Origination of yam

The main mystery about Yam is its exact location. There are some clues in the ancient sources as to where Yam is situated. Nevertheless, these are not clear enough, and modern scholars have disagreed with each other as to how such clues should be understood. The clues for Yam’s whereabouts can be found in a tomb inscription known as the Autobiography of Harkhuf .

Artifacts dated about 50,000 BC from West Africa indicate wild yams were used for food prior to that time. The growing of yams appears to have developed independently in two regions, in West Africa and Southeast Asia about 3000 BC. The present-day D. bulbifera, which is grown for the aerial tubers, is indigenous to both regions.

There are two centres of yam cultivation worldwide. The first is the high rainfall region of western Africa, from the Ivory Coast to Cameroon, while the second centre is Vietnam, Cambodia, Laos and neighbouring regions where the most commonly cultivated species is the Asiatic yam (D. Alata). Secondary arears of yam cultivation are the West Indies, Pacific Islands, and Southeastern United States from Louisiana to Georgia. Only the cush-cush yam (D. Trifida), is native of the New World

Yams (family Dioscoreaceae) are native to Africa and Asia and other tropical regions. Yams are starchy tubers that have an almost black bark-like skin and white, purple or reddish flesh and come in many varieties. The tubers can be as small as regular potatoes or grow upwards of five feet long.

Today yams are widely used as an important food  in Africa, Asia, the Caribbean, Pacific islands, and South America. 96 %, most yams are derived from West Africa, with Nigeria alone producing 71 % of that total, Côte d’Ivoire 8.1%; Benin 4.3% and Ghana 3.5% In the humid tropical countries of West Africa yams are one of the most highly regarded food products and are closely integrated into the social, cultural, economic and religious aspects of life.
There are many varieties of yam species widespread throughout the humid tropics but the edible yams are derived mainly from about ten. The most economically important species are:
·         White yam (Dioscorea rotundata Poir). Originated in Africa and is the most widely grown and preferred yam species. The tuber is roughly cylindrical in shape, the skin is smooth and brown and the flesh usually white and firm. A large number of white yam cultivars exist with differences in their production and post-harvest characteristics. 
·         Yellow yam (Dioscorea cayenensis Lam.). Derives its common name from its yellow flesh, which is caused by the presence of carotenoids. It is also native to West Africa and very similar to the white yam in appearance. Apart from some morphological differences (the tuber skin is firm and less extensively grooved), the yellow yam has a longer period of vegetation and a shorter dormancy than white yam. 
·         Water yam (Dioscorea alata L.) Originates from South East Asia, it is the species most widely spread throughout the world and in Africa is second only to white yam in popularity. The tuber shape is generally cylindrical, but can be extremely variable. Tuber flesh is white and “watery” in texture. 
·         Bitter yam (Dioscorea dumetorum). Also called trifoliate yam because of its leaves. Originates in Africa where wild cultivars also exist. One marked characteristic of the bitter yam is the bitter flavour of its tubers. Another undesired characteristic is that the flesh hardens if not cooked soon after harvest. Some wild cultivars are highly poisonous.

https://link.springer.com/referenceworkentry/10.1007%2F978-1-4419-0465-2_2193
http://science.jrank.org/pages/7437/Yam.html

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