The findings show agriculture took almost 12,000 years to emerge from ancient traditions of plant use, Li Liu of Stanford University and colleagues said.
They studied three grinding stones from China's Yellow River region displaying residues showing they were used to process millet and other grains, but the stones date from 23,000 to 19,500 years ago, late in the last ice age, while the earliest archaeological evidence for crop cultivation in China is 11,000 years old, NewScientist.com reported.
Other researchers say the findings support studies that seems to indicate a worldwide pattern of a delay between human consumption of grains and efforts to domesticate and farm them.
In the Middle East there is also "evidence of cereals at that 23,000-year point," Robin Allaby of the University of Warwick in Britain said -- long before people were farming them.
"Although this period is around the late glacial maximum, there is a blip at 23,000 years during which time it was milder."
Millet and the other food plants could have flourished in the warmth, tempting people to start exploiting them as a food source, Allaby said.