The common bean (Phaseolus vulgaris L.) is one of the most important domestic legumes in the world, because of its high concentrations of protein, fiber and complex carbohydrates. The global harvest today has been estimated at ~18.7 million tons and it is grown in nearly 150 countries on an estimated 27.7 million hectares. Beans are one of the "three sisters" of traditional agricultural cropping methods reported by European colonists in North America: Native Americans planted maize, squash and beans in the same place, providing a healthful and environmentally sound way of capitalizing on their various characteristics.
Beans come in a wide variety of shapes, sizes, and colors, from pinto to pink to black to white. Interestingly enough, despite this diversity in color and size, the wild and domestic beans belong to the same species, as do all of the colorful varieties ("landraces") of beans, which are believed to be the result of a mixture of population bottlenecks and purposeful selection.
The main difference between wild and cultivated beans are, well, domestic beans are less exciting. Of course, there is a significant increase in seed weight, and the seed pods are less likely to shatter than wild forms: but the primary change is a decrease in variability of grain size, seed coat thickness and water intake during cooking. Domestic plants are also annuals rather than perennials, increasing their reliability. The domestic bean is much more, well, predictable.
Two Centers Of Domestication?
Scholarly research indicates that beans were domesticated in two places: the Andes mountains of Peru, and the Lerma-Santiago basin of Mexico. The wild common bean grows today in the Andes and Guatemala: two separate large gene pools of the wild types have been identified, based on the variation in the type of phaseolin (seed protein) in the seed, DNA marker diversity, mitochondrial DNA variation and amplified fragment length polymorphism, and short sequence repeats marker data.
The Middle American gene pool extends from Mexico through Central America and into Venezuela; the Andean gene pool is found from southern Peru to northwestern Argentina. The two gene pools diverged some 11,000 years ago. In general, Mesoamerican seeds are small (<25 grams per 100 seed weight) or medium (25-40 gm/100 seed weight), with one type of phaseolin, the major seed-storage protein of the common bean. The Andean form has much larger seeds (>40 gm/100 seed weight), with a different type phaseolin.
Recognized landraces in Mesaomerica include Jalisco (coastal Mexico near Jalisco state) and Durango (central Mexican highlands, which includes pinto, great northern, small red and pink beans) and the Mesoamerican (lowland tropical Central American, which includes black, navy and small white). Andean cultivars have larger seeds, and include the Peruvian (Peruvian highlands), Chilean (northern Chile and Argentina) and Nueva Granada (Colombia) varieties, which last includes the commercial forms of dark and light red kidney, white kidney, and cranberry beans.
Origins in Mesoamerica
In March 2012, work by a group of geneticists led by Roberto Papa was published in the Proceedings of the National Academy of Sciences (Bitocchi et al. 2012), making an argument for a Mesoamerican origin of all beans. Papa and colleagues examined the nucleotide diversity for five different genes found in all forms--wild and domesticated, and including examples from the Andes, Mesoamerica and an intermediary location between Peru and Ecuador--and looked at the geographic distribution of the genes.
This study suggests that the wild form spread from Mesoamerica, into Ecuador and Columbia and then into the Andes, where a severe bottleneck reduced the gene diversity, at some time before domestication. Domestication later took place in the Andes and in Mesoamerica, independently. The main importance to the original location of beans is the wild adaptability of the original plant, which allowed it to move into a wide variety of climatic regimes, from the lowland tropics of Mesoamerica into the Andean highlands.
Dating the Domestication
While the exact date of domestication for beans has not yet been determined, wild landraces have been discovered in archaeological sites dated to 10,000 years ago in Argentina and 7,000 years ago in Mexico. In Mesoamerica, the earliest cultivation of domestic common beans occurred before ~2500 in the Tehuacan valley (at Coxcatlan), 1300 BP in Tamaulipas (at (Romero's and Valenzuela's Caves near Ocampo), 2100 BP in the Oaxaca valley (at Guila Naquitz). Starch grains from Phaseolus were recovered from human teeth from Las Pircas phase sites in Andean Peru dated between ~6970-8210 RCYBP (about 7800-9600 calendar years before the present).
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