I found :
Coffee beans were chewed raw for centuries in Ethiopia and Yemen. Excavations in the Ethiopian highlands where coffee grows wild indicates human gathers have been eating coffee berries over a hundred thousand years. The fleshy pulp around the coffee bean in Ethiopian coffee has a high sugar content. Being sweet, being nutritious, and seeds, nuts, grapes and berries being generally eaten by humans for over a million years sort of supports this theory.
Ugandans were noticed chewing dried coffee beans when the first explorers from Europe were searching for the origin of the Nile river. Green coffee beans were ground up and mixed with fat, then made into small balls, which were eaten by travelers on long journeys. Some say this is the first trail mix.