Don McCullin is one of the great photojournalists of our times. In recent years, his travels have taken him to some of the world's remotest regions, and his skill and empathy have gained him access to tribes on the edge of civilization. Over the last two years, he has traveled from Addis Ababa in Ethiopia to the valley of the Omo River leading to the border with Sudan. There he entered the tribal lands of the Suma, the Gheleb, the Bume, the Erbore,...