Expansion, extension, or retreating? A meta-archaeobotany approach to patterns in crop dispersal

Abstract The spread of agriculture and its later diversification involved domesticated crops expanding their range beyond the limiting factors that restricted their wild progenitors. The growth of archaeobotanical databases in recent years allows us to frame these questions of why and how some species have been more widely or quickly spread in new, quantitative ways across a large geographical and temporal canvas, i.e. a meta-archaeobotany. The approach introduced here offers simple metrics by which patterns and rates of geographical change can be estimated and compared across crops: expansion, an estimate of total geographical range expansion is based on great-circle distance between the highest latitude/longitude and lowest latitude/longitude, and extension, the total range of degrees of latitude. Based on an Old World Crops Archaeobotanical Database (OWCAD), 12 cereals, 6 pulses, and 3 oilseeds, from across Eurasia and Africa, are compared in terms of expansion and extension patterns and rates. It is found that initial expansion involved little or no extension across latitudes, which was rather delayed, implying a need for ecological adaptation. More regionally localised crops and those which have experienced declines had slower rates of expansion and extension.