Developmental plasticity under human management shaped cereal evolution prior to domestication in the Early Holocene...

The domestication of plants in southwest Asia was an evolutionary process that took place over several millennia in the Early Holocene. During this time, domestic species developed distinct traits that distinguished them from their wild counterparts. Current models of plant domestication emphasize the role of genetic selection in the evolution of these traits, viewing these as heritable adaptations that arose in response to selective pressures associated with human cultivation. In cereals, domestication resulted in the evolution of nonshattering rachis and increased grain size, two traits that can be tracked directly in the archaeobotanical record. Analyses of Early Neolithic cereal remains demonstrate that grain size increase occurred prior to the evolution of nonshattering rachis, a sequence that is often interpreted as evidence of selection for larger grains under cultivation, specifically tillage. Here, we combine morphological and metrical analyses of cereal remains, stable carbon isotope analysis, and weed ecology to test this hypothesis, using three assemblages from the southern Levant: Pre-Pottery Neolithic A Sharara (c. 9250-9200 cal BCE), Pre-Pottery Neolithic A el-Hemmeh (c. 9400-8700 cal BCE), and Late Pre-Pottery Neolithic B el-Hemmeh (c. 7500-7000 cal BCE). Our findings indicate that increased grain size in the Early Holocene can be better understood as a plastic response to variation in growing conditions, specifically moisture, rather than as a result of genetic selection for increased grain size under cultivation. We propose that cereal evolution in southwest Asia was initially driven by developmental plasticity, followed by genetic selection.