Cajanus cajan (L.) Millsp. origins and domestication: the South and Southeast Asian archaeobotanical evidence

Cajanus cajan (L.) Millsp. commonly known as pigeonpea, red gram or gungo pea is an important grain legume crop, particularly in rain-fed agricultural regions in the semi-arid tropics, including Asia, Africa and the Caribbean. This paper provides a baseline for the study of the domestication and early history of C. cajan, through reviewing its modern wild distribution, seed morphometrics of wild and domesticated populations, historical linguistics and the archaeological record. The distribution of wild populations, including published records and additional herbarium collections, suggest that the wild habitats of pigeonpea were at the interface of the forest-edge areas and more open savanna plains in eastern Peninsular India (e.g. Telangana, Chattisgarh, Odisha). Early archaeological finds presented here have been recovered from both the Southern peninsula and Odisha. Historical linguistic data suggests early differentiation into longer and shorter growing season varieties, namely arhar and tuar types, in prehistory. Pigeonpea had spread to Thailand more than 2000 years ago. Measurements of seeds from modern populations provide a baseline for studying domestication from archaeological seeds. Available measurements taken on archaeological Cajanus spp. suggest that all archaeological collections thus far fall into a domesticated Length:Width ratio, while they may also pick up the very end of the trend towards evolution of larger size (the end of the domestication episode) between 3700 and 3200 years BP. This suggests a trend over time indicating selection under domestication had begun before 3700 years ago and can be inferred to have started 5000–4500 years ago.