About thirty million years ago, the area was covered by extensive rainforest. During a cooler and drier period some ten million years ago, the lowland forests were converted to savanna, leaving the mountain ranges as islands where the tropical forests continued to flourish. The long-term persistence of a humid climate and the isolation of each mountain range has led to a great deal of endemism, and a very diverse flora and fauna. The Nguru and other Eastern Arc mountains have extremely high biodiversity with numerous endemic species (more than 25 percent of the vertebrate species).