Carbon and nitrogen isotopes from a Xinjiang cemetery, set against nearly 1,500 published records from six regions, trace a synchronised dietary shift and its reversal.

What people ate thousands of years ago is readable in their bones. Carbon isotope ratios distinguish two photosynthetic pathways — C3 plants such as wheat and barley against C4 plants such as millet — because the two discriminate differently against the heavier carbon-13, and the signature passes up the food chain into collagen. Nitrogen isotopes rise with trophic level, so they track how much animal protein a diet contained.

A study in Science China Earth Sciences applies both to a cemetery in Xinjiang and then sets the result against the published record for the whole central-eastern Eurasian steppe. The pattern that emerges is a dietary shift and, unusually, its reversal.

The study was led by Dr Wei Wang of Nanjing University of Information Science and Technology and Professor Chengbang An of Fudan University, with Futao Duan of NUIST and Xuemei Yan, associate research curator at the Institute of Cultural Relics and Archaeology in Xinjiang.

One cemetery, then fifteen hundred records

The new data come from the Baiyanghe cemetery, in the northern foothills of the Tianshan Mountains, where human and animal bones spanning the Late Bronze Age to the Tang Dynasty were analysed for carbon and nitrogen isotopes alongside radiocarbon dating.

Those results were then combined with nearly 1,500 previously published isotopic records from six regions: Baiyanghe itself, the Minusinsk Basin, the Altai Mountains, central Kazakhstan, southeastern Central Asia and central Mongolia.

Pooling published data this way is what turns a site report into a regional history, and it carries a characteristic weakness worth naming. Records assembled from decades of separate studies were produced with different sampling strategies, different bone preparation methods and different burial populations, and apparent regional differences can reflect how the samples were collected as much as what the people ate.

The sequence

Three phases emerge. In the Bronze Age, from roughly 5000 to 3000 years before present, populations ate mainly C3-based foods — wheat and barley — with meat and dairy from domesticated animals.

From the Early Iron Age through the Han Dynasty, about 3000 to 2000 BP, consumption of C4 crops rose substantially, principally broomcorn millet, and animal protein intake increased alongside it.

Then between about 2000 and 1000 BP, from the Han to the Tang, millet consumption dropped significantly and diets shifted back toward C3 staples.

The reversal is the interesting half. Crop adoptions are usually described as one-way: a better or more reliable crop arrives and stays. A staple that spreads across six regions and then recedes a millennium later points at conditions that changed rather than at a discovery that was made.

Millet as a staple, and millet as a status food

The regional breakdown shows that millet did not mean the same thing everywhere. The Minusinsk Basin, southeastern Central Asia and the northern Tianshan were high-intensity zones where millet was a staple for most people. Central Kazakhstan, the Altai and central Mongolia were limited-use zones, where millet was largely restricted to elites and functioned as a status marker.

That division makes sense of how the crop travelled. Broomcorn millet is drought-tolerant and extremely fast — it can mature in sixty to ninety days — which suits it to marginal rainfall and to mobile populations who cannot wait out a long growing season. Where it could be grown in quantity it fed everyone; where it had to be traded in, it fed whoever could obtain it.

At Baiyanghe the status dimension appears directly. Individuals buried with abundant grave goods or gold artefacts generally had higher nitrogen-15 values, indicating more animal protein, though the authors note exceptions and do not treat the link as fixed.

Those exceptions deserve to be kept. Nitrogen isotope values also vary with local soil and water conditions, with the species of animal consumed, and with physiological factors including nursing and nutritional stress, so an individual at odds with the pattern is not necessarily a poor person who ate well.

What drove it

The proposed explanations differ for the two transitions. The Bronze-Age-to-Han spread of millet is attributed partly to a warm, humid climate that is likely to have increased exchange among pastoralist groups — a wetter steppe supports more livestock, more movement and more contact, and crops travel with contact.

The retreat from millet is attributed to a cooling, drying trend together with advances in grain processing technology, which is the more surprising half of the argument. Improved milling makes wheat substantially more attractive: wheat grain is hard and its bran difficult to separate, so getting flour from it requires rotary querns or mills, whereas millet can be cooked whole as porridge with minimal processing. Better mills make a harder grain usable, and a population with good milling has less reason to grow the easy crop.

Both are hypotheses fitted to a correlation between climate reconstructions and isotope trends across a 4,000-year span, and the release reports no statistical test of either. A synchronous shift across six regions is a striking observation; attributing it to specific causes is interpretation.

Two gaps in the release are worth noting. It contains no direct quotations from any researcher, so the interpretations above follow its own summary rather than the authors’ words, and it states neither funding sources nor any conflicts of interest.

The paper appeared in volume 69, issue 6 of Science China Earth Sciences, pages 2102–2117.

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Notes

1. “Diachronic dietary evolution from the Bronze Age to the Tang Dynasty in the central-eastern Eurasian steppe revealed by stable isotopes of the Baiyanghe cemetery, Xinjiang.” Science China Earth Sciences, volume 69, issue 6, 2026, pages 2102–2117. DOI: 10.1007/s11430-025-1861-y.
2. Science China Press. “4,000 years of diet on the Eurasian Steppe: climate, mobility, and milling technology drove synchronized shifts.” EurekAlert!, 10 August 2026. https://www.eurekalert.org/news-releases/1139495.

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