Peanut

The peanut (*Arachis hypogaea*) is a legume grown for its edible seeds, which form in pods underground. It goes by many names, including groundnut, goober, goober pea, pindar, and monkey nut. Farmers in tropical and subtropical regions cultivate it on both small and large scales, using it as a grain legume and for oil production. Its unusual habit of fruiting underground—called geocarpy—prompted botanist Carl Linnaeus to give it the species name *hypogaea*, from Greek for "under the earth." The plant is part of the Fabaceae family, also known as the legume or pea family, and is native to South America. Like most legumes, peanuts host nitrogen-fixing bacteria in their root nodules, which enriches the soil and makes them useful in crop rotations.

Edible products from peanuts include oil and peanut butter. Peanut sauces appear in Latin American and Southeast Asian cuisines, while in the Indian subcontinent, peanuts are added to salads and stews. In North America, they go into candies, cakes, and cookies, and peanut butter is widely consumed. Some people have severe, sometimes fatal, allergic reactions to peanuts. The plant tops can be used for silage, and the oilcake meal serves as animal feed or fertilizer. Industrially, peanut oil is used in paint, varnish, and furniture polish.

Botanically, *Arachis hypogaea* is an annual herb that grows 30 to 50 centimeters tall. Its leaves are opposite and pinnate, with four leaflets arranged in two opposite pairs and no terminal leaflet. Each leaflet ranges from 1 to 7 centimeters long and 1 to 3 centimeters wide. Like many legumes, the leaves are nyctinastic—they close at night. The flowers are yellowish orange with reddish veining, measuring 1 to 1.5 centimeters across, and grow in axillary clusters on the stems. After fertilization, a stalk at the base of the ovary, called a gynophore or "peg," elongates and pushes into the soil, allowing the fruit to develop underground. These pods, technically legumes, are 3 to 7 centimeters long and usually contain one to four seeds. The shell is mostly mesocarp with several large veins running its length. Despite the name, peanuts are not true nuts, which are fruits with a hard ovary wall. Peanuts contain polyphenols, polyunsaturated and monounsaturated fats, phytosterols, and dietary fiber in amounts similar to tree nuts, and their skins contain resveratrol.

The *Arachis* genus originated in South America, east of the Andes, in regions that include Peru, Bolivia, Argentina, and Brazil. Cultivated peanuts arose from a hybrid between two wild species, likely *A. duranensis* and *A. ipaensis*. The initial hybrid was sterile, but spontaneous chromosome doubling restored fertility, creating an amphidiploid or allotetraploid. Genetic analysis suggests this hybridization happened only once, giving rise to *A. monticola*, a wild form found in limited areas of northwestern Argentina or southeastern Bolivia. From there, artificial selection produced *A. hypogaea*. Domesticated plants are bushier, more compact, and have different pod structures and larger seeds than their wild relatives. Cultivation spread from this center of origin, forming secondary and tertiary diversity centers in Peru, Ecuador, Brazil, Paraguay, and Uruguay. Over time, thousands of landraces evolved, classified into six botanical varieties and two subspecies. Subspecies *A. h. fastigiata* types are upright with shorter crop cycles, while *A. h. hypogaea* types spread along the ground and have longer cycles.

The oldest known archaeological peanut remains—pods about 7,600 years old—were found in Peru, where dry conditions preserved them. These may represent a wild species under cultivation or an early domesticated form. Peanut cultivation likely predates this in the moister center of origin. Pre-Columbian cultures, including the Moche, depicted peanuts in their art. By the time Spanish conquistadors arrived, peanuts were well-established in Mesoamerica, sold in the marketplace of Tenochtitlan under the Nahuatl name *tlālcacahuatl* (giving Spanish *cacahuate*). Cultivation reached Europe in the 19th century via Spain, especially Valencia, where it is still grown on a small scale. European traders then spread peanuts worldwide, and they are now widely cultivated in tropical and subtropical regions. In West Africa, peanuts largely replaced the Bambara groundnut, a related legume with underground pods. Peanuts were introduced to the United States during the colonial period as a garden crop. From 1870 onward, they were used as animal feed until human consumption grew in the 1930s. George Washington Carver promoted the peanut through agricultural extension efforts in the American South.