I happened to find a site where you can put qeustions to an expert on yams.
I asked from where the orignate, and if they are edible raw.
Here is the answear I got:

Dear Mr. Hans Kylberg:

Artifacts from West Africa indicate that wild yams were used for food there
prior to 50,000 BC.

The alkaloid, dioscorine, is contained in some species of yam making some,
like "D. hispida" poisonous raw.  Before use, the tuber is boiled so that
the alkaloid can be leached from the tuber.

You may find this book as a helpful reference:  "World Vegetables" by Mas
Yamaguchi an AVI book published in 1983 by Van Nostrand Reinhold Company,
New Yory, NY.
        Thank you for your interest!
        Phil Loretan, Tuskegee University,Tuskegee, AL 36088