Variability and preservation biases in the archaeobotanical record of Eleusine coracana (finger millet): evidence from Iron Age Kenya

Eleusine coracana (finger millet) is a nutritious and easily storable grain that can be grown in unfavourable environments and is important to the food security of millions of farmers in Africa and South Asia. Despite its importance and promise as a sustainable crop for smallholders in the Global South, its history remains poorly understood. Eleusine coracana has only rarely been recovered from archaeological sites in the region of Africa where it was domesticated and never in quantities large enough to study its evolution under cultivation. Here we report on a large assemblage of Iron Age (ca. 900–700 cal bp) E. coracana grains recovered from Kakapel rock shelter in western Kenya. We also carried out carbonization experiments on modern grains in order to directly compare these archaeological specimens to extant landraces. We found that finger millet is only well preserved when carbonized at temperatures lower than 220 °C, which may contribute to its scarcity in the archaeological record. Eleusine coracana shrinks but does not significantly change shape when carbonized. When corrected for the effects of carbonization, the E. coracana grown by Iron Age farmers at Kakapel was smaller grained than modern landraces, but is nonetheless identifiable as domesticated on the basis of grain shape and surface texture. A comparison with other Iron Age E. coracana reveals considerable variation in the grain size of landraces cultivated during this era. This is the largest quantitative morphometric analysis of E. coracana grains ever conducted, and provides a basis for the interpretation of other archaeological populations. This assemblage is also the first evidence for E. coracana cultivation in western Kenya, a biodiversity hotspot for landraces of this crop today.