Archaeology is hard

Even though rice rarely survives in archaeological deposits, all plants absorb tiny amounts of silica from groundwater. The silica fills some of the plant's cells and when the plant decays it leaves microscopic cell-shaped stones, called phytoliths, in the soil. Careful study of phytoliths can reveal not just whether rice was being eaten but also whether it was domesticated.

Yan and MacNeish dug a sixteen-foot trench in Diaotonghuan cave near the Yangzi valley...

From Why the West Rules- for now