11,000-Year-Old Grains Suggest Water, Not Human Selection, First Enlarged Cereal Seeds

A new isotope study of 11,000-year-old cereal grains from the southern Levant challenges a long-standing assumption used to identify early crop domestication: that larger grain size signals the start of deliberate human selection. The research, led by Dr. Jade Whitlam of the University of Oxford with senior author Professor Cheryl Makarewicz, examined barley and emmer wheat from the Early Neolithic sites of el-Hemmeh and Sharara.

Wild cereals have a brittle "rachis" — the structure holding seeds to the ear — that lets ripe seeds scatter naturally but makes harvesting difficult for people; over time, cultivation has been thought to favor a tougher, non-shattering rachis and larger grains that were easier to collect. At el-Hemmeh and Sharara, however, the barley and emmer wheat remained morphologically wild, with no non-shattering forms, even though grain size varied widely, from wild-sized to grains comparable in size to domesticated cereals.

To find out what was driving that size variation, the researchers analyzed stable carbon isotopes preserved in the charred grains, which change according to how much moisture was available while the plant grew. Barley grains within the size range of domesticated cereals showed isotope values indicating they grew under wetter conditions, while the smaller, wild-sized grains showed isotope values indicating drier growing conditions.

Combined with archaeobotanical evidence and the ecology of associated weedy plants — which indicated the cereals grew in low-disturbance environments, not systematically tilled soil — the isotope results point to environmental conditions, not genetic selection, as the explanation for the early increase in grain size. The researchers say genetic selection for larger grains may therefore have occurred later than previously thought, possibly only after selection for the non-shattering rachis.