Letter A represents where it is believed that the earliest humans lived, as this is the region of Africa where evidence of the oldest known human fossils and stone tools have been found. This area, known as the Great Rift Valley, spans several countries including Ethiopia, Kenya, and Tanzania. The early humans who lived here were likely part of the genus Homo, and they evolved and migrated out of Africa to colonize the rest of the world over the course of thousands of years.