The second largest continent, Africa is dominated in the north by the vast Sahara desert and in the east by the Great Rift Valley. Most of Africa is a high plateau covered with deserts, lush rainforest and dry grassland. Africa contains the world's longest river—the 6650km-long Nile River running from Burundi to Egypt. Modern humans, Homo sapiens, are believed to have originated in East Africa somewhere between Ethiopia and Kenya. Africa is the second most populous content, home to roughly 12 percent of all humans or 840 million people. Cairo is the largest city with over nine million inhabitants. Africa has eight percent of the world's oil reserves and produces nearly 50 percent of the gold in the world and also produces 50 percent of the diamonds in the world. Arabs put their mark on Africa starting in the seventh century A.D. and spread Islam throughout the country. In the eighteenth and nineteenth centuries, European powers began to dominate and eventually colonize much of Africa. In the mid to late twentieth century, most African nations gained independence and is currently considered the poorest continent on Earth.