The systematic and economic botany of American beans is discussed. Four species have been important food plants the main dietary role of which has been as a complementary ammo acid source in combination with corn. Beans were prominent among agricultural products cited in tribute lists in pre-Hispanic times.
Some important morphological features distinguishing the domesticates from the wild species are: increase in seed size; decrease in impermeability of seeds to water intake; reduction in fleshiness of the root system and loss of perennialism; reduction in shattering of the pods and violent seed dissemination. For the most part, the archeological materials now available do not document these changes. Excavations in the Pacific highlands and coastal areas of Mexico may be expected to yield transitional bean materials.
Archeological bean distributions do show that, unlike maize, varietal characteristics of beans have remained remarkably stable from their earliest records to their most recent. P. vulgaris, the common bean, was domesticated in Mexico by 7000 years ago; I’. coccineus, the runner bean, by 2200 years ago. P. acutifolius var. latifolius, the tepary, by 5000 years ago; P. lunatus, the sievas, or small Limas by 1400–1800 years ago, the big Limas by 5300 years ago in Peru. The present distribution of the tepary is a much contracted relic one. The tepary has been largely replaced by common beans. This process of replacement continues at the present time. Mexican sievas and Peruvian big Limas are separated throughout their archeological records, a fact that adds weight to the view that these are independently domesticated conspecific geographic races.
In the Tehuacan Valley, some kinds of beans and corn may have been domesticated in association with one another. Elsewhere, early records of corn and beans do not coincide. Although beans are pre-ceramic in the Southwest, Tamaulipas, Tehuacan and Coastal Peru, they did not become abundant in the Southwest and Middle America until agriculture was well established for some time.