Developmental plasticity and genetic selection shaped cereal evolution in the Early Holocene southern Levant

Abstract

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 distinguish them from their wild counterparts. Current models of plant domestication emphasise 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 non-shattering rachis and increased grain size, two traits that can be tracked directly in the archaeobotanical record. Measurements of cereal grains from Early Neolithic sites indicate that grain size increase occurred prior to the evolution of non-shattering rachis, and it has been proposed that this reflects selection for larger grains under tillage, signifying pre-domestication cultivation. Here we combine morphological and metrical analysis 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 is better understood as a plastic response to variation in growing conditions (specifically moisture), rather than a result of genetic selection for increased grain size under cultivation (i.e., tillage). We argue that cereal evolution in southwest Asia was initially driven by developmental plasticity, followed by genetic selection. Significance StatementCereals were amongst the earliest crops domesticated in southwest Asia and remain central to modern agriculture. We can track cereal domestication archaeologically using a suite of morphological traits, which distinguish domestic species from their wild counterparts. The process by which these traits evolved is still not fully understood but has widely been framed as genetic adaptation to cultivation. Here we seek to disentangle how two key domestication traits relating to seed dispersal and size evolved. By taking a multi-stranded approach to new archaeobotanical evidence, we demonstrate that in the Early Holocene southern Levant, cereal domestication occurred in a sequence where developmental plasticity preceded genetic selection.