence dates and 14 bulk organic carbon-14 dates combined with biomarker analysis, the team produced the first high-resolution continuous soil temperature record yet available for East Asia.
According to the study, early Chinese civilization experienced a notable slowdown between its origins around 8,000 years ago and its acceleration about 6,000 years ago. Millet farming followed a similar pattern.
The researchers noted that millet agriculture became an important source of subsistence in the Yanshan-Liaoning region around 8,000 years ago, but it did not expand rapidly until roughly 6,000 years ago, when its center shifted toward the Loess Plateau.
The slowdown puzzled researchers because both temperature and precipitation in northern China were relatively favorable during that period, and the environmental engine behind it remains unclear.
The newly reconstructed record appears to resolve that paradox. Between 8,000 and 7,500 years ago, relatively warm soil conditions created favorable environments for millet cultivation in the Yanshan-Liaoning region, a semiarid area spanning parts of present-day Liaoning and Hebei provinces and the Inner Mongolia autonomous region. In those conditions, communities facing subsistence pressure increasingly adopted millet farming, while populations on the Loess Plateau still relied largely on hunting.
Then came a critical shift. From 7,500 to 6,000 years ago, expanding vegetation cover and soil moisture significantly lowered soil temperatures across the Yanshan-Liaoning region. Simulation models of millet habitat suitability conducted for the study indicate that large areas gradually became unsuitable for millet cultivation during this period. As a result, the center of millet farming shifted southward to the Loess Plateau, where soil temperatures remained warmer and more suitable for agriculture.
Afterward, around 6,000 years ago, a return to relatively warm and stable soil temperatures, coupled with reduced vegetation cover, triggered the rapid expansion and intensification of millet farming across northern China. This agricultural boom, the researchers argue, laid the economic foundation for the flourishing of late Neolithic cultures in the region.
Millet agriculture, dominated by foxtail millet and broomcorn millet, known in Chinese as su and shu, is widely recognized as the primary economic basis for the emergence and early development of Chinese civilization.
The findings may also contribute to broader research tracing the origins of Chinese civilization. By establishing a direct link between subsurface thermal conditions and agricultural decision-making in prehistory, the study adds a new dimension to paleoenvironmental reconstruction, an area that has traditionally focused more heavily on air temperature and monsoon rainfall.
