I have seen it in several magazine photo captions and I wonder it the've put "maize" as a mistaken equivalent of "corn" (the pictures do not clearly show the plant).
Maize (Zea mays, common name "corn" in American English) is a very common commercial crop in many parts of Ethiopia. I don't know when it was first introduced (probably quite a long time ago), but it is fast replacing many traditional crops because of its higher market value.
Maize has become a staple crop in Africa in general. The Portuguese likely introduced maize to Africa in the 16th century. Not sure where you might find information on the Omo Valley, specifically.