Who discovered East Africa?

Johannes Rebmann, (born Jan. 16, 1820, Gerlingen, Württemberg—died Oct. 4, 1876, Korntal, near Stuttgart), German missionary and explorer, the first European to penetrate Africa from its Indian Ocean coast.

How did he discover? I mean separating from West to East or choosing countries on which side?

In 1885, European leaders met at the infamous Berlin Conference to divide Africa and arbitrarily draw up borders that exist to this day. The map on the wall in the Reich Chancellery in Berlin was five meters (16.4 feet) tall. It showed Africa with rivers, lakes, a few place names and many white spots.