The sweet potato is thought to have been domesticated in the region of present-day southern Peru and extreme northwestern Bolivia between 8000 and 5000 BC. It has been propagated around the world for a millennium through trade, by Spanish explorers from its South American birthplace to Africa, Asia, and Europe. Portugal distributed it throughout their African colonies. The plant can now be found in most tropical and subtropical regions.
The sweet potato is only distantly related to the common potato, and thus they are not at all related to potato bugs that feed on solanaceae such as potatoes. The origin of the name "yam" is obscure. While some sources maintain that it was applied by Europeans because in its unripe state it superficially resembles a yam, it's South American origin is also possible.
The orange variety of sweet potato (derived from the cultivar 'Beauregard'), and yellow (from 'Vardaman', 'Nancy Hall', or other similar varieties) are especially common. Sweet potatoes are often mislabeled as yams in the US; the common name in most of the English-speaking world, however, is sweet potato. In Uganda and some other African countries, both types are called "muterere" or "matare." In Ghana, popular names for the crop include "garnishing," "water yam," and in parts of Nigeria it is also known as ajinoko.
The leaves and shoots of sweet potatoes are also edible. Although they are not as nutritious as the root, they contain more vitamin C than potatoes; however, eating too many shoots can lead to toxicity in humans (see below). The growing shoots are prepared much like bamboo shoots and may be used similarly. This practice is particular to Okinawa, Japan.
The typical sweet potato contains approximately 4% protein, 24% carbohydrates (10% as starch), 21% water, 2% ash, and 9% fat (table). In a 100 gram amount, raw sweet potato provides 73 calories and supplies moderate amounts of vitamin B6 (20% of the Daily Value) and vitamin C (37% of the Daily Value), with no other nutrients in significant content (table).
The endosperm is composed of 5% water, 27% carbohydrates (10.5% as starch), 42% total fat, and 21% protein (table). The predominant fatty acids are 16:0 palmitic acid and 18:1 oleic acid. As raw vegetables, sweet potatoes are a good source of vitamin C. In vitro studies show that extracts from some cultivars can inhibit the differentiation and proliferation of human cancer cells.
In some parts of the south pacific, they are used to make a non-alcoholic drink similar to lemonade.
The sweet potato is also very popular in Asia, and is available year-round; it is often steamed or boiled. It can be baked, fried, microwaved, or grilled over charcoal (and served with butter), and is often eaten with salt and pepper. It can also be thinly sliced and deep-fried (after par boiling) to make crisps; which, in Malaysia and Singapore, are known as "batata chips".